Countermeasures to Protect Bridge Piers from Scour (2007)

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11 2.1 Review of Current Practice 2.1.1 Introduction Under NCHRP Project 24-07 (Phase 1), Parker et al. (1998) provided a review of the literature on pier scour and the state-of-the-practice knowledge on the design and place- ment of countermeasures around bridge piers to minimize scour. This section provides a review of the results of addi- tional research that has been conducted since the Phase 1 review (post-1995). References are cited in Chapter 5 and a complete bibliography is provided in Appendix A. In addition, a thorough review was conducted of current practice, performance data, research findings, and other in- formation related to pier scour countermeasures, in general, with specific emphasis on riprap, partially grouted riprap, ar- ticulating concrete block systems, gabions, grout-filled bags and mattresses, geotextile sand containers, countermeasure filters, and the use of combinations of countermeasures. An example of a countermeasure combination is the use of grout-filled tubes (similar to those featured in Design Guide- line 7, HEC-23 [Lagasse et al. 2001]) to seal the interface between a pier and an articulating concrete block counter- measure (similar to that described in Design Guideline 4, HEC-23). An alternative would be to consider the use of an articulating grout-filled mattress (as described in Design Guideline 5, HEC-23). This information was assembled from technical literature and from unpublished experiences of engineers, bridge owners, and others. This review included a detailed analysis of the testing, evaluation, and results of NCHRP Project 24-07 as discussed in Section 1.2.2. The review also included all the material ob- tained by the scanning review team in 1998, some (but not all) of which is discussed in NCHRP Research Results Digest 241 (Lagasse 1999). The foreign literature was reviewed as well, particularly guidelines, specifications, laboratory test- ing, and field evaluation results from Germany, as outlined in Section 1.2.3. 2.1.2 Scour at Bridge Piers The basic mechanism causing local scour at piers is the for- mation of vortices (known as the horseshoe vortex) at their base (Figure 2.1). The horseshoe vortex results from the pileup of water on the upstream surface of the obstruction and subsequent acceleration of the flow around the nose of the pier or abutment. The action of the vortex removes bed material from around the base of the obstruction. The trans- port rate of sediment away from the base region is greater than the transport rate into the region, and, consequently, a scour hole develops. As the depth of scour increases, the strength of the horseshoe vortex is reduced, thereby reducing the trans- port rate from the base region. Eventually, for live-bed local scour, equilibrium is re-established between bed material in- flow and outflow and scouring ceases. For clear-water scour, scouring ceases when the shear stress caused by the horseshoe vortex equals the critical shear stress of the sediment particles at the bottom of the scour hole (Richardson and Davis 2001). In addition to the horseshoe vortex around the base of a pier, there are vertical vortices downstream of the pier called the wake vortex (Figure 2.1). Both the horseshoe and wake vortices remove material from the pier base region. However, the intensity of wake vortices diminishes rapidly as the distance downstream of the pier increases. Therefore, immediately downstream of a long pier there is often deposition of material. Factors that affect the magnitude of local scour depth at piers and abutments are (1) velocity of the approach flow, (2) depth of flow, (3) width of the pier, (4) length of the pier if skewed to flow, (5) size and gradation of bed material, (6) angle of attack of the approach flow to the pier, (7) shape of the pier, (8) bed configuration, and (9) ice formation or jams and debris. Parker et al. (1998) provided a thorough review of the lit- erature on the existing knowledge of scour around bridge piers. In addition to this review, comprehensive reviews on the causes and effects of pier scour are provided by the Cen- tre for Civil Engineering Research and Codes (CUR) (1995), C H A P T E R 2 Findings

Lauchlan (1999), Melville and Coleman (2000), Richardson and Davis (2001), and Richardson et al. (2001). Melville and Coleman (2000) have compiled a summary table showing 20 different pier scour equations. 2.1.3 Riprap as a Pier Scour Countermeasure An extensive review of experiments, model studies, and laboratory tests conducted prior to 1996 on the use of riprap as a scour countermeasure around bridge piers is provided in Parker et al. (1998). However, most of the research, model studies, and laboratory tests were conducted at small scales using clear-water conditions. The ratio of the typical riprap size to the bed sediment size was also considerably smaller than that found under field conditions. Additionally, very few of these studies provided practical guidelines for the de- sign and placement of riprap around bridge piers. Typically riprap used for pier scour protection is placed on the surface of the channel bed (Figure 2.2a), in a pre-existing scour hole, or in a hole excavated around the pier (Fig- ure 2.2b). However, recent studies as described in the follow- ing sections, recommend placing the riprap layer at depth below the average bed level (Figure 2.2c). Subsequent to the Phase 1 review, additional studies were conducted under both clear-water and live-bed conditions and added a wealth of information on the causes of riprap failure. Most of these studies modeled live-bed conditions, because a live-bed condition with the presence of mobile bed forms is very likely to occur during floods. Many of these studies provide guidelines on the stone size, placement, thickness, coverage, and filter requirements for installation of riprap layers around bridge piers based on additional labora- tory experiments. Modes of Pier Riprap Failure Most of the early work on the stability of pier riprap is based on the size of the riprap stones and their ability to withstand high approach velocities and buoyant forces. Parola (1995) notes that secondary currents induced by bridge piers cause high local boundary shear stresses, high local seepage gradients, and sediment diversion from the streambed surrounding the pier, and that the addition of riprap also changes the boundary stresses. Due to the sen- sitivity of riprap size to velocity, Parola (1995) recom- mends that the stone size should be based on an acceptable flood level that would initiate riprap instability and that stone size should be determined for plane bed conditions, which were the most severe conditions found in model studies to that point. 12 Figure 2.1. Schematic representation of scour at a cylindrical pier. PIER RIPRAPCHANNEL BED (a) Surface Placement PIER RIPRAP CHANNEL BED (b) Excavated or Scour Hole Placement PIER RIPRAP AVERAGE BED LEVEL (c) Placement at Depth PLACEMENT DEPTH (Y) COVERAGE (C) THICKNESS (t) Figure 2.2. Typical pier riprap configurations.

However, a subsequent study of the causes of riprap failure at model bridge piers conducted by Chiew (1995) under clear-water conditions with gradually increasing approach flow velocities defined three modes of failure: • Riprap shear failure, whereby the riprap stones cannot withstand the downflow and horseshoe vortex associated with the pier scour mechanism. • Winnowing failure, whereby the underlying finer bed material is removed through voids or interstices in the riprap layer. • Edge failure, whereby instability at the edge of the coarse riprap layer and the bed sediment initiates a scour hole beginning at the perimeter and working inward until it ultimately destabilizes the entire layer. Because live-bed conditions are more likely to occur during flood flows, Lim and Chiew (1996) conducted experiments to evaluate the stability of pier riprap under live-bed conditions with migrating bed forms. Subsequent research conducted by Melville et al. (1997), Lim and Chiew (1997, 2001), Parker et al. (1998), Lauchlan (1999), Chiew and Lim (2000), and Lauchlan and Melville (2001) indicates that bed-form under- mining is the controlling failure mechanism at bridge piers on rivers with mobile bed forms, especially sand bed rivers. The most important factors affecting the stability of the riprap layer under live-bed conditions were the turbulent flow field around the pier and the fluctuations of the bed level caused by migrating bed forms (e.g., dunes) past the pier. Lim and Chiew (1996) find that the three failure modes defined by Chiew (1995) under clear-water conditions also exist under live-bed conditions and that they may act independently or jointly with migrating bed forms to destabilize the riprap layer. Once sediment transport starts and bed forms associated with the lower flow regime (i.e., ripples and dunes) begin to form, the movement of sediments at the edge of the riprap layer removes the support of the edge stones and allows the edge stones to be entrained in the flow (Lim and Chiew 1996). When the trough of a bed feature migrates past the riprap layer, stones slide into the trough, causing the riprap layer to thin. Depending on the thickness of the remaining riprap layer following stone sliding and layer thinning, winnowing may occur as a result of exposure of the underlying fine sedi- ments to the flow. Winnowing can cause the entire remaining riprap layer to subside into the bed. With thicker riprap layers winnowing is not a factor and there is no subsidence. Once the bed feature passes, the riprap layer may become buried, with the maximum depth of burial being dependent on the maximum size of the dunes. Thus, the maximum riprap scour level is closely related to the maximum scour depth (for a given flow), which is the sum of the equilibrium scour depth and the additional bed lowering contributed by the bed forms. The implication of Lim and Chiew’s (1996) work is that a riprap layer cannot offer any resistance against scour when large bed forms are present. Chiew (1995) shows that, under steady flow conditions, the inherent flexibility of a riprap layer can provide a self- healing process. As scour occurs and sediment is removed from around the riprap layer through the three modes of erosion described previously, the riprap layer, if it has suffi- cient thickness, can adjust itself to the mobile channel bed and remain relatively intact while providing continued scour protection for the pier. When flow velocity is steadily increased, Lim and Chiew (1997) and Chiew and Lim (2000) note that riprap shear, win- nowing, and edge erosion combine to cause either a total dis- integration or embedment failure of the riprap layer in the absence of an underlying filter (either geotextile or granular). Total disintegration, which is characterized by a complete breakup of the riprap layer whereby the stones are washed away by the flow field, occurs when the self-healing ability of the riprap layer is exceeded by the erosive power created by higher flow velocity (Lim and Chiew 1997). According to Chiew and Lim (2000), embedment failure occurs when (1) the riprap stones are large compared to the bed sediment and local erosion around the individual stones causes them to embed into the channel bed (i.e., differential mobility) and (2) the riprap stones lose their stability as bed forms pass and the stones drop into the troughs of the migrating bed forms (i.e., bed feature destabilization). Lim and Chiew (1997) propose a semi-empirical equation based on the critical shear velocity for bed sediment entrainment to distinguish between the total disintegration and embedment modes of failure. Toro-Escobar et al. (1998) present the results of experi- ments conducted by three cooperating research groups (Uni- versity of Auckland, Nanyang University, and St. Anthony Falls Laboratory) under NCHRP Project 24-07 (Parker et al. 1998), which verified the four modes of riprap failure (i.e., riprap shear, winnowing, edge failure, and embedment or settlement due to bed-form passage) defined by Lim and Chiew (1996, 1997). The experiments indicated that these processes, which occur even though the flow is unable to entrain the riprap, can produce less effective protection than that assumed in existing designs. In some cases, the riprap settled to the level of the ambient bottom of the bed-form troughs, and, in other cases, the riprap settled to levels slightly above those that would prevail in the complete ab- sence of riprap. Lauchlan (1999), Lauchlan and Melville (2001), and Lim and Chiew (2001) provide the most comprehensive paramet- ric studies to date on the four modes of pier riprap failure. The conditions under which the failure mechanisms for riprap protection at bridge piers occur are summarized in 13

Figure 2.3. The figure shows that riprap shear, winnowing, and edge failures are observed in all flow conditions, whereas bed-form undermining or destabilization occurs only under live-bed conditions. The potential for winnowing failure increases with U*/*cs, while the potential for edge failures in- creases with U*/U*cr. Riprap shear failure occurs only for U*/U*cr > 0.35 and winnowing is more likely at larger relative riprap size to bed sediment size ratios (dr/d). Sizing of Pier Riprap In addition to the literature review conducted by Parker et al. (1998), comprehensive reviews of the literature on sizing of riprap for bridge piers have been conducted by Fotherby (1995), CUR (1995), Lauchlan (1999), Melville and Coleman (2000), and Lauchlan and Melville (2001). Riprap, which is the most commonly used pier scour coun- termeasure, often consists of large stones placed around a pier to armor the bed at the pier. This armoring prevents the strong vortex flow at the front of the pier from entraining bed sediment and forming a scour hole. The ability of the riprap layer to provide scour protection is, in part, a function of stone size, which is a critical factor in terms of shear failure. The stability of riprap is typically expressed in terms of the stability number, Nsc, which is used in numerous equations to size riprap. Riprap stone size is designed using the critical velocity near the boundary where the riprap is placed. How- ever, many of the pier riprap sizing equations are modified versions of bank or channel protection equations and, there- fore, the use of this approach has limitations when applied at bridge piers because of the strongly turbulent flows near the base of a pier. Most of the remaining equations are based on threshold of motion criteria or empirical results of small- scale laboratory studies conducted under clear-water condi- tions with steady uniform flow. Table 2.1 provides a summary of most of the available equations, reduced to a common form, for sizing riprap to protect bridge piers against scour. A comparison of the var- ious equations for a range of Froude numbers from 0.2 to 0.6 with coefficients for round-nosed piers and sediment parti- cle specific gravity (Ss) of 2.65 indicates that there is a wide range of predicted riprap sizes for any given flow conditions (Figure 2.4). Lauchlan (1999), Melville and Coleman (2000), and Lauchlan et al. (2000a) compare these equations in detail. Since there is a lack of consistency among the meth- ods, Melville and Coleman (2000) recommend the use of the 14 U✶ /U✶ cs Li ve - B ed C on di tio ns Cl ea r- W at er C on di tio ns 1.0 U✶ /U✶ cr Riprap Shear Failure Unlikely ≈ 0.35 Riprap Shear Failure Likely • Bed-Form Undermining • Winnowing - edge failure Larger dr/d • Bed-Form Undermining • Shear Failure • Winnowing • Edge Failure • Shear Failure • Edge Failure - winnowing - edge failure - winnowing Smaller dr/d “Stable” Riprap U✶ = bed shear velocity U✶ cs = critical bed shear velocity for sediment of size d U✶ cr = critical bed shear velocity for riprap of size dr Source: modified from Lauchlan (1999) Figure 2.3. Summary of pier riprap failure conditions for clear-water and live-bed regimes.

Richardson and Davis (1995) (see Lagasse et al. 2001) and Lauchlan (1999) methods for sizing suitable riprap for bridge pier protection, because they lead to conservatively large riprap relative to the other methods. Melville and Lauchlan (1998) use these methods to assess riprap size re- quirements for the Hutt Estuary Bridge in New Zealand and were found to provide good agreement with model study results (Lauchlan et al. 2000b). Only recently have studies been conducted to address riprap size with regard to stability at bridge piers under live-bed con- ditions. Stone size affects shear failure because this failure mode occurs when high flow velocity results in entrainment of the riprap stones. Stone size also influences winnowing, be- cause an increase in stone size produces a concomitant increase in the size of the voids through which bed material is easily winnowed, particularly in thinner riprap layers. This ef- fect decreases with increasing riprap layer thickness. In terms of edge failure and bed-form destabilization, increasing stone size requires increasing bed-form size to cause the same level of damage for a given layer configuration. In a comprehensive parametric study, Lim and Chiew (2001) note that the use of very large stones in pier riprap, which has been shown to be beneficial in clear-water condi- tions, provides little benefit under live-bed conditions, 15 Reference Equation Standard Format (for comparison) Comments Bonasoundas (1973) dr50 (cm) = 6 – 3.3V + 4V2 Equation applies to stones with Ss = 2.65 V = mean approach velocity (m/s) Quazi and Peterson (1973) 0.2 r50 sc y d1.14N − ⎟⎟ ⎞ ⎞ ⎟⎟ ⎞ ⎞ ⎟⎟ ⎞ ⎞ ⎟⎟ ⎞ ⎞ = ( ) 2.5 1.25 r50 Fr 1S 0.85 y d − = Nsc = critical stability number = V2/[g(Ss-1)dr50] Fr = Froude number of the approach flow = V/(gy)0.5 Breusers et al. (1977) r50s 1)d2g(S0.42V −= ( ) 2 s r50 Fr 1S 2.83 y d − = Ss = specific gravity of riprap stones y = mean approach flow depth Farraday and Charlton (1983) 3r50 0.547Fr y d = 3r50 0.547Fr y d = Parola et al. (1989) ( ) 2 s r50 Fr 1S *C y d − = ( ) 2 s r50 Fr 1S *C y d − = C* = coefficient for pier shape; C* = 1.0 (rectangular), 0.61 (round-nose) Breusers and Raudkivi (1991) V = 4.8(Ss – 1)0.5dr501/3y1/6 ( ) 3 1.5 s r50 Fr 1S 0.278 y d − = Austroads (1994) ( ) 2 s vpr50 Fr 1S K0.58K y d − = ( ) 2 s vpr50 Fr 1S K0.58K y d − = Kp = factor for pier shape; Kp = 2.25 (round-nose), 2.89 (rectangular) Kv = velocity factor, varying from 0.81 for a pier near the bank of a straight channel to 2.89 for a pier at the outside of a bend in the main channel Richardson and Davis (1995) ( ) ( )2g1S V0.692d s 2 r50 − = 21 ff ( ) 2 s 2 2 2 1r50 Fr 1S ff0.346 y d − = ƒ1 = factor for pier shape; ƒ1 = 1.5 (round-nose), 1.7 (rectangular) ƒ2 = factor ranging from 0.9 for a pier near the bank in a straight reach to 1.7 for a pier in the main current of a bend Chiew (1995) ( ) 3 s r50 g1SU V y 0.168d − = ∗ ( ) yd 3 31.5 s r50 KK 0.3U Fr U1S 0.168 y d = − = ∗ ∗ 0.106 b y0.783K 0.322 y −= 0≤(y/b)<3 1Ky = (y/b)≥3 2 r50r50 d d bln0.034 d b0.398lnK ⎥⎥⎦ ⎤ ⎥⎥⎦ ⎤ −= 1≤(b/dr50)<50 1Kd = (b/dr50)≥50 Ky = flow depth factor Kd = sediment size factor ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞⎞⎟⎞⎞ Table 2.1. Equation for sizing riprap at bridge piers.

especially at the upper end of the dune regime where large stones offer no additional protection against pier scour. This finding is in contrast to clear-water experiments con- ducted by Parola (1995) who suggests that large riprap may act to dissipate pier-induced vortices, especially when riprap size approaches the size of the vortices. He reasons that because pier-induced vortices are a function of pier di- ameter, the stability number, Nsc, should increase when the rock size approaches the pier diameter. However, experi- mental observations by Lim and Chiew (2001) under live- bed conditions show that large riprap stones, once they are exposed to the flow, act as additional blockages to flow, thereby generating high local turbulence at the pier and resulting in significant riprap degradation. Lim and Chiew (2001) also show that no matter how large the riprap stones are, they will invariably become embedded 16 Reference Equation Standard Format (for comparison) Comments Parola (1993, 1995) Rectangular: Nsc = 0.8 20<(bp/dr50)<33 Nsc = 1.0 7<(bp/dr50)<14 Nsc = 1.0 4<(bp/dr50)<7 Aligned Round-Nose: Nsc = 1.4 ( ) 2 s r50 Fr 1Sy d − = 31 ff bp = projected width of pier ƒ1 = pier shape factor; ƒ1 = 1.0 (rectangular), 0.71 (round-nose if aligned) ƒ3 = pier size factor = ƒ(bp/dr50): ƒ3 = 0.83 4<(bp/dr50)<7 ƒ3 = 1.0 7<(bp/dr50)<14 ƒ3 = 1.25 20<(bp/dr50)<33 Croad (1997) 1/6 r50 r50s 2 d y1.16 1)gd(SA V − = − dr50 =17db50 ( ) 3 1.5 s 3 0.5 r50r50 Fr 1SA 0.641 y 2d1 y d − = − dr50 = 17db50 Use larger of dr50 sizes given by the two equations A = acceleration factor; A = 0.45 (circular and slab piers), A = 0.35 (square and sharp-edged piers) db50 = median size of bed material. Equation given for factor of safety = 1.25, as recommended by Croad (1997) Lauchlan (1999) 1.2 2.75 r f r50 Fr y Y10.3S y d −= 1.2 2.75 r f r50 Fr y Y10.3S y d −= Sf = safety factor, with a minimum recommended value of 1.1 Yr = placement depth below bed level Source: Melville and Coleman (2000) ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ ⎟⎞ ⎞ Table 2.1. (Continued). d r 50 /y o Fr 0.2 0.3 0.4 0.5 0.6 0.2 0.15 0.1 0.05 0 (1977) Croad (1997) Chiew (1995) b/dr50 = 4 Ss = 2.65 Austroads (1994) Kv=2.89 Breusers et al. Austroads (1994) Kv=0.81 Chiew (1995) b/dr50 = 10 Chiew (1995) b/dr50 = 33 Parola (1995) b/dr50 = 33 Richardson & Davis (1995) f2 = 1.0 Lauchlan (1999) Yr/y = 0 Parola (1995) b/dr50 = 10 Parola (1995) b/dr50 = 4 Quazi & Peterson (1973) Farraday & Charlton (1983) Breusers & Raudkivi (1991) Round-nose piers Source: modified from Lauchlan (1999) Figure 2.4. Comparison of equations for sizing riprap at round-nose bridge piers.

into the scour hole at the upper end of the dune regime as a result of bed-form passage. As bed forms pass, the riprap layer composed of large stones deforms and the stones slip or slide into the trough, thus increasing the number and spacing of voids which, in turn, contributes to winnowing of the bed material and, ultimately, embedment of the stones. Lauchlan and Melville (2001) conducted experiments on surface-placed riprap of various sizes where the depth of local scour was recorded for each riprap size at specific flow veloc- ities. Riprap failure was considered to have taken place when more than 20% of the maximum unprotected scour depth occurred in the riprap layer (i.e., dr/dsmax > 20%) over the experimental period. Past practices have been to size riprap such that no movement of the material would occur at the de- sign flow velocity, which has led to oversizing of riprap. How- ever, the data from Lauchlan and Melville (2001) provide larger critical stone sizes for particular flow velocities than many of the previous investigations because of the effects of bed-form destabilization of riprap, which was not evaluated in the fixed-bed flume models of many previous researchers. Recent studies by Lauchlan and Melville (2001) and Lim and Chiew (2001) provide additional information on sizing of riprap around bridge piers under live-bed conditions. Based on the results of their study, Lauchlan and Melville (2001) refined the equation for the minimum critical stone size in relation to flow velocity as defined by Lauchlan (1999). The equation for the minimum stone size is (2.1) where d50 = Median riprap size, ft (m) yo = Undisturbed approach flow depth, ft (m) KD = K-factor for pier diameter-to-bed material ratio (D) KS = K-factor for pier shape (S) Kα = K-factor for pier alignment KY = K-factor for riprap placement depth (Y) F = Froude number Because inadequate data were available to determine KS, KD, and Kα from the study, these factors were set to unity. However, Fotherby and Ruff (1998a) have shown KD (K- factor for pier diameter-to-bed material ratio) to be a signif- icant factor, especially when riprap diameter is comparable to pier width. Since Lauchlan and Melville (2001) used surface- placed riprap, the KY factor was not valid. They used the data from their study to estimate the riprap placed at depth, which allows Equation 2.1 to be rewritten as (2.2) d y Y y F o o 50 2 75 1 20 3 1= − ⎛ ⎝⎜ ⎞ ⎠⎟. . . d y K K K K F o D S Y 50 1 20 3= α . . For high Froude numbers (Figure 2.4), the riprap sizes predicted by the Lauchlan and Melville (2001) equation are similar to those given by equations from Richardson and Davis (1995) and Parola (1995). Their data also indicate that riprap size for a given Froude number decreases with increasing placement depth. To determine the d50 size of pier riprap, HEC-18 (Richard- son and Davis 1995) and HEC-23 (Lagasse et al. 2001) rec- ommend using the rearranged Isbash equation to solve for stone diameter for fresh water: (2.3) where d50 = Median stone diameter, ft (m) K = Coefficient for pier shape (1.5 for round-nose pier, 1.7 for rectangular pier) V = Velocity on pier, ft/s (m/s) Ss = Specific gravity of riprap (normally 2.65) g = Acceleration due to gravity, ft/s2 (m/s2) To determine the velocity on the pier, the average channel velocity (Q/A) is multiplied by a coefficient that ranges from 0.9 for a pier near the bank in a straight uniform reach of the stream to 1.7 for a pier in the main current of flow around a sharp bend. Riprap Placement Level, Coverage, Thickness, and Grading Specifications and guidance on the placement level, areal coverage, thickness, and gradation of a riprap layer placed around a bridge pier vary widely. Table 2.2 summarizes many of the methods used to estimate the extent of coverage, thick- ness, level of placement, and gradation requirements for pier riprap. Placement Level. As previously discussed, most studies of pier riprap failure were conducted under clear-water condi- tions. In most of these studies, the riprap layer was placed on top of the bed surface or buried with the top of the riprap layer flush with the bed surface. Many of the guidelines for place- ment of riprap are based on considerations of riprap for bank protection. Parker et al. (1998) note that even though the placement level of the riprap layer with respect to the channel bed is believed to be an important factor in the stability of the layer, there are no generally accepted design criteria available for this factor and, in particular, there are conflicting recom- mendations for the finished level of riprap protection. Riprap used for pier scour protection is usually placed on the surface of the channel bed (Figure 2.2a) because of the ease d KV S gs 50 20 692 1 2 = − . ( ) ( ) 17

and lower cost of placement and because it is more easily inspected. Parola (1995) hypothesized that mounded riprap on the bed surface may have an increased capacity to resist erosion because it alters the approach flow vertical velocity distribution such that the vortex systems created by the pier have a lower capacity to destabilize the riprap. However, mounding riprap around a bridge pier is unac- ceptable for design, in most cases, because it constricts flow, captures debris, and increases scour at the margins of the pier protection. Many studies suggest that riprap be placed in a flat layer on the bed surface, in an existing scour hole with the top nearly flush with the bed, or in a pre-excavated hole around the pier with the top of the layer level with the bed. The FHWA (Lagasse et al. 2001; Richardson and Davis 1995) recom- mends placing the top of the riprap layer flush with the chan- 18 Riprap Extent Reference Coverage (C) Thickness (t) Level Gradation Bonasoundas (1973) Semi-circular upstream shape (radius 3b), semi- elliptical downstream shape; overall length 7b b/3 Neill (1973) Project around the nose of the pier by a distance = 1.5b >2d r5 0 Posey (1974) 1.5b to 2.5b in all directions from the pier face Hjorth (1975) Length = 6.25b, width = 3b, circular arc upstream, triangular shape downstream Breusers et al. (1977) 2b from pier face 3d r50 Some distance below bed level to prevent excessive exposure Lagasse et al. (2001) Width > 5b > 3d r50 Top of riprap at bed level d r5 0 ≥0.5d rm ax Chiew (1995) 2.75 V V 12.5 D C c r − ≥ D = pier diameter Parola (1995) Semi-circular upstream (radius b p ), triangular downstream; overall length = 7b p Croad (1997) >5.5b p , of which 1.5b p is upstream of the upstream face of the pier 2d r50 d rm ax ≤ 2d r5 0 d r5 0 ≤ 2d r15 Lauchlan (1999) 1b to 1.5 b in all directions from the pier face. Sy nthetic filter (if placed) should have lateral extent about 75% of the lateral extent of the riprap layer 2d r5 0 to 3d r50 A factor for level of placement (Y r ) included in riprap sizing equation 0.5d rm ax <d r5 0 d r5 0 <2d r15 Brown and Clyde (1989) 2b from pier face ≥3d r50 Place mat below streambed a depth equivalent to the expected scour Fotherby (1995) Fotherby and Ruff (1998a) 1.5b a minimum (b a = adjusted pier width) 2D u min. (D u = riprap unit diameter) Top of riprap installed level with streambed or wi thin 2D u if approach flow velocit y is adjusted CUR (1995) 3b in the upstream direction and 4b on both sides and in the downstream direction (as measured from the pier face) 2b On or flush with the streambed surface Parker et al. (1998) Total lateral coverage (edge to edge) = 4b for excavated or existing scour hole = 5b for placement on streambed at least 3d r50 Lim and Chiew (2001) FHW A coverage of 2b from pier face (extent of coverage has no effect at upper dune regime) >1.5d r5 0 or d r100 Source: modified from Melville and Coleman (2000) Table 2.2. Methods to estimate riprap extent, gradation, and filter requirements for riprap at bridge piers.

nel bed for inspection purposes (Figure 2.2b). The European practice and the preferred practice of many state DOT main- tenance departments in the United States is to place the layer on top of the bed surface (Figure 2.2a), preferably with an underlying filter layer or geotextile to deter the effects of win- nowing of the underlying bed sediments. Most of the studies on the stability of riprap around bridge piers prior to the Phase 1 study by Parker et al. (1998) were conducted under clear-water conditions with the top of the riprap layer placed level with the channel bed. Many of these studies concentrate primarily on riprap size, layer thickness, and filter requirements when evaluating pier riprap stability (Parola 1995; Fotherby 1995; Lim and Chiew 1996; Yoon and Yoon 1997; Fotherby and Ruff 1998a, 1998b; Ruff and Nickelson 1998). The pioneering study by Laursen and Toch (1956) was one of the first studies to propose that riprap used at bridge piers should be placed well below the streambed. Breusers et al. (1977) recommends that riprap near bridge piers would perform most successfully when placed at the trough elevation of the largest bed forms. As discussed earlier, a live-bed condition with migrating bed forms is more likely to occur during floods and is now believed to be the most important contributor to pier riprap failure. Therefore, many of the experimental studies con- ducted over the last several years have been concerned with the processes of pier riprap failure under live-bed conditions, and several have addressed the placement level of the riprap layer with regard to the passage of mobile bed forms. Lim and Chiew (1996) propose an empirical equation to compute the maximum displaced riprap level, which is the level con- tributed jointly by the pier (i.e., equilibrium pier scour depth) and by the passage of the largest dunes (i.e., the dune trough level) just prior to the arrival of the transition to a plane bed (i.e., the transition to upper regime conditions). Studies by Parker et al. (1998) note that riprap performance improved when the top of the riprap layer was buried below the bed surface, but do not provide any guidance on recom- mended depth of burial. The comprehensive study conducted by Lauchlan (1999) indicates that placing the riprap layer at depth (Figure 2.2c) was shown to improve the performance of the layer for a spe- cific flow velocity, and that the deepest placement level tested provided the greatest reduction in local scour depths in the majority of tests. Based on experimental results, Lauchlan recommends the use of a placement depth factor, KY, to de- scribe the improved performance of riprap when it is placed below the average bed level (see Equations 2.1 and 2.2 for def- inition of KY). Lauchlan suggests that KY be used when the ratio of the depth of placement, Y, to the mean flow depth, yo, is between 0.0 and 0.6. Based on these results, Lauchlan (1999) and Melville and Coleman (2000) recommend that the riprap layer should be placed at about the lowest dune trough level expected. Although Lim and Chiew (2001) find that riprap layer degradation decreases with greater depth of placement, they indicate that the placement level of a riprap layer ceases to provide any benefit to riprap layer stability at approximately the upper end of the dune regime. Areal Coverage. As shown in Table 2.2, the recom- mended coverage varies with pier shape and can extend as lit- tle as one pier width from the pier face to as much as seven times the pier width depending on location around the pier. Most studies recommend that the coverage of the riprap layer extend at least to the edges of the predicted or existing scour hole. Various studies suggest shaping the riprap layer into a rectangle, pear, teardrop, or horseshoe shape. According to Lauchlan (1999) in most of the studies conducted using riprap filter layers, “it is unclear as to whether testing of the recommendations [for filter layer shape] was undertaken, which is doubtful, and little reasoning for the proposed shapes is given.” Layer Thickness. Most of the studies reviewed in the previous paragraphs suggest that thickness of the riprap layer placed around bridge piers should be between two to three times the median stone size of the riprap (Table 2.2). Riprap performance was found to increase significantly with an in- crease in thickness from 2dr50 to 3dr50 (Parker et al. 1998). Melville and Coleman (2000) indicate that there is as much as a 70% reduction in local scour associated with an increase in thickness from 1dr50 to 3dr50. Thin layers tend to fail under the process of winnowing of the underlying bed sediments and the passage of mobile bed forms (Chiew 1995; Lim and Chiew 1996; Parker et al. 1998). Experiments by Lim and Chiew (1996) indicate that thick riprap layers still become thin at the edges, but will not sub- side into the bed under live-bed conditions. They also found that thicker layers are able to self-heal under the modes of failure previously described. A thick riprap layer behaves similarly to a riprap layer of regular thickness with an under- lying filter; winnowing and subsidence are unable to take place because flow is unable to pass through the interstices of the riprap layer. However, riprap stones can still slide into the trough of passing dunes and may be swept away under higher velocities. The parametric study by Lim and Chiew (2001) indicates that riprap layer thickness has no influence on the stability of the layer with the passage of very large dunes. Gradation. Very few of the previously discussed studies have specifically examined the effects of riprap gradation on riprap layer stability. However, most studies suggest that a graded riprap layer will be more likely to withstand the effects of bed sediment winnowing than one composed of equi- dimensional stones. A few studies shown in Table 2.2 provide some guidance on riprap gradation. Brown and Clyde (1989) provide gradation limits and classes, and CUR (1995) pro- vides gradation class requirements and grading curves for general use in riprap revetments. 19

Summary. Based on much of the information in Table 2.2, Melville and Coleman (2000) provide the following recom- mendations for riprap protection at bridge piers: • Riprap size: based on Lauchlan (1999) equation (Equation 2.2) for sizing riprap • Riprap layer thickness: t = 2dr50 to 3dr50 • Coverage of riprap layer: width = 3 to 4 pier widths, or 1 to 1.5 pier widths from pier face • Placement level: at about lowest dune trough level • Grading: 0.5dr max < dr50 < 2dr15 • Synthetic filter layer: lateral extent should be about 75% of lateral extent of riprap layer • Inverted stone filter layer: t = dr50 with grading according to Terzaghi criteria Riprap Filter Requirements Two kinds of filters are used in conjunction with bridge pier riprap: granular (stone) filters and geotextile filters. Stone filters are composed of rock that may or may not be graded, and have a median size that is smaller than the over- lying riprap but large enough to be more permeable than the underlying bed material. Geotextiles are permeable textiles, meshes, and nets that are either synthetic or biodegradable (not recommended). Geotextiles can be woven, non-woven, or knitted. Woven geotextiles have evenly spaced fibers that are at right angles to form regularly spaced holes. Non-woven geotextiles have fibers or filaments that are randomly placed to form a wide range of hole sizes. Knitted geotextiles consist of immovable fibers that confer a high degree of strength and flexibility to the fabric. The durability of a geotextile is dependent on the type of fiber used and its mechanical, fil- tration, and chemical properties. The importance of the filter component of a riprap in- stallation should not be underestimated. Geotextiles and/or aggregate underlayers are used to perform the filtration function. Some situations call for a composite filter consist- ing of both a granular layer and a geotextile. The specific characteristics of the base soil determine the need for, and design considerations of the filter layer. In cases where the base soil is composed primarily of relatively large parti- cles (coarse sands and gravels), a filter layer may not be necessary. Careful design, selection, and installation of the appropriate filter material all play an important role in the overall per- formance of riprap. Figure 2.5 provides schematic illustrations of the three most typical types of riprap filter configurations. 20 Granular filter b) Granular filter Granular filter Geotextile c) Granular transition layer with geotextile (composite filter) Geotextile Base soil Riprap Design high water Freeboard a) Geotextile filter Figure 2.5. Channel cross sections showing common riprap/filter configuration.

The primary roles of a filter component are to (1) retain the soil particles, while (2) providing a zone for the free flow of water through the interface between the riprap armor and the under- lying soil. The soil retention function argues for very small pores in the filter, whereas maintaining a large permeability of the fil- ter argues for larger pores, and lots of them. Both of these two contrary objectives must be met to achieve an effective func- tional balance between retention and permeability. Filters assist in maintaining intimate contact between the revetment and the base soil by creating a stable interface. Depending upon the internal stability of the soil, several processes can occur over time at this interface. The filter pore size and the base soil stability influence these processes. As an example, consider the process of “piping.” Piping is basically the washing away of very fine particles, resulting in greater void space in the underlying soil structure. Piping is more likely to occur in non-cohesive/unstable soils that are in contact with a filter that has large openings. The large open- ings do not retain the smaller particles and therefore these particles are removed by seepage and pressure fluctuations, leaving only the larger particles. This process increases the potential for soil erosion by weakening the underlying soil structure. The reverse can occur when the pores of the filter are so small that they retain virtually all the particles of the base soil. If the base soil is internally unstable, the finest particles will continue to migrate with the seepage flow until a clogged layer is built up against the filter. This lower permeability zone will eventually create a barrier to flow, and excess uplift pressures can be created beneath the filter. A detailed discus- sion of the filter requirements is presented in Section 3.12. In Europe, it is common practice to use fascine mats as a means of placing a geotextile filter in deep water. Fascine mattresses are composed of natural woody material woven in bundles to form a matrix that is placed over a geotextile and then floated into position and sunk into place by dropping riprap on it from a barge (Pagán-Ortiz and Lagasse 1999; Lagasse et al. 2001). Lauchlan (1999) provides a comprehensive review of the literature on the use of granular and synthetic filters and the criteria for their use with pier riprap. General guidelines on the design and use of granular and fabric filters are provided in Brown and Clyde (1989). Escarameia (1998), Holtz et al. (1995), and Pilarczyk (2000) provide detailed information on the types of filters, potential applications, and specific guide- lines on the selection and installation of geotextile filters. CUR (1995) also provides detailed information on the prop- erties, design, and placement of filters used in conjunction with riprap in Europe. Brauns et al. (1993) provide a com- prehensive review of the design, placement, applications, and problems associated with the use of filters in geotechnical and hydraulic engineering. Some studies suggest that a filter may be unnecessary if the riprap layer is of sufficient thickness (Lim and Chiew 1996, 1997; Toro-Escobar et al. 1998; Lauchlan 1999). Yet, a majority of the research on the stability of riprap at bridge piers to date indicates that the use of an underlying filter layer significantly increases the stability of the riprap layer. Many of the more recent experimental studies have evaluated the effects of a filter layer placed below a riprap layer on the sta- bility of the riprap layer under live-bed conditions. In general, granular filter layers should be of a gradation, size, and thickness sufficient to deter the effects of winnow- ing of the underlying bed sediments. Geotextiles should also have an effective pore size sufficiently small to block the passage of bed sediments, but have large enough permeabil- ity to deter or withstand buoyant forces and potential pres- sure gradients in the surface and subsurface in the area of the pier. Parker et al. (1998) determined that using a geotextile with the same areal coverage as the riprap layer it is placed under results in relatively poor performance of the riprap at bridge piers. As a result of the effects of live-bed conditions de- scribed previously, the riprap at the edges tended to roll, slide, or be plucked off exposing the underlying geotextile and ultimately resulting in failure of the riprap layer as suc- cessive bed forms pass and pluck more stones from the riprap layer. The failure of the geotextile was due in part to the im- permeability of the fabric leading to the buildup of uplift forces and the creation of a bulge under the fabric, which contributed to the loss of riprap stones. In addition, the loss of the edge riprap and exposure of the geotextile allowed the geotextile to fold back on itself further reducing the stability of the riprap. If the geotextile was not sealed to the pier face, winnowing around the pier face resulted in a scour hole around the pier face and caused the geotextile and stones at the interface to fall into the scour hole. For bridge piers, Parker et al. (1998) determined that the tendency for riprap to settle was arrested when (1) the geot- extile has two-thirds the areal coverage of the riprap, (2) the geotextile is sufficiently permeable, and (c) the geotextile is sealed to the pier. Lauchlan (1999) recommends that the geotextile have an areal coverage of 75% of the riprap layer so that the edges of the geotextile will be anchored when the edge stone of the riprap layer slide into the trough of passing bed forms. However, placement of a filter layer at a bridge pier under riverine or tidal conditions can be very difficult and is greatly dependent on the type of filter used, the availability of appropriate equipment, accessibility, and flow condi- tions. Granular filters can be partially or completely washed away by stream flow when being installed around piers. A geotextile must be able to remain relatively intact and withstand ripping or tearing and displacement during in- 21

stallation in order to provide stability to the overlying riprap layer. Many European countries have developed spe- cial equipment and installation procedures to counter most of these problems (CUR 1995). According to Pagán-Ortiz and Lagasse (1999), a significant investment has been made in Germany and the Netherlands in the development and testing of geosynthetic materials, and innovative installa- tion techniques have been developed that could find appli- cation for bridge pier countermeasures in the United States. Heibaum (2000) describes the types of filter materials and systems used and the methods of placement under water, including the use of geotextile containers. European Applications Although riprap is used extensively as a pier scour counter- measure in the United States, it is not considered a permanent solution to scour around bridge piers. In contrast, riprap (armor stone in Europe), often in combination with either a geotextile or granular filter layer, is considered a permanent countermeasure for scour and stream instability in European countries (Pagán-Ortiz and Lagasse 1999; Bryson et al. 2000; Lagasse et al. 2001). Because of riprap’s highly desirable char- acteristics of availability, economy, ease of installation, and flexibility, considerable effort has been devoted to techniques for determining the size, gradation, layer thickness, horizon- tal coverage, filter design, and construction methods for use in riverine and coastal applications. Heibaum (2000) describes several methods used in Europe to counter scour around various structures, including bridge piers, and provides a summary of the types of materials and systems used and the methods of placement (under water) under both mild and strong currents. The difference between U.S. and European practice is not necessarily derived from the availability of better techniques for sizing riprap, but rather from the European practice of providing a higher standard of care and quality control in placing the stone, and providing an appropriate filter on sand bed channels. In addition, European practice includes in- spection and monitoring to verify that riprap is performing properly. European hydraulic engineers have developed innovative techniques for placing an effective filter beneath the riprap in flowing or deep water including the use of large geotextile sand containers, geotextile mattresses filled with granular filter material, and fascine sinker mats. Fascine sinker mats are used for water depths generally greater than about 60 ft (18 m). According to Bryson et al. (2000), most European engi- neers recommend the use of the Manual on the Use of Rock in Hydraulic Engineering (CUR 1995) as a reference for riprap and filter design. However, most of the applications in Europe are used to counteract erosion caused by wave wash, tidal fluctuations, and storm surges, and the majority of the design guidelines are based on the use of riprap primarily as bed and bank protection, not as a pier scour countermeasure. Partially Grouted Riprap Current practice in the United States discourages the use of grouted riprap, primarily because the voids within the riprap are, in most cases, nearly completely filled with grout, which creates rigidity and impermeability that often leads to failure. Guidelines on the construction of grouted riprap in the United States are associated almost entirely with riprap bed and bank protection (for example, Brown and Clyde 1989). Total grouting converts a flexible revetment material like a riprap layer into a rigid mass and reduces the perme- ability of the layer. This may cause the entire riprap layer to fail as a result of either undermining or uplift and thus negates the natural benefit caused by raveling of loose riprap into the scour hole or trough of migrating bed forms. This rigidity and reduced permeability may also suggest why a sur- vey of U.S. field engineers conducted by Parker et al. (1998) on the feasibility, effectiveness, constructability, durability, maintainability, and cost of various types of countermeasures ranked grouted riprap fairly low. Partially grouted riprap provides a more suitable alterna- tive to total grouting because it alleviates the concerns and problems associated with completely filling the surface voids with grout. Partial grouting increases the stability of the riprap unit without sacrificing flexibility and allows for the use of smaller rock and thinner riprap layers in areas where the re- quired stone size for loose riprap is unavailable. In the United Kingdom, grouting is used primarily at the edges of revetments and at transitions with hydraulic struc- tures (Escarameia 1998). There are two common types of grout material used in Europe: bituminous and cementitious. Bituminous grout consists of a chemically inert and viscous mixture of hydrocarbons and provides considerably more flexibility to the revetment compared to cement. Bituminous grout is the most commonly used material in the Netherlands because it reduces the stone sizes required. Cementitious grout is commonly used with “hand-pitched stone” and, in contrast to bituminous grout, confers rigidity and imperme- ability to a revetment. Design manuals by CUR (1995) and Escarameia (1998) pro- vide guidance for grouting stone revetments with both bituminous and cementitious grouts; bitumen is the most commonly used material with riprap. BAW in Germany has developed guidelines for the testing of bitumen-bonded mate- rials used in the grouting of riprap revetments (BAW 1991— see Section 2.1.5 of this report). Wave tank experiments at BAW in Germany, experience on German inland waterways, 22

and development of design guidance for partial bitumen- and cement-grouted riprap in the United Kingdom provide a wealth of information on the design and installation of par- tially grouted riprap. Various degrees of grouting are possible, but the most effective solutions are produced when the bituminous mor- tar envelops the loose stone and leaves relatively large voids between the stones. The degrees of bituminous grouting follow: • Surface grouting (grout does not penetrate the whole revetment thickness and fills about one-third of the voids) • Various forms of pattern grouting (where only part of the surface area of the revetment is filled, between 50% to 80% of voids) • Full grouting (an impermeable type of revetment) The two types of pattern grouting procedures, line-by-line and spot-by-spot, produce conglomerate-like elements in the riprap. With the proper grout, partial grouting can be done under water. Grout can be placed by hand only in water less than 1 meter deep. Special devices are required for placement in deeper water. Various European countries have developed special grout mixes and construction methods for underwa- ter installation of partially grouted riprap (Lagasse 1999; BAW 1990, 1993a, 1993b—see Reference Document [www.trb.org/TRBNet/ProjectDisplay.asp?ProjectID=702]). Partial pattern grouting is obtained when the grout is placed on the riprap leaving significant voids in the riprap matrix and considerable open space on the surface. No grout should penetrate deep enough to come in contact with any underlying filter. Construction methods must be closely monitored to ensure that the appropriate voids and surface openings are provided. Contractors in Germany have developed techniques and special equipment to achieve the desired grout coverage and the right grout penetration. Heibaum (2000) indicates that grouting has proven its long-term stability and ability to keep costs low; for example, laboratory tests at Braunschweig University in Germany proved that partially grouted riprap is stable up to a flow ve- locity of 26 ft/s (8 m/s). Also, because the riprap is dumped or placed as needed and only then is the layer grouted, a close contact to structural elements such as bridge piers can be achieved. In almost all cases, a geotextile filter is recommended or required in conjunction with partially grouted riprap because of the potential for winnowing of underlying bed material. BAW (1993a), CUR (1995), and Escarameia (1998) all pro- vide guidelines on the design and installation of filters used with partially grouted riprap. 2.1.4 Alternatives to Riprap In most cases where pier scour countermeasures are required, riprap is often the countermeasure of choice. How- ever, in some cases, riprap may not be an option for a num- ber of reasons. In some areas, riprap may be unavailable due to a lack of supply of durable stone. Where it is available the stone may not be in the size ranges required to provide the necessary protection against scour, or the stone may be pro- hibitively expensive to use. In some areas, environmental or aesthetic restrictions may preclude its use. The following review of alternatives to riprap includes articulating concrete block systems, concrete armor units, gabions, grout-filled bags and mattresses, and sand-filled geotextile containers. Articulating Concrete Block Systems Articulating concrete block (ACB) systems provide a flex- ible alternative to riprap and rigid revetments. These systems consist of preformed units that interlock, are held together by steel rods or cables, are bonded to a geotextile or filter fabric, or abut together to form a continuous blanket or mat (Figure 2.6). Data sheets for a number of the more common proprietary ACB revetment systems can be found in Escarameia (1998). Parker et al. (1998) provides a brief 23 (a) (b) Source: (a) American Excelsior Company, (b) Armortec Figure 2.6. Examples of interlocking block (a) and cable-tied block (b) systems.

review of the limited studies conducted on the use of ACBs for pier scour protection. There is limited experience with the use of ACB systems as a scour countermeasure for bridge piers alone. More frequently, these systems have been used for bank revetments and channel armoring where the mat is placed across the entire channel width and keyed into the abutments or bank protection. For this reason, guidelines for placing ACB systems along banklines and in channels are well documented (e.g., Ayres Associates 2001), but there are few published guidelines on the installation of these systems around bridge piers. There are two failure mechanisms for ACBs: (1) overturn- ing and rollup of the leading edge of the mat where it is not adequately anchored or toed in and (2) uplift at the center of the mat where the leading edge is adequately anchored. Although no additional hydraulic stability is attributed to the presence of cables, they can prevent individual blocks from being plucked out of the matrix when failure is imminent. In the absence of a filter or geotextile, winnowing can still occur and can result in subsidence of all or a portion of the ACB mat. Studies conducted on the effectiveness of ACBs as a countermeasure have determined that the use of a filter fabric or geotextile was important to the overall effectiveness and stability of the ACB. In some cases, a geotextile, usually composed of a non- woven, needle-punched synthetic, is bonded to the under- side of the ACB mat. The gap that separates the blocks in a geotextile-bonded system produces a more flexible mat be- cause of the lack of inter-block friction that can result with interlocking block geometries. By bonding the blocks to the geotextile, winnowing is eliminated; the blocks are not sep- arated from the underlying geotextile by shear forces, uplift pressures, or subsidence; and the blocks and geotextile together can fold down with the bed during the passage of bed forms. Although applied as a bed and bank revetment, this type of system has not been tested as a pier scour coun- termeasure and it is not known how this type of system will respond to the passage of deep troughs. Specifications and design guidelines for installation and anchoring of ACBs as bed and bank revetment are docu- mented in HEC-11 (Brown and Clyde 1989) and guidelines on the selection and design of filter material can be found in HEC-11 and Holtz et al. (1995). HEC-11 directs the de- signer to the manufacturer’s literature for the selection of appropriate block sizes for a given hydraulic condition. Be- cause ACBs vary in shape and performance from one pro- prietary system to the next, each system will have unique performance properties. Manufacturers of ACBs must test their products and develop design criteria based on the re- sults from these tests. HEC-23 (Lagasse et al. 2001) provides equations for determining the factor of safety in determin- ing block sizes and computing the potential effects of pro- jecting blocks. In Europe, ACBs are designed using guide- lines similar to those provided in HEC-23 and are used as bed and bank revetment primarily on relatively straight channels under normal flow conditions and low turbu- lence. Escarameia (1998) recommends that modeling should be conducted prior to installation for applications where turbulent or other extreme hydraulic conditions are expected. Parker et al. (1998) provide some design recom- mendations for the use of cable-tied blocks as pier scour countermeasures. The design procedure for ACBs for revetment and bed armor provided in HEC-23 (Lagasse et al. 2001) quantifies the hydraulic stability of revetment block systems using a “discrete particle” approach (like many riprap sizing meth- ods). The design approach is similar to that introduced by Stevens (1968) to derive the “factor of safety” method of riprap design as described in Hydraulic Design Series (HDS) 6 (Richardson et al. 2001). The force balance has been recomputed considering the properties of concrete blocks, and the Shields relationship utilized in the HDS 6 approach to compute the critical shear stress has been replaced with actual test results (Richardson et al. 2001). The design procedure incorporates results from hydraulic tests into a method that is based on fundamental principles of open channel flow and rigid body mechanics. The ratio of resisting to overturning moments (the “force balance” approach) is analyzed based on the size and weight charac- teristics of each class and type of block system and includes performance data from full-scale laboratory testing. This ratio is then used to determine the factor of safety against the initiation of uplift and rotation about the most critical axis of the block. Also incorporated into the design procedure in HEC-23 (Lagasse et al. 2001) are considerations that can account for the additional forces generated on a block that protrudes above the surrounding matrix because of subgrade irregular- ities or imprecise placement. Because finite movement consti- tutes failure, the analysis methodology provided in HEC-23 purposely contains no explicit attempt to account for resistive forces due to cables or rods. Similarly, the additional stability that may arise from vegetative root anchorage or mechanical anchoring devices, while recognized as significant, is ignored in the analysis procedures for the sake of conservatism in selection and design. According to HEC-23, the designer must determine what factor of safety should be used for a particular design. Risks associated with a failure of the project, the uncertainty of hy- draulic values used in the design, and uncertainties associated with installation practices are some of the variables that should affect the selection of the factor of safety used for final design. Typically, a minimum factor of safety of 1.5 is used for revetment design when the project hydraulic conditions 24

are well known and variations in the installation can be ac- counted for. Higher factors of safety are typically used for protection at bridge piers, abutments, and channel bends be- cause of the complexity in computing hydraulic stresses at these locations. Although the hydraulic stability of ACB systems at bridge piers can be assessed using the factor of safety method, uncer- tainties in the hydraulic conditions around bridge piers war- rant increasing the factor of safety in lieu of a more rigorous hydraulic analysis. Experience and judgment are required when quantifying the factor of safety to be used for scour pro- tection at an obstruction in the flow. In addition, when both contraction scour and pier scour are expected, design consid- erations for an ACB mat placed around a pier become more complex. The guidelines in HEC-23 (Lagasse et al. 2001) reflect recommendations developed by McCorquodale et al. (1993, 1998), the Minnesota Department of Transportation (Mn/DOT), and the Maine Department of Transportation (MDOT) for application of ACBs as a countermeasure for pier scour. Specific studies on the use of cable-tied blocks as a means of protection for bridge piers can be found in Jones et al. (1995), Bertoldi et al. (1996), Stein et al. (1998), McCorquodale et al. (1993, 1998), and Parker et al. (1998). Much of the guidance in these reports has been superseded by the findings of this study (see Appendix E). As part of NCHRP Project 24-07, Parker et al. (1998) eval- uated flow altering and armoring alternatives to standard riprap installations as pier scour countermeasures. Based on laboratory testing they conclude that a mattress of cable-tied blocks underlain by a geotextile tied to the pier provides “excellent protection” and present suggestions for the design of cable-tied blocks. An observed key point of failure for ACB systems at bridge piers occurs at the seal where the mat meets the bridge pier (McCorquodale et al. 1993, 1998; Stein et al. 1998). During the flume studies by McCorquodale et al. (1993, 1998) and Stein et al. (1998), the mat was sealed to the pier to prevent scouring of the sediments adjacent to the pier. This procedure was highly successful in the laboratory; however, in the field the transfer of moments from the mat to the pier may affect the structural stability of the pier. When the mat is attached to the pier, the increased loading on the pier must be considered. Mn/DOT has installed a cable-tied mat for a pier at TH 32 over the Clearwater River at Red Lake Falls. In addition to grout, Mn/DOT recommends the use of tension anchors around the pier seal. Anchors can provide additional support for the mat and grout at the pier seal will reduce scouring at the mat/pier interface. Mn/DOT provided the following specifications: • Anchors: Mn/DOT recommends the use duckbill anchors, 0.9 to 1.2 m (3 to 4 ft) deep, at corners and about every 2.4 m (8 ft) around pier footings. McCorquodale et al. (1998) rec- ommend an anchor spacing of 4 ft (1.2 m) along the edges. • Pier Seal: Research conducted by the FHWA indicates that the space between the pier and the cable-tied concrete blocks must be filled or scour may occur under the blocks. To provide this seal, Mn/DOT proposed that concrete be placed around the pier. Mn/DOT suggested that the riverbed could be excavated around the piers to the top of the footing. The mat could be put directly on top of the footing and next to the pier with concrete placed under- neath, on top of, or both, to provide a seal between mat and pier. A 1998 review of European practice for bridge scour coun- termeasures (Lagasse 1999) identifies two approaches for solv- ing the problem of providing a seal between the bridge pier and ACB or grout-filled mattress systems. The review references a proprietary system in Germany for installing a collar and tying the geotextile filter underlying a mat (or mattress) to the bridge pier using a flexible tie (Figure 2.7). This approach appears fea- sible for circular piers. Considering possible settlement of the mat relative to the structure (pile), a steel sleeve and a “top hat” of filter fabric were proposed with a collar of Fabriform® laid on top of the mat and tied to the sleeve as indicated in Figure 2.7. As relative settlement occurs, the sleeve is expected to slide down the pile and the top hat to expand, bellows fashion, with a collar for protection. This approach may be limited in areas where the top hat could be damaged by abrasion. In the Netherlands, the recommended approach to the problem of sealing the joint between a mat and a bridge pier is to place granular filter material to a depth of about 25 Source: Lagasse (1999) Figure 2.7. Flexible collar arrangement at a pile to seal the joint with a mattress.

3 ft (1 m) below the streambed for about 16 ft (5 m) around the pier. The geotextile filter and ACB mat placed on the streambed overlap this granular filter layer and the remaining gap between the mat and the pier is filled with riprap (Figure 2.8). Concrete Armor Units The group of concrete armor units, also known as artificial riprap, consists of individual pre-cast concrete units with complex shapes that are placed individually or in intercon- nected groups. These units were originally developed for shore protection to resist wave action during extreme storms. All are designed to give a maximum amount of interlocking using a minimum amount of material. These devices are used where natural riprap is unavailable or is more costly to obtain than fabrication of the artificial riprap units. Parker et al. (1998) provide a review of studies conducted on the use of concrete armor units as pier scour countermeasures. Various designs for size and shape of concrete armor units are available (Figure 2.9). Because concrete armor units are similar to riprap, they can be susceptible to the same failure mechanisms as riprap. However, the use of a filter layer or geotextile in conjunction with these types of devices is often required, especially in coastal applications, and a geotextile or filter may be critical to the stability of these devices when used as pier scour protection. The primary advantage of armor units is that they usually have greater stability compared to riprap particles of equiva- lent weight. This greater stability is due to the interlocking characteristics of their complex shapes. The increased stabil- ity allows their placement on steeper slopes or the use of lighter weight units for equivalent flow conditions as com- pared to riprap. This characteristic is significant when riprap of a required size is not available. The design of armor units in open channels is based on the selection of appropriate sizes and placement patterns to be stable in flowing water. The armor units should be able to withstand the flow velocities without being displaced. Hydraulic testing is used to measure the hydraulic conditions at which the armor units begin to move or “fail,” and dimen- sional analysis allows extrapolation of the results to other hydraulic conditions. Although a standard approach to the stability analysis has not been established, design criteria have been developed for various armor units using the following dimensionless parameters: • Isbash stability number (Parola 1993; Ruff and Fotherby 1995; Bertoldi et al. 1996) • Shields parameter (Bertoldi et al. 1996) • Froude number (Brown and Clyde 1989) The Isbash stability number and Shields parameter are indicative of the interlocking characteristics of the armor units. Froude number scaling is based on similitude of stabi- lizing and destabilizing forces. Quantification of these parameters requires hydraulic testing and, typically, regres- sion analysis of the data. Prior research and hydraulic testing have provided guidance on the selection of the Isbash stabil- ity number and Shields parameter for riprap and river sedi- ment particles, but stability values are not available for all concrete armor units. Therefore, manufacturers of concrete armor units have a responsibility to test their products and to develop design criteria based on the results of these tests. 26 Granular Filter 1 m 5 m Riprap pier Bed level Geotextile filter Block Mattress Source: Lagasse (1999) Figure 2.8. Use of granular filter and riprap to seal the joint between a bridge pier and ACB mattress.

Because armor units vary in shape and performance from one proprietary system to the next, each system will have unique performance properties. Installation guidelines for concrete armor units in stream- bank revetment and channel armor applications should consider subgrade preparation, edge treatment (toe down and flank) details, armor layer thickness, and filter requirements. Subgrade preparation and edge treatment for armor units are similar to that required for riprap, and general guidelines are documented in HEC-11 (Brown and Clyde 1989). Consider- ations for armor layer thickness and filter requirements are product specific and should be provided by the armor unit manufacturer. Concrete armor units have shown potential for mitigating the effects of local scour in the laboratory; however, only limited data are available on their performance in the field. Research efforts are currently being conducted to test the performance of concrete armor units as pier scour counter- measures in the field. Design methods that incorporate velocity (a variable that can be directly measured) are commonly used to select local scour countermeasures. Normally an approach velocity is used in the design equation (generally a modified Isbash equation) with a correction factor for flow acceleration around the pier or abutment (Lagasse et al. 2001). Although tetrahedrons are currently used for bank protec- tion (Fotherby 1995), they have garnered very little interest with regard to pier scour protection in the United States. This may be primarily related to their lack of appendages and interlock (i.e., their simple, compact shape is similar to riprap and spheres). Dolosse also have not been seriously consid- ered for use as pier scour protection because they have no inherent interlocking property to resist movement under steady state turbulent flow (Brebner 1978). Extensive testing and research have been conducted on the Core-Loc™ system, which was developed by the U.S. Army Engineer Waterways Experiment Station, but the testing was limited exclusively to coastal applications. Accropode™ and tribar systems are used almost exclusively in coastal applications as well. In contrast, tetrapods have been extensively studied and evaluated for use as pier scour protection (Fotherby 1992, 1993; Bertoldi et al. 1996; Jones et al. 1995; Bertoldi and Kilgore 1993). Fotherby (1992, 1993) and Stein et al. (1998) suggest that tetrapods offer little advantage compared to riprap in terms of stability. Layering and density had no appreciable effect on the stability of the tetrapods, although the stability increased with the size of the tetrapod pad. Work by Bertoldi et al. (1996) and Stein et al. (1998) indi- cates that riprap and tetrapods behaved comparably when both stability number and spherical stability number were compared and also suggests that fixing the perimeter and varying the number of tetrapod layers may have an effect on stability. A specific design procedure for Toskanes has been devel- oped for application at bridge piers and abutments and is described in HEC-23 (Lagasse et al. 2001) to illustrate a gen- eral design approach where the Toskanes are installed as individual, interlocking units. The design procedures for Toskanes are based on extensive research conducted at Col- orado State University (Ruff and Fotherby 1995; Fotherby 1995; Burns et al. 1996; Fotherby and Ruff 1998a, 1998b). Based on hydraulic model studies conducted at CSU for the Pennsylvania Department of Transportation, Burns et al. (1996) presented procedures for the design of Toskane pads, provided criteria for sizing Toskanes, and suggested tech- niques for installation of Toskanes. No other concrete armor unit has been as extensively tested and evaluated for use as a pier scour countermeasure. 27 Tetrapod Tetrahedron Toskane A-Jacks® Core-LocTM Accropode Dolos Tribar Figure 2.9. Concrete armor units.

Another approach to using concrete armor units for pier scour protection has been investigated by the Armortec Company and involves the installation of banded modules of the A-Jacks® armor unit (Ayres Associates 1999; Thornton et al. 1999). Laboratory testing results and installation guide- lines developed at CSU by Ayres Associates (1999) for the A- Jacks® system are also presented in HEC-23 (Lagasse et al. 2001) and illustrate the “modular” design approach in con- trast with the “discrete particle” approach for Toskanes. The discrete particle design approach illustrated by the Toskane design guidelines in HEC-23 concentrates on the size, shape, and weight of individual armor units, whether randomly placed or in stacked or interlocked configurations. In contrast, the basic construction element of A-Jacks® for pier scour applications is a “module” composed of a mini- mum of 14 individual A-Jacks® banded together in a densely interlocked cluster, described as a 5x4x5 module. The banded module thus forms the individual design element as illus- trated in Figure 2.10. It should also be noted that concrete armor units, depend- ing on their size, may be very susceptible to vandalism. In ad- dition, there may be maintenance and degradation issues as- sociated with any cables used to tie groups of concrete armor units together. Gabions Gabion systems, which include box gabions, gabion mat- tresses, and sack gabions, are containers constructed of wire mesh or other material and filled with loose stones or other similar material (Figure 2.11). The stones used to fill the con- tainers can be either angular rock or large cobbles. Unlike cobbles, angular rocks used to fill the gabions interlock natu- rally, which provides additional strength to the unit. Gabions have been used for streambank protection for more than 100 years in Europe and have gained increasing popularity in the United States, especially in the desert Southwest. Like riprap, they are porous, being composed of loose rock, and are not susceptible to uplift forces. They can be stacked to form a wall or joined together to form a large mattress. If the configuration is undermined or becomes un- stable, the inherent flexibility of the wire mesh allows them to mold themselves to the bed or bank, thus restoring stability to the unit. In addition, the use of a wire mesh allows for the use of relatively small stones, which can yield the same amount of protection characteristic of much larger units in loose configurations. Maccaferri, Inc. first developed the gabion in 1884 and has since compiled a considerable body of information on the gen- eral design and use of gabions in the field. However, much of the information is essentially anecdotal and few independent tests and quantitative design guidelines exist (Parker et al. 1998). Parker et al. (1998) and Lauchlan (1999) provide com- prehensive reviews of the literature on gabions. Brown and Clyde (1989), CUR (1995), Maynord (1995), and Escarameia (1998) all provide guidelines on design and installation of gabions as bed and bank revetment. The model testing con- ducted by Simons et al. (1984) is probably the most substantial attempt to obtain quantitative design guidelines and criteria for gabion mattresses in the fluvial environment. However, their experiments do not provide a direct test of the perform- ance of gabion mattresses as pier scour countermeasures. Information on the design and use of gabions as a pier scour countermeasure is scarce. Parker et al. (1998) provide some design recommendations for the installation of gabions around bridge piers. Yoon and Kim (1999) conducted experiments under clear-water conditions to investigate the effectiveness of a sack gabion as a scour countermeasure at bridge piers and used the results to derive formulas for sizing the gabions. The effectiveness and stability of gabions as pier scour coun- termeasures appear to vary. According to Parker et al. (1998), a report from New York State suggests that they have not per- formed well in the field there. Lauchlan (1999) indicates that gabion mattresses with an underlying geotextile filter performed poorly as a pier scour countermeasure at the Whakatane River Bridge on State Highway 30 in New Zealand. Yet in a survey conducted by Parker et al. (1998), gabions re- ceived a favorable review from most state engineers surveyed. It may seem intuitive that gabions should be effective as pier scour countermeasures, especially if they are installed with an underlying filter or geotextile and a seal at the pier is provided. However, the passage of bed forms could cause the wire mesh to break under tension during deformation of the gabion and allow the fill stones to be removed from the basket. In addition, uplift forces or piezometric gradients below the geotextile may cause warping of the gabion mattress and cause it to pull away from the pier, thus inducing or enhancing scour around the pier face and further destabilizing the gabion unit. The gabion mattress may also pull away from the pier face if there is sig- nificant edge settlement associated with winnowing or the pas- sage of bed forms. These factors appear to have contributed to the failure of the gabions used to counter pier scour at the Whakatane River Bridge on State Highway 30 in New Zealand (Lauchlan 1999). Anchoring the gabion with long steel rods may partially or completely alleviate these problems. Finally, the maintenance requirements for gabions may be somewhat higher than for other forms of revetment because the wire mesh used to construct the gabion is susceptible to abra- sion and corrosion, and because gabions are also very suscepti- ble to vandalism. Based on field studies conducted for Caltrans, Racin and Hoover (2001) have developed standard plans and material specifications for mesh types and corrosion-resistant coatings for use in gabions. Parker et al. (1998) also provide general design recommendations for the use of gabions. 28

29 Source: Ayres Associates (1999) Figure 2.10. A-Jacks® modules for pier scour protection.

30 GABION RENO MATTRESS SACK GABION Source: modified from Hemphill and Bramley (1989) Figure 2.11. Types of gabions and typical dimensions.

Grout-Filled Bags and Mattresses Grout-filled bags (including sacked concrete) and mat- tresses are fabric shells that are filled with concrete. These countermeasures may be the simplest and most cost-effective alternatives to riprap. They are used in areas where the avail- ability of riprap is limited or where it is expensive to use, where there are environmental restrictions that limit the use of riprap, where the size of the bridge opening and channel are small, or where equipment access is limited. Concrete also has the advantage of being a well-known and often-used construction material familiar to bridge engineers. Parker et al. (1998) and Lauchlan (1999) provide a compre- hensive review on the use of grout-filled bags and mattresses as pier scour countermeasures. The bulk of the literature on research pertaining to the use of grout-filled bags and mat- tresses as pier scour countermeasures is contained in Fotherby (1992, 1993), Bertoldi et al. (1996), Jones et al. (1995), and Stein et al. (1998). The researchers determined that properly installed grout-filled bags and mattresses reduce scour depth to a degree generally comparable with riprap. Grout-Filled Mattresses. The grout-filled mattress is a single, continuous layer of strong synthetic fabric sewn into a series of compartments that are connected internally by ducts. The compartments are then filled with a concrete grout that, when set, forms a mat made up of a grid of connected blocks or pillows. While the individual blocks may articulate within the mattress and the mattress remains structurally sound, the general design approach is to consider the mattress as a rigid monolithic layer. In some cases, the mattress may be strength- ened with cables installed similar to those used in articulating concrete blocks. Depending upon the proprietary system, fil- ter points or weep holes allow for pressure relief through the mattress. Grout-filled mattress systems can range from very smooth, uniform surface conditions approaching cast-in- place concrete in terms of surface roughness, to extremely ir- regular surfaces exhibiting substantial projections into the flow, resulting in boundary roughness approaching that of moderate size rock riprap. Because this type of revetment is quite specialized, comprehensive technical information on specific mattress types and configurations is available from a number of major manufacturers of this type of revetment. In a survey of bridge engineers conducted by Parker et al. (1998), grout-filled mattresses were ranked poorly in terms of cost and maintenance but were ranked favorably with regard to debris susceptibility and environmental disruption. In contrast, grout-filled bags were ranked favorably because of their minimal need for expertise or equipment, their rapid installation, and their cost effectiveness. Problems with water quality, climatic conditions, aesthetics, and social acceptabil- ity were defined as drawbacks to the use of grout-filled bags. The primary failure mechanisms for grout-filled mattresses consist of rolling, undermining, and scouring at gaps (Fotherby 1992). Rolling, the most severe form of failure, is re- lated to uplift forces created by flow over the mattress. This flow allows the mattress at midsection to be “lifted up” slightly and then pushed loose by the force of the current or allows the edges of the mattress to be rolled back. Undercutting is a grad- ual process arising from local scour at the mattress edges and from the main horseshoe vortex. Scouring at the gaps between the mattress and the pier wall allows the horseshoe vortex to generate a scour hole beneath the front edge or side sections of the mattress. The research to date on the use of grout-filled mattresses as a bridge scour countermeasure found that placement is ex- tremely important for successful performance and effective- ness. Properly placed grout-filled mattresses extending 1.5 to 2 pier widths were found to provide significant protection to bridge piers. Fotherby (1992) recommends that grout-filled mattresses should be placed at bed level and suggests that toe- ing in the mattress may increase stability with regard to potential rolling failure and undercutting, especially under live-bed conditions. Bertoldi et al. (1996) recommend that anchors be used to protect the leading edge against uplift forces when the mattress is placed on the surface of a loose, erodible channel bed. Jones et al. (1995) and Stein et al. (1998) stress the importance of a tight seal around the pier-mattress interface to inhibit scour and undermining beneath the mat- tress. Lagasse et al. (2001) provide mattress selection and sizing criteria based on analysis of sliding stability. Grout-Filled Bags. Grout/cement-filled bags have been used extensively as bank protection and are gaining in popu- larity as a countermeasure against scour at bridges. Histori- cally they have been used to fill undermined areas around bridge piers and abutments. As scour awareness increases, grout-filled bags are being used to armor channels where scour is anticipated or where scour is detected, such as around bridge piers. They are relatively easy to install and come in a wide range of sizes, depending on the application. Engineer- ing judgment is often used to select a bag size that will not be removed by the flow. As in the United States, grout-filled bags in the United Kingdom and Europe are used primarily as an emergency or temporary scour countermeasure. Failure of grout-filled bags can occur from undersized bags, local scour around the bags, a shift in the grout bags, and undercutting of the filter fabric when used with the grout bags. Undersized bags can be swept away by currents. The bags may shift or slide either by winnowing and scour around the bags or the passage of bed forms. If the bags protrude into the flow, they create their own local scour pattern, which contributes to the undermining of the filter fabric where 31

used. Undermining of the underlying geotextile, where used, can occur as a result of local scour induced by grout-filled bags protruding into the flow, by edge erosion, or by the pas- sage of bed forms. As bags slide off the underlying fabric, more fabric is exposed, which contributes to additional un- dermining and instability. Research also indicates that the effectiveness of grout-filled bags as a pier scour countermeasure is dependent upon the size, placement, use of a filter fabric or geotextile, tightness of the seal to the pier face, and the lateral extent of the revet- ment apron. As with riprap and grout-filled mattresses, cur- rent practice indicates that the grout-filled bag protection should extend 1.5 to 2 pier widths out from the pier. Bags placed along the side of the pier aligned flush with the front of the pier tend to be prone to failure; a staggered placement provides better protection and greater stability. The use of a geotextile or filter fabric, preferably toed in, is recommended. Studies show that the use of grout-filled bags without a geot- extile or filter fabric results in settlement of the bags into the bed and formation of a scour hole beneath them at the front of the pier. A single layer of properly sized grout-filled bags with an appropriate lateral extent was found to be more ef- fective than stacked bags. Undersized grout-filled bags can be washed away and sound engineering judgment should be used in sizing bags. Because of problems of comparison using a unit diameter (d50) to determine particle stability, studies conducted by Bertoldi et al. (1996), Jones et al. (1995), and Stein et al. (1998) used the height of the grout-filled bags for d50 in applying the Shields and Isbash criteria. Fotherby (1992) provides the fol- lowing limiting criteria for sizing grout-filled bags that are not rigidly connected: • Shorter height produces less scour when the bag is exposed to the flow field. • Shorter height in a rectangular bag is better able to resist overturning. • Under incipient motion tests, length contributes to failure when the bags are aligned perpendicular to flow; the longer bags fail first. • Longer bags are less able to adjust to bed elevation changes and tend to span scour holes rather than conform to the channel bed • Wider bags reduce labor and installation costs when cov- ering a large area. • Increased width helps reduce overturning. • Wider bags do not adjust as well to bed elevation changes and lose their ability to conform to changes in the channel bed. Based on a scour evaluation program developed by the Maryland State Highway Administration (MDSHA), Thorn- ton (1998) documents the few problems and multiple benefits associated with the use of grout-filled bags at small, relatively inaccessible bridges. Based on field experiences with the use of grout-filled bags for scour countermeasures, Thornton (1998) provides tips on their installation. These tips and additional rec- ommendations on the specifications, design, and installation of grout-filled bags, based on information provided by MDSHA, are included in HEC-23 (Lagasse et al. 2001). Parker et al. (1998) suggest that the stability and performance of grout-filled bags can potentially be improved by imbricating (i.e., shin- gling) the bags or increasing the effective weight of the concrete by spiking it with high-density material. Rigidly connecting the bags by cable or rods or sewing the bags together should be avoided because these techniques significantly reduce the flexi- bility of the system. Geotextile Containers In Europe, a significant investment has been made in the development and testing of geosynthetic materials, and in- novative installation techniques have been developed that could find application for bridge pier and abutment counter- measures in the United States. Highly specialized laboratory equipment is available for testing a wide range of geotextile characteristics. For example, BAW published “Code of Prac- tice – Use of Geotextile Filters on Waterways” (MAG) (BAW 1993a) and has issued a complete report on the testing of geotextiles (RPG), including (1) impact tests (to determine punching resistance, e.g., when large stone is dropped on the geotextile); (2) abrasion tests; (3) permeability, clay clogging, and sand clogging tests; and (4) tests of material characteris- tics such as elongation and strength (BAW 1994). Through this testing program, geotextile materials have been devel- oped that permit innovative approaches to filter placement for riprap and other countermeasures (Lagasse 1999). Because of the extensive testing program in Europe, geotex- tile filters can be manufactured with consistent quality and in accordance with the requirements of a specific application. The wide choice in synthetics also allows the use of an inert material that will not interact with the environment. While the filtration capacity of woven geotextiles is restricted to narrowly graded grain size distributions, a non-woven fabric can be designed for nearly any given grain size distribution of the subsoil. It is also possible to combine a woven and a non-woven geotextile to combine, for example, good filtration capacity with high strength. The main function of geosynthetics in scour counter- measures is that of a filter, but they also can be used as contain- ment or as reinforcement (Heibaum 2000, 2001, 2004). The development in Germany of geotextile containers, or geocontainers, as a filter or as a stand-alone countermeasure is one of the concepts that has benefited from the geosynthetic material testing program. Geotextile containers are large bags 32

made of mechanically bonded non-woven fabrics up to 44 ft3 (1.25 m3) in volume partially filled with sand and gravel filter material (Figure 2.12). They have been used to provide a filter layer for riprap installation at a number of large projects in Germany (Heibaum 2000). The containers are sewn on three sides at a factory and filled on site to approximately 80% of capacity with sand/gravel filter material using a hopper sys- tem. The final seam is sewn on site. The containers are placed in layers using a side-dump pontoon or bottom-dump split barge. The flexibility of the fabric and partial filling allow the containers to conform to irregularities in the channel bed at the installation site, especially where very large scour holes have developed (Lagasse 1999; Heibaum 2000). Riprap or partially grouted riprap can then be placed over the layer of geotextile containers as an armor layer. Because geotextile containers are designed as filters for a specific subsoil, it is essential that there are no gaps between the individual containers. Usually at least two layers of con- tainers are required. Figure 2.13 shows a schematic installation of two layers of geotextile containers and riprap as a pier scour countermeasure. Thus, geocontainers are multi-purpose ele- ments. They can be manufactured to site-specific size, shape, filtration capacity, and strength and, according to the demands of a specific site, only a few containers may be necessary, or many may be required (Heibaum 2000). Heibaum (2000) provides general guidelines on the design and installation of geotextile containers. Pilarczyk (2000) presents the state of the practice in the design and installation of geocontainers including general design considerations, analysis of dumping process, stresses associated with opening the barge and during free fall and impact on the floor, the final shape of the geocontainer, deformations due to lateral loading and wave attack after placement, scaling rules for model tests, and calculation methods that can be used as design rules. 2.1.5 Federal Waterways Engineering and Research Institute (BAW) Guidelines and Codes Task 1 included preparing an English translation of three BAW documents formerly available only in German: • “Code of Practice – Use of Cement Bonded and Bitumi- nous Materials for Grouting of Armor Stones on Water- ways” (MAV) (BAW 1990) • “Guidelines for Testing of Cement and Bitumen Bonded Materials for the Grouting of Armor Stones on Water- ways” (RPV) (BAW 1991) • “Code of Practice – Use of Standard Construction Meth- ods for Bank and Bottom Protection on Waterways” (MAR) (BAW 1993b) The initial translation of these documents was accom- plished by Dr. Kornel Kerenyi of GKY and Associates. Final translation was completed by Dr. Michael Heibaum of BAW in collaboration with Dr. P.F. Lagasse, the project principal investigator. The translations of these documents are included in the Reference Document (available on the TRB website: http://www.trb.org/TRBNet/ProjectDisplay.asp?ProjectID= 702). A fourth BAW document, “Code of Practice – Use of Geotextile Filters on Waterways” (MAG) (BAW 1993a), was obtained in English during the TRB/FHWA 1998 scanning review (Lagasse 1999). These documents form the basis for many of the guidelines and specifications in Chapter 3 for the application of partially 33 Source: Heibaum (2000) Figure 2.12. Batch plant for filling numerous geotextile containers on site. FLOW Sand - filled geocontainers Rock riprap placed flush with channel bed Pier Source: modified from Heibaum (2000) Figure 2.13. Schematic of pier scour repair using geocontainers as filter and fill with riprap as a cover layer.

grouted riprap and specialized geotextiles as pier scour coun- termeasures. 2.2 Performance Evaluation at Existing Sites 2.2.1 Introduction During NCHRP Project 24-07, the University of Min- nesota research team conducted a survey of field sites in 18 states representing different physiographic regions of the contiguous United States. The survey during 1996 was designed to develop first-hand knowledge of installation and inspection requirements for bridge scour countermeasures. Whenever possible the team identified the actual mode(s) of failure for existing installations. Of paramount importance was the identification of the controlling hydraulic, geomor- phic, geotechnical, aesthetic, and environmental parameters that can affect constructability, reliability, maintainability, and cost. Key findings from visits to 88 field sites are dis- cussed in Section 2.2.2. An additional 15 project sites were visited and evaluated by the NCHRP Project 24-07(2) research team in 2001. Of these 15 sites, 9 specifically involved scour at piers; the remainder consisted of abutment scour protection, bed or bank revet- ment, or other scour prevention applications where the use of specific materials or placement equipment of interest to the project team was investigated and/or demonstrated. Table 2.3 provides a brief summary of the field sites evaluated during NCHRP Project 24-07(2), organized by countermea- sure type. A discussion of the NCHRP Project 24-07(2) site visits in the United States—which included installations with riprap, articulating concrete blocks, and grout-filled mattresses as pier scour countermeasures as well as gabions and gabion mattresses installed for abutment scour protection—is pro- vided in Section 2.2.3. The findings from the NCHRP Project 24-07(2) site visits in Germany to investigate partially grouted riprap installations and geotextile sand containers are also presented in Section 2.2.3. The NCHRP Project 24- 07(2) interim report included a discussion of concrete armor units installed as a pier scour countermeasure, but no further investigations were made. 2.2.2 Key Findings: Phase 1 Site Visits The Phase 1 site visit findings are reported in Chapter 7 of Parker et al. (1998) and are summarized briefly here. Two pri- 34 Countermeasure Type/ Application Location/Structure I.D. Comment Riprap Multiple piers California Wash, Nevada Bridge B-839S, Interstate 15 near Moapa, Nevada Placed dry Multiple piers Piute Wash, Nevada Bridge B-420, U.S. Hwy 95 near Cal-Nev-Ari, Nevada Placed dry Single pier Colorado River, Colorado Bridge G-04-BA, Interstate 70 near De Beque, Colorado Placed under water Partially Grouted Riprap Bed and bank revetment Dortmund-Ems Canal, Germany Canal lining Placed under water Harbor bankline revetment Wilhelmshaven Harbor, Germany Shore protection Placed dry Roof protection for highway tunnel beneath river Elbe River, Hamburg, Germany Tunnel protection on channel bed Placed under water Scour protection for surge gate sills River Ems Storm Surge Barrier, Germany Scour Protection Placed under water Articulating Concrete Blocks Piers and bed Guadalupe River, California Bridge 37-0176, I-880 near Santa Clara, California Placed dry Grout-Filled Bags None Grout-Filled Mattresses Piers, bed, and abutments Gila River, Yuma County, Arizona Three Bridges: Avenue 20E, 45E, 64E Placed dry Geotextile Sand Containers Materials and equipment demonstration Colcrete–von Essen equipment yard, Germany Filling, lifting, and dropping 1.0 m3 non-woven geotextile sand-filled containers Gabions and Gabion Mattresses Abutment scour protection Guadalupe River, California Bridge 37-0176, I-880 near Santa Clara, California Concrete Armor Units Piers Bridge 45, Marshall County, Kentucky Placed dry Piers Hillsborough County, Florida Placed dry Table 2.3. Summary of Phase 2 scour countermeasure sites evaluated.

mary methods of failure were noted for riprap aside from direct entrainment by the flow. These are failures caused by (1) insta- bility of the river bed and (2) failure caused by an inadequate fil- ter. Stream instability affects countermeasure performance by altering the hydrodynamic conditions the countermeasure ex- periences. Other types of instability can occur when a bridge opening either significantly increases or decreases the con- veyance capacity of the river. Typically, bridge openings are de- signed to not restrict flow past the bridge under flood condi- tions. During typical river discharges however, this design practice may lead to sediment deposition affecting counter- measure performance by locally altering flow patterns. Adequate filtering should be placed under countermea- sures to prevent subsidence-related failures of the counter- measures. Riprap in particular can sink well below the bed surface. This subsidence only becomes a serious problem if riprap sinks to a level below which it provides adequate pro- tection; however, it becomes a maintenance problem as soon as riprap sinks to a level preventing routine detection during inspection. Winnowing of fines for other countermeasures can lead to voids underneath the countermeasure and gen- eral undercutting of the countermeasure if the countermea- sure is not flexible. A brief summary of the issues relevant to designing and installing pier scour countermeasures follows (Parker et al. 1998): • Two primary methods of failure were noted for properly sized riprap – Instability of the river bed – Failure caused by an inadequate filter • Countermeasure failure due to stream instability was con- sistently reported by the host engineers in most states. Many designers and nearly all maintenance personnel simply do not have the tools to effectively address stream stability issues. • When dumped riprap is placed, caution must be exercised to ensure that segregation of the riprap does not occur and areal coverage is sufficient. • The effect of localized drainage on countermeasure per- formance must be considered. Roadway ditches often dis- charge at 90˚ to the river channel and can subtly undercut scour protection, rendering the countermeasure less effec- tive when a large flood arrives. Mitigation requires that drainage and bridge engineers work together to ensure that designs integrate well and are mutually effective. • Geotextile must be placed so that no gaps are present, or can form, between geotextile and any structure it is protecting. • Wire and cabling selection for gabions, gabion mattresses, and cable-tied blocks should be limited to non-corrosive materials. Field experience indicated that even well-specified galvanized, coated wire was subject to internal corrosion. • Gabions should be inspected for basket tearing caused by riverborne debris following floods that exceed bankfull. • Grout-filled bags were effective for small bridges, but un- dercutting was observed at the sides and end of bags when bags were too large to settle effectively. 2.2.3 Key Findings: Phase 2 Site Visits The Phase 2 research team selected existing installations in the United States of several countermeasure types, counter- measure filters, and combinations of countermeasures listed in Task 1 and conducted a systematic analysis of the performance of the installations (Table 2.3). The analysis of performance included an in-depth documentation of the hydraulic and structural design of the installations as well as documentation of the construction, maintenance, and inspection considerations of each installation as well as an evaluation of the performance of each installation to date. The installations were geomorphically diverse and included underwater and dry placements. Articulating Concrete Block Performance in the United States The Phase 2 site visits in the United States are described in detail in a trip report supplied to the NCHRP Project 24-07(2) panel. One site on Guadalupe River at Interstate 880 near San Jose, California, with an ACB countermeasure was visited during Phase 1 (1996) and again during Phase 2 (2001). The comparison of performance over a 5-year period, which in- cluded several significant floods, is summarized here. The I-880 bridge over the Guadalupe River is a two-span structure with a mid-channel wall pier. The structure was identified as scour critical in early 1992. The 100-year dis- charge at the site is 17,000 ft3/s (482 m3/s); however, the Stan- dard Project Flood (SPF) in-channel discharge of 24,000 ft3/s (680 m3/s) was used for scour analysis and countermeasure design, with an associated maximum design velocity esti- mated at 13.4 ft/s (4.1 m/s). Unprotected scour depth was estimated at 15.5 ft (4.7 m) at the pier and 20.5 ft (6.25 m) at the abutment. An ACB scour countermeasure was designed and con- structed in mid-1992 under the direction of the U.S. Army Corps of Engineers, Sacramento District. The design utilized the factor of safety method as described in HEC-23 (Lagasse et al. 2001) with a target safety factor of 2.0. Overall project costs for improvements to the channel reach, excluding util- ity relocates, were $2.9 million. Of that total, approximately half pertained to clearing, grading, materials, and installation costs associated with scour protection (ACB and gabion mat- tress). The cost for the materials and installation of 4,700 yd2 (3930 m2) of ACB alone was $324,000. Figure 2.14 provides a 35

photograph of the installation. Figure 2.15 provides a schematic diagram showing relevant dimensions of the in- stallation, while Figure 2.16 shows a close-up of the interface between the ACB system and the gabion mattress. Initial field inspection was performed by the NCHRP Proj- ect 24-07 research team in June 1996 (4 years after construc- tion), and the site was re-examined by the NCHRP Project 24- 07(2) research team in December 2001. The condition of the countermeasure in December 2001 appeared to be identical to that described in the NCHRP Project 24-07 report. The ma- jority of the installation is in excellent condition and during the period between these examinations has withstood at least three events of approximately 3,000 ft3/s (85 m3/s) in magni- tude with no further deterioration. Gaging station data are available from U.S. Geological Survey (USGS) gage near San Jose, California. A record from 1992 is provided in Figure 2.17. Minor areas of local subsidence through the reach were noted in the NCHRP Project 24-07 report; in these areas, the ACB mat responded as intended and remained in intimate contact with the subgrade as revealed by the December 2001 examination. The interface between the ACB system and the pier was grouted adjacent to the edge of the concrete pile cap (Figure 2.18). Downstream of the bridge, two areas of subgrade subsi- dence that were bridged by the ACB, causing local voids beneath the revetment, were noted in the inspection reports by both research teams. These areas are located at the down- stream edge of the ACB mat at a plunging transition back into the natural channel bed. At this point, a continuous dis- charge of effluent enters the river from an outlet structure on the left bank. The blocks terminate in a plunging transition at the downstream edge of the installation and are toed down into the bed of the natural channel at that point. These void areas have apparently never been repaired since they were first observed and in December 2001 appeared to be qualitatively identical to the conditions described in June 1996. In addition, the gabion mattresses on the overbanks and abutments were intact and in good condition. The wire of the baskets did not show evidence of corrosion, abrasion, or distortion from vegetative growth that has begun to in- 36 Figure 2.14. Interstate 880 bridge over the Guadalupe River, San Jose, CA (looking downstream). Profile 20’ min Bike / 18 inch gabion 4.75 inch cabled ACB Cross Section Figure 2.15. Guadalupe River pier/abutment scour countermeasure schematic diagram. Figure 2.16. Close-up of interface between cabled ACB and gabion mattress.

37 GUADALUPE RIVER NEAR SAN JOSE, CA I-880 Pier Scour Countermeasure 0 1000 2000 3000 4000 5000 M ea n da ily d is ch ar ge , f t3 / s 6000 7000 8000 9000 Jan-1992 Jan-1993 Jan-1994 Jan-1995 Jan-1996 Jan-1997 Jan-1998 Jan-1999 Jan-2000 Jan-2001 Jan-2002 Jan-2003 Year USGS Gaging Station 11169000 Articulating concrete block Q100 = 17,000 ft3/s Figure 2.17. Mean daily hydrograph of Guadalupe River near San Jose, CA. trude into the edges of the construction works both upstream and downstream of the bridge. Site Visits in Germany As discussed in Section 1.2.3, two research team members visited installations in Germany to evaluate the potential for application of geotextile containers and partially grouted riprap as a pier scour countermeasure and the performance of partially grouted riprap in a high-velocity, high-turbulence en- vironment. The visit provided an opportunity to evaluate field performance of these countermeasures and the potential for adapting this technology for application in the United States. The site visit was coordinated by Dr. Heibaum of the BAW laboratory and included evaluation of specialized equipment developed by contractors for placing grout for various specifica- tions of extent of coverage and penetration into the riprap ma- trix, above or under water. Mr. Trentmann of Gewatech, who is an industry expert in partial grouting techniques, assisted with the site visit. The visit was conducted from September 22–29, 2001, and involved the principal investigator (PI) and Co-PI from the Phase 2 research team and an FHWA representative. Partially Grouted Riprap. Although the state of the prac- tice in partial grouting of riprap has achieved a high level of reliability and sophistication through methods advanced in Germany, the research team did not identify a site to examine where partially grouted riprap was used as a specific applica- tion for mitigating pier scour at a bridge. Four project sites in Germany were examined where partial grouting of riprap was used. Two of these sites were actively under construction at the time of the visit. The nature of the de- sign loading at the four sites included barge-induced draw- down, barge-induced wave attack, propeller wash, coastal wave attack, high-velocity currents associated with the operation of gates at a tidal storm surge barrier, and anchor drag. In addi- tion, design methods, laboratory test procedures, and field placement quality assurance/quality control (QA/QC) proce- dures were reviewed with German researchers and contractors. The following four sites using partially grouted riprap were examined: • Dortmund-Ems Canal (bed and bank revetment, prima- rily to mitigate barge-induced hydraulic loading) • Wilhelmshaven Harbor (coastal wave attack environment)

38 Figure 2.19. Larger effective aggregate size of partially grouted riprap. Source: Colcrete–Von Essen Inc. Figure 2.20. Grouting frame used for underwater placement. • Elbe River highway tunnel beneath waterway (protection against accidental anchor drag) • River Ems storm surge barrier (high-velocity currents cre- ated during gate operation) Partial grouting of riprap results in a revetment matrix that achieves greater stability than an ungrouted installation. This greater stability implies that for a given hydraulic loading, a smaller class of riprap can be used when partial grouting is incorporated in the design. The grout creates a larger effective aggregate size, while maintaining a suitable degree of porosity and permeability of the installation (Figure 2.19). This charac- teristic is obviously desirable under conditions where large hydraulic gradients can occur. Partially grouted riprap also maintains a degree of flexibility compared to rigid, fully grouted rock and can therefore withstand moderate amounts of differ- ential settlement or frost heave without losing integrity. Partial grouting can be performed both in the dry and under water. In the latter case, polymer admixtures are included in the mix design to prevent segregation during placement; the generic term for this specialized grout mix is “anti-wash” or “sticky” concrete. Underwater placement is typically accomplished by a global positioning system (GPS)–positioned frame that holds multiple injection noz- zles approximately 1 ft above the surface of the riprap (Figure 2.20). Considerable guidance for the design, installation, and as- sociated testing (both laboratory and field) of partially grouted riprap has been developed by the BAW. Addition- ally, considerable effort has gone into the understanding of filter requirements and the development of guidelines for the testing, selection, and placement of filters for revetment and scour protection applications. The NCHRP Project 24-07 research team identified a lack of guidelines for the selection and placement of filters for scour protection works in the United States, and developed prelimi- nary recommendations to accommodate this need. More recent work accomplished in the United States under NCHRP Project 24-23, “Riprap Design Criteria, Specifications, and Quality Control” (Lagasse et al. 2006), combined with the guidance and information derived from the German experience, has effec- tively bridged this gap. A demonstration project in the United States of placement of partially grouted riprap during Phase 2 represented an ideal vehicle for importing German technology to the United States (see discussion in Section 3.5). Figures 2.21, 2.22, and 2.23 show an installation in Germany being placed in the dry using a small, mobile batch plant and five-man crew. Geotextile Containers. Geotextile sand containers are made of very thick, high-strength non-woven geotextile fabric Figure 2.18. Close-up of grout interface between ACB system and pile cap at pier.

rupture. The research team was not able to visit and examine any field sites utilizing sand-filled geocontainers, although the filling, sewing, and placement techniques were demonstrated in the construction yard of Colcrete-Von Essen in Rastede, Germany (Figure 2.24). The geocontainers are filled to 80% capacity with sand and sewn shut. Because they are not filled completely, they remain flexible and deformable. The containers may represent a par- ticularly well-suited means of filling existing scour holes under water, where the site cannot be dewatered or where strong currents prevail. Placement by conventional construc- tion equipment is readily achieved (Figure 2.25). Although the BAW has developed laboratory tests and design guidelines for strength, abrasion resistance, and puncture resistance, the long-term survivability of the geotextile containers as a stand- alone countermeasure is not known, particularly in a high- bedload environment. 2.3 Merits and Deficiencies of Pier Scour Countermeasures 2.3.1 Life-Cycle Factors This section presents an evaluation of merits and deficien- cies of life-cycle factors for each scour countermeasure type as assessed in the context of pier scour applications. The evalua- tion considers existing design, installation, and maintenance guidance derived primarily from HEC-23 and modified, where applicable, by the NCHRP Project 24-07 final report, review of more current literature and studies, the investigation of field sites both in the United States and Germany, and the experi- ence and judgment of the research team. 39 Source: Gewatech-Soil and Hydraulic Engineering Figure 2.21. Partial grouting of riprap performed in the dry. Source: Gewatech-Soil and Hydraulic Engineering Figure 2.22. Close-up of completed partial grout installation. Source: Gewatech-Soil and Hydraulic Engineering Figure 2.23. Mobile batch plant used for partial grouting in dry conditions. that is premanufactured as an open-ended “pillow,” filled with sand, and sewn shut at the end. Standard sizes manufactured in Germany are 0.25, 0.50, and 1.0 m3 in volume. The non- woven geotextile, typically 4.6 to 6 mm or thicker, provides exceptional elongation (stretching) before it begins to tear or

Design Specifications and Guidelines and Performance Evaluation Guidelines These factors were used to form the basis for structuring the laboratory testing activities for this project. These two criteria must incorporate the differences in functional application of the various countermeasures as well as the failure mechanisms unique to each countermeasure. Construction Specifications and Guidelines Construction specifications and guidelines consider the different needs and challenges required for placing a coun- termeasure in the dry as well as installing it under water, or in flowing water. In addition, the requirement for specialized equipment must be addressed. For example, the equipment requirements, placement techniques, and construction QA/QC sampling and testing procedures for partially grouted riprap are quite straightforward when working in the dry; however, placement under water requires much more specialized equipment and greater degree of sophistication. Grading requirements and placement tolerances also vary among countermeasure types. For example, a relatively thin veneer of articulating concrete blocks requires finer grading techniques than an equivalent, and much thicker, riprap layer. Alternative placement techniques, particularly for rock riprap, typically dictate the strength requirements for geotex- tiles in order to meet construction survivability criteria. Maintenance and Inspection Guidelines Maintenance and inspection guidelines will vary greatly among countermeasure types. Underwater or buried installa- 40 Source: Colcrete–Von Essen Inc. Figure 2.25. Conventional construction equipment handling sand-filled geocontainer. The factors considered include the following: • Selection criteria • Design specifications and guidelines • Construction specifications and guidelines • Maintenance and inspection guidelines • Performance evaluation guidelines • Life-cycle cost information Selection Criteria This factor was considered from the technical, if some- what qualitative, standpoint of functional application in a specific river environment under a given set of design hydraulic, geometric, and sediment transport conditions. Other evaluation criteria, such as constructability, mainte- nance, and cost, also play an important role in the selection of a pier scour countermeasure for application at a specific site. The HEC-23 countermeasure suitability matrix provides a valuable background for this guidance (Lagasse et al. 2001). Source: Colcrete–Von Essen Inc. Figure 2.24. Geotextile sand container (1.0 m3) being filled from hopper.

tions require different considerations to ensure that the coun- termeasure can be adequately inspected, compared to surficial treatments in ephemeral or intermittent stream environments. Maintenance requirements can range from “dump more and larger” in the case of riprap to “remove, redesign, repair, and replace” activities for manufactured systems. Even for a single countermeasure type, such as articulating concrete blocks, maintenance requirements may differ depending on the type of damage or deterioration the countermeasure has suffered. Life-Cycle Cost Life-cycle cost information is difficult to quantify. Initial construction costs are relatively easy to develop; however, even for a specific countermeasure, these costs can vary widely depending on regional availability of materials, site condi- tions, and access constraints. Therefore, a countermeasure type can be very cost effective in one locale and prohibitively expensive in another. Extending these issues to life-cycle maintenance requirements requires an even broader set of assumptions. Riprap, for example, is a standard countermea- sure type in many states; however, alternatives to riprap may need to be investigated because of cost and availability limita- tions. The risks and consequences of failure at any given site further complicate the issue. For these reasons, life-cycle costs were not considered in the tabulation of merits and deficien- cies, but are the focus of the countermeasure selection methodology developed in Chapter 3. 2.3.2 Merits and Deficiencies by Life-Cycle Factors The factors are presented and discussed in Tables 2.4 through 2.9, organized by countermeasure type. Because the 41 Factor Merits Deficiencies Selection criteria HEC-23 suitability matrix provides qualitative guidance Flexible and porous Layer thickness allows self- healing Often used as default countermeasure with little or no design Required rock size not always economically available Design specifications and guidelines Sizing and gradation criteria well established Rock suitability requirements well established Filter characteristics well established (geotextile and/or granular) HEC-23 provides baseline design guidance Layout dimensions preliminary Filter often overlooked Prior excavation recommended Relative impact of various scour mechanisms not well understood Contraction scour and long-term degradation not considered when determining extent Construction specifications and guidelines Standard construction equipment typically used for placement Can be placed under water or dry Can accommodate irregular subgrade conditions Geotextile filter preferred but difficult to place under water (see geotextile sand containers, Table 2.6) Effective filter seal against pier required Difficult to place larger stone in areas with limited access beneath bridge deck Maintenance and inspection guidelines Standard 2-year inspection frequency, and recommended inspection after a flood event Inspectors are familiar with this countermeasure Maintenance consists essentially of "dumping more" Visual inspection not always reliable because of launching and subsequent redeposition; may require supplemental probing Performance evaluation guidelines HEC-23 provides current design recommendations, including prior excavation Riprap will provide laboratory benchmark to determine performance for this research Modes of failure well known (typically include particle dislodgement, substrate winnowing, or edge deterioration) Phase 2 laboratory testing required to establish benchmark Phase 2 laboratory testing required to determine adequacy of filter recommendations Phase 2 laboratory testing required to confirm edge details Contraction scour and long-term degradation not considered when determining extent Life-cycle cost information Not considered Not considered Table 2.4. Riprap merits and deficiencies.

long-term survivability of geotextile sand containers is not known, particularly in a high bedload riverine environment, the application considered in this study will be as a filter for either a riprap or partially grouted riprap armor layer. No attempt has been made to rank the factors or provide any quantitative measure for comparison between and among countermeasures. The objective was to identify gaps in the current state of the practice for use in developing a prelimi- nary set of recommended guidelines for each countermea- sure, and to lay the groundwork for structuring a laboratory investigation program to address these gaps. Deficiencies addressed in this study that are common to most of the countermeasures described in these tables in- clude (1) countermeasure extent and edge details, (2) filter extent requirements, and (3) application to pier scour condi- tions (in comparison to riprap). Deficiencies addressed that are specific to individual countermeasures include (1) riprap stability as a benchmark for comparison and (2) guidance for selecting an appropriate target factor of safety for pier scour applications using ACB or grout-filled mattress systems. Deficiencies that are not addressed in this study that are common to most of the countermeasures described in the ta- bles include (1) relative impact of various combinations of scour mechanisms (i.e., only local pier scour will be consid- ered); (2) countermeasure extent and/or thickness require- ments related to contraction scour and degradation; and (3) testing of proprietary systems. 2.3.3 Summary The information presented in Tables 2.4 through 2.9 is qualitative in nature. The discussion of merits and deficien- cies is not intended to provide a ranking or prioritization of countermeasures; however, the information can be used to 42 Factor Merits Deficiencies Selection criteria Porous More flexible than fully grouted riprap Allows smaller riprap class to be used compared to standard (ungrouted) riprap Less flexible than standard riprap General lack of familiarity with product in the United States Design specifications and guidelines German manuals (MAR, MAV, RPV) provide materials, testing, and installation specifications HEC-23 provides baseline design guidance for standard riprap Sizing and gradation criteria well established Rock suitability requirements well established Filter characteristics well established Grout mix specifications well established Layout dimensions preliminary Termination/edge treatment uncertain Filter extent uncertain Prior excavation preferred Hydraulic loading conditions for pier scour application not well defined Relative impact of various scour mechanisms not well understood Contraction scour and long-term degradation not considered when determining extent and layer thickness Construction specifications and guidelines Can be placed under water or dry For dry placement, grout can be installed by hand or by mechanized injection frames Can accommodate irregular subgrade conditions Specialized equipment required for underwater grout placement Specialized grout mix required Geotextile filter preferred but difficult to place under water (see geotextile sand containers, Table 2.6) Effective filter seal against pier required Maintenance and inspection guidelines Standard 2-year inspection frequency, and recommended inspection after a flood event Repair is straightforward May be difficult to detect voids beneath bridged areas under water Inspectors not likely to be familiar with this countermeasure Performance evaluation guidelines Exposure of filter or bedding underlayer generally recognized as threshold of performance Phase 2 laboratory tests needed to benchmark against standard riprap specifically for pier scour applications Life-cycle cost information Not considered Not considered Table 2.5. Partially grouted riprap merits and deficiencies.

distinguish differences between countermeasures with re- spect to certain attributes. The information provided in this section is derived from previous studies, drawing from the NCHRP Project 24-07 research effort, and supplemented by the experience and judg- ment of the research team. The primary objective of this sec- tion is to identify those factors that are suitably developed for recommending baseline design, installation, and maintenance guidelines for pier scour countermeasures; conversely, the sec- ondary objective is to identify gaps in the state of practice where additional laboratory and field work are warranted. The results of this section were used to develop a recom- mended course of laboratory work to bridge any knowledge gaps or shortcomings of a particular countermeasure prior to the development of implementation guidelines for field application. Chapter 3 presents an overview of the laboratory testing program and test results for the following pier scour countermeasures: • Riprap • Partially grouted riprap and geotextile containers • ACB systems • Gabion mattresses • Grout-filled mattresses Interpretation and appraisal of the findings of this chapter and the laboratory testing results are combined in Chapter 3 to develop guidelines and specifications for design and con- struction; guidelines for inspection, maintenance, and per- formance evaluation; and practical selection criteria for each countermeasure type. Factor Merits Deficiencies Selection criteria Can be used to repair/fill existing scour hole Flexible and porous Geotextile can be selected for compatibility with riverbed sediments Can provide a filter layer(s) underneath a more durable top layer General lack of familiarity with product in the United States General lack of experience in applications specific to pier scour Armor top layer recommended Design specifications and guidelines Filter characteristics well established German manual (RPG) provides specifications for laboratory testing methods for required physical properties of geotextile Suitability for use without an armor top layer has not been demonstrated; therefore, combination approach is recommended (see Tables 2.4 and 2.5) Relative impact of various scour mechanisms not well understood Construction specifications and guidelines Can be placed under water or dry Effective seal against pier easily accomplished Use of heavy (> 4 mm) non- woven fabric provides protection against puncture or tear during installation Multiple layering of containers ensures overlap to prevent substrate leaching/winnowing Can accommodate irregular subgrade conditions Finished surface of containers may not be suitable for use with armor that requires a fine finish grade (e.g., articulating concrete blocks) Maintenance and inspection guidelines Standard 2-year inspection frequency, and recommended inspection after a flood event Flexibility and overlap minimizes potential for voids within system Inspection of armor top layer commensurate with that particular countermeasure type Inspectors not likely to be familiar with this countermeasure Performance evaluation guidelines NCHRP 24-07(Phase 1) research team recommended the investigation of sand-filled bags as a flexible, non-rigid alternative to grout-filled bags Phase 2 testing recommended Life-cycle cost information Not considered Not considered Table 2.6. Geotextile sand container merits and deficiencies.

44 Factor Merits Deficiencies Selection criteria HEC-23 suitability matrix provides qualitative guidance Relatively flexible and porous Can accept vegetation where desired Thinner layer provides equivalent protection compared to standard riprap Single-layer "veneer" does not allow for self-healing General lack of familiarity with product Full-scale, product-specific performance testing required Typically proprietary Design specifications and guidelines Stability design criteria (sizing requirements) well established Material characteristics per ASTM D 6684 Filter characteristics well established HEC-23 provides baseline design guidance In general, little or no prior excavation required because of low profile Target Factor of Safety can be adjusted for site-specific conditions Layout dimensions preliminary Termination/edge treatment uncertain. Pre-excavated turndowns at edges recommended Filter extent uncertain Guidance for selecting target factor of safety not established Contraction scour and long-term degradation not considered when determining extent Construction specifications and guidelines Can be placed under water or dry Can be individually hand placed or placed as pre-cabled mats Hand placement allows access to confined areas, but typically limited to dry installations Geotextile may be attached directly to pre-cabled mat Geotextile filter preferred but difficult to place under water Effective filter seal against pier required Underwater placement of pre- cabled ACB mats may be difficult where access directly beneath bridge deck is limited Subgrade preparation and block placement more stringent than riprap Maintenance and inspection guidelines Standard 2-year inspection frequency, and recommended inspection after a flood event Inspectors not likely to be familiar with this countermeasure May be difficult to detect voids under bridged areas of blocks if these areas are under water Single-layer "veneer" does not allow for self-healing Underwater repair difficult Performance evaluation guidelines NCHRP 24-07 (Phase 1) report provided favorable performance review Overturning of mat or individual blocks most typical mode of failure (mode of failure well known) Phase 2 laboratory testing required to determine adequacy of filter recommendations and edge details to supplement 24-07(1) findings Life-cycle cost information Not considered Not considered Table 2.7. Articulating concrete block merits and deficiencies.

45 Factor Merits Deficiencies Selection criteria Porosity provided by pre- manufactured filter points Thinner layer provides equivalent protection compared to standard riprap Materials and methods of construction allow placement in areas of restricted access Typically used to provide continuous protection across full width of crossing HEC-23 suitability matrix does not address applications at piers Single-layer "veneer" does not allow for self-healing General lack of familiarity with product Essentially rigid Cannot support vegetation unless pre-excavated and buried below rooting depth Typically proprietary Design specifications and guidelines Stability design criteria (sizing requirements) well established Grout characteristics per ASTM D 6449 Fabric characteristics per ASTM D 6685 Filter characteristics well established HEC-23 provides baseline design guidance Target factor of safety can be adjusted for site-specific conditions Layout dimensions preliminary Termination/edge treatment uncertain. Pre-excavated turndowns at edges recommended Filter extent uncertain Guidance for selecting target factor of safety not established Rigidity requires calculation of load transfer to piers unless tension anchors provided on upstream edge Contraction scour and long-term degradation not considered when determining extent Construction specifications and guidelines Can be placed under water or dry Typically can accommodate irregular subgrade conditions Geotextile filter preferred but difficult to place under water May be difficult to place and secure fabric in flowing water Effective filter seal against pier required Maintenance and inspection guidelines Standard 2-year inspection frequency, and recommended inspection after a flood event Inspectors not likely to be familiar with this countermeasure Very difficult to detect voids under bridged areas of mattress, whether under water or not Single-layer "veneer" does not allow for self-healing Underwater repair difficult Performance evaluation guidelines Undermining or flanking are most typical modes of failure (mode of failure well known) Not investigated under NCHRP 24- 07 (Phase 1) Flanking, undermining, and uplift most likely modes of failure Phase 2 laboratory testing required to determine adequacy of filter recommendations and edge details Life-cycle cost information Not considered Not considered Table 2.8. Grout-filled mattress merits and deficiencies.

46 Factor Merits Deficiencies Selection criteria HEC-23 suitability matrix provides qualitative guidance Porous Gabion mattresses flexible; gabions less so Thinner layer provides equivalent protection compared to standard riprap Not recommended for coarse bedload environments because of potential for abrasion of wire Even galvanized or PVC-coated wire baskets not proven for long- term use in moderately saline environments Typically proprietary Design specifications and guidelines NCHRP 24-07 (Phase 1) report provides baseline design guidance Wire basket characteristics per ASTM A 974 (welded wire) or A 975 (twisted wire) Filter characteristics well established Rock fill per ASTM D 6711 Sizing requirements for pier scour applications are preliminary Layout dimensions lacking Termination/edge treatment uncertain. Pre-excavated turndowns at edges recommended Filter extent uncertain Relative impact of various scour mechanisms not well understood Contraction scour and long-term degradation not considered when determining extent Construction specifications and guidelines Can be placed under water or dry Can be filled individually or placed as pre-filled mattresses Individual filling allows access to confined areas, but typically limited to dry installations Geotextile may be attached directly to pre-assembled basket prior to filling Typically can accommodate irregular subgrade conditions Effective filter seal against pier required Prior excavation recommended Maintenance and inspection guidelines Standard 2-year inspection frequency, and recommended inspection after a flood event Flexibility of mattresses minimizes potential for bridging over voids Inspectors not likely to be familiar with this countermeasure Underwater repair difficult Vandalism of wire baskets has been reported as a concern, especially in urban areas Repair often requires total replacement of the individual baskets involved Performance evaluation guidelines Performance threshold typically associated with excessive movement of rockfill within basket, exposing filter layer (mode of failure well known) NCHRP 24-07 (Phase 1) did not perform laboratory testing on this type of countermeasure Phase 2 laboratory testing required to determine adequacy of filter recommendations and edge details Other performance issues include general flanking/undermining Life-cycle cost information Not considered Not considered Table 2.9. Gabions and gabion mattress merits and deficiencies.