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Publication Number: FHWA-HRT-11-026
Date: June 2012

 

Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide

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

Location

Correction

Page 9, section 2.1

Added “RFglobal: Global reduction factor for the geosynthetic to account for long-term strength losses due to installation damage, creep, and durability [dimensionless]

Page 17, last paragraph

Change “gradations are shown in sections 3.3.2.1 and 3.3.2.2” to “gradations are shown in sections 3.3.1.1 and 3.3.1.2”

Page 34, step 6

Change “see section 4.4.7.3.1” to “see section 4.4.7.3”

Page 42, third paragraph

Add “Direct sliding should also be checked at the interface between the RSF and the foundation soils.” at end of paragraph.

Page 44, section 4.3.7

Change “refer to appendix B” to “refer to appendix C”

Page 49, section 4.3.7.3

Change “(2) it must be less than the strength at 2 percent reinforcement strain (Reinforcement strength at 2 percent reinforcement strain [F/L]).” to “(2) it must be less than the strength at 2 percent reinforcement strain (Reinforcement strength at 2 percent reinforcement strain [F/L]) in the direction perpendicular to the abutment wall face.”

Page 72, first paragraph

Delete two instances of “(see section 4.5.3)”

Page 73, first paragraph

Change “Section 6.5 discusses drainage details” to “Section 7.11 discusses drainage details.”

Page 88, last paragraph

Change “Overlapping between sheets is required.” to “Overlapping between sheets is not required.”

Page 90, fourth paragraph

Add “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.” after second sentence.

Page 137, first paragraph

Change “Refer to section 4.4 for discussion … and to section 4.5 for the ASD calculation” to ““Refer to section 4.3 for discussion … and to section 4.4 for the ASD calculation”

Page 141, first paragraph

Change “A resistance factor for reinforcement strength (Φreinf) of 0.4 should be applied to the ultimate strength (Tf) to determine the factored reinforcement strength (Tf,f).” to  “In addition to a global reduction factor of 2.25 accounting for long-term strength losses (RFglobal) of the geosynthetic, a resistance factor for reinforcement strength (Φreinf) of 0.9 should be applied to the ultimate strenth (Tf) to determine the factored reinforcement strength (Tf,f).”

Page 141, equation 93

Change

The ratio of T subscript f,f and T subscript req,f equals the product phi subscript reinf and T subscript f divided by T subscript req,f and equals the product of 0.4 and T subscript f divided by T subscript req, f and is greater than or equal to 1.
to

Page 141, section C.3

Change “design example in the ASD format contained in section 4.5.” to “sections of the design example in the ASD format contained in section 4.4.”

Page 145, section C.3.7.3

Change “Applying a resistance factor ( Φreinf ) of 0.4, the factored reinforcement strength (Tf,f) is 1,920 lb/ft.” to “Applying the resistance and global reduction factors of 0.9 and 2.25, respectively, the factored reinforcement strength (Tf,f) is 1,920 lb/ft.”


Rn Nominal resisting force for direct sliding calculations [F/L]
RR Factored resisting force for direct sliding calculations [F/L]
RFglobal Global reduction factor for the geosynthetic to account for long-term strength losses due to installation damage, creep, and durability [dimensionless]
Se Superelevation angle [deg]
Sk Skew angle [deg]
Sv Reinforcement spacing [L]

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).
AMENDED May 24, 2012


Where γr 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.
AMENDED May 24, 2012


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.
AMENDED May 24, 2012


The required reinforcement strength (Treq) must satisfy two criteria: (1) it must be less than the allowable reinforcement strength (Tallow), and (2) it must be less than the strength at 2 percent reinforcement strain (Reinforcement strength at 2 percent reinforcement strain [F/L]) in the direction perpendicular to the abutment wall face.
AMENDED May 24, 2012


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.
AMENDED May 24, 2012


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.
AMENDED May 24, 2012


For abutments, a minimum wide width tensile strength (Tf) of 4,800 lb/ft is required. A resistance factor for reinforcement strength ( Φreinf ) of 0.4 should be applied to the ultimate strength (Tf) to determine the factored reinforcement strength (Tf,f). In addition to a global reduction factor of 2.25 accounting for long-term strength losses (RFglobal) of the geosynthetic, a resistance factor for reinforcement strength (Φreinf) of 0.9 should be applied to the ultimate strength (Tf) to determine the factored reinforcement strength (Tf,f). The factored required reinforcement strength (Treq,f) must be less than this factored reinforcement strength (Tf,f), as shown in equation 93.
AMENDED May 24, 2012


The ratio of T subscript f,f and T subscript req,f equals the product phi subscript reinf and T subscript f divided by T subscript req,f and equals the product of 0.4 and T subscript f divided by T subscript req, f and is greater than or equal to 1. (93)
(93)

AMENDED May 24, 2012


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 ( Φreinf ) of 0.4, the factored reinforcement strength (Tf,f) is 1,920 lb/ft Applying the resistance and global reduction factors of 0.9 and 2.25, respectively, the factored reinforcement strength (Tf,f) is 1,920 lb/ft. According to the manufacturer, Reinforcement strength at 2 percent reinforcement strain [F/L]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

 

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