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Federal Highway Administration > Publications > Research > Infrastructure > Structures > Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide
Publication Number: FHWA-HRT-11-026
Date: January 2011

Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide

CHAPTER 3. MATERIALS

3.1 INTRODUCTION

Building GRS is as easy as 1–2–3: (1) a row of blocks (the facing elements), (2) a layer of compacted granular fill to the height of the facing blocks, and (3) a layer of geosynthetic reinforcement. The materials used for each step of the process need not be proprietary and are readily available. Recommendations are made to optimize the design based on numerous case histories and field experiments. There are also several miscellaneous materials needed for the details of GRS–IBS.

 

3.2 FACING ELEMENTS

The facing element is not a structural member of GRS–IBS. Its purpose is to provide a form for compaction, serve as a façade, and protect the granular fill from outside weathering. Since the facing is not a structural element of a GRS mass, it is up to the user to define the type of facing used. It may be made of various materials, including concrete, timber, natural rock, metal, automobile tires, shotcrete, and gabion baskets. While some of the facing elements shown would not be appropriate for use in GRS–IBS bridges, figure 2 shows various facing elements that have been used in the construction of GRS walls.

Figure 2. Illustration. GRS walls with different facings. Drawing showing nine different types of facing elements an engineer can use for the integrated bridge system (IBS): wrapped-face, wrapped-face with shotcrete cover, articulated concrete facing, full-height concrete facing, full-height concrete MSB, timber-faced, modular block, tire-faced, and gabion-faced.
Figure 2. Illustration. GRS walls with different facings.(4)

The most commonly used facing element for GRS walls and abutments is the split face concrete masonry unit (CMU) with nominal dimensions of 8 inches by 8 inches by 16 inches and actual dimensions of 75/8 inches by 75/8 inches by 155/8 inches (see figure 3 and figure 4). It is important to use the actual dimensions in designing and detailing GRS–IBS. CMU blocks are lightweight, easy to place, and ensure compaction at every 8–inch lift before placement of the next geosynthetic layer. As seen in figure 3, the reinforcement extends directly beneath each layer of CMU blocks as a frictional connection.

Figure 3. Photo. Split face CMU blocks. Photo showing split face concrete masonry unit (CMU) blocks on a geosynthetic reinforced soil (GRS) abutment being constructed. The photo shows an isometric view.
Figure 3. Photo. Split face CMU blocks.

 

Figure 4. Photo. Detail view of split face CMU blocks. Photo showing split face concrete masonry unit (CMU) blocks on a geosynthetic reinforced soil (GRS) abutment being constructed. The photo shows a top-down view.
Figure 4. Photo. Detail view of split face CMU blocks.

The CMU should have a minimum compressive strength of 4,000 psi and a water absorption limit of 5 percent. In colder climates, a freeze–thaw test (ASTM C1262–10) should be conducted to assess the durability of the CMU and ensure it follows the standard specification (ASTM C1372). One method to ensure the overall quality of the CMU is to review the QA/QC process of a particular producer.

There are several types of CMU that are commonly used in GRS–IBS construction: solid face, hollow core, and corner block. All of these blocks come in the standard dimensions previously described. In addition to the 7 5⁄8–inch height, there are a 3 5⁄8–inch solid CMU blocks that can be used as spacers to form the beam seat (see chapter 7). CMU blocks have been used for GRS construction because they are readily available and inexpensive. They are also compatible with the frictional connection to the recommended reinforcement. Since the facing element is not structural in a GRS wall or abutment, any facing element can be used. With other facing elements, however, special design considerations may apply, and such considerations are beyond the scope of this guide.

3.3 BACKFILL MATERIAL

Backfill selection for GRS–IBS is important because it is a major structural component for the abutment. The backfill must be properly compacted to a minimum of 95 percent of maximum dry density according to AASHTO T–99. Other procedures to determine the degree of compaction can also be used (e.g., modulus–based test methods), as discussed in chapter 7. In GRS–IBS construction, other areas to consider for backfill selections are the RSF and the integrated approach.

Locally sourced aggregates, as long as they meet the material qualifications, are the most economical choice for GRS construction. Most State specifications for aggregate, which are usually met by local quarries and aggregate suppliers, will satisfy the material requirements. Recommendations are provided in this section for GRS abutment, RSF, and approach–way backfills.

It should be noted that some backfill materials are easier to work with than others. Certain backfills are more suitable for compacting behind a given facing element than others. These factors need to be considered when selecting the backfill for a given project.

It has been observed that some fine–grained sands and open–graded coarse aggregates with a maximum grain size greater than 2 inches are difficult to compact directly behind the face of a frictionally connected split face CMU block. The selection of a compatible fill and facing element is therefore necessary for the following purposes:

  • Ensure adequate compaction directly behind the face.

  • Control face alignment.

  • Limit post construction lateral deformation.

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.
AMENDED May 24, 2012

Lower quality granular or natural fill materials can be used if the amount of fines is limited to less than 12 percent for drainage. However, a performance test must be conducted (see appendix B) to quantify the deformation and composite behavior of the mass. The engineer should be cautious when using fills of a lower quality than specified, as the allowable load may be significantly reduced. Safety factors for reinforcement strength and ultimate capacity will also deviate from what is specified in design (see chapter 4). It is therefore recommended to follow the abutment backfill specifications outlined in this chapter.

In addition to the gradation requirement, the backfill selection is dependent on the following factors:

  • Ability to ensure compaction.

  • Drainage (open–graded backfill is recommended for an abutment located in a flood zone to facilitate the flow of water out of the abutment).

  • Workability (open–graded fine aggregates (about 0.5 inches) are easier to spread, level, and compact than well–graded fill).

  • Angular particles are recommended to maximize the shear strength of the GRS mass.

3.3.1.1 Well–Graded Backfill

Most State transportation department subbase aggregates have a specification for well–graded backfill. A maximum grain size of 2 inches is recommended for efficient compaction behind the abutment wall facing. An example of this type of aggregate is shown in table 1 and in figure 5. The exact gradation is not required. As long as the maximum aggregate size is not exceeded, the amount of fines passing the No. 200 sieve is not greater than 12 percent, and the friction angle is at least 38 degrees, the backfill material will be adequate for GRS–IBS.

Table 1. GRS abutment well–graded backfill (VDOT 21–A).

Gradation
(VDOT 21–A)

U.S. Sieve Size

Percent Passing

2 inch

100

1 inch

94–100

3/8 inch

63–72

No. 10

32–41

No. 40

14–24

No. 200

6–12

Plasticity Index (PI)
(AASSHTO T–90)

PI ≤ 6

Soundness
(AASHTO T–104)

The backfill shall be substantially free of shale or other poor durability particles. The material shall have a magnesium sulfate loss of less than 30 percent after four cycles (or a sodium value less than 15 percent after five cycles).

 

 Figure 5. Photo. Sample of VDOT 21-A gravel. Photo showing a sample of a VDOT 21-A well-graded gravel used in the construction of a geosynthetic reinforced soil (GRS) wall. A ruler is shown to give an idea of scale. The pieces are angular and of varying sizes, with the largest pieces about 0.5 inch by 1 inch.
Figure 5. Photo. Sample of VDOT 21–A gravel.

 

3.3.1.2 Open–Graded Backfill

Recommended open–graded backfill material consists of clean, crushed angular (not rounded) stone. The minimum maximum grain size to efficiently achieve compaction behind the abutment wall face is 0.5 inches. An example of a typical open–graded abutment backfill is shown in table 2 and in figure 6 . The amount of fines passing the No. 200 sieve should be as close to 0 percent as possible and no more than 5 percent.

Table 2. GRS abutment open–graded backfill (AASHTO No. 89).

Gradation
(AASHTO M–43)

U.S. Sieve Size

Percent Passing

1/2 inch

100

3/8 inch

90–100

No. 4

20–55

No. 8

5–30

No. 16

0–10

No. 50

0–5

Plasticity Index (PI)
(AASHTO T–90)

PI ≤ 6

Soundness
(AASHTO T–104)

The backfill shall be substantially free of shale or other poor durability particles. The material shall have a magnesium sulfate loss of less than 30 percent after four cycles (or a sodium value less than 15 percent after five cycles).

 

Photo showing a sample of a typical AASHTO No. 89 open-graded gravel used in the construction of a geosynthetic reinforced soil (GRS) wall. A ruler is shown to give an idea of scale. The pieces are angular and of varying sizes, with the largest pieces about 0.25 inch by 0.5 inch.
Figure 6. Photo. Sample of AASHTO No. 89 gravel.

 

3.3.2 RSF Backfill

The backfill for the RSF should be well–graded so a dense packing can occur during compaction. The recommended backfill is the same as that used in abutment construction (see table 1).

3.3.2.1 Riprap Protection

Riprap protection should be sized appropriately for the class of stone specified. The stone used should be hard, durable, angular, free of organic and spoil material, and resistant to weathering and water action. It should be free of clay or soft shale seams that can slake when exposed to water. Hydraulic Engineering Circular 23 (HEC–23) should be used to adequately size riprap or other scour countermeasures.(5)

3.3.3 Integrated Approach Backfill

The GRS located directly behind the beam end is necessary to provide a smooth, integrated transition from the approach way to the bridge deck. This area of GRS–IBS is called the approach–way transition. The fill material used for this transition should be a well–graded gravel similar to that used for the RSF backfill (see table 1).

3.4 GEOSYNTHETIC

Since GRS is generic, there are many types of geosynthetic materials of various strengths available for abutment construction. At the time of this report, all in–service GRS–IBSs have used a biaxial, woven polypropylene (PP) geotextile in the abutment. This geotextile was used for several reasons, including cost, ease of placement, and compatibility with the friction connection that is used between the block facing and the GRS mass. While any geosynthetic meeting the requirements outlined in this section can be used in the abutment, a geotextile must be used for the RSF and the integrated approach to encapsulate the material.

An ultimate strength of at least 4,800 lb/ft is used for GRS load–bearing applications. In some cases, it might be appropriate to specify stronger reinforcement strength depending on the design requirements. Chapter 4 provides design guidance on the required reinforcement strength for a particular application, which is a function of the lateral stress, reinforcement spacing, and backfill properties.

The reinforcement strength at 2 percent strain is also an important consideration in the performance of GRS–IBS (see chapter 4). Limiting the required reinforcement strength to less than the reinforcement strength at 2 percent strain will ensure long–term performance and serviceability.

In some situations, the permittivity and apparent opening size of a geosynthetic need to be considered to ensure adequate long–term drainage, particularly when the abutment may be submerged at any point. Since the use of a free–draining backfill is recommended in this situation, a rapid release of water from the reinforced soil fill can occur. Nevertheless, the impact of water on wall design needs to be considered, particularly in situations where rapid drawdown can occur as the result of receding floodwaters. It is also important to ensure that the geosynthetic material is capable within its specific environment.

Geosynthetics can be either uniaxial or biaxial, meaning the reinforcement either has more strength in one direction or it has equal strength in both directions along its length. The term machine direction (or warp direction) refers to the strength along the length of the roll, and the term cross–machine direction (or fill or weft direction) refers to the strength along the width of the roll (see figure 7 ). If a uniaxial reinforcement is used, having greater strength in the cross–machine direction allows for easy placement, as the geosynthetic can be rolled out parallel to the wall. When using geosynthetics that are uniaxial in the machine direction, the placement must be perpendicular to the wall, adding to construction time. It is recommended, however, that biaxial reinforcement be used to eliminate construction placement errors and ensure approximately equal strength in both directions.

Drawing showing a roll of geosynthetic with cross-machine and machine directions labeled. The machine direction is along the length of the roll, whereas the cross-machine direction is along the width of the roll.
Figure 7. Illustration. Geosynthetic roll direction.

 

It is important to properly select the geosynthetic for the specific site conditions. The following should be specified for geosynthetic reinforcement:

  • Laboratory test results documenting ultimate strength in accordance with ASTM D4595 for geotextiles or ASTM D6637 for geogrids. Tests should be conducted at a strain rate of 10 percent per minute.

  • Follow industry standards on the hydrolysis resistance of polyester (PET), oxidative resistance of PP and high density polyethylene (HDPE), and stress cracking resistance of HDPE for all components of the geosynthetic

  • Laboratory tests documenting direct sliding coefficients for various soil types or project specific soils in accordance with ASTM D5321.

  • Manufacturing QC program and data indicating minimum test requirements, test methods, test frequency, and lot size for each product. Further minimum conformance requirements as prescribed by the manufacturer shall be indicated. Table 3 shows the minimum conformance criteria required for approval.

3. Conformance criteria.

Test Test Procedure
Wide Width Tensile (geotextiles) ASTM D4595
Wide Width Tensile (geogrids) ASTM D6637
Specific Gravity (HDPE only) ASTM D1505
Melt Flow Index (PP and HDPE) ASTM D1238
Inherent Viscosity (PET only) ASTM D4603
Carboxyl End Group (PET only) ASTM D2455
Single Rib Tensile (geogrids) ASTM D6637
  • The primary resin used in manufacturing shall be identified as to its ASTM type, class, grade, and category.

    • For HDPE resin, type, class, grade, and category in accordance with ASTM D1248 shall be identified. For example: Type III, Class A, Grade E5, Category 5.

    • For PP resins, group, class, and grade in accordance with ASTM D4101 shall be identified. For example: Group 1, Class 1, Grade 4.

    • For PET resins, minimum production inherent viscosity (ASTM D4603) and maximum carboxyl end groups (ASTM D2455) shall be identified.

  • For all products, the minimum UV resistance as measured by ASTM D4355 shall be identified.

3.5 MISCELLANEOUS MATERIALS

The three main materials involved in GRS construction are the facing element, the backfill, and the geosynthetic reinforcement. Other miscellaneous materials are also necessary during construction, including the following:

  • Concrete block wall fill: Concrete block wall fill, along with rebar, is used to fill in and bind together the top three courses of facing blocks (see chapter 7). It is also used for runoff coping and, if necessary, to connect the wing wall to the abutment face when a vertical seam is located at the corners. The concrete used should be ASTM Class A concrete with 4,000 psi compressive strength.

  • Rebar: No. 4 rebar (0.5–inch diameter), preferably epoxy–coated, is used in the concrete block wall fill to pin the top three courses of facing blocks. If necessary, it can also be used to connect the wing wall to the abutment face at the corners (see chapter 7).

  • Flashing: Flashing (e.g., aluminum flashing) can be used for two main purposes: (1) to serve as a drip edge under the superstructure within the clear space to shed potentially corrosive fluids off of the dry cast block as a precaution and (2) to prevent animals from burrowing into the abutment (see chapter 7). Typical dimensions of the aluminum fascia are 4 inches by 1.5 inches. This may not be necessary and is a decision left to the engineer.

  • Foam board: A rigid foam insulation board is used to provide setback and to create a bearing buffer between the superstructure and the wall face (see chapter 7). The foam board is 2–inches thick by 12–inches wide.

  • Bitumen coating: A bitumen coating is often shop–installed on a concrete beam where it will be embedded within the GRS abutment and wing walls to prevent corrosion of the embedded concrete (see figure 8).

Photo showing black bitumen coating on the end of several concrete beams being placed on the geosynthetic reinforced soil (GRS) abutment.
Figure 8. Photo. Bitumen coating on concrete beam ends.

 

 

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