Countermeasures to Protect Bridge Piers from Scour (2007)

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Introduction, F-2 1 Design and Specification, F-3 2 Construction, F-15 3 Inspection, Maintenance, and Performance Evaluation, F-21 References, F-25 F-1 A P P E N D I X F Guidelines for Pier Scour Countermeasures Using Gabion Mattresses

Introduction Gabion mattresses are containers constructed of wire mesh and filled with rocks. The length of a gabion mattress is greater than the width, and the width is greater than the thickness. Diaphragms are inserted widthwise into the mattress to create compartments (Figure F1.1). Wire is typically galvanized or coated with polyvinyl chloride to resist corrosion, and either welded or twisted into a lattice. Stones used to fill the containers can be either angular rock or rounded cob- bles; however, angular rock is preferred because of the higher degree of natural interlocking of the stone fill. During installation, individual mattresses are connected together by lacing wire or other connectors to form a continuous structure. The wire mesh allows the gabions to deform and adapt to changes in the bed while maintaining stability. Additionally, when compared to riprap, less excavation of the bed is required and smaller, more economical stone can be used. The obvious benefit of gabion mattresses is that the size of the individual stones used to fill the mattress can be smaller than stone that would individually be too small to withstand the hydraulic forces of a stream (Freeman and Fischenich 2000). This design guideline considers the application of gabion mattresses as a pier scour countermeasure. There is limited field experience with the use of gabion mattresses systems as a scour coun- termeasure for bridge piers alone. More frequently, these systems have been used for structures such as dams or dikes, or for channel slope stabilization. The guidance for pier scour applica- tions provided in this document has been developed primarily from the results of Federal High- way Administration (FHWA) Hydraulic Engineering Circular No. 23 (HEC-23) (Lagasse et al. 2001), NCHRP Project 24-07 (Parker et al. 1998), and NCHRP Project 24-07(2) (Lagasse et al. 2007). Durability of the wire mesh under long-term exposure to the flow conditions at bridge piers has not been demonstrated; therefore, the use of gabion mattresses as a bridge pier scour countermeasure has an element of uncertainty (Parker et al. 1998). Successful long-term performance of gabion mattresses depends on the integrity of the wire. Because of the potential for abrasion by coarse bed load, gabion mattresses are not appropri- ate for gravel bed streams and should be considered for use only in sand- or fine-bed streams. Additionally, water quality of the stream must be non-corrosive (i.e., relatively non-saline and F-2 Source: modified from Hemphill and Bramley (1989) Figure F1.1. Gabion mattress showing typical dimensions.

non-acidic). A polyvinyl chloride (PVC) coating should be used for applications where the potential for corrosion exists. This document is organized into three parts: • Part 1 provides design and specification guidelines for gabion mattress systems. • Part 2 presents construction guidelines. • Part 3 provides guidance for inspection, maintenance, and performance evaluation of gabion mattress systems used as a pier scour countermeasure. Part 1: Design and Specification 1.1 Materials 1.1.1 Rock Fill Standard test methods relating to material type, characteristics, and testing of rock and aggre- gates typically associated with riprap installations (e.g., filter stone and bedding layers) are pro- vided in this section and are recommended for specifying the rock fill used in gabion mattresses. In general, the test methods recommended in this section are intended to ensure that the stone is dense and durable and will not degrade significantly over time. Rocks used for gabion mattresses should break only with difficulty, have no earthy odor, not have closely spaced discontinuities (joints or bedding planes), and not absorb water easily. Rocks composed of appreciable amounts of clay, such as shales, mudstones, and claystones, are never acceptable for use as fill for gabion mattresses. Table F1.1 summarizes the recommended tests and allowable values for rock and aggregate. 1.1.2 Gabion Mattresses and Components Successful gabion performance depends not only on properly sizing and filling the baskets, but also on the quality and integrity of the wire comprising the basket compartments, diaphragms, lids, and lacing wire. Investigations conducted under NCHRP Project 24-07 (Parker et al. 1998) concluded that the lacing wire in particular proved to be the weakest link of gabion mattress systems. Wire should be single strand galvanized steel; a PVC coating may be added to protect against corrosion where required. The wire mesh may be formed with a double twist hexagonal pattern or can be made of welded wire fabric. Fasteners, such as ring binders or spiral binders, must be of the same quality and strength as that specified for the gabion mattresses. The following recommendations are pro- vided for twisted-wire and welded-wire gabions, respectively: • Twisted-wire gabion mattresses. A producer’s or supplier’s certification shall be furnished to the Purchaser that the material comprising the gabion mattress components and lacing wire was manufactured, sampled, tested, and inspected in accordance with the specifications of ASTM A 975, “Standard Specification for Double-Twisted Hexagonal Mesh Gabions and Revet Mattresses (Metallic-Coated Steel Wire or Metallic-Coated Steel Wire with Poly Vinyl Chloride (PVC) Coating).” The certification must indicate that the minimum requirements of this standard have been met. • Welded-wire gabion mattresses. A Producer’s or Supplier’s certification shall be furnished to the Purchaser that the material comprising the gabion mattress components and lacing wire was manufactured, sampled, tested, and inspected in accordance with the specifications of ASTM A 974, “Standard Specification for Welded Wire Fabric Gabions and Gabion Mattresses (Metallic-Coated or Poly Vinyl Chloride (PVC) Coated).” The certification must indicate that the minimum requirements of this standard have been met. F-3

Flexibility of the gabion mattress units is a major factor in the successful performance of these systems as a pier scour countermeasure. The ability to adjust to changes in the environment around a bridge pier is desirable, particularly settling around the perimeter if scour at the edges of the system occurs. Rigid systems are more prone to undermining and subsequent damage to the mesh and are therefore less suitable for use at bridge piers. Designers are encouraged to famil- iarize themselves with the flexibility and performance of various materials and proprietary prod- ucts for use in riverine environments. 1.2 Hydraulic Stability Design Procedure 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. The Harris County Flood Control District (HCFCD), Texas, has developed a simple flowchart approach that considers the type of application, uncertainty in the hydraulic and hydrologic F-4 Test Designation Property Allowable value Frequency (1) Comments AASHTO TP 61 Percentage of Fracture < 5% 1 per 20,000 tons Percentage of pieces that have fewer than 50% fractured surfaces AASHTO T 85 Specific Gravity and Water Absorption Average of 10 pieces: Sg > 2.5 Absorption < 1.0% 1 per year If any individual piece exhibits an Sg less than 2.3 or water absorption greater than 3.0%, an additional 10 pieces shall be tested. If the second series of tests also exhibits pieces that do not pass, the riprap shall be rejected. AASHTO T 103 Soundness by Freezing and Thawing Maximum of 10 pieces after 25 cycles: < 0.5% 1 per 2 years Recommended only if water absorption is greater than 0.5% and the freeze-thaw severity index is greater than 15 per ASTM D 5312. AASHTO T 104 Soundness by Use of Sodium Sulfate or Magnesium Sulfate Average of 10 pieces: < 17.5% 1 per year If any individual piece exhibits a value greater than 25%, an additional 10 pieces shall be tested. If the second series of tests also exhibits pieces that do not pass, the riprap shall be rejected. AASHTO TP 58 Durability Index Using the Micro- Deval Apparatus Value > 90 > 80 > 70 Application Severe Moderate Mild 1 per year Severity of application per Section 5.4, CEN (2002). Most riverine applications are considered mild or moderate. ASTM D 3967 Splitting Tensile Strength of Intact Rock Core Specimens Average of 10 pieces: > 6 MPa 1 per year If any individual piece exhibits a value less than 4 MPa, an additional 10 pieces shall be tested. If the second series of tests also exhibits pieces that do not pass, the riprap shall be rejected. ASTM D 5873 Rock Hardness by Rebound Hammer See Note (2) 1 per 20,000 tons See Note (2) Shape Length to Thickness Ratio A/C < 10%, d50 < 24 in < 5%, d50 > 24 in 1 per 20,000 tons Percentage of pieces that exhibit A/C ratio greater than 3.0 using the Wolman count method (Lagasse et al., 2006) ASTM D 5519 Particle Size Analysis of Natural and Man-Made Riprap Materials 1 per year See Note (3) Gradation Particle Size Distribution Curve 1 per 20,000 tons Determined by the Wolman count method (Lagasse et al., 2006), where particle size, d, is based on the intermediate (B) axis. (1) Testing frequency for acceptance of riprap from certified quarries, unless otherwise noted. Project-specific tests exceeding quarry certification requirements, either in performance value or frequency of testing, must be specified by the Engineer. (2) Test results from D 5873 should be calibrated to D 3967 results before specifying quarry-specific minimum allowable values. (3) Test results from D 5519 should be calibrated to Wolman count (Lagasse et al., 2006) results before developing quarry-specific relationships between size and weight; otherwise, assume W = 85% that of a cube of dimension d having a specific gravity of Sg. Table F1.1. Recommended tests for rock quality.

F-5 Source: Ayres Associates (2001) Figure F1.2. Selecting a target factor of safety. models used to calculate design conditions, and consequences of failure to select an appropriate target factor of safety to use when designing an articulating concrete block (ACB) installation (Ayres Associates 2001). In this approach, the minimum allowable factor of safety for ACBs at bridge piers is 1.5. This value is then multiplied by two factors, each equal to or greater than 1.0, to account for risk and uncertainty. Figure F1.2 shows the HCFCD flowchart method. The method is also considered appropriate for the use of gabion mattresses at piers. 1.2.2 Design Method Gabion mattress design methods typically yield a required d50 stone size that will result in sta- ble performance under the design hydraulic loading. Because stone is produced and delivered in a range of sizes and shapes, the required size of stone is often stated in terms of a minimum

and maximum allowable size. For example, ASTM D 6711, “Standard Practice for Specifying Rock to Fill Gabions, Revet Mattresses, and Gabion Mattresses,” recommends the ranges in Table F1.2. ASTM D 6711 also indicates that the fill should be well graded with a full range of sizes between the upper and lower limits. The rocks used to fill gabion mattresses should be hard, dense, and durable. In general, rocks used for filling gabion mattresses should be of the same material qual- ity as would be used for riprap, as described in Section 1.1 of this appendix. The recommended procedure for determining the permissible shear stress for a gabion mat- tress is determined using the relationship provided in Hydraulic Engineering Circular No. 15 (HEC-15) third edition (Kilgore and Cotton 2005): τp = Cs(γs − γw)d50 (F1.1) where τp = Permissible shear stress, lb/ft2 (N/m2) Cs = Stability coefficient for rock-filled gabion mattress equal to 0.10 γs = Unit weight of stone, lb/ft3 (N/m3) γw = Unit weight of water, 62.4 lb/ft3 (9,810 N/m3) d50 = Median diameter of rockfill in mattress, ft (m) The coefficient Cs is an empirical coefficient developed by Maynord (1995) from test data pre- sented in Simons et al. (1984). Use of Cs = 0.1 is limited to the conditions of the testing program, which used angular rock and a ratio of maximum to minimum stone size of 1.5 to 2.0. 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 HEC-23 (Lagasse et al. 2001), the section-average approach velocity, Vavg, must be multiplied by factors that are a function of the shape of the pier and its location in the channel: Vdes = K1K2Vavg (F1.2) 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 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 velocity is recommended since the channel could shift and the maximum velocity could impact any pier: Vdes = K1Vlocal (F1.3) F-6 Mattress thickness, inches (cm) Range of stone sizes, inches (cm) 6 (15) 3 to 5 (7.6 to 12.7) 9 (23) 3 to 5 (7.6 to 12.7) 12 (30) 4 to 8 (10 to 20) Table F1.2. Size ranges for rock to fill gabion mattresses.

The local shear stress at the pier, τdes, is calculated using a rearranged form of Manning’s equation: (F1.4) where τdes = Design shear stress, lb/ft2 (N/m2) γw = Unit weight of water, 62.4 lb/ft3 (9,810 N/m3) y = Depth of flow at pier, ft (m) n = Manning’s n for the gabion mattress (typical range 0.025–0.035) Ku = 1.486 for U.S. customary units, 1 for SI units The factor of safety can be calculated as the ratio of the permissible shear stress divided by the applied shear stress: (F1.5) Minimum rock size should be at least 1.25 times larger than the aperture size of the wire mesh that comprises the mattress (Parker et al. 1998). Rock should be well graded between the mini- mum and maximum sizes to minimize the size of the voids in the matrix. If design criteria and economic criteria permit, standard gradations may be selected. The thickness of the gabion mattress should be at least twice the average diameter of the rock fill, T ≥ d50. If the computed thickness does not match that of a standard gabion thickness, the next larger thickness of mattress should be used (Maynord 1995). At a minimum, the thickness should be 0.15 m (6 in.) (Parker et al. 1998). 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 gabion mattresses as a pier scour countermeasure was obtained when the mattresses 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 structure 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): (F1.6) Gabion mattresses should be placed so that the long axis is parallel to the direction of flow (Yoon 2005). Where only clear-water scour is present, the gabion mattresses may be placed hor- izontally such that the top of the mattress is flush with the bed elevation; however, when other types of scour are present, the mattresses must be sloped away from the pier in all directions such that the depth of the system at its periphery is greater than the depth of contraction scour and long-term degradation, or the depth of bed-form troughs, whichever is greater (Figure F1.3). The mattresses should not be laid on a slope steeper than 1V:2H (50%). In some cases, this cri- terion may result in gabions being placed further than two pier widths away from the pier. K a L a α α α = +⎛⎝⎜ ⎞⎠⎟ cos sin .0 65 F S p des . . = τ τ τ γ des w des uy nV K = ⎛⎝⎜ ⎞⎠⎟1 3 2 / F-7

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 gabion mattresses at bridge piers. The filter should not be extended fully beneath the gabions; instead, it should be terminated two-third of the distance from the pier to the edge of the gabion mattress. When using a granular stone filter, 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 granular filter layer thickness should be increased by 50% when plac- ing under water. 1.4 Filter Requirements The importance of the filter component of a gabion installation should not be underestimated. Two kinds of filters are used in conjunction with gabion mattresses: granular filters and geotex- tile filters. Some situations call for a composite filter consisting of both a granular layer and a geotextile. The specific characteristics of the base soil determine the need for, and design con- siderations of, the filter layer. In cases where dune-type bed forms may be present, it is strongly recommended that only a geotextile filter be considered. F-8 a. Profile Width = a Gabion extent = 2a (minimum, all around) FLOW b. Plan PIER 2a 2a FLOW Extend filter 2/3 the distance from the pier face to the periphery of the gabions Toe down to maximum scour depth or depth of bedform trough, whichever is greater Figure F1.3. Gabion mattress layout diagram for pier scour countermeasures.

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 filter, 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 abil- ity of a geotextile to transmit water across its thickness. It is typically reported in units of cen- timeters per second (cm/s). This property is directly related to the filtration function that a geo- textile 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 geotextile industry to more readily compare geotextiles of different thicknesses. Permit- tivity, ψ, is defined as K divided by the geotextile thickness, t, in centimeters; therefore, per- mittivity has a value of (s)−1. Permeability (and permittivity) is extremely important in gabion mattress filter design. For gabion mattress installations, the permeability of the geotextile should be at least 10 times greater than that of the underlying material: Kg > 10Ks (F1.7) 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 gabion mattress filter design. • Apparent opening size (AOS). Also known as equivalent opening size, this measure is gener- ally 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. • Thickness. As mentioned above, thickness is used to calculate traditional permeability. It is typically reported in millimeters or mils (thousandths of an inch). F-9

• Grab strength and elongation. Grab strength is the force required to initiate a tear in the fab- ric when pulled in tension. It is typically reported in Newtons or pounds as measured in a test- ing 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 F1.3 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 gabion mattress installation at piers have been discussed here. Geotex- tiles 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 cov- ered by gabion mattresses, these stresses do not represent the environment that the geotextile will experience in the long term. F-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 F1.3. Recommended requirements for geotextile properties.

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. A decision tree is provided as Figure F1.4 to assist in determining the appropriate soil retention criterion for the geotextile. The figure includes guidance when a granular transition layer (i.e., composite filter) is necessary. A com- posite 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). F-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 F1.4. Geotextile selection based on soil retention.

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 geotextile. 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 F1.3 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 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 minimize 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 F1.5 provides a design chart based on the Cistin–Ziems approach. F-12

• 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 gabion mattress 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. 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. F-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 F1.5. Granular filter design chart according to Cistin and Ziems.

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 F1.5) 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 distribu- tion 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 Gabion Mattress Rock. Repeat steps 1 through 4 above, considering that the filter material is now the “finer” soil and the rock is the “coarser” material. If the Cistin–Ziems criterion is not met, then multiple 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 large rock fill particle sizes are used. 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). NOTE: In cases where dune-type bed forms may be present or underwater installation, it is strongly recommended that only a geotextile filter be considered. 1.5 Guidelines for Seal Around the Pier An observed key point of failure for gabion mattress systems at bridge piers during laboratory studies occurs at the joint where the mattress meets the bridge pier. During NCHRP Project 24- 07(2), securing the geotextile to the pier prevented the leaching of the bed material from around the pier. This procedure worked successfully in the laboratory, but there are constructability implications that must be considered when this technique is used in the field, particularly when the mattress is placed under water. A grout seal between the mattress and the pier is recommended. A grout seal is not intended to provide a structural attachment between the mattress and the pier, but instead is a simple method for plugging gaps to prevent bed sediments from winnowing out from beneath the sys- tem. In fact, structural attachment of the mattress to the pier is strongly discouraged. The trans- fer of moments from the mattress 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. 1.6 Anchors Anchors are not typically used with gabion mattress systems; however, 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 degradation, or bed-form troughs, whichever is greater. Where such toedown depth cannot be achieved, for example where bedrock is encountered at shallow depth, a gabion mattress system with anchors along the front F-14

(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 resistance based on calculated drag on the exposed leading edge. Spacing between anchors of no more than 4 ft (1.3 m) is rec- ommended. The following example is provided: Given ρ = Mass density of water, slugs/ft3 = 1.94 V = Approach velocity, ft/s = 10 Δz = Height of gabion mattress, ft = 0.5 b = Width of mattress 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 the corners of the 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 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 gabion mattress systems to achieve suitable hydraulic performance and maintain stability against hydraulic loading. The proper installation of gabion mattress systems is essential to the adequate functioning and performance of the system during the design hydrologic event. Gabion mattress installation can be labor intensive, requiring manual attachment of the lacing wire and/or con- nectors on mattress components and also for mattress-to-mattress connection. 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, gabion mattress 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 through 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 gabion mattress placement are included in the project plans and specifications. Standard gabion mattress specifications and layout guidance are found in Part 1 of this appendix. Recommended requirements for the stone and wire mesh, including the tests F-15

necessary to ensure that the physical and mechanical properties meet the requirements of the project specifications, are provided. Field tests can be performed at the quarry and/or on the job site, or representative samples can be obtained for laboratory testing to ensure that the rock fill is of suitable quality. Inspection of gabion mattress placement consists of visual inspection of the operation and the finished surface. Inspection and quality assurance must be carefully organized and conducted in case potential problems or questions arise over acceptance of stone or wire mesh material. Accep- tance should not be made until measurement for payment has been completed. The engineer and inspectors reserve the right to reject stone at the quarry, at the job site or stockpile, and in place in the structures throughout the duration of the contract. Stone rejected at the job site should be removed from the project site. Stone rejected at the quarry should be disposed of or otherwise prevented from mixing with satisfactory stone. Construction techniques can vary tremendously because of the following factors: • Size and scope of the overall project • Size and weight of the stone fill particles • 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- mation regarding construction of gabion mattress installations and some basic information and description of techniques and processes involved. 2.2 Materials 2.2.1 Mattresses Materials composing the gabion mattress system shall be in accordance with the guidance pro- vided in Part 1 of this appendix. Mattresses shall be sound and free of defects that would inter- fere with proper placement or that would impair the integrity of the system. Wire mesh should be inspected upon arrival to ensure no structural damage occurred during transport. 2.2.2 Wire Wire shall not be kinked or broken. Kinks may be stretched or stamped out if integrity of the gabion is not compromised in doing so. PVC coated wire shall not show sign of cracks, splits, or color changes. 2.2.3 Filter Geotextile. Either woven or non-woven, needle-punched geotextiles 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. Each roll of geotextile shall be labeled with the manufacturer’s name, product identification, roll dimensions, lot number, and date of manufacture. Geotextiles shall not be exposed to sunlight prior to placement. Granular filters. Samples of granular filter material shall be tested for grain size distribution to ensure compliance with the gradation specification used in design. Sampling and testing fre- quency shall be in accordance with the owner or owner’s authorized representative. F-16

2.2.4 Subgrade Soils When placement is in the dry, the gabion mattress system shall be placed on undisturbed native soil, on an excavated and prepared subgrade, or on acceptably placed and compacted fill. 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 shall be removed, and the excavation backfilled with approved mate- rial that is compacted prior to placement of the mattress 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 geotechnical engineer’s recommendations. When gabion mattresses are 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 system and the subgrade. Under no circumstances may gabion mattresses be draped over boulders, bridged over subgrade voids, or placed over other irregularities that would prevent achievement of intimate contact between the system and the subgrade. Placing a layer of bed- ding stone may assist in achieving a suitable surface on which to place the gabion mattress 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 facili- tate the development of intimate contact between the gabion mattress system and the underlying grade. Termination between the gabion mattress system and a concrete slab, footer, pier, wall, or similar 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 gabion mattresses. Soils not meeting the requirements shall be removed and replaced with acceptable material. When placement is in the dry, the areas to receive the gabion mattress 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 gabion mat- tress system. The subgrade should be uniformly compacted to the geotechnical engineer’s site- specific requirements. If the subgrade surface for any reason becomes rough, corrugated, uneven, textured, or traffic marked prior to gabion mattress installation, such unsatisfactory portion shall be scarified, reworked, recompacted, or replaced as directed by the engineer. When placement is under water, 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 mattress placement or that would create voids beneath the system in accordance with Section 2.2.4. Immediately prior to placement of the filter and gabion mattress system, the prepared sub- grade must be inspected. 2.3.2 Placing the Filter Whether the filter comprises one or more layers of granular material or is made of geotextile, its placement should result in a continuous installation that maintains intimate contact with the soil beneath. Voids, gaps, tears, or other holes in the filter must be avoided to the extent practi- cable, and the filter must be replaced or repaired when they occur. Placement of Geotextile. The geotextile shall be placed directly on the prepared area, in inti- mate contact with the subgrade. When a geotextile is placed, it should be rolled or spread out F-17

directly on the prepared area and be free of folds or wrinkles. The rolls shall not be dragged, lifted by one end, or dropped. The geotextile should be placed in such a manner that placement of the overlying materials will not excessively stretch or tear the geotextile. After geotextile placement, the work area shall not be trafficked or disturbed in a manner that might result in a loss of intimate contact between the gabion mattress, the geotextile, and the subgrade. The geotextile shall not be left exposed longer than the manufacturer’s recommenda- tion to minimize potential damage due to ultraviolet radiation; therefore, the overlying materi- als should be placed as soon as practicable. The geotextile shall be placed so that upstream strips overlap downstream strips. Overlaps shall be in the direction of flow wherever possible. The longitudinal and transverse joints shall be overlapped at least 1.5 ft (46 cm) for dry installations and at least 3 ft (91 cm) for below-water 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 radi- ation. If necessary to expedite construction and to maintain the recommended overlaps, anchor- ing pins, U-staples, or temporary weights such as sandbags shall be used. Placing Geotextiles Under Water. Placing geotextiles under water can be problematic for a number of reasons. Most geotextiles that are used as filters beneath gabion mattresses are made of polyethylene or polypropylene. These materials have specific gravities ranging from 0.90 to 0.96, meaning that they will float unless weighted down or otherwise anchored to the subgrade prior to placement of the countermeasure (Koerner 1998). Flow velocities greater than about 1.0 ft/s (0.3 m/s) create large forces on the geotextile. These forces cause the geotextile to act like a sail, often resulting in wavelike undulations of the fabric (a condition that contractors refer to as “galloping”) that are extremely difficult to control. The preferred method of controlling geotextile placement is to isolate the work area from river cur- rents by a temporary cofferdam. In mild currents, geotextiles precut to length can be placed by divers, with sandbags to hold the fabric temporarily. In the past, geotextiles have been affixed to the base of each mattress or placed on the bottom of the mattress compartments prior to their being filled with stone; however, this method will not result in a good overlap of fabric between individual mattresses and is not recommended. At bridge piers, sand-filled geocontainers made of non-woven, needle-punched fabric are particularly effective for placement under water as shown in Figure F2.1. The geotextile fabric F-18 FLOW Sand -filled geocontainers Gabion mattresses Pier Figure F2.1. Schematic diagram showing the use of sand-filled geocontainers as a filter.

and sand fill that compose the geocontainers should be selected in accordance with the filter design criteria presented in Part 1 and placed such that they overlap to cover the required area. Geocontainers can be fabricated in a variety of dimensions and weights. Each geocontainer should be filled with sand to no more than 80% its total volume so that it remains flexible and “floppy.” The geocontainers also can serve to fill a pre-existing scour hole around a pier prior to placement of the gabion mattresses, as shown in Figure F2.1. For more detail, see Lagasse et al. (2006, 2007). Placement of Granular Filter. For placing a granular filter, front-end loaders are the preferred method for dumping and spreading the material on slopes milder than approximately 4H:1V. A typical minimum thickness for granular filters is 0.5 to 1.0 ft (0.15 to 0.3 m). Placing granular media under water around a bridge pier is best accomplished using a large-diameter tremie pipe to control the placement location and thickness, while minimizing the potential for segregation. When a granular filter is placed under water, the thickness should be increased by 50%. NOTE: For riverine applications where dune-type bed forms may be present, it is strongly recom- mended that only a geotextile filter be considered. 2.3.3 Placing the Gabion Mattress System Manufacturer’s assembly instructions should be followed. Mattresses shall be placed on the filter layer and assembled so that the wire does not kink or bend. Mattresses shall be placed so that the longitudinal axis is parallel to the flow and internal diaphragms are perpendicular to the flow. Prior to filling, adjacent mattresses should be connected along the vertical edges and the top selvedges by lacing, fasteners, or spiral binding. Custom fitting of mattresses around corners or curves should be done according to manufacturer’s recommendations. Care shall be taken during installation so as to avoid damage to the geotextile or subgrade during the installation process. Mattresses should not be pushed or pulled laterally once they are on the geotextile. Preferably, the mattress placement and filling should begin at the upstream section and proceed downstream. If a mattress 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 mattress system to protect against flow that could otherwise undermine the system during flow events that may occur during construction. On sloped sections where practical, placement and filling shall begin at the toe of the slope and proceed upslope. Gabion Mattress Placement Under Water. Gabion mattresses placed in water require close observation and increased quality control to ensure a continuous countermeasure system. A sys- tematic process for placing and continuous monitoring to verify the quantity and layer thickness is important. Excavation, grading, and placement of gabion mattresses and filter under water require addi- tional measures. 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. Gabions can be assembled, filled, and closed 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 mat- tresses can be laced together or otherwise connected by divers. 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 the gabion mattress placement under water. F-19

2.3.4 Filling of Mattresses Gabions should be filled carefully, either by machine or by hand placement. Machine place- ment will typically require some hand positioning to minimize voids and avoid bulging. Com- partments shall be filled simultaneously so that the depth of rock in one compartment is never significantly greater than the depth in adjacent compartments. Gabions can be overfilled slightly to account for settling, but overfilling should not cause the lid to bulge or become separated from the sides of the basket. Any wire damaged during assembly, filling, and/or placement should be promptly repaired or replaced according to manufacturer’s instructions. Excessive damage may require replacement of the gabion mattress. 2.3.5 Closing the Gabion Mattresses Lids shall be tightly stretched over the rock fill and secured using manufacturer-recommended tools. The lids shall then be secured to the gabion mattresses along all top selvedges. The wire fabric shall be drawn tightly against the rock on all sides and tied with galvanized wire, locking clips, hog rings, or connectors. When ties, locking clips, hog rings, or connectors are used for tying mesh sections and selvedges together, they shall be spaced 3 in. or less apart as specified in the plans. Galvanized wire ties shall be spaced approximately 2 ft on center and shall be anchored to the bottom layer of wire fabric (Lagasse et al. 2001). 2.4 Finishing 2.4.1 System Termination Termination of the gabion mattress 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 gabions or (2) abutted to a structural feature such as a pier, footing, or pile cap. In the case of gabions abutting a structural feature, any gaps between the mattress and the structure shall be filled with cast-in-place concrete or grout and finished flush with the top surface of the gabion mattress. 2.4.2 Backfilling the Gabion Mattress System The gabion mattress system can be either backfilled with suitable soil for revegetation, or with 3/8- to 3/4-in. (10- to 20-mm) crushed rock aggregate. Backfilling with soil or granular fill 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 gabion system should be overfilled by 1 to 2 in. (25 to 50 mm) to account for backfill material consolidation. 2.4.3 Inspection The subgrade preparation, geotextile placement, gabion mattress system installation, and over- all finished condition including termination trenches shall be inspected before work acceptance. Inspection guidelines for the gabion installation are presented in detail in Part 3 of this appendix. 2.5 Measurement and Payment Measurement of the gabion 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 mattresses and/or mattresses placed in termination trenches. The finished surface of the gabion mattresses should be surveyed to ensure that the as-built lines and grades meet the design plans within the specified tolerance. Survey cross sections perpendicular to the axis of the structure are usually taken at specified intervals. Payment will be full compensation for all material, labor, and equipment to complete the work. F-20

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 a gabion mattress system shall be only performed 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 subgrade should be clean and free of projections, debris, construction materials, or other foreign objects that would prevent the filter from being properly placed. Likewise, there should be no potholes, rills, or other voids that the filter material might 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 project 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 from 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 or slit-film geotextiles should never be used in gabion mattress applications. The geotextile must be stored so that it is out of direct sunlight, as damage can occur from exposure to ultraviolet radiation. When placed, it must be free of wrinkles, folds, or tears. Sand- bags, anchor pins, or U-shaped soil staples may be used to hold the geotextile in position while the gabion mattresses are being placed. The gabions should be placed within 48 hours after the geotextile is placed unless unusual circumstances warrant otherwise. 3.1.3 Gabion Mattresses Inspection of gabion mattress placement typically consists of visual inspection of the opera- tion and the finished surface. Inspection must ensure that the mattresses are sound and have been connected on all vertical surfaces and top selvedges to form a continuous unit, that the rock fill is compact and voids are minimal within each compartment, and that the wire has not been broken or kinked. 3.2 Periodic and Post-Flood Inspection Bridge pier gabion mattress systems are typically inspected during the biennial bridge inspec- tion 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. Underwater inspection of a gabion mattress system shall be performed only by divers specif- ically trained and certified for such work. Whether visually or by feel, the diver should pay par- ticular attention to the areas where the gabion abuts structural elements to ensure that no gaps exist and that subgrade material is not being removed from beneath the gabion mattress F-21

system. The diver should also ensure that individual mattresses abut one another such that there are no gaps between mattresses, that mattresses have not been placed on top of one another, and that the mattresses have been joined together by lacing wire or other approved connectors. An important aspect of inspecting a gabion mattress installation is to determine if the system has been repositioned by the flow to the extent that the subgrade or filter has become exposed. The mattresses and filling stone should be examined for evidence of downstream migration. If filling stone is observed to migrate downstream within each compartment, the height difference between the highest and lowest rock surface should be measured as shown in Figure F3.1. Some movement of the stone fill within mattress compartments after exposure to high shear stresses is acceptable. The following relationship should be maintained to ensure that underlying subgrade remains protected and unexposed (Maccafferi, undated): (F3.1) where ΔZ = Height difference between the highest and lowest rock surface within a mattress, ft (m) d50 = Median diameter of rock fill, ft (m) t = Thickness of mattress, ft (m) Finally, the mesh and wire should be checked for signs of deterioration. Waterborne sediment and debris can abrade PVC and galvanized coating, resulting in increased corrosion and thin- ning of the wire. When damage is detected, the damaged wire should be replaced. 3.3 Maintenance Deficiencies noted during the inspection should be corrected as soon as possible. Because gabion mattress systems are essentially an armor layer that is only one particle thick, any localized ( )ΔZ d t d ≤ −⎛⎝⎜ ⎞⎠⎟2 150 50 F-22 Gabion mattress compartment Gabion mattress lid FLOW t t Source: Maccafferi (undated) Z Figure F3.1. Downstream migration of stones in a gabion mattress.

area of displaced mattresses 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. Some opportunity may exist to repair gabion mattresses in place by using custom-fit mat- tresses, wire mesh, and rock (Parker et al. 1998). Any voids underneath the system must be filled. Depending on the size of the void and the nature of the gabion mattress system, voids can be filled by the following actions: • Removing the mattress and geotextile, filling the void area with proper fill material, provid- ing proper compaction of the filled area, replacing the geotextile and mattress, and relacing the system • Removing the mattresses, filling the void with sand-filled geocontainers having the same fil- tration capacity as the original geotextile, and then replacing the mattresses and relacing the system • Filling the void by pumping sand and gravel, concrete, or grout into the void via tremie pipe 3.4 Performance Evaluation The evaluation of any gabion mattress system’s performance should be based on its design parameters as compared to actual field experience, longevity, and inspection/maintenance his- tory. To properly assess the performance of gabion mattresses, the history of hydraulic loading 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 gabion mattresses. Present-day channel cross-section geometry and planform should be compared to those at the time of installation. Both lateral and vertical instability of the channel can significantly alter hydraulic conditions at the piers. Approach flows may exhibit an increas- ingly severe angle of attack (skew) over time, increasing the hydraulic loading on the gabion mattress. 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 to allow compar- ison to the initial capital improvement cost. 3.4.1 Performance Rating Guide To guide the performance evaluation for gabion mattress systems as a pier scour counter- measure, a rating 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 F3.1 through F3.3 present a rating system for gabion mattress pier scour countermea- sures. A single 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. F-23

F-24 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 F3.1. Rating system for gabion mattress: hydraulic history. 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 F3.2. Rating system for gabion mattress: maintenance history. Code Description of Current Condition Code Description of Current Condition U The gabion system is uninspectable, due to burial by sediment, debris, or other circumstance. 3 Fair: Stone fill in mattress has shifted but filter and subgrade are not exposed. No stone fill has been lost. Broken or corroded wire has been replaced or repaired. 6 Excellent: The system is in excellent condition, with no shifting of stone fill, and no broken, corroded, or kinked wire. 2 Poor: Stone fill has shifted severely and the filter or subgrade is exposed. Wire has been broken or corroded and stone fill has been removed. 5 Very Good: The system exhibits only minor deterioration in localized areas. 1 Badly Damaged: The system has experienced substantial deterioration in terms of loss of stone fill and/or undermining. Wire is broken or corroded. 4 Good: Minor shifting of stone fill, no evidence of loss of stone fill. Some broken or corroded wire noticed, but mattresses are intact and functional. 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 gabion mattress 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 gabion mattress 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 gabion mattresses might be considered. Codes 1 or 0: The maintenance engineer’s office should be notified immediately. Depending upon the nature of the gabion application, other local officials and/or law enforcement agencies may also need to be notified. Table F3.3. Rating system for gabion mattress: current condition

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