Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide

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Editorial corrections were made to this report after it was originally published. The following table shows the modifications that were made to this report.

3.3.1 GRS Abutment Backfill

Because a GRS abutment is designed to support load, the backfill is considered a structural component. Abutment backfill should consist of crushed, hard, durable particles or fragments of stone or gravel. These materials should be free from organic matter or deleterious material such as shale or other soft particles that have poor durability. The backfill should follow the size and quality requirements for crushed aggregate material normally used locally in the construction and maintenance of highways by Federal or State agencies.

Abutment backfill typically consists of either well–graded or open–graded aggregates (example gradations are shown in sections 3.3.2.1 and 3.3.2.2 gradations are shown in sections 3.3.1.1 and 3.3.1.2, respectively). It is recommended that either one of these gradations or a blend in between the two be used as backfill behind GRS abutments. At the time of this report, open–graded aggregates had been selected on all GRS–IBS projects due to the relative ease of construction and favorable drainage characteristics (see appendix A). If the abutment will be submerged at any point in time, open–graded gravel should be used because it is free–draining. The friction angle of the backfill should be no less than 38 degrees.

Add a bearing reinforcement zone underneath the bridge seat to support the increased loads due to the bridge (see figure 13). This bearing bed reinforcement serves as an embedded footing in the reinforced soil mass. The bearing bed reinforcement spacing directly underneath the beam seat should be, at a minimum, half the primary spacing (e.g., for an 8–inch primary spacing, the bearing bed reinforcement spacing will equal 4 inches). In general, the minimum length of the bearing bed reinforcement should be twice the setback plus the width of the bridge seat. The depth of the bearing reinforcement zone is determined based on internal stability design for required reinforcement strength (see section 4.4.7.3.1 4.4.7.3). At a minimum, there should be five bearing bed reinforcement layers (see figure 13).
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Where γ_r is the unit weight of the reinforced fill, H is the height of the GRS abutment including the clear space distance, and B is the base width of the GRS abutment not including the wall facing. Direct sliding should also be checked at the interface between the RSF and the foundation soils.
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4.3.7 Step 7–Conduct Internal Stability Analysis

The internal stability analysis will vary slightly depending on the whether ASD or LRFD is the chosen design method. ASD is presented in this chapter. For guidance on LRFD, refer to appendix B C.
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The required reinforcement strength (T_req) must satisfy two criteria: (1) it must be less than the allowable reinforcement strength (T_allow), and (2) it must be less than the strength at 2 percent reinforcement strain () in the direction perpendicular to the abutment wall face.
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• Scour countermeasures: When scour depth is calculated as described in this section, a designed scour countermeasure is included. Design scour countermeasures include riprap aprons, gabion mattresses, and articulating concrete blocks (see section 4.5.3). The purpose of installing a designed scour countermeasure is to prevent loss of soil from underneath a GRS abutment from scour that occurs at or near the abutment. Soil loss can reduce bearing capacity or lead to settlement, which can cause structural failure (see section 4.5.3). Figure 36 shows a cross section of a typical abutment riprap countermeasure recommended for smaller, more culvertlike structures (flow length through structure is longer than structure width). See HEC–23 for additional details regarding the specific requirements for the design and configuration of this countermeasure.^{( 5 )} Larger, more bridgelike structures (opening length is greater than the flow distance through the structure) must be evaluated for scour using the procedures outlined in HEC–18 and HEC–20 and use a designed countermeasure as outlined in HEC–23.^{( 14, 15, 5 )}
  

AMENDED May 24, 2012

Another hydraulic consideration is drainage. The potential for unbalanced water pressure exists when a wall can become partially submerged by a flood or when surface drainage is not controlled. All GRS structures should include consideration for surface and subsurface drainage. Critical areas are behind the wall at the interface between the GRS mass and the retained fill, at the base of the wall, and any location where a fill slope meets the wall face. For example, the design needs to include provisions for surface drainage along the fill slope adjacent to the wing walls. Section 6.5 7.11 discusses drainage details.
AMENDED May 24, 2012

7.6 REINFORCEMENT

Generally, the length of the reinforcement layers will follow the cut slope, as shown in figure 50. While the reinforcement layers in the GRS abutment can be any geosynthetic, the RSF and integrated approach should be constructed and encapsulated with a geotextile to confine the compacted granular fill. The geosynthetic should be placed so that the strongest direction is perpendicular to the abutment face, as shown in figure 51. Where the roll ends, the next roll should begin. Overlapping between sheets is required Overlapping between sheets is not required. The geosynthetic reinforcement should extend between layers of CMU block to provide a frictional connection. The geosynthetic reinforcement should cover a minimum of 85 percent of the top surface of the CMU block; any excess can be removed by either burning with a propane torch or cutting with a razor knife.
AMENDED May 24, 2012

7.6.1 Operating Equipment on Geosynthetic Reinforcement

Driving should not be allowed directly on the geosynthetic reinforcement. Place a minimum 6-inch layer of granular fill prior to operating any vehicles or equipment over the geosynthetic reinforcement. In the bearing reinforcement zone, hand-operated compaction equipment should be used over the 4-inch lifts to prevent excessive installation damage of the reinforcement. Rubber–tired equipment may pass over the geosynthetic reinforcement at speeds less than 5 mi/h. Skid steers and tracked vehicles can cause considerable damage to the geosynthetic. On one occasion, a track hoe operating on a GRS structure turned and pulled the fabric causing deformation to the wall face. For this reason, it is recommended to restrict the use of these vehicles on GRS structures. If absolutely necessary, use may be permitted provided no sudden braking or sharp turning occur and a minimum 6–inch cover is placed.
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Once these steps are accomplished, the GRS–IBS can be constructed. The basic design guidelines are the same whether using ASD or LRFD. However, the detailed equations within step 6 and step 7 will differ between the two design methods. In this appendix, only the differences in step 6 and step 7 that result from conversion to the LRFD format are presented. Refer to section 4.4 section 4.3 for discussion on each of these design elements and the equivalent ASD equations and to section 4.5 section 4.4 for the ASD calculation.
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For abutments, a minimum wide width tensile strength (T_f) of 4,800 lb/ft is required. A resistance factor for reinforcement strength ( Φ_reinf ) of 0.4 should be applied to the ultimate strength (T_f) to determine the factored reinforcement strength (T_f,f). In addition to a global reduction factor of 2.25 accounting for long-term strength losses (RF_global) of the geosynthetic, a resistance factor for reinforcement strength (Φ_reinf) of 0.9 should be applied to the ultimate strength (T_f) to determine the factored reinforcement strength (T_f,f). The factored required reinforcement strength (T_req,f) must be less than this factored reinforcement strength (T_f,f), as shown in equation 93.
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(93)

(93)

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C.3 DESIGN EXAMPLE (LRFD): BOWMAN ROAD BRIDGE, DEFIANCE COUNTY, OH

In this section, the equations formatted for the LRFD method in section C.2 are demonstrated. For additional details and discussion in support of these calculations, see the corresponding sections of the design example in the ASD format contained in section 4.5 section 4.4.
AMENDED May 24, 2012

C.3.7.3 Required Reinforcement Strength

The strength of the reinforcement used at Bowman Road Bridge is 4,800 lb/ft. Applying a resistance factor ( Φ_reinf ) of 0.4, the factored reinforcement strength (T_f,f) is 1,920 lb/ft Applying the resistance and global reduction factors of 0.9 and 2.25, respectively, the factored reinforcement strength (T_f,f) is 1,920 lb/ft. According to the manufacturer, is equal to 1,370 lb/ft. The maximum required reinforcement strength is found as a function of depth, as shown in equation 115.
AMENDED May 24, 2012