Countermeasures to Protect Bridge Piers from Scour (2007)

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Introduction, E-2 1 Design and Specification, E-3 2 Construction, E-16 3 Inspection, Maintenance, and Performance Evaluation, E-21 References, E-30 E-1 A P P E N D I X E Guidelines for Pier Scour Countermeasures Using Articulating Concrete Block (ACB) Systems

Introduction Articulating concrete block (ACBs) systems provide a flexible armor for use as a pier scour countermeasure. These systems consist of preformed concrete units that either interlock, are held together by cables, or both (Figure E1.1). After installation is complete, the units form a continuous blanket or mat. This design guideline considers the application of ACB systems as a pier scour countermeasure. The term “articulating,” as used in this document, implies the ability of individual blocks of the system to conform to changes in the subgrade while remaining interconnected by virtue of block interlock and/or additional system components such as cables, ropes, geotextiles, or geogrids. ACB systems include interlocking and non-interlocking block geometries; cable-tied and non-cable-tied systems; and vegetated and non-vegetated systems. Block systems are typi- cally available in both open-cell and closed-cell varieties. There is little field experience with the use of ACB systems as a scour countermeasure for bridge piers alone. More frequently, these systems have been used for bank revetment and channel armoring where the mat is placed across the entire channel width and keyed into the abutments or bank protection. The guidance for pier scour applications provided in this doc- ument has been developed primarily from the results of NCHRP Project 24-07(2) (Lagasse et al. 2007). It should be noted that manufacturers of ACB systems must test their products and develop performance data from the test results. Since ACB systems vary in shape and performance from one proprietary system to the next, each system will have unique performance characteristics. In all cases, successful performance of ACBs depends on maintaining intimate contact between the block system and the subgrade under the hydraulic loading associated with the design event. This document is organized into three parts: • Part 1 provides design and specification guidelines for ACBs, given the appropriate perform- ance data for any particular block system. • Part 2 presents construction guidelines. • Part 3 provides guidance for inspection, maintenance, and performance evaluation of ACB systems used as a pier scour countermeasure. E-2 (a) (b) Source: (a) American Excelsior Company, (b) Armortec Figure E1.1. Examples of interlocking block (a) and cable-tied block (b) systems.

Part 1: Design and Specification 1.1 Materials An ACB system consists of a matrix of individual concrete blocks placed together to form an erosion-resistant armor layer with specific hydraulic performance characteristics. The system includes a filter layer, typically a geotextile, specifically selected for compatibility with the sub- soil. The filter allows infiltration and exfiltration to occur while providing particle retention. The individual blocks must be dense and durable, and the matrix must be flexible and porous. ASTM International has published D 6684 (2005) specifically for ACB systems. Table E1.1 lists some concrete properties required by this standard. ASTM D 6684 also specifies minimum strength properties of geotextiles according to the sever- ity of the conditions during installation. Harsh installation conditions (vehicular traffic, repeated lifting, realignment, and replacement of mattress sections, etc.) require stronger geotextiles. 1.2 Hydraulic Stability Design Procedure The hydraulic stability of ACB systems is analyzed using a “discrete particle” approach. The design approach is similar to that introduced by Stevens and Simons (1971) as modified by Julien (1995) in the derivation of the factor of safety method for sizing rock riprap. In that method, a calculated factor of safety of 1.0 or greater indicates that the particles will be stable under the given hydraulic conditions. For ACBs, the factor of safety force balance has been recomputed considering the weight and geometry of the blocks, and the Shields relationship for estimating the particle’s critical shear stress is replaced with actual test results (Clopper 1989, 1992). Considerations are also incorporated into the design procedure to account for the additional forces generated on a block that protrudes above the surrounding matrix because of subgrade irregularities or imprecise placement. The analysis methodology purposely omits any restrain- ing forces due to cables, because any possible benefit that cables might provide are reflected in the performance testing of the block. Cables may prevent blocks from being lost entirely but they do not prevent a block system from failing through loss of intimate contact with the subgrade. Similarly, the additional stability afforded by vegetative root anchorage or mechanical anchor- ing devices, while recognized as potentially significant, is ignored in the analysis procedures for the sake of conservatism in block selection and design. 1.2.1 Selecting a Target Factor of Safety The designer must determine what factor of safety should be used for a particular application. Typically, a minimum allowable factor of safety of 1.2 is used for revetment (bank protection) when the project hydraulic conditions are well known and the installation can be conducted under well-controlled conditions. Higher factors of safety are typically used for protection at bridge piers, abutments, and channel bends because of the complexity in computing hydraulic conditions at these locations. E-3 Property Average of 3 Units Individual Unit Minimum allowable compressive strength, lb/in2 4,000 3,500 Maximum allowable water absorption, lb/ft3, (%) 9.1 ( 7.0%) 11.7 ( 9.4%) Minimum allowable density in air, lb/ft3 130 125 Freeze-thaw durability As specified by owner in accordance withASTM C 67, C 666, or C 1262 Table E1.1. Concrete properties required by ASTM D 6684.

The Harris County Flood Control District, Texas, (Ayres Associates 2001) has developed a simple flow chart approach that considers the type of application, uncertainty in the hydraulic and hydrologic models used to calculate design conditions, and consequences of failure to select an appropriate target factor of safety to use when designing an ACB installation. In this approach, the minimum allowable factor of safety for ACBs at bridge piers is 1.5. This value is then multi- plied by two factors, each greater than 1.0, to account for risk and uncertainty. Figure E1.2 shows the HCFCD flowchart method. 1.2.2 Design Method The stability of a single block is a function of the applied hydraulic conditions (velocity and shear stress), the angle of the inclined surface on which it rests, and the weight and geometry of the block. Considering flow along a channel bank as shown in Figure E1.3, the forces acting on E-4 Source: modified from Ayres Associates (2001) Figure E1.2. Selecting a target factor of safety.

a concrete block are the lift force, FL; the drag force, FD; and the submerged weight of the block, WS. Block stability is determined by evaluating the moments about the point, O, where rotation can take place. The components of forces are shown in Figure E1.3. The safety factor (SF) for a single block in an ACB matrix is defined as the ratio of restraining moments to overturning moments (terms are defined in Table E1.2): (E1.1) Note that additional lift and drag forces F′L and F′D are included to account for protruding blocks that incur larger forces due to impact. Dividing Equation E1.1 by l1WS and substituting terms yields the final form of the factor of safety equations as presented in Table E1.2. The equa- tions can be used with any consistent set of units; however, variables are indicated here in U.S. customary units. The moment arms l1, l2, l3, and l4 are determined from the block dimensions shown in Fig- ure E1.4. In the general case, the pivot point of overturning will be at the downstream corner of the block; therefore, the distance from the center of the block to the corner should be used for both l2 and l4. Since the weight vector acts through the center of gravity, one half the block height should be used for l1. The drag force acts both on the top surface of the block (shear drag) and on the body of the block (form drag). Considering both elements of drag, eight-tenths the height of the block is considered a reasonable estimate of l3. While charts have been developed to aid in the design of ACB systems, the charts generally are based on the assumption of a “perfect” installation (i.e., no individual blocks protrude into the flow). In reality, some placement tolerance must be anticipated and the factor of safety equation modified to account for protruding blocks. Because poor installation can cause blocks to exceed the design placement tolerance, the actual factor of safety can be greatly reduced and may lead to failure. There- fore, construction inspection becomes critical to successful performance of ACB systems. SF W a W a F F F S S D L D = − + + + ′ l l l l l 2 1 2 3 4 31 θ θ β δcos cos cosδ + ′l 4 FL E-5 Figure E1.3. Three-dimensional view of a block on a channel side slope with factor of safety variables defined.

E-6 Equation Term Definitions 2 desDL )V)(z(b5.0'F'F (E1.2) C des 0 (E1.3) 1 0 tan tan arctan (E1.4) 2 0 2 1 )(sin)(cosa (E1.5) )sin()/( a1 1 )cos( arctan 0 120 2 3 4 0 (E1.6) o90 (E1.7) 1)/( )sin()/( 34 034 01 (E1.8) c wc s WW (E1.9) s1 L4D3 121 2 12 W )'Fcos'F()/()a1(cos a)/(SF (E1.10) a = Projection of WS into plane of subgrade b = Block width normal to flow (ft) F D, F L = added drag and lift forces due to protruding block (lb) x = Block moment arms (ft) c = Concrete density, lb/ft3 w = Density of water, lb/ft3 Vdes = Design velocity (ft/s) W = Weight of block in air (lb) WS = Submerged block weight (lb) z = Height of block protrusion above ACB matrix (ft) = Angle between block motion and the vertical = Angle between drag force and block motion 0 = Stability number for a block on a horizontal surface 1 = Stability number for a block on a sloped surface = Angle between side slope projection of WS and the vertical 0 = Channel bed slope (degrees) 1 = Side slope of block installation (degrees) = Mass density of water (slugs/ft3) c = Critical shear stress for block on a horizontal surface (lb/ft2) des = Design shear stress (lb/ft2) SF = Calculated factor of safety Note: The equations cannot be solved for 1 = 0 (i.e., division by 0 in Equation E1.4); therefore, a very small but non-zero side slope must be entered for the case of 1 = 0. Table E1.2. Design equations for ACB systems. Figure E1.4. Schematic diagram of a block showing moment arms 1, 2, 3, and 4.

The design conditions in the immediate vicinity of a bridge pier are more severe than the approach conditions upstream; therefore, the local velocity and shear stress should be used in the design equations. As recommended in Hydraulic Engineering Circular No. 23 (Lagasse et al. 2001), the section-average approach velocity, Vavg, must be multiplied by factors that are a func- tion of the shape of the pier and its location in the channel: Vdes = K1K2Vavg (E1.11) where Vdes = Design velocity for local conditions at the pier, ft/s (m/s) K1 = Shape factor equal to 1.5 for round-nose piers and 1.7 for square-edged piers K2 = Velocity adjustment factor for location in the channel (ranges from 0.9 for pier near the bank in a straight reach to 1.7 for pier located in the main current of flow around a sharp bend) Vavg = Section-average approach velocity (Q/A) upstream of bridge, ft/s (m/s) If the local velocity, Vlocal, is available from stream tube or flow distribution output from a one- dimensional (1-D) model, or directly computed from a two-dimensional (2-D) model, then only the pier shape coefficient should be used to determine the design velocity. The maximum local veloc- ity is recommended since the channel could shift and the maximum velocity could impact any pier: Vdes = K1Vlocal (E1.12) The local shear stress at a pier should be calculated as (E1.13) where τdes = Design shear stress for local conditions at pier, lb/ft2 (N/m2) n = Manning’s “n” value for block system Vdes = Design velocity as defined by Equation E1.11 or E1.12, ft/s (m/s) γw = Density of water, 62.4 lb/ft3 (9,810 N/m3) for fresh water y = Depth of flow at pier, ft (m) Ku = 1.486 for U.S. customary units, 1 for SI units 1.3 Layout Dimensions Based on small-scale laboratory studies performed for NCHRP Project 24-07(2) (Lagasse et al. 2007), the optimum performance of ACBs as a pier scour countermeasure was obtained when the blocks were extended a distance of at least 2 times the pier width in all directions around the pier. In the case of wall piers or pile bents consisting of multiple columns where the axis of the struc- ture is skewed to the flow direction, the lateral extent of the protection should be increased in proportion to the additional scour potential caused by the skew. While there is no definitive guidance for pier scour countermeasures, it is recommended that the extent of the armor layer should be multiplied by a factor Kα, which is a function of the width, a, and length, L, of the pier (or pile bents) and the skew angle, α, as given below (after Richardson and Davis 2001): (E1.14) Where only clear-water scour is present, the ACB system may be placed horizontally such that the top of the blocks are flush with the bed elevation, with turndowns provided at the system periphery. However, when other processes or types of scour are present, the block system must be sloped away from the pier in all directions such that the depth of the system at its periphery K a L a α α α = +⎛⎝⎜ ⎞⎠⎟ cos sin .0 65 τ γ des des u wnV K y = ⎛⎝⎜ ⎞⎠⎟ 2 1 3/ E-7

is greater than the depth of contraction scour and long-term degradation, or the depth of bed- form troughs, whichever is greater (Figure E1.5). The blocks should not be laid on a slope steeper than 1V:2H (50%). In some cases, this limitation may result in blocks being placed further than two pier widths away from the pier. In river systems where dune-type bed forms are present during flood flows, the depth of the trough below the ambient bed elevation should be estimated using the methods of Karim (1999) and/or van Rijn (1984). In general, an upper limit on the crest-to-trough height, Δ, is provided by Bennet (1997) as Δ < 0.4y where y is the depth of flow. This limit suggests that the maximum depth of the bed-form trough below ambient bed elevation will not exceed 0.2 times the depth of flow. A filter is typically required for ACB systems at bridge piers. The filter should be extended fully beneath the ACB system. When a granular stone filter is used, the layer should have a minimum thickness of 4 times the d50 of the filter stone or 6 in. (15 cm), whichever is greater. The d50 size of the granular filter should be greater than one half the smallest dimension of the open cells in the block system. The granular filter layer thickness should be increased by 50% when placing under water. 1.4 Filter Requirements The importance of the filter component of an ACB installation should not be underestimated. Geotextile filters are most commonly used with ACB systems. Some situations call for a com- posite filter consisting 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 E-8 2a 2a PIER Toe down to maximum scour depth or bedform trough depth, whichever is greater Extend geotextile to edge of blocks around entire perimeter FLOW Slope blocks no greater than 1V:2H (50 percent) a. Profile FLOW Pier width = a ACB extent = 2 (minimum, all around) b. Plan Figure E1.5. ACB layout diagram for pier scour countermeasures.

dune-type bed forms may be present, it is strongly recommended that only a geotextile filter be considered. The filter must retain the coarser particles of the subgrade while remaining permeable enough to allow infiltration and exfiltration to occur freely. It is not necessary to retain all the particle sizes in the subgrade; in fact, it is beneficial to allow the smaller particles to pass through the fil- ter, leaving a coarser substrate behind. 1.4.1 Geotextile Filter Properties Either woven or non-woven, needle-punched fabrics may be used. If a non-woven fabric is used, it must have a mass density greater than 12 oz/yd2 (400 g/m2). Under no circumstances may spun-bond or slit-film fabrics be allowed. For compatibility with site-specific soils, geotextiles must exhibit the appropriate values of permeability, pore size (otherwise known as apparent opening size), and porosity (or percent open area). In addition, geotextiles must be sufficiently strong to withstand stresses during instal- lation. These values are available from manufacturers. The following list briefly describes the most relevant properties: • Permeability. The permeability, K, of a geotextile is a calculated value that indicates the ability of a geotextile to transmit water across its thickness. It is typically reported in units of centime- ters per second (cm/s). This property is directly related to the filtration function that a geotex- tile must perform, where water flows perpendicularly through the geotextile into a crushed stone bedding layer, perforated pipe, or other more permeable medium. The geotextile must allow this flow to occur without being impeded. A value known as the permittivity, ψ, is used by the geo- textile industry to more readily compare geotextiles of different thicknesses. Permittivity, ψ, is defined as K divided by the geotextile thickness, t, in centimeters; therefore, permittivity has a value of (s)−1. Permeability (and permittivity) is extremely important in filter design. For ACB installations, the permeability of the geotextile should be at least 10 times greater than that of the underlying material: Kg > 10Ks (E1.15) where Kg = Permeability of geotextile (cm/s) Ks = Permeability of subgrade soil (cm/s) • Transmissivity. The transmissivity, θ, of a geotextile is a calculated value that indicates the ability of a geotextile to transmit water within the plane of the fabric. It is typically reported in units of square centimeters per second. This property is directly related to the drainage function, and is most often used for high-flow drainage nets and geocomposites, not geotex- tiles. Woven monofilament geotextiles have very little capacity to transmit water in the plane of the fabric, whereas non-woven, needle-punched fabrics have a much greater capacity due to their three-dimensional (3-D) microstructure. Transmissivity is not particularly relevant to filter design. • Apparent opening size (AOS). Also known as equivalent opening size, this measure is generally reported as O95, which represents the aperture size such that 95% of the openings are smaller. In similar fashion to a soil gradation curve, a geotextile hole distribution curve can be derived. The AOS is typically reported in millimeters, or in equivalent U.S. standard sieve size. • Porosity. Porosity is a comparison of the total volume of voids to the total volume of geotex- tile. This measure is applicable to non-woven geotextiles only. Porosity is used to estimate the potential for long-term clogging and is typically reported as a percentage. • Percent open area (POA). POA is a comparison of the total open area to the total geotextile area. This measure is applicable to woven geotextiles only. POA is used to estimate the poten- tial for long-term clogging and is typically reported as a percentage. E-9

• Thickness. As mentioned above, thickness is used to calculate traditional permeability. It is typically reported in millimeters or mils (thousandths of an inch). • Grab strength and elongation. Grab strength is the force required to initiate a tear in the fabric when pulled in tension. It is typically reported in Newtons or pounds as measured in a testing apparatus having standardized dimensions. The elongation measures the amount the material stretches before it tears and is reported as a percentage of its original (unstretched) length. • Tear strength. Tear strength is the force required to propagate a tear once initiated. It is typ- ically reported in Newtons or pounds. • Puncture strength. Puncture strength is the force required to puncture a geotextile using a standard penetration apparatus. It is typically reported in Newtons or pounds. Table E1.3 provides the recommended characteristics for geotextile filters. There are many other tests to determine various characteristics of geotextiles; only those deemed most relevant to applications involving ACBs at piers have been discussed here. As previously mentioned, geo- textiles should be able to withstand the rigors of installation without suffering degradation of any kind. Long-term endurance to stresses such as ultraviolet solar radiation or continual abrasion are considered of secondary importance, because once the geotextile has been installed and E-10 Allowable value (1)Test Designation Property Elongation < 50%(2) Elongation > 50%(2) Comments ASTM D 4632 Grab Strength > 315 lbs (Class 1) > 250 lbs (Class 2) > 180 lbs (Class 3) > 200 lbs (Class 1) > 160 lbs (Class 2) > 110 lbs (Class 3) From AASHTO M 288 ASTM D 4632 Sewn Seam Strength (3) > 270 lbs (Class 1) > 220 lbs (Class 2) > 160 lbs (Class 3) > 180 lbs (Class 1) > 140 lbs (Class 2) > 100 lbs (Class 3) From AASHTO M 288 ASTM D 4533 Tear Strength (4) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) From AASHTO M 288 ASTM D 4833 Puncture Strength > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) From AASHTO M 288 ASTM D 4751 Apparent Opening Size Per design criteria (Section 1.4 of this design guide) Maximum allowable value ASTM D 4491 Permittivity and Permeability Per design criteria (Section 1.4 of this design guide) Minimum allowable value ASTM D 4355 Degradation by Ultraviolet Light > 50% strength retained after 500 hours of exposure Minimum allowable value ASTM D 4873 Guide for Identification, Storage, and Handling Provides information on identification, storage, and handling of geotextiles. ASTM D 4759 Practice for the Specification Conformance of Geosynthetics Provides information on procedures for ensuring that geotextiles at the jobsite meet the design specifications. (1) Required geotextile class for permanent erosion control design is designated below for the indicated application. The severity of installation conditions generally dictates the required geotextile class. The following descriptions have been modified from AASHTO M 288: • Class 1 is recommended for harsh or severe installation conditions where there is a greater potential for geotextile damage, including when placement of riprap must occur in multiple lifts, when drop heights may exceed 1 ft (0.3 m) or when repeated vehicular traffic on the installation is anticipated. • Class 2 is recommended for installation conditions where placement in regular, single lifts are expected and little or no vehicular traffic on the installation will occur, or when placing individual rocks by clamshell, orange-peel grapple or specially equipped hydraulic excavator with drop heights less than 1 ft. • Class 3 is specified for the least severe installation environments, with drop heights less than 1 ft onto a bedding layer of select sand, gravel or other select imported material. (2) As measured in accordance with ASTM D 4632. (3) When seams are required. (4) The required Minimum Average Roll Value (MARV) tear strength for woven monofilament geotextiles is 55 lbs. The MARV corresponds to a statistical measure whereby 2.5% of the tested values are less than the mean value minus two standard deviations (Koerner 1998). Table E1.3. Recommended requirements for geotextile properties.

covered by the ACB system, these stresses do not represent the environment that the geotextile will experience in the long term. 1.4.2 Geotextile Filter Design Procedure Step 1. Obtain Base Soil Information. Typically, the required base soil information consists simply of a grain size distribution curve, a measurement (or estimate) of permeability, and the plasticity index (PI is required only if the base soil is more than 20% clay). Step 2. Determine Particle Retention Criterion. The decision tree approach provided in Figure E1.6 assists in determining the appropriate soil retention criterion for the geotextile. The figure E-11 FROM SOIL PROPERTY TESTS MORE THAN 30% CLAY (D30 < 0.002 mm) LESS THAN 30% CLAY AND MORE THAN 50% FINES (d30 > 0.002 mm, AND d50 < 0.075 mm) LESS THAN 50% FINES AND LESS THAN 90% GRAVEL (d50 > 0.075 mm, AND d90 < 4.8 mm) MORE THAN 90% GRAVEL (d90 > 4.8 mm) USE CISTIN – ZIEMS METHOD TO DESIGN A GRANULAR TRANSITION LAYER, THEN DESIGN GEOTEXTILE AS A FILTER FOR THE GRANULAR LAYER O95 < d50WIDELY GRADED (CU > 5) O95 < 2.5d50 and O95 < d90 UNIFORMLY GRADED (CU ≤5) d50 < O95 < d90 WAVE ATTACK OPEN CHANNEL FLOW Definition of Terms dx = particle size for which x percent is smaller PI = plasticity index of the base soil K = permeability of the base soil O95 = the AOS of the geotextile c = Undrained shear strength Cu = Coefficient of Uniformity, d60/d10 Note If the required O95 is smaller than that of available geotextiles, then a granular transition layer is needed. O95 ≤ #70 SIEVE (0.2 mm) YES NO PI > 5 ? YES NO K < 10-7 cm/s, and c > 10 kPa, and PI > 15 ? Source: modified from Koerner (1998) Figure E1.6. Geotextile selection based on soil retention.

includes guidance when a granular transition layer (i.e., composite filter) is necessary. A compos- ite filter is typically required when the base soil is greater than 30% clay or is predominantly fine- grained soil (more than 50% passing the #200 sieve). If a granular transition layer is required, the geotextile should be designed to be compatible with the properties of the granular layer. If the required AOS is smaller than that of available geo- textiles, then a granular transition layer is required. However, this requirement can be waived if the base soil exhibits the following conditions for hydraulic conductivity, K; plasticity index, PI; and undrained shear strength, c: K < 1 × 10−7 cm/s PI > 15 c > 10 kPa Under these soil conditions there is sufficient cohesion to prevent soil loss through the geot- extile. A geotextile with an AOS less than a #70 sieve (approximately 0.2 mm) can be used with soils meeting these conditions and essentially functions more as a separation layer than a filter. Step 3. Determine Permeability Criterion. The permeability criterion requires that the filter exhibit a permeability at least 4 times greater than that of the base soil (Koerner 1998) and, for critical or severe applications, at least 10 times greater (Holtz et al. 1995). Generally speaking, if the permeability of the base soil or granular filter has been determined from laboratory testing, that value should be used. If laboratory testing was not conducted, then an estimate of perme- ability based on the particle size distribution should be used. To obtain the permeability of a geotextile in cm/s, multiply the thickness of the geotextile in cm by its permittivity in s−1. Typically, the designer will need to contact the geotextile manufac- turer to obtain values of permeability, permittivity, and thickness. Step 4. Select a Geotextile that Meets the Required Strength Criteria. Strength and durabil- ity requirements depend on the installation environment and the construction equipment that is being used. See Table E1.3 for recommended values based on AASHTO M 288, “Geotextile Specification for Highway Construction,” which provides guidance on allowable strength and elongation values for three categories of installation severity. For additional guidelines regard- ing the selection of durability test methods, refer to ASTM D 5819, “Standard Guide for Select- ing Test Methods for Experimental Evaluation of Geosynthetic Durability.” Step 5. Minimize Long-Term Clogging Potential. When a woven geotextile is used, its POA should be greater than 4% by area. If a non-woven geotextile is used, its porosity should be greater than 30% by volume. A good rule of thumb suggests that the geotextile having the largest AOS that satisfies the particle retention criteria should be used (provided of course that all other minimum allowable values described in this section are met as well). 1.4.3 Granular Filter Properties Granular filters are not often used with ACB systems, unless the bed material is coarse sand and gravel such that the granular filter can be composed of stones large enough to resist win- nowing through the open cells of the blocks. However, in some circumstances the bed material is composed of fine silt and/or clays with low cohesion, such that the particle retention require- ments of Figure E1.6 cannot be met with standard geotextiles. In these cases, it is recommended that a transition layer of 6 in. (15 cm) of sand be placed on the fine bed material and a geotextile selected for compatibility with the sand layer. The method of Cistin and Ziems as described in NCHRP Report 568 (Lagasse et al. 2006) is recommended for selecting a sand transition layer that is compatible with very fine, non- or low- cohesive bed sediments. This method is based on the coefficients of uniformity of the bed sedi- E-12

ment and the sand transition layer to determine the maximum allowable ratio d50(sand) to d50(bed). The design chart for selecting a sand transition layer is provided as Figure E1.7. Generally speaking, most required granular filter properties can be obtained from the parti- cle size distribution curve for the material. Granular filters can be used alone or can serve as a transitional layer between a predominantly fine-grained base soil and a geotextile. The follow- ing list briefly describes the most relevant properties: • Particle size distribution. As a rule of thumb, the gradation curve of the granular filter mate- rial should be approximately parallel to that of the base soil. Parallel gradation curves mini- mize the migration of particles from the finer material into the coarser material. Heibaum (2004) presents a summary of a procedure originally developed by Cistin and Ziems whereby the d50 size of the filter is selected based on the coefficients of uniformity (d60/d10) of both the base soil and the filter material. With this method, the grain size distribution curves do not necessarily need to be approximately parallel. Figure E1.7 provides a design chart based on the Cistin–Ziems approach. • Permeability. Permeability of a granular filter material is determined by laboratory test or estimated using relationships relating permeability to the particle size distribution. The per- meability of a granular layer is used to select a geotextile when designing a composite filter. For ACB installations, the permeability of the granular filter should be at least 10 times greater than that of the underlying material. • Porosity. Porosity is that portion of a representative volume of soil that is interconnected void space. It is typically reported as a dimensionless fraction or a percentage. The porosity of soils is affected by the particle size distribution, the particle shape (e.g., round vs. angular), and degree of compaction and/or cementation. • Thickness. Practical issues of placement indicate that a typical minimum thickness of 6 to 8 in. is specified. For placement under water, thickness should be increased by 50%. • Quality and durability. Aggregate used for a granular filter should be hard, dense, and durable. E-13 M ax im u m A 50 = d 50 f/d 50 s Coefficient of Uniformity (filter) Cuf = d60f/d10f Coefficient of Uniformity (soil) Cus = d60s/d10s Source: Heibaum (2004) Cuf = 18 Cuf = 14 Cuf = 4 Cuf = 2 Cuf = 1 Cuf = 10 Cuf = 6 Figure E1.7. Granular filter design chart according to Cistin and Ziems.

1.4.4 Granular Filter Design Procedure Numerous texts and handbooks provide details on the well-known Terzaghi approach to designing a granular filter. That approach was developed for subsoils consisting of well-graded sands and may not be widely applicable to other soil types. An alternative approach that is con- sidered more robust in this regard is the Cistin–Ziems method. The suggested steps for proper design of a granular filter using this method are outlined below. Note that the subscript “s” is used to represent the base (finer) soil, and “f” is used to represent the filter (coarser) layer. Step 1. Obtain Base Soil Information. Typically, the required base soil information consists simply of a grain size distribution curve, a measurement (or estimate) of permeability, and the plasticity index (PI is required only if the base soil is more than 20% clay). Step 2. Determine Key Indices for Base Soil. From the grain size information, determine the median grain size, d50, and the coefficient of uniformity, Cus = d60/d10, of the base soil. Step 3. Determine Key Indices for Granular Filter. One or more locally available aggregates should be identified as potential candidates for use as a filter material. The d50 and coefficient of uniformity, Cuf = d60/d10, should be determined for each candidate filter material. Step 4. Determine Maximum Allowable d50 for Filter. Enter the Cistin–Ziems design chart (Figure E1.7) with the coefficient of uniformity, Cus, for the base soil on the x-axis. Find the curve that corresponds to the coefficient of uniformity, Cuf, for the filter in the body of the chart and, from that point, determine the maximum allowable A50 from the y-axis. Compute the maximum allowable d50f of the filter using d50fmax equals A50max times d50s. Check to see if the candidate filter material conforms to this requirement. If it does not, continue checking alternative candidates until a suitable material is identified. Step 5. Check for Permeability. From laboratory permeameter tests or the grain size distri- bution of the candidate filter material, determine whether the hydraulic conductivity of the filter is at least 10 times greater than that of the subsoil. Step 6. Check for Compatibility with the ACB System. The accepted rule of thumb for gran- ular material that directly underlies the ACB system is that it should have a d50 greater than 1/2 the smallest dimension of the open cells of the ACB system. This criterion will minimize win- nowing of the granular material out through the open cells. If this criterion is not met, then mul- tiple layers of granular filter materials should be considered. Step 7. Filter Layer Thickness. For practicality of placement, the nominal thickness of a sin- gle filter layer should not be less than 6 in. (15 cm). Single-layer thicknesses up to 15 in. (38 cm) may be warranted where irregularities in the subgrade need to be smoothed out prior to place- ment of the ACB system. When multiple filter layers are required, each individual layer should range from 4 to 8 in. (10 to 20 cm) in thickness (Brown and Clyde 1989). 1.5 Guidelines for Seal Around the Pier An observed point of failure for ACB systems at bridge piers occurs at the seal where the mat meets the bridge pier. During NCHRP Project 24-07, securing the geotextile to the pier pre- vented the leaching of the bed material from around the pier (Parker et al. 1998). This procedure worked successfully in the laboratory, but constructability implications must be considered when this technique is used in the field, particularly when the mat is placed under water. Dur- ing flume studies at the University of Windsor (McCorquodale et al. 1993) and for the NCHRP Project 24-07(2) study (Lagasse et al. 2007), the mat was grouted to the pier. A grout seal is not intended to provide a structural attachment between the mat and the pier, but instead is a simple method for plugging gaps to prevent bed sediments from winnowing out E-14

from beneath the system. In fact, structural attachment of the mat to the pier is strongly dis- couraged. The transfer of moments from the mat to the pier may affect the structural stability of the pier, and the potential for increased loadings on the pier must be considered. When a grout seal is placed under water, an anti-washout additive is required. The State of Minnesota Department of Transportation (Mn/DOT) has installed a cabled ACB mat system for a pier at TH 32 over Clearwater River at Red Lake Falls, Minnesota. 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. The State of Maine Department of Transportation (MDOT) has designed an ACB system for a pier at Tukey’s Bridge over Back Cove. MDOT recommended a design in which grout bags were placed on top of the mat at the pier location to provide the necessary seal. 1.6 Anchors Mn/DOT also recommends the use of anchors when installing a cabled ACB mat, although, as dis- cussed in Section 1.2, no additional stability is attributed to the cables themselves. Mn/DOT requires duckbill-type soil anchors placed 3 to 4 ft deep at the corners of the ACB mats, and at regular inter- vals of approximately 8 ft on center-to-center spacing throughout the area of the installation. In reality, if uplift forces on an ACB system were great enough to create tension in the cables, then soil anchors could provide a restraining force that is transmitted to a group of blocks in the matrix. Using the same reasoning, anchors would be of no use in an uncabled system, unless there was a positive physical vertical interlock from block to block in the matrix. It should be noted that the stability analysis procedure presented in Section 1.2.2 is intended to ensure that uplift forces do not exceed the ACB system’s capability, irrespective of cables. The layout guidance presented in Section 1.3 indicates that the system should be toed down to a termination depth at least as deep as any expected contraction scour and long-term degra- dation, or bed form troughs, whichever is greater. Where such toedown depth cannot be achieved, for example where bedrock is encountered at shallow depth, a cabled system with anchors along the front (upstream) and sides of the installation is recommended. The spacing of the anchors should be determined based on a factor of safety of at least 5.0 for pullout resist- ance based on calculated drag on the exposed leading edge. Spacing between anchors of no more than 4 ft (1.3 m) is recommended. The following example is provided: Given ρ = Mass density of water (slugs/ft3) = 1.94 V = Approach velocity, ft/s = 10 Δz = Height of block system, ft = 0.5 b = Width of block installation (perpendicular to flow), ft = 40 Step 1: Calculate total drag force, Fd, on leading edge of system: Fd = 0.5ρV2(Δz)(b) = 0.5(1.94)(102)(0.5)(40) = 1,940 lb Step 2: Calculate required uplift restraint using 5.0 safety factor: Frestraint = 5.0(1,940) = 9,700 lb Step 3: Counting anchors at corners of system, calculate required pullout resistance per anchor (rounded to nearest 10 lb): a) Assume 11 anchors at 4-ft spacing: 9,700 lb/11 anchors = 880 lb/anchor b) Assume 21 anchors at 2-ft spacing: 9,700 lb/21 anchors = 460 lb/anchor E-15

Anchors should never be used as a means to avoid toeing the system down to the full required extent where alluvial materials are present at depth. In this case, scour or bed-form troughs will simply undermine the anchors as well as the system in general. Part 2: Construction The guidance in this section has been developed to facilitate the proper installation of ACB sys- tems to achieve suitable hydraulic performance and maintain stability against hydraulic loads. The proper installation of ACB systems is essential to the adequate functioning and performance of the system during the design hydrologic event. Guidelines are provided herein for maximizing the correspondence between the design intent and the actual field-finished conditions of the project. This section addresses the preparation of the subgrade, geotextile placement, ACB system placement, backfilling and finishing, and measurement and payment. 2.1 General Guidelines The contractor is responsible for constructing the project according to the plans and specifi- cations; however, ensuring conformance with the project plans and specifications is the respon- sibility of the owner. This responsibility is typically performed by the owner’s engineer and inspectors. Inspectors observe and document the construction progress and performance of the contractor. Prior to construction, the contractor should provide a quality control plan to the owner (for example, see ER 1180-1-6 [U.S. Army Corps of Engineers 1995]) and provide labor and equipment to perform tests as required by the project specifications. Construction requirements for ACB placement are included in the project plans and speci- fications. Standard ACB mat specifications and layout guidance are found in Part 1 of this appendix. Recommended requirements for the ACBs, including the tests necessary to ensure that the physical and mechanical properties meet the requirements of the project specifications are provided. Inspection of ACB placement consists of visual inspection of the placement operation and the finished surface. Inspection and quality assurance must be carefully organized to ensure that materials delivered to the job site meet the specifications. Acceptance of the work should not be made until measurement for payment has been completed. The engineer and inspectors reserve the right to reject incorrect or unsuitable materials (e.g., broken blocks, wrong geotextile, etc.) at the job site. Material that has been improperly placed should also be rejected throughout the duration of the contract, and require removal and replacement at the contractor’s expense. Rejected material should be removed from the project site. Construction techniques can vary tremendously due to the following factors: • Size and scope of the overall project • Size and weight of the materials • Placement under water or in the dry • Physical constraints to access and/or staging areas • Noise limitations • Traffic management and road weight restrictions • Environmental restrictions • Type of construction equipment available Competency in construction techniques and management in all their aspects cannot be acquired from a book. Training on a variety of job sites and project types under the guidance of experienced senior personnel is required. The following sections provide some general infor- E-16

mation regarding construction of ACB installations and some basic information and description of techniques and processes involved. 2.2 Materials 2.2.1 Blocks Materials composing the ACB system shall be in accordance with the guidance provided in Part 1 of this appendix. Blocks shall be sound and free of defects that would interfere with proper placement or that would impair the integrity of the system. Blocks with the following defects shall be discarded: • Broken blocks • Blocks having chips larger than 2 in. (50 mm) in any dimension • Blocks having cracks wider than 0.02 in. (0.5 mm) and longer than one-third the nominal height of the block Minor cracks incidental to the usual method of manufacture or chipping that results from customary handling during shipping, delivery, and placement will not be deemed grounds for rejection. 2.2.2 Geotextile Each roll of geotextile shall be labeled with the manufacturer’s name, product identification, roll dimensions, lot number, and date of manufacture. The rolls shall not be dragged, lifted by one end, or dropped. Geotextiles shall not be exposed to sunlight prior to placement. 2.2.3 Cable Cable may be composed of polyester, stainless steel, or galvanized steel unless otherwise spec- ified. Cable used for preassembled mats shall be sufficiently sized and fastened for the size and weight of the assembled mats such that the mats can be placed in compliance with Occupational Safety and Health Administration (OSHA) requirements. The manufacturer shall be responsi- ble for determining the minimum allowable cable strength compatible with mat size and weight to assure safe handling. The cable strength shall be based on a minimum factor of safety of 5 for mat lifting and shall include appropriate reduction factors for mechanically crimped cables, clamps, or other fasteners. Any systems that rely on the geotextile as a carrier fabric instead of cables must also meet the applicable portions of this section, with particular attention given to the grab points. 2.2.4 Subgrade Soils When placement is in the dry, the ACB system shall be placed on undisturbed native soil, on an excavated and prepared subgrade, or on acceptably placed and compacted fill. Smoothing the subgrade prior to block placement is required. Unsatisfactory soils shall be considered those soils having excessive in-place moisture content; soils containing roots, sod, brush, or other organic materials; soils containing turf clods or rocks; or frozen soil. These soils must be removed, and the excavation backfilled with approved material that is compacted prior to placement of the block system. Unsatisfactory soils may also be defined as soils such as very fine non-cohesive soils with uniform particle size, gap-graded soils, laminated soils, and dispersive clays, per the geot- echnical engineer’s recommendations. When a block system is placed under water, compaction of the subgrade is impractical. How- ever, the surface must be relatively smooth, with no abrupt irregularities that would prevent intimate contact between the ACB system and the subgrade. Under no circumstances may an ACB system be draped over boulders, bridged over subgrade voids, or placed over other irreg- ularities that would prevent achievement of intimate contact between the system and the E-17

subgrade. Placing a layer of bedding stone may assist in achieving a suitable surface on which to place the block system. 2.3 Installation 2.3.1 Subgrade Preparation Stable and compacted subgrade soil shall be prepared to the lines, grades, and cross sections shown on the contract drawings. Termination trenches and transitions between slopes, embank- ment crests, benches, berms, and toes shall be compacted, shaped, and uniformly graded to facil- itate the development of intimate contact between the ACB system and the underlying grade. Termination between the ACB revetment system and a concrete slab, footer, pier, wall, or sim- ilar structure shall be sealed in a manner that prevents soil migration. The subgrade soil conditions shall meet or exceed the required material properties described in Section 2.2.4 prior to placement of the block. Soils not meeting the requirements shall be removed and replaced with acceptable material. When placement is in the dry, the areas to receive the ACB system shall be graded to establish a smooth surface and ensure that intimate contact is achieved between the subgrade surface and the geotextile, and between the geotextile and the bottom surface of the ACB system. It is rec- ommended that the subgrade be uniformly compacted to a minimum of 90% of Standard Proc- tor density (ASTM D 698). If the subgrade surface for any reason becomes rough, corrugated, uneven, textured, or traffic marked prior to ACB installation, such unsatisfactory portion shall be scarified, reworked, recompacted, or replaced as directed by the engineer. Grading tolerance shall be within 2 in. (50 mm) from the prescribed elevations, with no abrupt variations that would cause unacceptable projections of individual blocks. When placing underwater, divers shall be used to ensure that the bed is free of logs, large rocks, construction materials, or other blocky materials that would create irregularities in the block surface, or that would create voids beneath the system, in accordance with section 2.2.4. Immediately prior to placing the geotextile and ACB system, the prepared subgrade shall be inspected. 2.3.2 Placement of the Geotextile The geotextile shall be placed directly on the prepared area, in intimate contact with the sub- grade and free of folds or wrinkles. The geotextile shall be placed in such a manner that place- ment of the overlying materials will not excessively stretch or tear the geotextile. After geotextile placement, the work area shall not be trafficked or disturbed so as to result in a loss of intimate contact between the concrete block, the geotextile, and the subgrade. The geotextile shall not be left exposed longer than the manufacturer’s recommendation to minimize potential damage due to ultraviolet radiation. The geotextile shall be placed so that upstream strips overlap downstream strips and so that upslope strips overlap downslope strips. Overlaps shall be in the direction of flow wherever pos- sible. The longitudinal and transverse joints shall be overlapped at least 3 ft (91cm) for below- water installations and at least 1.5 ft (46 cm) for dry installations. If a sewn seam is to be used for the seaming of the geotextile, the thread to be used shall consist of high-strength polypropylene or polyester and shall be resistant to ultraviolet radiation. For bank protection, the geotextile shall extend beyond the top, toe and side termination points of the revetment. If necessary to expedite construction and to maintain the recommended overlaps, anchoring pins, U-staples, or weights shall be used. If the system is to be placed under water, the geotextile shall be securely attached to the bot- tom of the preassembled ACB mat prior to lifting with crane and spreader bar. In shallow water E-18

where velocities are low, the geotextile may be placed under water and held in place temporar- ily with weights until the blocks are placed. 2.3.3 Placement of the ACB System General ACB Placement. Placement of the ACB system, whether as mats or as individual blocks by hand, shall be performed to ensure that each block lies in intimate contact with the geotextile and subgrade. For blocks within a mat and individual blocks that are hand placed, the joint spacing between adjacent blocks is to be maintained so that binding of blocks does not occur and so that block-to-block interconnection is achieved. In areas of curvature or grade change, alignment of an individual block with adjacent blocks shall be oriented such that inti- mate contact between the block, geotextile, and subgrade is maintained and block-to-block interconnection is achieved. Care shall be taken during block installation to avoid damage to the geotextile or subgrade during the installation process. Mats or individual blocks shall not be pushed or pulled later- ally once they are on the geotextile. Preferably, where the geotextile is laid on the ground prior to the ACB installation, the ACB placement shall begin at the upstream section and proceed downstream. If an ACB system is to be installed starting downstream and proceeding in the upstream direction, a contractor option is to construct a temporary toe trench at the front edge of the ACB system to protect against flow that could otherwise undermine the system during flow events that may occur during construction. On sloped sections where practical, place- ment shall begin at the toe of the slope and proceed upslope. Block placement shall not bring block-to-block interconnections into tension. Individual blocks within the plane of the fin- ished system shall not exceed a protrusion greater than the tolerance referenced in the con- tract documents. If assembled and placed as mats, the ACB mats can be attached to a spreader bar to aid in the lifting and placing of the mats in their proper position with a crane or backhoe. The mats shall be placed side by side and/or end to end, so that the mats abut each other. Mat seams or openings between mats that are 2 in. (50mm) or greater in the matrix shall be filled with grout. Whether the blocks are placed individually or as mats, distinct grade changes shall be accommodated with a well-rounded transition (i.e., minimum radius per specific system characteristics). However, if a discontinuous revetment surface exists in the direction of flow, a grout seam at the grade change location shall be provided to produce a continuous, flush finished surface. Mats may be cut using a concrete saw where mitered joints are required. Partial blocks less than one half of a full-size block shall be removed, and the resulting gaps along the joint shall be filled with grout. Mats must never be overlapped on top of one another. ACB Placement Under Water. ACB systems placed in water require close observation and increased quality control to ensure a continuous countermeasure system. A systematic process for placing and continuous monitoring to verify the quantity and layer thickness is important. Excavation, grading, and placement of ACBs and filter under water require additional meas- ures. For installations of a relatively small scale, the stream around the work area may be diverted during the low-flow season. For installations on larger rivers or in deeper water, the area can be temporarily enclosed by a cofferdam, which allows for construction dewatering if necessary. Alternatively, a silt curtain made of plastic sheeting may be suspended by buoys around the work area to minimize environmental degradation during construction. ACBs can be assembled in the dry, and a crane and spreader bar can be used to lift and place the system under water. Once under water and in the correct positions, the individual mats can be cabled together by divers. E-19

Depending on the depth and velocity of the water, sounding surveys using a sounding pole or sounding basket on a lead line, divers, sonar bottom profiles, and remote operated vehicles (ROVs) can provide some information about ACB placement under water. 2.4 Finishing 2.4.1 System Termination Termination of the ACB system shall be either (1) in excavated trenches that are properly backfilled with approved material flush with the top of the finished surface of the blocks or (2) abutted to a structural feature such as a pier, footing, or pile cap. In the case of blocks abutting a structural feature, the gap between the blocks and the structure shall be filled with cast-in-place concrete or grout, and finished flush with the top surface of the ACB system. 2.4.2 Concrete Joints The use of cast-in-place concrete joints shall be minimized to the extent practicable. The fol- lowing joints shall require concrete: • Joints between cabled mats where the joint is more than 2 in. (50 mm) wider than the nomi- nal joint of the particular ACB system • Joints where block interlock is discontinuous, for example where mats are saw cut to accom- modate bends or structural features • Locations where the ACB system abuts a structural feature • Areas where there are partial blocks (to avoid small elements that have reduced hydraulic stability) 2.4.3 Anchors If soil anchors are used, they may be either helical or duckbill type. Anchors must be capable of being attached directly to the blocks, or to the ACB system cable. Anchors shall have the capa- bility of being load tested to ensure that the specified pullout capacity is achieved. Anchor pen- etrations through the geotextile shall be sealed with cast-in-place concrete or structural grout to prevent migration of subsoil through the penetration point. 2.4.4 Backfilling the ACB System The open area of the ACB system is typically either backfilled with suitable soil for revegeta- tion, or with 3/8- to 3/4-inch (10-mm to 20-mm) crushed rock aggregate. Backfilling with soil or granular fill within the cells of the system shall be completed as soon as practicable after the revetment has been installed. When topsoil is used as a fill material above the normal waterline, the ACB system should be overfilled by 1 to 2 in. (25 to 50mm) to account for backfill material consolidation. 2.4.5 Inspection The subgrade preparation, geotextile placement, ACB system installation, and overall finished condition including termination trenches shall be inspected before work acceptance. Inspection guidelines are presented in detail in Part 3 of this appendix. 2.5 Measurement and Payment Measurement of the ACB system for payment shall be made on the basis of surface area. The pay lines will be neat lines taken off the contract drawings and will include embedded blocks and/or blocks placed in termination trenches. The work includes grading and preparatory work, furnishing and installing the geotextile and ACB system, backfilling the system, securing cables and fasteners, installing soil anchors, and seeding (where specified). Payment will be made at the E-20

respective unit price per square foot. Payment will be full compensation for all material, labor, and equipment to complete the work. Part 3: Inspection, Maintenance, and Performance Evaluation 3.1 Inspection During Construction Inspection during construction shall be conducted by qualified personnel who are independ- ent of the contractor. Underwater inspection of an ACB system shall be performed only by divers specifically trained and certified for such work. 3.1.1 Subgrade Inspection of the subgrade shall be performed immediately prior to geotextile placement. The construction inspector should be alert to any condition that could cause the ACB system to not be in intimate contact with the subgrade, even if only in small, localized areas. The subgrade should be clean and free of debris, rocks, construction materials, or other foreign objects that would prevent the blocks from being firmly seated. Likewise, there should be no potholes, rills, or other voids that the blocks could bridge over. The subgrade material itself should not be muddy or frozen and should not contain organic material or other deleterious substances. Variations in subgrade characteristics over the proj- ect area shall be noted and photographed; observations of such should be brought to the attention of the project engineer as they may represent conditions that are different than those used for design. It is generally recommended that compaction testing be performed at a frequency of one test per 2,000 ft2 (186 m2) of surface area, unless project specifications require otherwise. 3.1.2 Geotextile Each roll of geotextile delivered to the job site must have a label with the manufacturer’s name and product designation. The inspector must check the labels to ensure that the geotextile is the same as that specified in the design. It is a good idea for inspectors to familiarize themselves with the different kinds of geotextiles on the market. Spun-bond fabrics and slit-film geotextiles should never be used in ACB applications. The geotextile must be stored so that it is out of direct sunlight, as damage can occur from expo- sure to ultraviolet radiation. When placed, it must be free of wrinkles, folds, or tears. Sandbags, extra concrete blocks, or U-shaped soil staples may be used to hold the geotextile in position while the blocks are being placed. The blocks should be placed within 48 hours after the geotextile is placed unless unusual circumstances warrant otherwise. 3.1.3 Blocks The inspector shall check the blocks to ensure that they are sound and are not excessively cracked or chipped. Interlocking blocks are typically hand placed and should be installed such that the interlock is not brought into tension. The block-to-block joints should be neutrally spaced such that there is equal free-play in all directions for the joint to be able to open and close. If the block pattern becomes skewed to an extent that blocks bind or protrude above the allow- able placement tolerance, the placed ACB that is determined to be out of tolerance shall be removed and replaced. The inspector must be aware that in cases where warped subgrade slopes or structural elements cause the joint pattern to become skewed, cast-in-place concrete joints may be field located in concurrence with the project engineer. E-21

E-22 When pre-assembled mats are placed, the mats should abut one another as tightly as practi- cable. Mats should never be dragged laterally across the geotextile. If the mattress is not aligned properly, it must be lifted before being repositioned. Mats should never be allowed to overlap one another. Gaps between mats, or between mats and structural features, that are more than 2 in. (50 mm) greater than the nominal system spacing shall be filled with cast-in-place concrete or structural grout. Unless specifically intended as part of the design, vehicle traffic should not be allowed on either the geotextile or the blocks. If the inspector notices vehicle traffic on the installation, the project engineer should be notified and should clearly identify which pieces of equipment are allowed on the block system and which pieces are not. Usually, light rubber-tired equipment can be tol- erated, whereas heavier vehicles and tracked vehicles cannot. 3.2 Periodic and Post-Flood Inspection As a pier scour countermeasure, the ACB system is typically inspected during the biennial bridge inspection program. However, more frequent inspection might be required by the Plan of Action for a particular bridge or group of bridges. In some cases, inspection may be required after every flood that exceeds a specified magnitude. The most important aspect of inspecting an ACB installation is to determine if the system is maintaining intimate contact with the subgrade. In the dry, this can be readily determined pro- vided the system has not been buried by sediment. The inspector should look for the following: • Cracked, broken, or missing blocks • Protruding blocks • Overturned blocks, particularly along the periphery of the system or near structural features such as piers, footers, or pile caps • Irregularities in the surface of the system (e.g., bumps or depressions) • Voids beneath the blocks or the geotextile • Deteriorated blocks (e.g., freeze-thaw or wet-dry weathering) A length of reinforcing bar is helpful in detecting voids beneath the system. It can be used as a probe to poke into the cells of the blocks or between individual blocks in a system. The inspec- tor can also use it to thump the blocks and listen for a hollow ringing sound that would indicate the presence of a void beneath the system. Underwater inspection of an ACB system shall be performed only by divers specifically trained and certified for such work. Again, the use of a bar as a probe or thumper is particularly helpful. Whether visually or by feel, the diver should pay particular attention to the areas where the ACB abuts structural elements to ensure that no gaps exist and that subgrade material is not being removed from beneath the ACB system. Also, the perimeter of the installation should be exam- ined to determine if there are any areas where mats, or portions of mats, have been undermined or overturned. Figures E3.1 through E3.5 are provided as examples of inspection-related topics specific to ACB systems, as discussed in this section. 3.3 Maintenance Deficiencies noted during the inspection should be corrected as soon as possible. Because ACB systems are essentially an armor layer that is only one particle thick, any localized area of dis- placed blocks or voids beneath the system is vulnerable to further destabilization during the next high-flow event. As with any armor system, progressive failure from successive flows must be avoided by providing timely maintenance intervention.

E-23 Note cast-in-place concrete joints at abutment. Figure E3.1. Protruding blocks, not in conformance with placement tolerance. Figure E3.2. Overturned cable-tied mats at pier.

E-24 Note ponded water at top of slope. Figure E3.4. Incomplete backfill in termination trench. Note voids beneath the overturned mats. Figure E3.3. Construction debris on subgrade. Any areas of overturned blocks must be removed and replaced. Cable-tied block mats should not be reused; for this reason, it is important to have a source of replacement blocks, geotextile, and cables/fasteners available. Non-cabled interlocking blocks may be reused if the individual blocks are undamaged. Where localized areas are limited to cracked, broken, or missing blocks, a patch consisting of cast-in-place concrete or structural grout will usually suffice. For larger

areas, the potential for uplift pressures to develop beneath impermeable patches will require weep holes to replace the lost permeability of the original block system. Any voids underneath the system must be filled. Depending on the size of the void and the nature of the ACB system, voids can be filled by the following actions: • Removing the blocks and geotextile, filling the void area with proper fill material, providing proper compaction of the filled area, and replacing the geotextile and blocks • Removing the blocks, filling the void with sand-filled geocontainers having the same filtration capacity as the original geotextile, and then replacing the blocks • Filling the void by pumping concrete or grout into the void via tremie pipe 3.4 Performance Evaluation The evaluation of any countermeasure’s performance should be based on its design parame- ters as compared to actual field experience, longevity, and inspection/maintenance history. To properly assess the performance of a pier scour countermeasure, the history of hydraulic load- ing on the installation, in terms of flood magnitudes and frequencies, must also be considered and compared to the design loading. Changes in channel morphology may have occurred over time subsequent to the installation of the pier scour countermeasure. Present-day channel cross-section geometry and planform should be compared to those at the time of countermeasure installation. Both lateral and verti- cal instability of the channel in the vicinity of the bridge can significantly alter hydraulic condi- tions at the piers. Approach flows may become skewed to the pier alignment, causing greater local and contraction scour. It is recognized that the person making the performance evaluation will probably not be the inspector; however, inspection records will be fundamental to the evaluation. Maintenance records must also be consulted so that costs can be documented and reported as a percentage of the initial capital improvement cost. E-25 Figure E3.5. Variations in subgrade conditions should be brought to the attention of the project engineer.

3.4.1 Performance Rating Guide To guide the performance evaluation for ACB systems as a pier scour countermeasure, a rat- ing system is presented in this section. It establishes numerical ratings from 0 (worst) to 6 (best) for each of three topical areas: • Hydraulic history: Has the countermeasure been subjected to severe hydraulic loading since it was constructed? • Maintenance history: Has the installation required a lot of attention and repair over its installed life to date? • Current condition: What is the current condition of the countermeasure? Tables E3.1 through E3.3 present a rating system for ACB pier scour countermeasures. A sin- gle numerical score is not intended; rather, an independent rating (0-6 or U) is given for each of the three topical areas. Recommended actions corresponding to the rating codes are also provided. 3.4.2 Example To illustrate the concepts involved in performance evaluation, a 14-year-old ACB project installed in 1991 in Fort Collins, Colorado, is described and evaluated in this section. • Project title: Abutment / bike trail scour protection • Location: Cache la Poudre River at Mulberry Street • Installation date: September 1991 • Design event frequency: 100 year • Design event discharge: 13,300 ft3/s • Side slope: 1V:4H (25%) • Surface area of project: 4,000 ft2 • System type: Armorflex 30-s, cabled • System weight (per unit area): 36 lb/ft2 • Installed cost (1991 dollars): $24,000 During construction, a temporary earth cofferdam isolated the work area from the river, and portable sump pumps allowed the ACB system to be placed in the dry. Because of limited head- room beneath the bridge deck, the system was not placed using pre-assembled mats. Instead, the contractor placed the blocks by hand and threaded the cables through the blocks later. Figure E3.6 shows the ACB system during installation in September 1991. At intervals, a row of blocks was left out of the matrix to make room for a cast-in-place concrete joint. There were a number of pre-existing structural features in the project area against which the ACB system abutted; those areas also received a cast-in-place concrete seal. E-26 Code Hydraulic History Code Hydraulic History U N/A 3 Moderate: The countermeasure has experienced one or more flows greater than the 10-year event. 6 Extreme: The countermeasure has experienced one or more flows greater than the 100-year event. 2 Low: The countermeasure has experienced one or more flows greater than the 5-year event. 5 Severe: The countermeasure has experienced one or more flows greater than the 50-year event. 1 Very Low: The countermeasure has experienced one or more flows greater than the 2-year event. 4 High: The countermeasure has experienced one or more flows greater than the 25-year event. 0 Negligible: The countermeasure has not experienced any flows greater than a 2- year event. Table E3.1. Rating system for articulating concrete blocks: hydraulic history.

The flow history is available from a U.S. Geological Survey gauging station located less than 1 mile upstream of the project site. The mean daily discharge is shown on Figure E3.7. In 1999, a flood of approximately 5,950 ft3/s passed the site. That flow was slightly greater than a 10-year event. To date, no maintenance has been necessary and the installation has remained in a stable condition with no cracked or displaced blocks at all. Figures E3.8 through E3.10 show the con- dition of the system in September 2005. E-27 Code Maintenance History Code Maintenance History U N/A 3 Moderate: The system has required occasional maintenance since installation. 6 None Required: No maintenance has been needed since installation. 2 High: Frequent maintenance has been required. 5 Very Low: The system has required maintenance for very small, local areas once or twice. 1 Very High: Significant maintenance is usually required after flood events. 4 Low: The system has required minor maintenance. 0 Excessive: The system typically requires maintenance every year. Table E3.2. Rating system for articulating concrete blocks: maintenance history. Code Description of Current Condition Code Description of Current Condition U The ACB system is uninspectable, due to burial by sediment, debris, or other circumstance. 3 Fair: The system exhibits some missing or overturned blocks. The surface of the system exhibits irregularities or protruding blocks. Localized voids beneath system. 6 Excellent: The system is in excellent condition, with no cracked, broken, or displaced blocks. Concrete joints and seals are intact. 2 Poor: The system exhibits a very irregular surface with many missing or overturned blocks. Voids under large areas of the system. 5 Very Good: The system exhibits only minor deterioration in localized areas. 1 Badly Damaged: The system has experienced substantial deterioration in terms of broken, missing, or displaced blocks, or overturned mats. 4 Good: The system exhibits few cracked or broken blocks. Only minor protrusions noted in localized areas. 0 Severe: The system has suffered damage such that it is no longer repairable. The only recourse is to remove the entire installation and replace it with a redesigned countermeasure. Recommended actions based on current condition rating: Code U: The articulating concrete block system cannot be inspected. A plan of action should be developed to determine the condition of the installation. Possible remedies may include removal of debris, excavation during low flow, probing, or non-destructive testing using ground-penetrating radar or seismic methods. Codes 6 or 5: Continue periodic inspection program at the specified interval. Codes 4 or 3: Increase inspection frequency. The rating history of the installation should be tracked to determine if a downward trend in the rating is evident. Depending on the nature of the ACB application, the installation of monitoring instruments might be considered. Code 2: The maintenance engineer’s office should be notified and maintenance should be scheduled. The cause of the low rating should be determined, and consideration given to redesign and replacement. Materials other than ACB might be considered as a replacement. Codes 1 or 0: The maintenance engineer’s office should be notified immediately. Depending upon the nature of the ACB application, other local officials and/or law enforcement agencies identified in the Plan of Action for the bridge may also need to be notified. Table E3.3. Rating system for articulating concrete blocks: current condition.

E-28 (a) (b) Figure E3.6. Mulberry Street ACB project during construction, September 1991. Cache la Poudre River at USGS Station0 6752260 Fort Collins, Colorado 0000 2000 4000 6000 8000 10000 12000 14000 1975 1979 1983 1987 1991 1995 1999 2003 Date M ea n da ily d is ch ar ge , f t3 / s ACB system installed September 1991 100 year 50 year 25 year 10 year 5 year 2 year Figure E3.7. Flow history at the Mulberry Street ACB project.

E-29 Figure E3.8. View beneath bridge deck of Mulberry Street ACB installation in September 2005. Note sediment infill and grassy vegetation in blocks. Figure E3.9. Mulberry Street ACB installation in September 2005 upstream of deck.

E-30 Note coarse bed material (3- to 6-in. cobbles) Figure E3.10. Close-up view of blocks at Mulberry Street ACB installation in September 2005. Based on the rating system presented in Section 3.4.1, the Mulberry Street ACB installation is rated as follows: • Hydraulic history: 3 (moderate) • Maintenance history: 6 (none required) • Present condition: 6 (excellent) References American Association of State Highway and Transportation Officials (AASHTO) (2003). “Geotextile Specifica- tion for Highway Construction,” AASHTO M 288, Washington, D.C. American Association of State Highway and Transportation Officials (AASHTO) (2003). “Standard Specifica- tions for Transportation Materials and Methods of Sampling and Testing,” Washington, D.C. ASTM International (2005). “Test Method for Laboratory Compaction Characteristics of Soil Using Standard Effort,” ASTM D 698, West Conshohocken, PA. ASTM International (2005). “Specification for Materials and Manufacture of Articulating Concrete Block (ACB) Revetment Systems,” ASTM D 6684, West Conshohocken, PA. ASTM International (2005). “Standard Guide for Selecting Test Methods for Experimental Evaluation of Geosynthetic Durability,” ASTM D 5819-05, West Conshohocken, PA. Ayres Associates (2001). “Design Manual for Articulating Concrete Block Systems,” prepared for Harris County Flood Control District under Project No. 32-0366.00, Fort Collins, CO. Bennett, J.P. (1997). “Resistance, Sediment Transport, and Bedform Geometry Relationships in Sand-Bed Chan- nels,” In: Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4–7. Brown, S.A., and Clyde, E.S. (1989). “Design of Riprap Revetment,” Hydraulic Engineering Circular No. 11 (HEC-11), FHWA-IP-89-016, Federal Highway Administration, Washington, D.C. Clopper, P.E. (1989). “Hydraulic Stability of Articulated Concrete Block Revetment Systems During Overtop- ping Flow,” FHWA-RD-89-199, Office of Engineering and Highway Operations R&D, McLean, VA.

Clopper, P.E. (1992). “Protecting Embankment Dams with Concrete Block Systems,” Hydro Review, Vol. X, No. 2. Heibaum, M.H. (2004). “Geotechnical Filters – the Important Link in Scour Protection,” Federal Waterways Engineering and Research Institute, Karlsruhe, Germany, 2nd International Conference on Scour and Ero- sion, Singapore. Holtz, D.H., Berry, P.E., Christopher, B.R., and Berg, R.R. (1995). “Geosynthetic Design and Construction Guidelines,” FHWA HI-95-038, Federal Highway Administration, Washington D.C. Julien, P.Y. (1995). Erosion and Sedimentation. Cambridge University Press, Cambridge, U.K. Karim, F. (1999). “Bed-Form Geometry in Sand-Bed Flows,” Journal of Hydraulic Engineering, Vol. 125, No. 12. Koerner, R.M. (1998). Designing with Geosynthetics, Fourth Edition, Prentice-Hall, Inc., Englewood Cliffs, NJ, 761 p. Lagasse, P.F., Zevenbergen, L.W., Schall, J.D., and Clopper, P.E. (2001). “Bridge Scour and Stream Instability Countermeasures,” Hydraulic Engineering Circular No. 23 (HEC-23), Second Edition, FHWA NHI -01- 003, Federal Highway Administration, Washington, D.C. Lagasse, P.F., Clopper, P.E., Zevenbergen, L.W., and Ruff, J.F. (2006). NCHRP Report 568: Riprap Design Crite- ria, Recommended Specifications, and Quality Control, Transportation Research Board of the National Acad- emies, Washington, D.C. Lagasse, P.F., Clopper, P.E., Zevenbergen, L.W., and Gerard, L.G. (2007). NCHRP Report 593: Countermeasures to Protect Bridge Piers from Scour, Transportation Research Board of the National Academies, Washington, D.C. McCorquodale, J.A., Moawad, A., and McCorquodale, A.C. (1993). “Cable-Tied Concrete Block Erosion Pro- tection,” In: Hydraulic Engineering, Vol. 2, 1993 National Conference on Hydraulic Engineering, 25–30 July, San Francisco, CA, ASCE, New York, pp. 1367–1373. Parker, G., Toro-Escobar, C., and Voight, R.L., Jr. (1998). “Countermeasures to Protect Bridge Piers from Scour,” Users Guide (revised 1999) and Final Report, NCHRP Project 24-7, prepared for National Coop- erative Highway Research Program, Transportation Research Board by St. Anthony Falls Hydraulic Labo- ratory, University of Minnesota, MN. Richardson, E.V., and Davis, S.R. (2001). “Evaluating Scour at Bridges,” Hydraulic Engineering Circular No. 18 (HEC-18), Fourth Edition, FHWA NHI 01-004, Federal Highway Administration, Washington, D.C. Stevens, M.A., and Simons, D.B. (1971). “Stability Analysis for Coarse Granular Material on Slopes,” In River Mechanics, H.W. Shen (Ed.), Water Resources Publications, Fort Collins, CO. U.S. Army Corps of Engineers (1995). “Construction Quality Management,” Engineering Regulation No. 1180-1-6, Washington D.C. van Rijn, L.C. (1984). “Sediment Transport, Part III: Bed Forms and Alluvial Roughness,” Journal of Hydraulic Engineering, Vol. 110, No. 12. E-31