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111 is Fs = 5.5 for reinforcement spacing 0.2 m, and Fs = 3.5 ment) and abutment settlement (i.e., settlement occurs for reinforcement spacing of 0.4 m. These combined within the abutment). The foundation settlement caused safety factors only apply to the condition where the back- by the self weight of a GRS abutment and subject to fill material and placement conditions satisfy those in the bridge loads can be estimated by using the conventional recommended construction guidelines. settlement computation methods in soils engineering text- books and reports (e.g., Terzaghi and Peck, 1967; Perloff, 1975; Poulos, 2000). The abutment settlement, under the Step 10: Design the back/upper wall recommended allowable bearing pressure determined in Step 4, can be estimated conservatively as 1.5 percent of If the back wall (or the upper wall) is to be a reinforced H1 (H1 = height of the loading bearing wall). soil wall, it should be designed similarly to the load- In situations where the heights of the load-bearing bearing wall. In most cases, the same fill, same reinforce- walls at the two ends of a bridge differ significantly, it is ment, and same fill placement conditions as those of the a good practice to preload or prestress the load-bearing load-bearing wall should be used, although the default walls. The proper magnitude of preloading or prestressing reinforcement spacing in the approach fill can be increased and the probable reduction in differential settlement caused somewhat (e.g., from 0.2 m to 0.3 m or even 0.4 m). The by preloading of a reinforced soil mass can be evaluated length of all the layers of reinforcement (at least in the top by a procedure established by Ketchart and Wu (2001 three layers, if there is a spacing constraint) should be and 2002). extended about 1.5 m beyond the end of the approach slab to produce a "smoother" surface subsidence profile over the entire design life of the abutment. RECOMMENDED CONSTRUCTION GUIDELINES If there is no significant space constraint, it is recom- mended that the reinforcement length of the top three lay- Earthwork construction control for GRS abutments is essen- ers (all the layers, if there is little space constraint) in the tially the same as that required for conventional bridge abut- lower wall be also extended about 1.5 m beyond the end ments, but with a few additional details that require special of the approach slab. Extending the reinforcement lengths attention. Field substitutions of backfill materials or changes beyond the approach slab promotes integration of the in construction sequence, procedures, or details should only be abutment wall with the approach embankment and the permitted with the express consent of the responsible geo- load-bearing abutment, so as to eliminate bridge "bumps." technical or preconstruction design engineer. The recom- mended construction guidelines focus on GRS abutments with a segmental concrete block facing. Only basic guidelines are Step 11: Check angular distortion between given for GRS abutments with other forms of flexible facing. abutments The angular distortion between abutments or between Segmental Concrete Block Facing GRS piers and abutments should be checked to ensure ride qual- Abutments ity and structural integrity. Angular distortion = (differ- ence in total settlements between abutments or between The construction guidelines presented below are established piers and abutments)/(span between the bridge-supporting based on the guidelines for construction of segmental GRS structures). The angular distortion should be limited to walls provided by various agencies (including AASHTO, 0.005 (or 1:200) for simple spans and 0.004 (or 1:250) for NCMA, FHWA, CTI, SAGP, and JR), as summarized in continuous spans. Appendix A, as well as the authors' and their colleagues' The total settlement of each abutment is the sum of foun- observations and experiences with construction of GRS walls dation settlement (i.e., settlement occurs beneath the abut- and abutments. Site and Foundation Before placement of the reinforcement, the ground should be graded to provide a Preparation smooth, fairly level surface. The surface should be clear of vegetation, large rocks, stumps, and the like. Depres- sions 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 soil, it 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 should be excavated and replaced with a reinforced soil foundation (compacted granular soil
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112 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 about 150 mm (6 in.) thick and 450 mm (18 in.) wide. Compaction of the leveling pad should be performed using a light-compactor to obtain a minimum of 95 percent of the maximum standard Proctor density (per ASTM D698). If excavation is needed, it should be carried out to the lines and grades shown on the project grading plans. Over-excavation should be minimized. In a stream environment, GRS abutments should be protected from possible scour and abrasion by using riprap or other protection measures. Reinforcement and Geosynthetic reinforcement should consist of high-tenacity geogrids or geotextiles Reinforcement Placement manufactured for soil reinforcement applications. Geosynthetics, especially geotex- tiles, 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 ten- sioning until the reinforcement has been covered by at least 150 mm (6 in.) 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 direc- tion is not permitted. Adjacent sections of geosynthetic reinforcement should be placed so as to ensure 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 (6 in.) is required before 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 interconnected, geosynthetic reinforcement should be wrapped at the wall face. The wrapped face will help reduce sloughing of fill caused by precipitation and the "gaps" that may form because of movement of the wall face. In the upper wall, the wrapped return should be extended at least 0.45 m (18 in.) in the horizontal direction and anchored in at least 0.1 m (4 in.) of fill material. The wrapped return should extend at least 1.5 m (5 ft) in the load bearing wall. The added reinforcement in the load-bearing wall will increase the safety margin of its load-carrying capacity. It is a good practice to place a compressible layer (e.g., a low- to medium-density expanded polystyrene sheet), of about 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 (2 ft) beyond the heel of the sill should be used to "attach" the facing with the reinforced fill (see Figure 3-3). The wrapped return of geosynthetic reinforcement at the top surface of each tier (top surfaces of the upper and lower walls) should extend to the full length (see Figure 3-3). For larger reinforcement spacing (e.g., 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 should consist of material free from organic or other unsuitable material as determined by the engineer.
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113 Unless otherwise specified, grading of the backfill should be as follows,: 100 percent passing 100 mm (4 in.) sieve, 0-60 percent passing No. 40 (0.425 mm) sieve, and 0-15 percent passing No. 200 (0.075mm) sieve; plasticity index (PI) as determined by AASHTO T90, should not exceed 6. The backfill should 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 percent of AASHTO T-99, Methods C or D, at optimum moisture content. No testing is required for backfills where 80 percent of sizes are greater than 19 mm. The backfill should be substantially free of shale or other soft, poor durability parti- cles and should have an organic content not larger than 1 percent. For permanent applications, the backfill should have a pH between 4.5 and 9. The pH limits may be increased to 3 and 11 respectively for temporary applications. Backfill Placement Reinforced fill should be placed as specified in construction plans in maximum com- pacted lift thickness of 250 mm (10 in.). Reinforced fill should be placed and compacted at or within 2 percent dry of the optimum moisture content. If the reinforced fill is free draining (i.e., with less than 5 percent passing a No. 200 sieve), water content of the fill may be within ±3 per- cent of the optimum. A minimum density of 100 percent of AASHTO T-99 (or 95 percent of AASHTO T-180) is highly recommended for abutments and approaches. A procedural specifi- cation is preferable where a significant percentage of coarse material (i.e., greater than 30 percent retained on the 19 mm, or 3/4 in., sieve) prevents the use of the AASHTO T-99 or T-180 test methods. For procedural specification, typically 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 percent 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 should be placed, spread, and compacted so as to prevent 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, espe- cially near the face of the wall. Only hand-operated compaction equipment should be allowed within 0.5 m (1.5 ft) of the front of the wall face. Compaction within 0.5 m (1.5 ft) of the back face of the facing units should 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 percent standard Proctor density. Sheepsfoot or grid-type rollers should not be used for compacting backfill within the limits of the soil reinforcement. Compaction control testing of the reinforced backfill should be performed regularly during the entire construction project. A minimum frequency of one test within the reinforced soil zone per 1.5 m (5 ft) of wall height for every 30 m (100 ft) 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, sur- face 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 percent. Facing blocks used in freeze-thaw prone areas should be tested for freeze-thaw resis- tance and survive 300 freeze-thaw cycles without failure per ASTM C666.
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114 Facing blocks should also meet the requirements of ASTM C90 and C140. All facing units should 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 de-iced pavements should be sealed after erection with a water-resistant coating or be manufactured with a coating or addi- tive to increase freeze-thaw resistance. Facing blocks should be placed and supported as necessary so 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 cement. If lightweight blocks are used, it is recommended that the three to four courses of blocks be filled with concrete mortar and reinforced with steel bars. 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 (1.25 in.) maximum over a 3 m (10 ft) distance; 75 mm (3 in.) maximum. 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 combina- tion of granular drain materials and geotextiles or a geocomposite drain be installed along the back and the base of the 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, increase load-carrying capacity, and reduce settlement. Other Flexible Facings · Typical lift thickness ranges from 0.2 to 0.45 m (8 in. to 18 in.). Lift thickness of 0.3 m is most common. For a flexible facing differing from the segmental concrete · Reinforcement spacing of 0.15 m is recommended as it block facing, the following construction guidelines about the is easy to work with and it will help minimize face facing should be observed: deformation. · Face alignment and compaction can be greatly facili- tated with the use of temporary forms, such as 50 mm x Wrapped-Faced Geotextile Facing 200 mm (2 in. x 8 in.) wooden boards. · When making a windrow, care must be exercised not to · If the geotextile is wide enough for the required rein- dig into the geotextile beneath or at the face of the wall. forcement length, it can be unrolled parallel to the wall · Before applying a coating to a vertical or near-vertical (i.e., in the longitudinal direction). Two rolls of geotextile wall, a wire mesh may need to be anchored to the geo- can be sewn together if a single roll is not wide enough. textile to keep the coating on the wall face. Alternatively, the geotextile can be deployed perpen- · It is usually necessary to have scaffolding in front of the dicularly to the abutment wall and adjacent sheets can be wall when the wall is higher than about 1.8 m (6 ft). overlapped or sewn. The stronger direction of a geotextile, usually in the machine direction, should be oriented in the maximum stress direction (i.e., the direction perpendicu- Timber Facing lar to the wall face). · Compaction shall be done with equipment that will not · The timber typically has a 150 mm x 200 mm (6 in. x 8 in.) damage the geotextile facing, and no compaction is or 150 mm x 150 mm (6 in. x 6 in.) cross-sectional dimen- allowed within 0.3 to 0.6 m (1 to 2 ft) from the wall face. sion and should be treated to an acceptable level with