Seismic Design of Geosynthetic-Reinforced Soil Bridge Abutments with Modular Block Facing (2012)

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CHAPTER 8 CONSTRUCTION GUIDELINES RECOMMENDED CONSTRUCTION GUIDELINES Earthwork construction control for Geosynthetic-Reinforced Soil (GRS) abutments under seismic loading is essentially the same as that required under static loading (as presented in NCHRP Report 556, Wu, et al., 2006). The recommended construction guidelines described below focus on GRS abutments with segmental concrete block facing. Only basic guidelines are given for GRS abutments with other forms of flexible facing. Segmental Concrete Block Facing GRS Abutments The construction guidelines presented below are established based on the guidelines for construction of segmental GRS walls provided by various agencies, including AASHTO (1998), National Concrete Masonry Association (2009), Federal Highway Administration (Elias, et al., 2001; Adams, et al., 2011), Colorado Transportation Institute (Wu, 1994), Swiss Association of Geotextile Professionals (1981), Japan Railway (1998), as summarized in NCHRP Report 556, as well as the authors’ and their colleagues’ observations and experiences with construction of GRS walls and abutments. Site and Foundation Preparation - Before placement of the reinforcement, the ground should be graded to provide a smooth, fairly level surface. - The surface should be clear of vegetation, large rocks, stumps, and the like. Depressions may need to be filled; soft spots may need to be excavated and replaced with backfill material; and the site may need to be proof rolled. - If the foundation contains frost susceptible soils, they should be excavated to at least the maximum frost penetration line and replaced with non-frost-susceptible soil. - If the foundation is only marginally competent, the top 1 m of the foundation may be excavated and replaced with a reinforced soil foundation (compacted granular soil reinforced with four equally- spaced layers of geosynthetic reinforcement, wide-width strength of reinforcement > 70 kN/m, per ASTM D 4595). - For abutment walls less than 10 m high, unless the ground surface is level and the foundation soil is stiff, a leveling pad should be constructed under the first course of the facing blocks. The leveling pad should be a compacted road base material of

240 approximately 150 mm thick and 450 mm wide. Compaction of the leveling pad should be performed using a light-compactor to obtain a minimum of 95% of the maximum standard Proctor density (AASHTO T-99). - If excavation is needed, it shall be carried out to the lines and grades shown on the project grading plans. Over-excavation shall be minimized. - In a stream environment, GRS abutment should also be protected from possible scour and abrasion by using riprap or other protection measures. Reinforcement and Reinforcement Placement - Geosynthetic reinforcement shall consist of high tenacity geogrids or geotextiles manufactured for soil reinforcement applications. Geosynthetics, especially geotextiles, should not be exposed to sunlight and extreme temperatures for an extended period of time. Damaged or improperly handled geosynthetic reinforcement should be rejected. - Geosynthetic reinforcement should be installed under tension. A nominal tension shall be applied to the reinforcement and maintained by staples, stakes or hand tensioning until the reinforcement has been covered by at least 150 mm of soil fill. - The geosynthetic reinforcement perpendicular to the wall face should consist of one continuous piece of material. Overlap of reinforcement in the design strength direction is not permitted. Adjacent sections of geosynthetic should be placed in a manner to assure that horizontal coverage shown on the plans is provided. - Tracked construction equipment shall not be operated directly on the geosynthetic reinforcement. A minimum backfill thickness of 150 mm is required prior to operation of tracked vehicles over the geosynthetic reinforcement. Turning of tracked vehicles should be kept to a minimum to prevent displacing the fill and damaging or moving the geosynthetic reinforcement. - Rubber-tired equipment may pass over the geosynthetic reinforcement at slow speeds less than 17 km/hr (10 miles/hr). Sudden braking and sharp turning should be avoided. - At any elevations where the facing is “rigid”, such as behind a rigid facing upper wall or the top two to three courses of the lower wall where the segmental facing blocks are inter- connected, geosynthetic reinforcement should be wrapped at the wall face. The wrapped face shall help reduce sloughing of fill due to precipitations and the “gaps” that may form as a result of movement of the wall face. In the upper wall, the wrapped return should be extended at least 0.45 m in the horizontal direction and anchored in at least 0.1 m of fill material. The wrapped return should extend at least 1.5 m in the load bearing wall. The added reinforcement in the load bearing wall will increase the safety margin of its load carrying capacity.

241 - It is a good practice to place a compressible layer (e.g., a low to medium density expanded polystyrene sheet), of approximately 50 mm in thickness, between the wrapped face reinforcement and the rigid abutment upper wall. Such a measure can effectively reduce lateral earth pressure and movement of the abutment wall (Monley and Wu, 1993). - A “tail” (a shortened reinforcement sheet with one end sandwiched between facing blocks) extending a minimum of 0.6 m beyond the heel of the sill should be used to “attach” the facing with the reinforced fill (see Figure 8.1). - The wrapped return of geosynthetic reinforcement at top surface of each tier (top surfaces of the upper and lower walls) should extend to the full length (see Figure 8.1). - For larger reinforcement spacing (say, 0.4 m or larger), it is a good practice to incorporate secondary reinforcement, of length about 1 m, between full-length reinforcement. Backfill - Structure backfill material shall consist of material that is free from organic material or other unsuitable material as determined by the engineer. - Unless otherwise specified, grading of the backfill shall be as follows,: 100% passing 100 mm sieve, 0-60% passing No. 40 (0.425 mm) sieve, and 0-15% passing No. 200 (0.075mm) U.S. Standard sieve; plasticity index (PI) as determined by AASHTO T90, shall not exceed 6. - The backfill shall exhibit an angle of internal friction of not less than 34 degrees, as determined by the standard direct shear test on the portion finer than 2 mm (No.10) sieve, using a sample compacted to 95% of AASHTO T-99, Methods C or D, at optimum moisture content. No testing is required for backfills where 80% of sizes are greater than 19 mm. - The backfill shall be substantially free of shale or other soft, poor durability particles, and shall have an organic content not larger than 1%. For permanent applications, the backfill shall have a pH between 4.5 and 9. Backfill Placement - Reinforced fill shall be placed as specified in construction plans in maximum compacted lift thickness of 250 mm. - Reinforced fill should be placed and compacted at or within 2% dry of the optimum moisture content. If the reinforced fill is free draining (i.e., with less than 5% passing a No. 200 sieve), water content of the fill may be within + 3% of the optimum. - A minimum density of 100% of AASHTO T-99 (or 95% of AASHTO T-180) is highly recommended for abutments and approaches. A procedural specification is preferable where a significant percentage of coarse material (i.e., greater than 30% retained on the 19 mm sieve) prevents the use of the AASHTO T- 99 or T-180 test methods. For procedural specification, typically

242 three to five passes with conventional vibratory roller compaction equipment may be adequate. The actual requirements should be determined based on field trials. - When compacting uniform medium to fine sands (in excess of 60% passing a No. 40 sieve), use a smooth-drum static roller or lightweight (walk-behind) vibratory roller. The use of large vibratory compaction equipment with this type of backfill material will make wall alignment control difficult. - Placement of the reinforced fill near the front should not lag behind the remainder of the structure by more than one lift. - Backfill shall be placed, spread and compacted in such a manner that eliminates the development of wrinkles or movement of the geosynthetic reinforcement and the wall facing units. - Special attention should be given to ensuring good compaction of the backfill, especially near the face of the wall. - Only hand-operated compaction equipment shall be allowed within 0.5 m of the front of the wall face. Compaction within 0.5 m of the back face of the facing units shall be achieved by at least three passes of a lightweight mechanical tamper, plate or roller. Soil density in this area should not be less than 90% standard Proctor density. - Sheepsfoot or grid-type rollers shall not be used for compacting backfill within the limits of the soil reinforcement. - Compaction control testing of the reinforced backfill should be performed on a regular basis during the entire construction project. A minimum frequency of one test within the reinforced soil zone per 1.5 m of wall height for every 30 m of wall is recommended. - At the end of each day’s operation, the last level of backfill should be sloped away from the wall facing to direct runoff of rainwater away from the wall face. In addition, surface runoff from adjacent areas to enter the wall construction site should be avoided. Facing - Masonry concrete facing should have a minimum compressive strength of 28 MPa (4,000 psi) and a water absorption limit of 5%. - Facing blocks used in freeze-thaw prone areas should be tested for freeze-thaw resistance and survive 300 freeze-thaw cycles without failure per ASTM C666. - Facing blocks should also meet the requirements of ASTM C90 and C140. All facing units shall be sound and free of cracks or other defects that would interfere with the proper placement of the unit or significantly impair the strength or permanence of the construction. - Facing blocks directly exposed to spray from deiced pavements shall be sealed after erection with a water resistance coating or be

243 manufactured with a coating or additive to increase freeze-thaw resistance. - Facing blocks shall be placed and supported as necessary to that their final position is vertical or battered as shown on the plans or the approved working drawings with a tolerance acceptable to the engineer. - It is recommended that the bottom of the top two to three courses of facing blocks be bonded with mortar cement (see Figure 8.1). If lightweight blocks are used, it is highly recommended that the core of the top three to four courses of blocks be filled with concrete mortar and reinforced with steel bars (e.g., with No. 4 rebar). - The cap block and/or top facing units should be bonded to the units below using cap adhesive that meets the requirements of the facing unit manufacturer. - The overall tolerance relative to the wall design verticality or batter shall not exceed + 30 mm maximum over a 3 m distance; 75 mm maximum. Bridge Sill - The bridge sill is constructed over the reinforced soil mass to “spread” bridge loads over a larger area. It also serves to provide necessary clear space between the bridge girder and facing so that the bridge girder is supported by the reinforced soil mass, and not the facing. - The clear space is 75 mm or the factored anticipated settlement of the girder (typically not greater than 2% of the load-bearing wall height), whichever is larger. - Alternatively, a bridge sill can be replaced by “beam seat” comprising two 100 mm lifts of wrapped GRS. Details of the beam seat have been described by Adams et al. (2011). Bearing Pads - The bearing pads, when used, should be designed to support and transfer vertical and horizontal loads from the bridge superstructure to the substructure. The design of the bearing pads should be based on Method B from AASHTO LRFD Bridge Design Specifications (2007). Expansion Joints - Expansion joint (50-mm width minimum) should be used at both ends of the bridge. These expansion joints are to be designed to serve two purposes: (1) allow for thermal expansion of bridge, and (2) allow bridge to oscillate horizontally via bearing (elastomeric) pads. Drainage - To reduce percolation of surface water into the backfill during the service life of an abutment wall, the crest should be graded to direct runoff away from the back slope. Interceptor drains on the back slope may also be used. Periodic maintenance may be necessary to minimize runoff infiltration. It is highly recommended that a combination of granular drain materials and geotextiles, or a geocomposite drain be installed along the back and the base of the

244 fill. - Geotextile reinforcement typically provides inherent drainage function; subsurface drainage at wall face is generally not needed. Construction Sequence - It is preferable to construct the upper wall and place fill behind the upper wall before placement of the bridge girder. This construction sequence tends to produce more favorable stress conditions in the load-bearing wall and increase load carrying capacity and reduce settlement. Other Flexible Facings For a flexible facing differs from the segmental concrete block facing, the following construction guidelines regarding the facing should be observed: Wrapped-Faced Geotextile Facing: • If the geotextile roll is wide enough, use a single sheet parallel the face and if not, cut the roll into prescribed lengths and place them normal to the face with just a butt connection, no overlap or sewing. • Compaction shall be done with equipment that will not damage the geotextile facing, and no compaction is allowed within 0.3 to 0.6 m from the wall face. • Typical lift thickness ranges from 0.2 to 0.45 m. Lift thickness of 0.3 m is most common. • Reinforcement spacing of 0.15 m is recommended as it is easy to work with and it will also help minimizing face deformation. • Face alignment and compaction can be greatly facilitated with the use of temporary forms, such as 50 mm x 200 mm wooden boards. • When making a windrow, care must be exercised not to dig into the geotextile beneath or at the face of the wall. • Before apply a coating to a vertical or near vertical wall, a wire mesh may need to be anchored to the geotextile to keep the coating on the wall face. • It is usually necessary to have scaffolding in front of the wall when the wall is higher than about 1.8 m.

245 Timber Facing: • The timber typically has a 150 mm x 200 mm or 150 mm x 150 mm cross-sectional dimension and shall be treated to an acceptable level with copper chromate or approved equivalent preservative. The bottom row of timber shall be treated for direct burial. The color may be green or brown, but not mixed. • Forming elements in the back of timber face may consist of wood (minimum 250 mm nominal thickness treated to an acceptable level with copper chromate or approved equivalent), fiberglass, plastic, or other approved material. • Typical reinforcement used is a nonwoven geotextile, although other geosynthetics that satisfy the design criteria can also be used. • Nails shall be 16d galvanized ring shank nails and shall be placed at the top and bottom of the timbers at 0.3 m intervals. • Compaction shall be consistent with project embankment specifications, except that no compaction is allowed within 0.3 to 0.6 m of the wall face. • Shimming of timber to maintain the verticality is permissible. • All reinforcement overlaps shall be at least 0.3-m (1-ft) wide and shall be perpendicular to the wall face. • All exposed fabric shall be painted with a latex paint matching the color of the timbers. • To improve connection strength on the top lifts, the geotextile can be wrapped around the facing timbers then covered/protected with wooden panels. This technique has been described by Keller and Devin (2003). Natural Rock Facing: • Do not exceed the height and slope angles delineated in the design without evidence that higher or steeper features will be stable. • Rocks should be placed by skilled operators and should be placed in fairly uniform lifts. • Care should be exercised in placing the infill. The infilling should be as complete as possible.

246 Figure 8.1 Details of reinforcement layout near the top of the load-bearing wall