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Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing (2006)

Chapter: Chapter 3 - Interpretation, Appraisal, and Applications

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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Washington, DC: The National Academies Press. doi: 10.17226/13936.
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96 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATIONS ASSESSMENT OF THE NCHRP TEST ABUTMENTS The NCHRP test abutments were assessed in two ways: (1) the measured performance and observed behavior were evaluated against existing performance criteria for bridge abutments; and (2) the safety factors and failure loads of the abutments were evaluated using the design method in the current NHI manual (Elias et al., 2001) through the computer program, MSEW. Assessment of Measured Performance and Observed Behavior The assessment of the measured performance and observed behavior of the two full-scale test abutments was made against performance criteria previously established based on experiences with real bridges. Load-carrying capacity and ductility, sill settlement and angular distortion, and maxi- mum lateral movement of the abutment wall were all assessed. The performance criteria, referred to as “the exist- ing criteria” in this chapter, are from studies conducted by Bozozuk (1978), Walkinshaw (1978), Grover (1978), Moul- ton et al. (1985), and Wahls (1990). The differences in performance between the two test abut- ments were caused by the difference in the geosynthetic rein- forcement used for the two test sections. The tensile strength of the reinforcement used was 70 kN/m for the Amoco test section and 21 kN/m for the Mirafi test section. Load-Carrying Capacity and Ductility • As the loading was being terminated (814 kPa for the Amoco test section and 414 kPa for the Mirafi test section), the Mirafi test section approached a bearing failure condition while the Amoco test section appeared to still be sufficiently stable. For a typical design pres- sure of 200 kPa, the safety margin in terms of the load- carrying capacity was “acceptable” for the Amoco test section and “marginally acceptable” for the Mirafi test section. • With a sufficiently strong reinforcement (Tult = 70 kN/m, per ASTM D 4595), the Amoco test section was still exhibiting a near-linear load-settlement relationship at 814 kPa, about four times the typical design pressure of 200 kPa, although the deformation had become fairly large (average sill settlement = 163 mm, maximum lateral movement = 82 mm)—an indication of high duc- tility of the abutment system. With a weak reinforce- ment (Tult = 21 kN/m, per ASTM D 4595), however, the ductility was significantly compromised. As the applied pressure increased beyond 200 kPa, the rate of settle- ment continued to increase with increasing applied pres- sure, and the failure state was only about twice the typ- ical design pressure of 200 kPa. Sill Settlement and Angular Distortion • The settlement of the sill was much smaller in the Amoco test section than in the Mirafi test section. Under an applied pressure of 200 kPa, the average sill settle- ment was 40 mm and 72 mm in the Amoco test section and the Mirafi test section, respectively. As the loading was being terminated (814 kPa for the Amoco test sec- tion and 414 kPa for the Mirafi test section), the average sill settlement of the two test sections was comparable, 163 mm and 170 mm, respectively. • The existing bridge settlement criteria for ride quality and structural distress range from 50 to 100 mm. The average sill settlement of 40 mm for the Amoco test section under a typical design pressure of 200 kPa met the settlement criterion. For the Mirafi test section, the average sill settlement of 72 mm under 200 kPa might have compromised ride quality, but was still considered “tolerable.” The performance criteria are for bridges resting on “conventional” abutments. Given that GRS abutments can provide a “smoother” transition between the approach fill and the bridge structure, the settlement criteria may be relaxed to some extent. • The typical tolerable maximum angular distortion for single-span bridges is 1:200. For an 18-m (60-ft)-long single-span bridge, the “maximum possible” angular distortion would be 1:450 for the Amoco test section and 1:250 for the Mirafi test section—both below 1:200. The test abutments were constructed over a rigid

97 foundation, thus the settlement that would occur in the foundation was not accounted for. • In the load tests, the sill was loaded in equal increments of 50 kPa average vertical pressure and maintained for about 30 minutes between increments. Maintaining each load increment for 30 minutes was intended to allow the stresses to be transferred to the entire soil mass, not to determine creep deformation. The AASHTO guideline suggests that creep tests be conducted for 10,000 hours and extrapolated for a maximum of two log cycles in time. • Based on the soil-geosynthetic interactive performance (SGIP) tests performed on Amoco 2044 with a road base material by Ketchart and Wu (2001 and 2002), the mag- nitude of creep deformation over the lifetime of a GRS mass under 200 kPa (30 psi) and in an “unconfined condition” (a conservative condition) would be about the same as the magnitude of the “immediate” settlement. This means that the Amoco 2044 test under 200 kPa would settle about 80 mm and the Mirafi test section would settle about 145 mm over the design life. This would render the Amoco test section only “margin- ally acceptable” and the Mirafi test section “unaccept- able” under a design pressure of 200 kPa. The measured data from the Founders/Meadows abutment (with a soil friction angle of 40 deg from the standard direct shear tests) showed that the sill settled 13 mm because of place- ment of bridge superstructure and an additional settle- ment of 11 mm occurred over 18 months after the bridge was opened to the traffic. • From the standpoint of angular distortion, the maximum possible long-term angular distortion for the Amoco and Mirafi test sections would be 1:225 and 1:125, respec- tively, for an 18-m (60-ft)-long single-span bridge under 200 kPa pressure. Again, the Amoco test section would only be “marginally acceptable” and the Mirafi test section would be “unacceptable” under a design pres- sure of 200 kPa. Lateral Movement of Abutment Wall • In both test sections, the abutment wall moved outward with the maximum movement occurring near the top of the wall. At an average applied pressure of 200 kPa, the maximum movements in the abutment wall were 24 mm in the Amoco test section and 36 mm in the Mirafi test section. The wing-walls also moved outward with the maximum movement occurring at about H/6 from the top of the wall. At an average applied pressure of 200 kPa, the maximum movement was 18 mm in the Amoco test section and 30 mm in the Mirafi test section. • The existing lateral movement criteria for ride quality and structural distress range from 25 to 50 mm. The maximum lateral displacements for both test sections (24 mm and 36 mm) were somewhat below the lateral movement criterion. • There is little information in the literature about long- term lateral movement of GRS bridge abutments. For the Founders/Meadows abutment, with a fill of 40° fric- tion angle, the maximum outward movement of the abutment wall caused by placement of bridge super- structure was about 9 mm. The additional outward movement over 18 months after the bridge was opened to the traffic was about 13 mm. Assuming that the long- term lateral movement is about the same magnitude as the “immediate” lateral movement, the Amoco test section would be “marginally acceptable” and the Mirafi test section would be “unacceptable.” Observed Behavior • A tension crack was observed on the wall crest in both load test sections. The tension crack was first observed at an applied pressure of 150 to 200 kPa. The distinct tension crack was parallel to the abutment wall face and located at end of the reinforcement. The location of the tension crack suggests that the assumption of rigid rein- forced soil mass in the existing design methods for evaluating external stability is a sound procedure. The tension cracks might be suppressed by lengthening the top few layers (e.g., three layers) of the reinforcement. If an upper wall had been constructed over the test abutment, as in the case of typical bridge abutments, the tension crack would not have been visible and perhaps would have been less likely to occur. • Under higher applied loads, the facing blocks in the top three courses were pushed outward as the sill tilted for- ward. This suggests (1) that the sill clear distance of 0.15 m, the minimum value stipulated by the NHI man- ual, may be too small, and (2) that it may be beneficial to increase the connection strengthening in the top three to four courses of the facing. The authors believe that the strengthening effect will be most effective if the facing blocks are “inter-connected” after all the facing units are in place. Assessment of Safety Factors and Failure Loads by the MSEW Program The NCHRP test abutments were evaluated by MSEW, a computer program developed by ADAMA Engineering, Inc., for design and analysis of mechanically stabilized earth walls. The MSEW program follows the design guidelines presented in the FHWA Demo-82 manual (Elias et al., 1997) and the NHI manual (Elias et al., 2001) and is completely compatible with AASHTO Standard Specifications for High- way Bridges, 16th edition, 1996; as amended by the 1998 interim revisions.

98 The MSEW program has two modes of operation: design and analysis. In the design mode, the program computes the required layout (length and vertical spacing) corresponding to the user’s prescribed safety factors. In the analysis mode, the program computes the factors of safety corresponding to the user’s prescribed reinforcement layout. The NCHRP test abutments were evaluated using the analysis mode by Michael Adams of the FHWA. Three loading conditions were considered: (1) soil self- weight only (i.e., no external load applied to the abutment), (2) soil self-weight plus self-weight of sill (an equivalent point load of 29 kN), and (3) loading condition of 2 plus a dis- tributed load of 200 kPa (an equivalent point load of 866 kN). The strength reduction factors for geosynthetic reinforce- ments (i.e., creep reduction factor, durability reduction factor, and installation damage reduction factor) were set equal to 1.0. The “overall factor of safety,” as defined in the NHI manual, was also set to 1.0. Therefore, Ta (design long-term rein- forcement tension load) = Tal (long-term tensile strength). The MSEW analysis indicated that all the calculated tensile forces were less than Ta, the design long-term reinforcement tension load (i.e., the safety factors against reinforcement rupture failure were greater than 1.0 under all three loading conditions for both test sections). The MSEW analysis also indicated that all the safety factors against pullout failure were greater than 1.0 for the first two loading conditions (i.e., no external load and sill self-weight only). However, for the third loading case (i.e., with an applied pressure of 200 kPa), the pullout safety factors in the top two reinforce- ment layers were less than 1.0 for both test sections, with the lowest value of pullout safety factor of 0.2 occurring at the very top layer. Trial and error revealed that a pullout safety factor of 1.0 would occur at an applied pressure of 33 kPa. Therefore, the failure pressure according to the MSEW pro- gram is 33 kPa. Note that 33 kPa reflects the “true” pre- dicted failure pressure by the MSEW program (hence by the NHI method) because all reduction factors for the re- inforcements and the overall safety factor have been set equal to 1.0. The performance of the full-scale tests, however, indicated that the test abutments were far from a failure condition at 33 kPa. LIMITATIONS OF THE DESIGN AND CONSTRUCTION GUIDELINES The recommended design and construction guidelines pre- sented later in this chapter apply only to GRS abutments and approaches that satisfy the following conditions: • The total abutment height is less than 10 m. • The facing comprises dry-stacked concrete modular blocks, timber, natural rocks, wrapped geosynthetic sheets, or gabions—with or without any mechanical connections (pins or lips) between vertically adjacent facing units. • No prop (temporary bracing) is used in constructing the abutment wall. • The backfill meets the following requirements: 100 per- cent passing 10 cm (4 in.) sieve, 0 to 60 percent passing 0.425 mm (No. 40) sieve, and 0 to 15 percent passing 0.075 mm (No. 200) sieve, free from organic material, plasticity index not greater than 6. • The backfill has an internal friction angle not less than 34 deg, 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, Method C or D, with oversize correction and at the opti- mum moisture content. • The backfill in the construction is compacted to at least 100 percent of AASHTO T-99 (i.e., 100 percent of the standard Proctor maximum dry density) or 95 percent of AASHTO T-180 (i.e., 95 percent of the modified Proc- tor maximum dry density) and the placement moisture is within ±2 percent of the optimum. • The foundation soil is “competent,” although the term “competent” is, to some extent, relative to the abutment height and the applied loads on the sill. For a medium height GRS abutment (e.g., with a total height of about 7 m) and under the maximum allowable sill pressure (see “The Recommended Design Method”), the foundation is considered “competent” if the in situ undrained shear strength is greater than about 140 kPa (3,000 lb/ft2) for a clayey foundation or the standard penetration blow count is not less than about 20 for a non-prestressed granular foundation. Specific checks of the foundation bearing pressure for a given bridge abutment are performed in Step 7 of the “The Recommended Design Method” below. RECOMMENDED DESIGN METHOD For ease of acceptance by the GRS design community and by AASHTO, the recommended design method adopts the format and methodology of the NHI design method. This sec- tion begins with a review of the NHI design method, followed by specific refinements and revisions to the NHI design method. Each refinement or revision is described in reference to the NHI method, and the basis for the refinement or revision is given in detail. The section ends with a complete descrip- tion of the recommended design method. The design method has been established primarily for highway bridges in critical and “permanent” applications. For low-cost applications of GRS bridge abutments, the recommended design method will be rather conservative. The NHI Design Method for MSE Abutments The FHWA NHI reference manual, FHWA-NHI-00-043: Mechanically Stabilized Earth Walls and Reinforced Soil

Slopes Design and Construction Guidelines, by Elias, Christo- pher, and Berg (2001), formerly known as Demo 82, provides a design method for MSE bridge abutments (Section 5.1 of the NHI manual). The design method can be described as follows: Step 1: Establish design height and external loads. Step 2: Establish engineering geotechnical properties (includ- ing unit weight and internal friction angle of the rein- forced fill and the retained earth and allowable bearing pressure of the foundation soil and the reinforced fill). Step 3: Establish design safety factors (including design life, external stability safety factors for sliding, allowable eccentricity, maximum foundation pressure, and inter- nal pullout). Step 4: Choose segmental facing type and reinforcement spacing. Step 5: Establish preliminary reinforcement length (typi- cally 0.7 * total abutment height). Step 6: Size abutment footing/sill (select an initial trial size for the sill and check sliding, eccentricity, and bear- ing pressure). Step 7: Check external stability with the preliminary rein- forcement selected in Step 5: (1) check eccentricity, e (e should be ≤ L/6), (2) check bearing pressure at the foundation level by considering the effective width because of eccentricity, and (3) check safety factor against sliding. Step 8: Determine internal stability at each reinforcement level and required horizontal spacing (for steel strip reinforcement). Step 9: Determine the required reinforcement strength based on consideration of internal stability at each rein- forcement level. In addition, the NHI manual suggests that the following conditions be implemented in the design of MSE abutments: • The tolerable angular distortions (i.e., limiting differen- tial settlement) between abutments or between piers and abutments should be limited to 0.005 for simple-span bridges and 0.004 for continuous-span bridges. • A minimum offset of 0.9 m (3 ft) from the front of the facing to the centerline of the bridge bearing is required. • A clear distance of 150 mm (6 in.) between the back face of the facing units and front edge of footing is required. • The abutment should be placed on a bed of compacted coarse aggregate 1 m (3 ft) thick where significant frost penetration is anticipated. • The bearing capacity on the reinforced volume should be limited to 200 kPa (4,000 lb/ft2). • The maximum horizontal force at each reinforcement level should be used for the design of connections to the facing units. • The density, length, and cross-section of reinforcements of the abutment should be extended to wing walls for a horizontal distance of 0.5 H (H = height of abutment wall). • The seismic design forces should also include seismic forces transferred from the bridge through bearing sup- ports that do not slide freely (e.g., elastomeric bearings). Refinements and Revisions to the NHI Design Method The recommended design method refines and revises the NHI’s bridge abutment design procedure while maintaining the format and basic methodology of the NHI design method. The refinements and revisions are based on findings of this study (including the literature study and the analytical study, both presented in Chapter 2) and the authors’ experiences and knowledge. There are 14 specific refinements and revi- sions, as described below. 1. Refinement/Revision of Step 1 Refinement/Revision: The height of the load-bearing wall (referred to as the “facing wall height” in the NHI manual) is defined as the height measured from the base of the embedment to the top of the load-bearing wall. The embedment of a GRS abutment wall need only be a nominal depth (e.g., one block height). If the foundation soil contains frost-susceptible soils, they should be excavated to at least the maximum frost pen- etration line and replaced with a non-frost-susceptible soil. If the GRS abutment is in a stream environment, scour/abrasion/channel protection should also be implemented. Examples of the scour/abrasion/channel protection for GRS abutments have been described by Keller and Devin (2003). Basis for the Refinement/Revision: Experiences from actual construction of GRS walls and bridge-supporting structures. 2. Refinement/Revision of Step 2 Refinement/Revision: The allowable bearing capacity of a bridge sill on the load-bearing wall (the lower wall) of a GRS abutment is a function of the soil stiffness/ strength, reinforcement vertical spacing, sill width, sill configuration, reinforcement stiffness/strength, foun- dation stiffness/strength, and so forth. For an abutment founded on a “competent foundation” (as defined ear- lier in Limitations of the Design and Construction Guidelines) and with a sufficiently strong reinforce- ment (to be examined quantitatively in Step 9 of the recommended design method), the allowable bearing pressure, qallow, can be determined by the three-step pro- cedure as follows: 99

TABLE 3-1 Recommended allowable bearing pressures of a GRS abutment, with an integrated sill (sill width = 1.5 m), on a competent foundation 100 (1) Use Table 3-1 to determine the allowable bearing pressure under the following condition: (a) an “integrated sill” configuration, (b) sill width = 1.5 m, (c) a sufficiently strong reinforcement, and (d) a competent foundation. (2) Use Figure 3-1 to determine a correction factor for the selected sill width. The allowable bearing pres- sure for the selected sill width is equal to the allow- able pressure determined in Step (1) multiplied by the correction factor. A minimum sill width of 0.6 m is recommended. (3) If an “isolated sill” is used, a reduction factor of 0.75 should be applied to the corrected bearing pressure determined in Step (2). “Isolated sill” refers to an isolated footing separated from the upper wall of the abutment; whereas an “integrated sill” refers to a sill integrated with the upper wall as an integrated structure. Figure 3-1. Relationship between sill width and the correction factor. Correction Factor vs. Sill Width 0 0.5 1 1.5 2 2.5 1 1.5 2 2.5 3 3.5 4 4.5 Sill Width (m) C o rr e c ti o n F a c to r 0.6 Design Friction Angle of Fill1,2 φ = 34° φ = 35° φ = 36° φ = 37° φ = 38° φ = 39° φ = 40° Reinforcement Spacing = 0.2 m (8 in.) 180 kPa (26 psi) 190 kPa (27.5 psi) 200 kPa (29 psi) 220 kPa (32 psi) 235 kPa (34 psi) 255 kPa (37 psi) 280 kPa (40.5 psi) Reinforcement Spacing = 0.4 m (16 in.) 125 kPa (18 psi) 140 kPa (20 psi) 155 kPa (22.5 psi) 175 kPa (25 psi) 195 kPa (28 psi) 215 kPa (31 psi) 240 kPa (34.5 psi) 1 The internal friction angle should be determined by the standard direct shear test on the portion finer than 2 mm (No.10) sieve, using specimens compacted to 95% of AASHTO T-99, Methods C or D, at optimum moisture content. 2 If multiple sets of direct shear tests are performed, the lowest friction angle should be used as the “design friction angle.” If a single set of valid shear tests is performed, the “design friction angle” will be one (1) degree lower than the value obtained from the tests.

If multiple direct shear tests are performed, the lowest friction angle should be used in design. If a single set of direct shear tests is performed, the “design friction angle” should be taken as 1 degree lower than the value obtained from the tests. For instance, if a single set of tests shows that a soil has a friction angle of 35 deg, the design fric- tion angle will be 34 deg; whereas, if two sets of tests are performed and both show a friction angle of 35 deg, then the design friction angle will be 35 deg. Standard direct shear tests performed on some presumably identical gran- ular soil specimens have suggested that a probable vari- ance of ±1 degree friction angle should be used for designs of critical earth structures (Aksharadananda and Wu, 2001). Although the soil specimens used in the tests for determining the friction angle are to be compacted to 95 percent of AASHTO T-99, it is stipulated that the fill in construction be compacted to 100 percent of AASHTO T-99. The additional 5 percent compaction is recommended to provide improved performance and an increased safety margin of a reinforced soil abutment. Basis for the Refinement/Revision: The recom- mended allowable bearing pressures in Table 3-1, the correction factors in Figure 3-1, and the reduction fac- tor for isolated sills are based on findings from the ana- lytical study (Chapter 2), especially the analysis results of allowable bearing pressures. Special emphases have been placed on the applied pressure at short-term sill settlement = 1 percent of lower abutment wall height and on the applied pressure corresponding to the condi- tion in which the critical shear strain has just reached a triangular distribution extending through the height of the load-bearing wall (for details, see Load-Carrying Capacity Analysis). The critical shear strain, γ (critical), is defined as: γ(critical) = (2/3)(1 − 3)failure and can be obtained from triaxial compression test results. The refinement/revision is also based on findings from the literature study, especially the performance characteris- tics of field experiments presented in Chapter 2 and the authors’ judgment of conservative, yet not overly con- servative, design values and their experiences with GRS walls and bridge-supporting structures. The fill is characterized by its friction angle in the design method. The friction angle of a soil relates directly to the “strength” of the soil but does not address its “stiffness,” which determines the deformation of a soil mass before failure. Given that different soils of a similar strength can have rather different values of stiff- ness, the characterization of a soil by its friction angle is generally considered inadequate in a deformation-based design method. The stiffness values used in the analyses presented in Chapter 2 had to be assumed. The assumed stiffness values are derived from the triaxial stress- strain-strength relationships of more than 120 soils for design purposes (Wong and Duncan, 1974). 3. Refinement/Revision of Step 3 Refinement/Revision: The default value for rein- forcement spacing should be 0.2 m. For wrap faced geotextile walls (temporary or adding future facing), reinforcement spacing of 0.15 m is recommended. Reinforcement spacing greater than 0.4 m is not recommended under any circumstances. Basis of Refinement/Revision: The benefits of smaller reinforcement spacing to improved perfor- mance of GRS walls and abutments (both in terms of deformation and ultimate load-carrying capacity) is shown in the analytical results presented in Chapter 2 and has been demonstrated in actual construction. The use of smaller reinforcement spacing will not only help create a more “coherent” reinforced soil mass (i.e., with greater soil-reinforcement interaction) for the abutment wall (as opposed to areas of reinforced soil sandwiched between unreinforced soil when larger reinforcement spacing is used), but will also improve the efficiency of compaction by increasing the “lock-in” lateral stresses of the soil next to the reinforcement surfaces. The finite element analysis results in Chapter 2 did not account for the lock-in lateral stress, thus the true benefits of reduced reinforcement spacing are likely to be even more pronounced than those indicated. For critical structures such as a bridge abutment, it is the authors’ view that the reinforcement spacing should be kept below 0.4 m in all cases to ensure satisfactory perfor- mance and an enhanced margin of stability. 4. Refinement/Revision of Step 4 Refinement/Revision: A minimum front batter (i.e., leaning backward from the vertical) of 1/35 to 1/40 is recommended for a segmental abutment wall facing to provide improved appearance and greater flexibility in construction. A typical minimum setback of 5 to 6 mm between successive courses of facing blocks is recom- mended for 200-mm-high blocks. Basis of the Refinement/Revision: This refinement/ revision is based on the findings of finite element analyses on lateral displacement of abutment wall faces. An in-depth examination of the analysis results reveals that the maximum displacement is typically about 0.03 H (H = wall height) and occurs at 0.7 to 0.8 H from the base at an applied pressure equal to two times the recommended design pressures as determined in Refinement/Revision 2, above. The typical batter needed to offset the lateral movement is calculated to be between 1/35 and 1/40. Averaging the needed batter over the distance between wall base and the point of maximum lateral movement, the setback for each course of facing block is 5 to 6 mm for a block height of 200 mm. 101

102 5. Refinement/Revision of Step 5 Refinement/Revision: The reinforcement length may be “truncated” in the bottom portion of the wall pro- vided that the foundation is “competent” (as defined in Limitations of the Design and Construction Guidelines earlier in this Chapter). The recommended configura- tion of the truncation is: reinforcement length = 0.35 H at the foundation level (H = total abutment height) and increases upward at 45° angle. The allowable bearing pressure of the sill, as determined in Refinement/Revi- sion 2, should be reduced by 10 percent for truncated- base walls. When reinforcement is truncated at the bot- tom portion, external stability of the wall (sliding failure, overall slope failure, and foundation bearing failure) must be checked thoroughly. Basis of the Refinement/Revision: Finite element analysis results of walls with 0.4 m reinforcement spac- ing show insignificant differences in general perfor- mance characteristics between a truncated-reinforcement wall and an un-truncated-reinforcement wall, except for maximum lateral displacement at the wall face. The max- imum lateral displacement of a truncated-reinforcement wall is about 10 percent higher than that of an un- truncated-reinforcement wall. 6. Refinement/Revision of Step 6 Refinement/Revision: A recommended clear distance between the back face of the facing and the front edge of sill is 0.3 m (12 in.). Basis for the Refinement/Revision: This refinement/ revision is based on the findings of the finite element analysis conducted in this study and the typical com- paction operation. The analysis results were for soils with φ = 34° and conservative values of soil stiffness. As the applied pressure increases beyond 100 kPa, settlement of the sill and rotation of sill tend to “increase” somewhat as the sill clear distance increases from 0 to 0.3 m. The max- imum lateral displacement of the load-bearing abutment wall also increases slightly with increasing sill clear distance. To reduce the cost of bridge girder and bridge deck, the sill clear distance should also be kept to a minimum. On the other hand, the soil immediately behind the facing (within about 0.3 to 0.5 m) should not be compacted by a heavy compactor during construction. As a result, the density of the fill within 0.3 m behind the wall face is generally lower than the rest of the fill. A sill with a clear distance less than 0.3 m, therefore, may expe- rience a larger sill settlement and larger sill rotation. 7. Refinement/Revision of Step 7 Refinement/Revision: This refinement/revision is needed only when D1, the “influence length” on the foun- dation level (note: D1 = d + B + H1/2, see Figure 3-2) is less than the length of the reinforcement (corrected with consideration of load eccentricity) in the load-bearing wall. In this case, the contact pressure on the foundation level, pcontact, should be computed as pcontact = (papplied * B / D1) + γH1 + γHe where papplied is the average applied pressure on the base of the sill (including the pressures caused by the self- weight of the sill, caused by the dead load and live load applied on the sill, and caused by the traffic loads); B is the width of the sill; d is the clear distance between the back face of the facing and front edge of the sill; B = B – 2e (e = eccentricity of the sill load); H1 is the height between the base of the sill and the foundation level; He is the “equivalent” height of the upper wall, He = H1H2/(2D1), in which H2 is the height of the upper wall; and γ is the unit weight of reinforced fill. The safety factor against bearing failure is evaluated by dividing the average foundation contact pressure, pcontact, by the allowable bearing pressure of the foundation. The allowable bearing pressure of the foundation can be evaluated by the method described in the NHI manual. If the reinforcements near the base of the lower wall are “truncated” (see Refinement/Revision 5), the re- inforcement length at the truncated base, if it is smaller than the influence length (D1), should be used when determining the average foundation contact pressure. Basis of Refinement/Revision: For most bridge abut- ments, a relatively high-intensity bridge load is applied close to the wall face. To ensure that the foundation soil beneath the abutment will have a sufficient safety mar- gin against bearing failure, it is important to examine the contact pressure over a more critical region (within the “influence length” D1 measured from the wall face, provided that D1 < reinforcement length in the lower wall), as opposed to the average pressure over the entire reinforced zone (with eccentricity correction)—the pro- cedure prescribed in the current NHI manual. Field measurement (e.g., Founders/Meadows abut- ment) has suggested that the vertical stress caused by concentrated vertical loads applied on a sill can be esti- mated by the 2V:1H pyramidal distribution (Figure 3- 2) as described in the NHI manual. The measured data of the NCHRP test abutments have also indicated that the 2V:1H pyramidal distribution yields a good aver- age value of the measured contact pressure on the foun- dation level (see Assessment of the NCHRP Test Abut- ments in this chapter). 8. Refinement/Revision of Step 8 Refinement/Revision: If the bearing capacity of the foundation soil supporting the bridge abutment is found only marginally acceptable or somewhat unacceptable,

a reinforced soil foundation (RSF) may be used to increase its bearing capacity and reduce potential set- tlement. A typical RSF is formed by excavating a pit that is 0.5L deep (L = reinforcement length) and replac- ing it with compacted road base material reinforced by the same reinforcement to be used in the reinforced abutment wall at 0.3 m vertical spacing. The lateral extent of the RSF should at least cover the vertical pro- jection of the reinforced soil area and should extend no less than 0.25L in front of the wall face. A procedure proposed by Barreire and Wu (2001) may be used as a guide for evaluating the bearing capacity and settle- ment of an RSF. Basis of the Refinement/Revision: This refinement/ revision is based on full-scale experiments by Adams at the Turner-Fairbank Highway Research Center, and recent research on bearing capacity of an RSF (e.g., Huang and Tatsuoka, 1990; Omar et al., 1993; Yetimoglu et al., 1994; Adams and Collin, 1997; Wayne et al., 1998). The use of an RSF typically adds only a small cost to the project but can produce signif- icant benefits. 9. Refinement/Revision of Step 9 Refinement/Revision: Both a minimum ultimate ten- sile strength and a minimum tensile stiffness of the reinforcement should be specified to ensure sufficient tensile resistance at the service load and to ensure a sufficient safety margin against rupture failure. The tensile stiffness is defined as the tensile resistance at the working strain (i.e., the strain at the working load). The maximum reinforcement strain under the work- ing load for an in-service GRS bridge-supporting structure typically ranges from 0.2 percent to 1.6 per- cent (see Chapter 2). It is recommended that the resis- tance at tensile strain of 1.0 percent be taken as the reference strain for specification of the required reinforcement stiffness. B - 2e' 2e' H2 B d H1 D1 1 2 sill (e' = eccentricity of sill load) Figure 3-2. Distribution of vertical stress from sill load and definition of D1, the influence length. 103

104 The minimum required reinforcement stiffness in the direction perpendicular to the wall face, T@ =1.0 percent, should be determined by T@ =1.0 percent ≥ σh (max) * s where σh (max) is the maximum lateral stress in the rein- forced fill and s is the vertical reinforcement spacing. For non-uniform reinforcement spacing, s = (1/2 dis- tance to reinforcement layer above) + (1/2 distance to reinforcement layer below). The lateral stress in the reinforced fill, σh, can be calculated as σh = Ka (γZ + Δσv) + Δσh, as suggested by the NHI Manual. The minimum value of the ultimate reinforcement strength in the direction perpendicular to the abutment wall face, Tult, should be determined by imposing a combined safety factor on T@ =1.0 percent to ensure satis- factory long-term performance, to ensure sufficient ductility of the abutment, and to account for various uncertainties, i.e., Tult ≥ Fs * T@ =1.0 percent The recommended combined safety factor is Fs = 5.5 for reinforcement spacing ≤ 0.2 m, and Fs = 3.5 for reinforcement spacing of 0.4 m. The combined safety factor only applies to the backfill material and place- ment conditions specified in Recommended Construc- tion Guidelines in this chapter. Basis of the Refinement/Revision: The maximum reinforcement strains measured in GRS walls, piers, and abutments under service loads typically are on the order of 0.1 percent to 2.0 percent; however, the ulti- mate strength of geosynthetic reinforcements typically occurs at a strain over 10 percent. Geosynthetic rein- forcements of a similar strength can have rather differ- ent load-deformation relationships. In design, it will be prudent to specify the resistance required at the work- ing load to ensure satisfactory performance under the in-service condition. In addition, a minimum value of the ultimate reinforcement strength is also needed to ensure adequate ductility and satisfactory long-term performance and to account for uncertainties. The rec- ommended combined safety factors are derived from the cumulative long-term reduction factors for GRS mass (see Wu, 2001) in conjunction with an overall uncertainty factor of 2.5. As an example, for a 10-m-high abutment (a 7.5-m- high lower wall plus a 2.5-m-high upper wall) with φ = 34°, the maximum vertical stress is about 200 kPa, and the maximum lateral stress, σh (max) = 56.6 kPa. For reinforcement spacing of 0.2 m, the minimum required tensile stiffness at 1 percent strain, T@ =1.0 percent ≥ σh (max) * s = 56.6 kPa * 0.2 m = 11.3 kN/m (or 65 lb/in.). In other words, the reinforcement must have a minimum “working” stiffness of at least 11.3 kN/m (or 65 lb/in.). In addition, the minimum ultimate tensile strength, Tult ≥ Fs * T@ =1.0 percent = 5.5 (11.3) = 62.1 kN/m (or 357 lb/in.). 10. General Revision: If the heights of the load-bearing walls at the two ends of a bridge differ significantly, the angular distortion between the abutments may exceed 0.005 (a limiting value recommended by the NHI manual for a single-span bridge); therefore, it is a good practice to preload or even prestress the load- bearing abutment walls. The proper magnitude of pre- loading or prestressing and the reduction in differential settlement caused by preloading of a reinforced soil mass may be evaluated by a procedure recommended by Ketchart and Wu (2001 and 2002). Preloading typ- ically reduces the vertical deformation of a reinforced soil mass by twofold to sixfold, depending on the field placement density, and the lateral deformation by about threefold, as evidenced by limited case histories (see Chapter 2). Basis for the Revision: The benefits to be gained by preloading and/or prestressing a GRS bridge- supporting structure have been demonstrated in in- service bridge abutments (e.g., Black Hawk bridge abutment), in full-scale experiments of bridge sup- porting structures (e.g., FHWA Turner-Fairbank bridge pier), and in GRS abutment walls constructed by the Japan Railway (e.g., Tatsuoka et al., 1997; Uchimura et al., 1998). Extensive research on the subject has been conducted by Tatsuoka et al. (1997) and Ketchart and Wu (2001). 11. General Refinement: The NHI manual does not address the design of the back wall (the upper wall). The back wall should be designed in a similar manner as the load-bearing wall. In most cases, the same fill, same reinforcement, and same fill placement condi- tions as those of the load-bearing wall should be used, although the default reinforcement spacing in the approach fill can be increased somewhat (e.g., from 0.2 m to 0.3 or 0.4 m). The length of all the layers of reinforcement (at least in the top three layers, if there is a significant space constraint) should be 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. Basis for the Refinement: Experiences from actual construction of GRS walls and bridge-supporting structures. 12. General Refinement: If there is no significant space constraint, it is recommended that the reinforcement length of the top three layers (all the layers, if there is

little space constraint) in the lower wall be extended to about 1.5 m beyond the end of the approach slab. Extending the reinforcement lengths beyond the approach slab tends to integrate the abutment wall with the approach embankment and the load-bearing abutment, so as to eliminate bridge “bumps”—a chronic problem in many bridges (Adams et al., 1999). The use of an integrated sill (i.e., integrating sill with the upper wall, see Refinement/Revision 2, above) is also a major part of an effective system for alleviating bridge bumps. Basis of the Refinement: Bridge bumps typically occur over time, because of factors such as traffic loads, temperature change, and soil moisture varia- tion. These effects cannot be examined realistically by any current analytical tools. The analytical study conducted in this study (see Chapter 2), however, did indicate that differential settlement occurring in the approach fill would be negligible under life loads. The Founders/Meadows abutment (Abu- Hejleh et al., 2000) extended the reinforcement lengths beyond the end of the approach slab in the load-bearing wall and has not experienced any notice- able bridge bumps 4 years into service. This recom- mended refinement is based on limited field experi- ence. Engineering judgment, however, suggests that integrating the abutment wall, the approach fill, and the load-bearing abutment should help reduce the differential settlement. 13. General Revision: Connection strength is not a design concern as long as the reinforcement spacing is kept to not more than 0.2 m, the selected fill is com- pacted to the specification, and the applied pressure does not exceed the recommended design pressures determined in Refinement/Revision 2, above. For reinforcement spacing of 0.4 m, long-term connection failure should be checked to ensure long-term stabil- ity. Moreover, a recommended practice that the hor- izontal interfaces in the top three to four courses of the facing block be strengthened to provide adequate interface shear resistance (see Recommended Con- struction Guidelines later in this chapter) should be observed to avoid potential facing failure. The inter- face strengthening effect will be more effective if the facing blocks are interconnected after all the facing units are in place. Basis for the Revision: The revision is based primarily on field experiences of very tall GRS walls (Wu, 2001). GRS walls of a height up to 16 m have been con- structed with dry-stacked split-faced concrete blocks without any interblock mechanical connections. These walls have performed satisfactorily without any sign of distress. A 15-m-high wall can be regarded as being equivalent to a 5-m-high wall with about 200 kPa surcharge. The finite element analysis results of short-term behavior of GRS walls with a segmental facing have indicated that a GRS abutment with rein- forcement spacing of 0.2 m will not suffer from any connection-related problems up to a sill pressure of 1,000 kPa. With reinforcement spacing of 0.4 m; however, connection failure may occur between 600 to 800 kPa (see Load-Carrying Capacity Analysis, Chapter 2). Also, with reinforcement spacing not greater than 0.2 m, the reinforced soil mass tends to behave as a “soil-reinforcement composite,” which will exert a far smaller lateral earth pressure against “flexible” facing (Wu, 2001). 14. General Refinement: The angular distortion between abutments or between piers and abutments should be checked to ensure ride quality and structural integrity. The angular distortion = (difference in settlement between abutments or between piers and abutments)/ (span between the bridge-supporting structures). The angular distortion should be limited to 0.005 (or 1:200) for simple spans and 0.004 (or 1:250) for con- tinuous spans. The settlement of each abutment is the sum of the foundation settlement and the abutment settlement. The foundation settlement of a GRS abutment subject to bridge loads can be estimated by using the conven- tional settlement computation methods found in soils engineering textbooks and reports (e.g., Terzaghi and Peck, 1967; Perloff, 1975; and Poulos, 2000). The abutment settlement with the recommended allowable bearing pressure presented in Refinement/Revision 2 (above) can be estimated conservatively as 1.5 per- cent of H1 (H1 = height of the loading bearing wall or the lower abutment wall). Basis for the Refinement: The NHI manual stipu- lates the angular distortion requirement, but does not include it explicitly as part of the design proce- dure. This refinement makes the design method more complete. As the allowable sill bearing pres- sures were based in part on abutment settlement of 1.0 percent of H1, it is recommended that the settle- ment within a GRS abutment, under the allowable sill bearing pressure, be estimated conservatively to be 1.5 percent of H1. The Recommended Design Method The recommended design method for GRS bridge abut- ments is presented step by step. Before using the recom- mended design method, the limitations described earlier in this Chapter (in Limitations of the Design and Construction Guidelines) should be checked thoroughly. 105

106 Step 1: Establish abutment geometry and external loads and trial design parameters • Establish abutment geometry and loads (see Figure 3-4): – Total abutment height, H (the sum of lower wall height and upper wall height) – Load-bearing wall (lower wall) height, H1, as mea- sured from the base of the embedment to the top of the load-bearing wall – Back wall (upper wall) height, H2 – Traffic surcharge, q – Bridge vertical dead load, DL – Bridge vertical live load, LL – Bridge horizontal load – Bridge span and type (simple or continuous span) – Length of approach slab – The embedment of a GRS abutment wall need only be a nominal depth (e.g., one block height). If the founda- tion contains frost-susceptible soils, they should be excavated to at least the maximum frost penetration line and replaced with a non-frost-susceptible soil. If the GRS abutment is in a stream environment, scour/abra- sion/channel protection measures should be undertaken. • Establish trial design parameters: – Sill width, B (a minimum sill width of 0.6 m is recommended). – Clear distance between the back face of the facing and the front edge of the sill, d (the recommended clear distance is 0.3 m). – Sill type (integrated sill or isolated sill). “Isolated sill” refers to a sill separated from the upper wall of the abutment; whereas “integrated sill” refers to a sill integrated with the upper wall as an integrated structure). – Facing type (dry-stacked concrete modular blocks, timber, natural rocks, wrapped geosynthetics, or gabions) and facing block size (for concrete modular block facing). – Batter of facing (a minimum front batter of 1/35 to 1/40 is recommended for segmental wall facing to provide improved appearance and greater flexi- bility in construction. A typical minimum setback of 5 to 6 mm between successive courses of facing blocks is recommended for blocks with height = 200 mm). – Reinforcement spacing (the default value for reinforce- ment spacing is 0.2 m). For wrapped-faced geotextile walls, temporary walls, or walls where facing may be added in the future, a reinforcement spacing of 0.15 m is recommended. Reinforcement spacing greater than 0.4 m is not recommended under any circumstances. Step 2: Establish soil properties • Check to ensure that the selected fill satisfies the fol- lowing criteria: 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 (0.075 mm) sieve; and plasticity index (PI) ≤ 6. • Establish reinforced fill parameters: – Wet unit weight of the reinforced fill. Figure 3-3. Details of reinforcement layout near the top of the load-bearing wall.

– The “design friction angle” of the reinforced fill, φdesign, is taken as 1 degree lower than the friction angle obtained from tests, φdesign = φtest − 1°, where φtest is determined by one set of the standard direct shear tests on portion finer than 2 mm (No. 10) sieve, using a sample compacted to 95 percent of AASHTO T-99, Methods C or D, at the optimum moisture content. – If multiple direct shear tests are performed, the smallest friction angle should be used in design. For instance, if two sets of tests are performed— both showing a friction angle of 35°—the “design friction angle” will be 35°. On the other hand, if a single set of tests shows that a soil has a friction angle of 35°, then the “design friction angle” will be taken as 34°. • Establish retained earth parameters: – Friction angle of the retained earth – Wet unit weight of the retained earth – Coefficient of active earth pressure of the retained earth • Establish foundation soil parameters: – Friction angle of the foundation soil – Wet unit weight of the foundation soil – Allowable bearing pressure of the foundation soil, qaf Step 3: Establish design requirements • Establish external stability design requirements: – Factor of safety against reinforced fill base sliding ≥ 1.5 – Eccentricity ≤ L/6 (L = length of reinforcement at base of the reinforced zone) – Average sill pressure ≤ allowable bearing pressure of the reinforced fill, qallow – Average contact pressure at the foundation level ≤ allowable bearing pressure of the foundation soil, qaf Figure 3-4. Design Example 1—configuration of the abutment. V1 V2 V3LL DL F2 Fq F1 B d H 1 = 7 .5 0 m H 2 = 2 .2 0 m H ' = T ot al A bu tm en t H ei gh t = 9 .7 0 m L - 2e 2e L = 7.00 m Traffic Surcharge: q = 9.4 kN/m2 F3 F4 V4 Reinforced Fill Soil Unit Wt. = 18.8 kN/m3 φrf = 34° K = 0.28 K = 0.33 Retained Earth Soil Unit Wt. = 18.8 kN/m3 φre = 30° z Vq V5 (L oa d-B ea rin g W all ) (B ac k W all ) Foundation Soil C a(rf) a(re) 107

108 • Establish internal stability design requirements: – Factor of safety against reinforcement pullout, FSpullout ≥1.5. – Connection strength will not be a design concern pro- vided that (a) the reinforcement spacing is kept no greater than 0.2 m, (b) the selected fill is compacted to the specifications in the Recommended Construc- tion Guidelines presented later in this chapter, and (c) the average applied pressure on the sill does not exceed the recommended allowable pressure deter- mined in Step 4, below. – For reinforcement spacing of 0.4 m, long-term con- nection failure should be checked to ensure long- term stability. Moreover, a recommended practice that “the horizontal interfaces in the top three to four courses of the facing block be strengthened to provide adequate interface shear resistance” should be observed to avoid potential facing failure (see Recommended Construction Guidelines later in this chapter). The interface strengthening effect will be even more effective if the facing blocks are interconnected “after” all the facing units are in place. Step 4: Determine allowable bearing pressure of reinforced fill The allowable bearing pressure of the reinforced fill, qallow, can be determined by the following three-step procedure: (1) Use Table 3-1 to determine the allowable bearing pressure under the following conditions: (a) an “integrated sill” configuration, (b) sill width = 1.5 m, (c) a sufficiently strong reinforcement (meeting the minimum required values of stiffness and strength as defined in Step 9, below) is used, and (d) the abutment is constructed over a compe- tent foundation (satisfying the bearing pressure requirement in Step 7, below). (2) Use Figure 3-1 to determine a correction factor for the selected sill width. The allowable bearing pressure for the selected sill width is equal to the allowable pressure determined in Step (1), above, multiplied by the correction factor. A minimum sill width of 0.6 m is recommended. (3) If an “isolated sill” is used, a reduction factor of 0.75 should be applied to the corrected allowable bearing pressure determined in Step (2), above. The allowable bearing pressure determined by the three-step procedure is for a GRS abutment founded on a “competent” foundation and with a sufficiently strong reinforcement. Example 1: the Founders/Meadows Abutment (see Chapter 2) Conditions: Fill: φdesign = 39° (note: φtest = 40.1° from a single set of standard direct shear tests) Reinforcement spacing = 0.4 m Integrated sill, sill width = 3.8 m Allowable bearing pressure: (1) From Table 3-1, for φ = 39° and reinforcement spacing = 0.4 m, allowable pressure = 215 kPa. (2) Extrapolating from Figure 3-1, the correction factor for a sill width of 3.8 m = 0.77; thus, the cor- rected allowable bearing pressure = 215 kPa x 0.77 = 166 kPa. (3) No reduction for an integrated sill. Thus, qallow = 166 kPa. Example 2: the NCHRP test abutments (see Chapter 2) Conditions: Fill: φdesign = 34° (note: φtest = 34.8° from a single set of standard direct shear tests) Reinforcement spacing = 0.2 m Isolated sill, sill width = 0.9 m Allowable bearing pressure: (1) From Table 3-1, for φdesign = 34° and reinforcement spacing = 0.2 m, allowable pressure = 180 kPa. (2) From Figure 3-1, the correction factor for sill width of 0.9 m = 1.4; thus, the corrected allowable bearing pressure = 180 kPa x 1.4 = 252 kPa. (3) Reduction factor for an isolated sill = 0.75; thus, qallow = 252 x 0.75 = 189 kPa. Step 5: Establish trial reinforcement length • A preliminary reinforcement length, L, can be taken as 0.7 × total abutment wall height (L = 0.7 × H). • The reinforcement length may be “truncated” in the bottom portion of the wall provided that the foundation is “competent” (as defined in Limitations of the Design and Construction Guidelines earlier in this chapter). The recommended configuration of the truncation is rein- forcement length = 0.35H at the foundation level (H = total abutment height) and increases upward at 45° angle. • The allowable bearing pressure of the sill, as deter- mined in Step 4, should be reduced by 10 percent for a truncated-base wall.

• When reinforcement is truncated at the bottom portion, external stability of the wall (i.e., sliding failure, overall slope failure, and foundation bearing failure) must be examined thoroughly. Step 6: Evaluate stability of footing/sill • Establish trial sill configuration (e.g., establishing the magnitude of B, d, H2, t, b, fw and fh in Figure 3-5). • Determine the forces acting on the sill (see, Figure 3-5 for example) and calculate the factor of safety against sliding, FSsliding. FSsliding should be ≥ 1.5. • Check sill eccentricity requirement: The load eccentric- ity at the base of the sill, e, should be ≤ B/6 (B = width of sill). • Check allowable bearing pressure of the reinforced fill; the applied contact pressure on base of the sill should be ≤ qallow determined in Step 4. Step 7: Check external stability of reinforced fill with the preliminary reinforcement length established in Step 5 • Determine the forces needed for evaluating the external stability of the abutment (e.g., V4, V5, Vq, F3, F4, and I1 in Figure 3-4, I1 is the influence depth caused by the hor- izontal forces in the back wall, as shown in Figure 3-6). • Check factor of safety against sliding of the reinforced volume, FSsliding, should be ≥ 1.5. • Check eccentricity requirement for the reinforced volume, e, should be ≤ L/6 (L = length of reinforcement). • Check allowable bearing pressure of the foundation soil: – Determine the “influence length” D1 at the foun- dation level (D1 = d + (B − 2e) + H1/2, see Fig- ure 3-2) and compare it with the effective rein- forcement length, L = L −2e. – The contact pressure on the foundation level, pcontact, is calculated by dividing the total vertical load in the reinforced volume by D1 or L, whichever is smaller. – pcontact should be ≤ qaf If the bearing capacity of the foundation soil sup- porting the bridge abutment is only marginally accept- able or somewhat unacceptable, an RSF may be used to increase its bearing capacity and to reduce potential settlement. A typical RSF is founded by excavating a pit that is 0.5L deep (L = reinforcement length) and replacing it with compacted road base material re- inforced by the same reinforcement to be used in the reinforced abutment wall at 0.3 m vertical spacing. The lateral extent of the RSF should at least cover the vertical projection of the reinforced fill and should extend no less than 0.25L in front of the wall face. Step 8: Evaluate internal stability at each reinforcement level When evaluating the internal stability, the coefficient of lateral earth pressure is assumed to be constant Fq b = 0.4 m d = 0.3 m 1.45 m fh = 0.1 m t = 0.65 m F2 H2 = 2.2 m fw = 0.8 m V1 V2 DL LL V3 B = 1.5 m F1 q ΣVa B' = B - 2e' 2e' A Figure 3-5. Design Example 1—dimensions and loads acting on the sill. 109

110 throughout the entire wall height. The internal stability is evaluated by checking the factor of safety against rein- forcement pullout failure at each reinforcement level. The factor of safety against pullout failure, FSpullout, at any given reinforcement level, is equal to pullout resis- tance at the reinforcement level divided by Tmax. Tmax is the maximum reinforcement tensile force at the reinforce- ment level where the pullout safety factor is being evalu- ated. Tmax is calculated as the product of the average active lateral earth pressure at the reinforcement level multiplied by the reinforcement vertical spacing. The pullout resis- tance, on the other hand, arises from the frictional resis- tance at soil-reinforcement interface along the portion of reinforcement lies beyond the potential failure plane. The potential failure plane is taken as the active Rankine fail- ure surface with a uniform vertical surcharge. FSpullout at all reinforcement levels should be ≥ 1.5. Step 9: Determine the required reinforcement stiffness and strength Both a minimum value of ultimate tensile stiffness and a minimum value of tensile strength of the geosynthetic reinforcement should be specified to ensure sufficient ten- sile resistance at the service load and a sufficient safety margin against rupture failure. The tensile stiffness is defined as the tensile resistance at the working strain. It is recommended that a tensile strain of 1.0 percent be taken as the reference working strain, and the resistance at strain = 1.0 percent be used for specification of the required reinforcement stiffness. The minimum required reinforcement stiffness in the direction perpendicular to the wall face, T@=1.0 percent, is to be determined as T@=1.0 percent ≥ σh(max)  s where σh(max) is the maximum lateral stress in the rein- forced fill, and s is the vertical reinforcement spacing. For non-uniform reinforcement spacing, s = (1⁄2 distance to reinforcement layer above) + (1⁄2 distance to reinforce- ment layer below). The required minimum value of the ultimate reinforce- ment strength in the direction perpendicular to the abut- ment wall face, Tult, should be determined by imposing a combined safety factor on T@=1.0 percent as Tult ≥ Fs  T@ =1.0 percent The combined safety factor is applied to ensure satis- factory long-term performance, to provide sufficient ductility of the abutment, and to account for various uncertainties. The recommended combined safety factor Figure 3-6. Design Example 1 - Notations of the quantities for internal stability evaluation. d = 0.3 m B' = 1.28 m D1 = 5.33 m 2e' = 0.22 m z 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 l 1 = 2 .9 7 m 45 + φrf /2 z Influence line (2V:1H slope) Failure surface Li = 0.17 m (at no. 25) La = 2.66 m (at no. 25) Le = 4.34 m (at no. 25) 0. 6 m 35 36 37

is Fs = 5.5 for reinforcement spacing ≤ 0.2 m, and Fs = 3.5 for reinforcement spacing of 0.4 m. These combined safety factors only apply to the condition where the back- fill material and placement conditions satisfy those in the recommended construction guidelines. Step 10: Design the back/upper wall If the back wall (or the upper wall) is to be a reinforced soil wall, it should be designed similarly to the load- bearing wall. In most cases, the same fill, same reinforce- ment, and same fill placement conditions as those of the load-bearing wall should be used, although the default reinforcement spacing in the approach fill can be increased somewhat (e.g., from 0.2 m to 0.3 m or even 0.4 m). The length of all the layers of reinforcement (at least in the top three layers, if there is a spacing constraint) should be 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. If there is no significant space constraint, it is recom- mended that the reinforcement length of the top three lay- ers (all the layers, if there is little space constraint) in the lower wall be also extended about 1.5 m beyond the end of the approach slab. Extending the reinforcement lengths beyond the approach slab promotes integration of the abutment wall with the approach embankment and the load-bearing abutment, so as to eliminate bridge “bumps.” Step 11: Check angular distortion between abutments The angular distortion between abutments or between piers and abutments should be checked to ensure ride qual- ity and structural integrity. Angular distortion = (differ- ence in total settlements between abutments or between piers and abutments)/(span between the bridge-supporting structures). The angular distortion should be limited to 0.005 (or 1:200) for simple spans and 0.004 (or 1:250) for continuous spans. The total settlement of each abutment is the sum of foun- dation settlement (i.e., settlement occurs beneath the abut- ment) and abutment settlement (i.e., settlement occurs within the abutment). The foundation settlement caused by the self weight of a GRS abutment and subject to bridge loads can be estimated by using the conventional 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 recommended allowable bearing pressure determined in Step 4, can be estimated conservatively as 1.5 percent of H1 (H1 = height of the loading bearing wall). In situations where the heights of the load-bearing walls at the two ends of a bridge differ significantly, it is a good practice to preload or prestress the load-bearing walls. The proper magnitude of preloading or prestressing and the probable reduction in differential settlement caused by preloading of a reinforced soil mass can be evaluated by a procedure established by Ketchart and Wu (2001 and 2002). RECOMMENDED CONSTRUCTION GUIDELINES Earthwork construction control for GRS abutments is essen- tially the same as that required for conventional bridge abut- ments, but with a few additional details that require special attention. Field substitutions of backfill materials or changes in construction sequence, procedures, or details should only be permitted with the express consent of the responsible geo- technical or preconstruction design engineer. The recom- mended construction guidelines focus on GRS abutments with a 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, NCMA, FHWA, CTI, SAGP, and JR), as summarized in Appendix A, as well as the authors’ and their colleagues’ observations and experiences with construction of GRS walls 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 111

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.

– 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. 113

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 For a flexible facing differing from the segmental concrete block facing, the following construction guidelines about the facing should be observed: Wrapped-Faced Geotextile Facing • If the geotextile is wide enough for the required rein- forcement length, it can be unrolled parallel to the wall (i.e., in the longitudinal direction). Two rolls of geotextile can be sewn together if a single roll is not wide enough. Alternatively, the geotextile can be deployed perpen- dicularly to the abutment wall and adjacent sheets can be 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- lar to the wall face). • 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 (1 to 2 ft) from the wall face. • 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. • Reinforcement spacing of 0.15 m is recommended as it is easy to work with and it will help minimize face deformation. • Face alignment and compaction can be greatly facili- tated with the use of temporary forms, such as 50 mm x 200 mm (2 in. x 8 in.) 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 applying a coating to a vertical or near-vertical wall, a wire mesh may need to be anchored to the geo- textile 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 (6 ft). Timber Facing • The timber typically has a 150 mm x 200 mm (6 in. x 8 in.) or 150 mm x 150 mm (6 in. x 6 in.) cross-sectional dimen- sion and should be treated to an acceptable level with

copper chromate or approved equivalent preservative. The bottom row of timber should be treated for direct burial. The color may be green or brown, but not mixed. • Forming elements in the back of the timber face may con- sist of wood (minimum 250 mm nominal thickness treated to an acceptable level with copper chromate or approved equivalent), fiberglass, plastic, or other approved material. • The typical reinforcement used is a nonwoven geotex- tile, although other geosynthetics that satisfy the design criteria can also be used. • Nails should be 16d galvanized ring shank nails and should be placed at the top and bottom of the timbers at 0.3 m (1 ft) intervals. • Compaction should be consistent with project embank- ment specifications, except that no compaction is allowed within 0.3 to 0.6 m (1 to 2 ft) of the wall face. • Shimming of timber to maintain verticality is permissible. • All reinforcement overlaps should be at least 0.3 m (1 ft) wide and should be perpendicular to the wall face. • All exposed fabric should be painted with a latex paint matching the color of the timbers. • To improve connection strength on the top lifts, the geo- textile can be wrapped around the facing timbers and then covered or 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 infill- ing should be as complete as possible. DESIGN EXAMPLES Two design examples are given here to illustrate the design computation procedure of the recommended design method. Design Example 1 has an integrated sill with a tall upper wall, whereas Design Example 2 has an isolated sill with a short upper wall. Design Example 1: GRS Abutment with an Integrated Sill and a Tall Upper Wall Step 1: Establish abutment geometry, external loads and trial design parameters Wall heights and external loads: Total abutment height, H 9.7 m Load-bearing wall height, H1 7.5 m Back wall height, H2 2.2 m Traffic surcharge, q 9.4 kN/m2 Bridge vertical dead load, DL 45 kN/m Bridge vertical live load, LL 50 kN/m Bridge horizontal load, F2 2.25 kN/m Span 24 m (simple span) Length of concrete approach slab 4.25 m Trial design parameters: Sill width, B 1.5 m Clear distance, d 0.3 m Sill type integrated sill Facing modular concrete blocks Facing block size 200 mm x 200 mm x 400 mm Batter of facing 1/35 (6 mm setback for each block) Reinforcement spacing 0.2 m Note: As the batter of 1/35 corresponds to an angle of 1.6°, less than 8°, the abutment wall is to be designed as a ver- tical wall, and the coefficient of earth pressure is to follow the general Rankine case, per Section 4.2d, NHI manual. The configuration of trial design for the GRS abutment is shown in Figure 3-4. Step 2: Establish soil properties Reinforced fill: The selected fill satisfies the following criteria: 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 (0.075 mm) sieve; PI ≤ 6. The friction angle of the fill = 35°, as determined by one set of the standard direct shear test on the portion finer than 2 mm (No. 10) sieve, using a sample com- pacted to 95 percent of AASHTO T-99, Methods C or D, at optimum moisture content. φtest = 35°, γrf = 18.8 kN/m3, Ka(rf) = tan2 (45° − φrf/2) = 0.28 Note: The “design friction angle” is taken as one degree lower than φtest, i.e., φdesign = φrf = 34° (see The Recom- mend Design Method Step 2). Retained earth: φre = 30°, γre = 18.8 kN/m3, Ka(re) = tan2 (45° − φre/2) = 0.33 Foundation soil: φfs = 30°, γfs = 20.0 kN/m3, qaf = 300 kN/m2 115

116 Step 3: Establish design requirements External stability design requirements: – Sliding ≥ 1.5 – Eccentricity ≤ l/6 – Sill pressure ≤ allowable bearing of the reinforced fill qallow = 180 kPa (as determined in Step 4 below) – Average contact pressure at the foundation level ≤ allow- able bearing pressure of the foundation soil, qaf = 300 kPa Internal stability design requirements: – Factor of safety against pullout FSpullout ≥ 1.5 – Facing connection strength is OK with reinforcement spacing = 0.2 m (see The Recommended Design Method, Step 3). Step 4: Determine allowable bearing pressure of reinforced fill Determine the allowable bearing pressure of the rein- forced fill, qallow, with the following conditions: φdesign = φrf = 34° Reinforcement spacing = 0.2 m (uniform spacing with no truncation) Integrated sill, sill width = 1.5 m (1) From Table 3-1, for φ = 34° and reinforcement spac- ing = 0.2 m, allowable bearing pressure = 180 kPa. (2) From Figure 3-1, the correction factor for a sill width of 1.5 m is 1.0; thus the corrected allowable bearing pressure = 180 kPa × 1.0 = 180 kPa. (3) No reduction for an integrated sill. Thus, qallow = 180 kPa. Step 5: Establish trial reinforcement length Select a preliminary reinforcement length = 0.7 * total abutment height L = 0.7 × H = 0.7 × 9.7 m = 6.8 m (use 7.0 m) Step 6: Evaluate stability of footing/sill The preliminary sill configuration and forces acting on the sill are shown in Figure 3-5. The dimensions of the sill are B 1.5 m d 0.3 m H2 2.2 m t 0.65 m b 0.4 m fw 0.8 m fh 0.1 m With unit weight of concrete, γconcrete = 23.6 kN/m3, the fol- lowing forces acting on the sill are determined: V1 = (B × t)  γconcrete V1= (1.5 m × 0.65 m)  23.6 kN/m3 = 23.01 kN/m V2 = [(fw + b) × fh]  γconcrete V2 = [(0.8 m + 0.4 m) × 0.1 m]  23.6 kN/m3 = 2.83 kN/m V3 = [b × (H2 − fh − t)]  γconcrete V3 = [0.4 m × (2.2 m − 0.1 m − 0.65 m)]  23.6 kN/m3 = 13.69 kN/m DL = 45 kN/m (from Step 1) LL = 50 kN/m (from Step 1) Fq = Ka(rf)  q  H2 Fq = 0.28  9.4 kN/m2  2.2 m = 5.79 kN/m F1 = 1/2  Ka(rf)  γrf  H22 F1 = 1/2  (0.28)  18.8 kN/m3  (2.2 m)2 = 12.74 kN/m F2 = 2.25 kN/m (from Step 1) Check factor of safety against sliding: ΣVa = sum of vertical forces acting on the sill ΣVa = V1 + V2 + V3 + DL + LL ΣVa = 23.01 kN/m + 2.83 kN/m + 13.69 kN/m + 45 kN/m + 50 kN/m = 134.53 kN/m ΣFa = sum of horizontal forces acting on the sill ΣFa = Fq + F1 + F2 ΣFa = 5.79 kN/m + 12.74 kN/m + 2.25 kN/m = 20.78 kN/m FSsliding = (134.53 kN/m − 50 kN/m)  tan 34° / 20.78 kN/m = 2.74 > 1.5 (OK) Check eccentricity requirement: ΣMOA = sum of overturning moments about point A ΣMOA = Fq  (H2/2) + F1  (H2/3) + F2  (t + fh) ΣMOA = 5.79 kN/m  (2.2 m/2) + 12.74 kN/m  (2.2 m/3) + 2.25 kN/m  (0.65 m + 0.1 m) = 17.40 kN/m  m ΣMRA = sum of resisting moments about point A ΣMRA = V1  (B/2) + V2  [(fw + b)/2 + (B − b − fw)] + V3  [(b/2) + (B − b)] + (DL + LL)  [(fw/2) + (B − b − fw)] FS Va LL Fasliding rf = −( )Σ Σ tan φ

ΣMRA = 23.01 kN/m  (1.5 m/2) + 2.83 kN/m  [(0.8 m + 0.4 m)/2 + (1.5 m − 0.4 m − 0.8 m)] + 13.69 kN/m  [(0.4 m/2) + (1.5 m − 0.4 m)] + (45 kN/m + 50 kN/m)  [(0.8 m/2) + (1.5 m − 0.4 m − 0.8 m)] = 104.10 kN/m  m e = eccentricity at the base of the sill e = (1.5 m/2) − (104.10 kN/m  m − 17.40 kN/m  m)/134.53 kN/m = 0.11 m B/6 = 1.5 m/6 = 0.25 m e < B/6 (OK) Check allowable bearing pressure of the reinforced fill: psill = applied pressure from the sill psill = 134.53 kN/m/[1.5 m − (2  0.11 m)] = 105.1 kN/m2 psill = 105.1 kPa < qallow = 180 kPa (OK) Step 7: Check external stability of reinforced fill with the preliminary reinforcement length established in Step 5 The forces needed to evaluate the external stability of the abutment are shown in Figure 3-4. These forces are calculated as follows. V4 = (L × H1)  γrf V4 = (7 m × 7.5 m)  18.8 kN/m3 = 987 kN/m V5 = [(L − d − B) × H2]  γrf V5 = [(7 m − 0.3 m − 1.5 m) × 2.2 m]  18.8 kN/m3 = 215.07 kN/m Vq = (L − d − B)  q Vq = (7 m − 0.3 m − 1.5 m)  9.4 kN/m2 = 48.88 kN/m F3 = [Ka(re)  (q + γre  H2)]  H1 F3 = [0.33  (9.4 kN/m2 + 18.8 kN/m3  2.2 m)]  7.5 m = 125.63 kN/m F4 = 1/2  Ka(re)  γre  H12 F4 = 1/2  (0.33)  18.8 kN/m3  (7.5 m)2 = 174.49 kN/m ΣVa = 134.53 kN/m (from Step 6) ΣFa = 20.78 kN/m (from Step 6) p Va B esill = − Σ 2 ' e B M M Va RA OA = − − 2 Σ Σ Σ I1 is the influence depth caused by the horizontal forces in the back wall (see Figure 3-6): I1 = (d + B − 2  e)  tan (45° + φrf/2) I1 = [0.3 m + 1.5 m − 2  (0.11 m)]  tan (45° + 34°/2) = 2.97 m Check factor of safety against sliding for the reinforced volume: ΣV = sum of vertical forces acting on the foundation soil ΣV = V4 + V5 + Vq + ΣVa ΣV = 987 kN/m + 215.07 kN/m + 48.88 kN/m + 134.53 kN/m = 1385.48 kN/m ΣF = sum of horizontal forces acting on the foundation soil ΣF = F3 + F4 + ΣFa ΣF = 125.63 kN/m + 174.49 kN/m + 20.78 kN/m = 320.90 kN/m FSsliding = (1385.48 kN/m − 50 kN/m − 48.88 kN/m)  tan (30°)/320.90 kN/m = 2.31 > 1.5 (OK) Check eccentricity requirement for the reinforced volume: ΣMO = sum of overturning moments about point C ΣMO = F3  (H1/2) + F4  (H1/3) + ΣFa  (H1 − I1/3) ΣMO = 125.63 kN/m  (7.5 m/2) + 174.49 kN/m  (7.5 m/3) + 20.78 kN/m  [7.5 m − (2.97 m/3)] = 1042.62 kN/m  m ΣMR = sum of resisting moments about point C ΣMR = V4  (L/2) + (V5 + Vq)  [(L − d − B)/2 + (d + B)] + (ΣMRA + ΣVa  d) ΣMR = 987 kN/m  (7 m/2) + (215.07 kN/m + 48.88 kN/m)  [(7 m − 0.3 m − 1.5 m)/2 + (0.3 m + 1.5 m)] + [104.10 kN/m  m + 134.53 kN/m  (0.3 m)] = 4760.34 kN/m  m MS = moment about point C caused by traffic surcharge MS = Vq  [(L − d − B)/2 + (d + B)] MS = 48.88 kN/m  [(7 m − 0.3 m − 1.5 m)/2 + (0.3 m + 1.5 m)] = 215.07 kN/m  m e = eccentricity at the base of the reinforced volume e = (7 m/2) − [(4760.34 kN/m  m − 215.07 kN/m  m) − 1042.62 kN/m  m]/(1385.48 kN/m − 48.88 kN/m) = 0.88 m e L M M M V Vq R S O = − −( ) − −2 Σ Σ Σ FS V LL Vq Fsliding fs = − −( )Σ Σ tan φ 117

118 L/6 = 7 m/6 = 1.17 m e < L/6 (OK) Check allowable bearing pressure of the foundation soil: Calculate the “influence length” D1 at the foundation level and compare with the effective reinforcement length, L. D1 = d + B + H1/2 = d + (B − 2e) + H1/2 D1 = 0.3 m + [1.5 m − 2  (0.11 m)] + 7.5 m/2 = 5.33 m L = L − 2e L = 7.0 m − 2  (0.88 m) = 5.24 m Because D1 at the foundation level is greater than L (L = L−2e), thus the contact pressure on the founda- tion level, pcontact, is calculated as follows. pcontact = 1385.48 kN/m/[7.0 m − 2  (0.88 m)] = 264.40 kN/m2 qaf = 300 kN/m2 (from Step 2) pcontact = 264.40 kN/m2 < qaf = 300 kN/m2 (OK) Step 8: Evaluate internal stability at each reinforcement level With geosynthetic reinforcement, the coefficient of lateral earth pressure is constant throughout the entire wall height, per Section 4.3b, NHI manual. The internal stability is evaluated by checking the rein- forcement pullout failure. Check reinforcement pullout failure: Pr = pullout resistance Pr = F*  α  (σv  Le)  C  Rc F* = pullout resistance factor F* = 2/3  tan φrf F* = 2/3  tan(34°) = 0.45 α = a scale effect correction factor ranging from 0.6 to 1.0 for geosynthetic reinforcement; for geotextile, α is defaulted to 0.6, per Section 3.3b, NHI manual. (σv  Le) = normal force at the soil-reinforcement inter- face at depth z (excluding traffic surcharge) (σv  Le) = (σvs  Le) + (Δσv  Li) p V L econtact = − Σ 2 Le = length of embedment in the resistant zone behind the failure surface at depth z Le = L − La La = length of embedment in the active zone at depth z La = (H1 − z) tan (45° − φrf/2) Li = length of embedment within the influence area inside the resistant zone; this length can be measured directly from the design drawing. C = reinforcement effective unit perimeter; C = 2 for strips, grids, and sheets. Rc = coverage ratio; Rc = 1.0 for 100 percent coverage of reinforcement. σh = horizontal pressure at depth z σh = Ka(rf)  (σvs + Δσv + q) + Δσh σvs = vertical soil pressure at depth z σvs = (γrf  H2) + (γrf  z) Δσv = distributed vertical pressure from sill Δσv = ΣVa / D D = effective width of applied load at depth z For z ≤ z2: D = (B − 2e) + z For z > z2: D = d + (B − 2e) + z/2 z2 = 2  d Δσh = supplement horizontal pressure at depth z For z ≤ I1: Δσh = 2  ΣFa  (I1 − z)/(I12) For z > I1: Δσh = 0 Tmax = maximum tensile force in the reinforcement at depth z Tmax = σh  s s = vertical reinforcement spacing FSpullout = factor of safety against reinforcement pullout FSpullout = Pr / Tmax Let depth z be measured from the top of the load-bearing wall. Reinforcement no. 25 at z = 2.5 m (see Figure 3-6) would serve as an example for determining the FSpullout. σvs = (γrf  H2) + (γrf  z) σvs = (18.8 kN/m3  2.2 m) + (18.8 kN/m3  2.5 m) = 88.36 kN/m2 z2 = 2  d = 2  (0.3 m) = 0.6 m

z = 2.5 m > z2 = 0.6 m, D = d + (B − 2e) + z/2 D = 0.3 m + [1.5 m − 2  (0.11 m)] + 2.5 m/2 = 2.83 m Δσv = ΣVa / D Δσv = 134.53 kN/m/ 2.83 m = 47.54 kN/m2 z = 2.5 m < I1 = 2.97 m, Δσh = 2  ΣFa  (I1 − z)/(I12) Δσh = 2  (20.78 kN/m)  (2.97 m − 2.5 m)/(2.97 m)2 = 2.21 kN/m2 σh = Ka(rf)  (σvs + Δσv + q) + Δσh σh = 0.28  (88.36 kN/m2 + 47.54 kN/m2 + 9.4 kN/m2) + 2.21 kN/m2 = 42.89 kN/m2 Tmax = σh  s Tmax = 42.89 kN/m2  (0.2 m) = 8.58 kN/m La = (H1 − z) tan (45° − φrf/2) La = (7.5 m − 2.5 m) tan (45° − 34°/2) = 2.66 m Le = L − La Le = 7 m − 2.66 m = 4.34 m Calculate the normal force at z = 2.5 m: (σv  Le) = (σvs  Le) + (Δσv  Li) (σv  Le) = (88.36 kN/m2  4.34 m) + (47.54 kN/m2  0.17 m) = 391.56 kN/m Pr = F*  α  (σv  Le)  C  Rc Pr = (0.45)  (0.6)  391.56 kN/m  (2)  (1) = 211.44 kN/m FSpullout = Pr / Tmax FSpullout = 211.44 kN/m/ 8.58 kN/m = 24.64 FSpullout = 24.64 > 1.5 (OK) The values of FSpullout for all the reinforcements in the load-bearing wall are summarized in Table 3-2. Step 9: Determine the required reinforcement stiffness and strength The minimum “working” reinforcement stiffness is deter- mined as T@=1.0 percent ≥ σh(max)  s The minimum ultimate reinforcement strength is deter- mined as Tult ≥ Fs  T@=1.0 percent The recommended combined safety factor is Fs = 5.5 for reinforcement ≤ 0.2 m and Fs = 3.5 for reinforcement spacing of 0.4 m. From Table 3-2, σh(max) is 59.84 kN/m2, which occurs at reinforcement No. 1 with depth z = 7.3 m and s = 0.2 m. T@=1.0 percent = σh(max)  s T@=1.0 percent = 59.84 kN/m2  (0.2 m) = 11.97 kN/m The uniform reinforcement spacing is 0.2 m, hence Fs = 5.5. Tult = Fs  T@=1.0 percent Tult = 5.5  (11.97 kN/m) = 65.84 kN/m A reinforcement with minimum “working” stiffness, T@=1.0 percent = 12.0 kN/m and minimum ultimate strength (per ASTM D4595), Tult = 65.8 kN/m is required. Step 10: Design of back/upper wall Reinforced fill: Same as that of the load- bearing wall Reinforcement: Same as that of the load- bearing wall Reinforcement length: 4.25 m (length of approach slab) + 1.5 m = 5.75 m (see The Recommended Design Method Step 10) Reinforcement layout: Vertical spacing = 0.3 m Wrapped-face with wrapped return at least 0.5 m (18 in.) in the horizontal direction and anchored in at least 100 mm of fill material. A compressible layer of about 50 mm thick should be installed between the wrapped face and the rigid back wall. Step 11: Check angular distortion between abutments δabutment = abutment settlement δabutment = 1.5 percent  H1 δabutment = 0.015  (7.5 m) = 0.1125 m δfoundation = foundation settlement (as determined by con- ventional settlement computation methods) Egδfoundation = 0.01 m δtotal = total settlement δtotal = δabutment + δfoundation δtotal = 0.1125 m + 0.01 m = 0.1225 m 119

120 No. Depth s σ Δσ Δσ σ σvs D v h h Tmax La Le Li ( v · Le) Pr FSpullout z (m) (m) (kN/m 2 ) (m) (kN/m 2 ) (kN/m 2 ) (kN/m 2 ) (kN/m) (m) (m) (m) (kN/m) (kN/m) 1 7.3 0.2 178.6 5.23 25.72 0.00 59.84 11.97 0.11 6.89 5.12 1363.00 735.49 61.45 2 7.1 0.2 174.84 5.13 26.22 0.00 58.93 11.79 0.21 6.79 4.92 1315.65 709.93 60.24 3 6.9 0.2 171.08 5.03 26.75 0.00 58.02 11.60 0.32 6.68 4.71 1268.98 684.75 59.01 4 6.7 0.2 167.32 4.93 27.29 0.00 57.12 11.42 0.43 6.57 4.50 1222.99 659.93 57.76 5 6.5 0.2 163.56 4.83 27.85 0.00 56.23 11.25 0.53 6.47 4.30 1177.67 635.48 56.51 6 6.3 0.2 159.8 4.73 28.44 0.00 55.34 11.07 0.64 6.36 4.09 1133.02 611.39 55.24 7 6.1 0.2 156.04 4.63 29.06 0.00 54.46 10.89 0.74 6.26 3.89 1089.03 587.65 53.95 8 5.9 0.2 152.28 4.53 29.70 0.00 53.59 10.72 0.85 6.15 3.68 1045.68 564.25 52.65 9 5.7 0.2 148.52 4.43 30.37 0.00 52.72 10.54 0.96 6.04 3.47 1002.96 541.20 51.33 10 5.5 0.2 144.76 4.33 31.07 0.00 51.86 10.37 1.06 5.94 3.27 960.87 518.49 49.99 11 5.3 0.2 141 4.23 31.80 0.00 51.02 10.20 1.17 5.83 3.06 919.39 496.11 48.62 12 5.1 0.2 137.24 4.13 32.57 0.00 50.18 10.04 1.28 5.72 2.85 878.51 474.05 47.24 13 4.9 0.2 133.48 4.03 33.38 0.00 49.35 9.87 1.38 5.62 2.65 838.21 452.31 45.82 14 4.7 0.2 129.72 3.93 34.23 0.00 48.54 9.71 1.49 5.51 2.44 798.48 430.87 44.38 15 4.5 0.2 125.96 3.83 35.13 0.00 47.74 9.55 1.60 5.40 2.23 759.30 409.72 42.92 16 4.3 0.2 122.2 3.73 36.07 0.00 46.95 9.39 1.70 5.30 2.03 720.64 388.86 41.42 17 4.1 0.2 118.44 3.63 37.06 0.00 46.17 9.23 1.81 5.19 1.82 682.49 368.28 39.88 18 3.9 0.2 114.68 3.53 38.11 0.00 45.41 9.08 1.91 5.09 1.62 644.82 347.95 38.31 19 3.7 0.2 110.92 3.43 39.22 0.00 44.67 8.93 2.02 4.98 1.41 607.61 327.87 36.70 20 3.5 0.2 107.16 3.33 40.40 0.00 43.95 8.79 2.13 4.87 1.20 570.82 308.02 35.04 21 3.3 0.2 103.4 3.23 41.65 0.00 43.25 8.65 2.23 4.77 1.00 534.41 288.37 33.34 22 3.1 0.2 99.64 3.13 42.98 0.00 42.57 8.51 2.34 4.66 0.79 498.35 268.91 31.59 23 2.9 0.2 95.88 3.03 44.40 0.34 42.25 8.45 2.45 4.55 0.58 462.58 249.61 29.54 24 2.7 0.2 92.12 2.93 45.91 1.28 42.56 8.51 2.55 4.45 0.38 427.08 230.45 27.07 25 2.5 0.2 88.36 2.83 47.54 2.22 42.90 8.58 2.66 4.34 0.17 391.76 211.40 24.64 26 2.3 0.2 84.6 2.73 49.28 3.16 43.28 8.66 2.76 4.24 0.00 358.29 193.34 22.34 27 2.1 0.2 80.84 2.63 51.15 4.10 43.69 8.74 2.87 4.13 0.00 333.77 180.10 20.61 28 1.9 0.2 77.08 2.53 53.17 5.04 44.15 8.83 2.98 4.02 0.00 310.05 167.30 18.95 29 1.7 0.2 73.32 2.43 55.36 5.98 44.65 8.93 3.08 3.92 0.00 287.13 154.94 17.35 30 1.5 0.2 69.56 2.33 57.74 6.93 45.20 9.04 3.19 3.81 0.00 265.01 143.00 15.82 31 1.3 0.2 65.8 2.23 60.33 7.87 45.81 9.16 3.30 3.70 0.00 243.68 131.49 14.35 32 1.1 0.2 62.04 2.13 63.16 8.81 46.50 9.30 3.40 3.60 0.00 223.16 120.42 12.95 33 0.9 0.2 58.28 2.03 66.27 9.75 47.26 9.45 3.51 3.49 0.00 203.44 109.78 11.62 34 0.7 0.2 54.52 1.93 69.70 10.69 48.11 9.62 3.62 3.38 0.00 184.52 99.57 10.35 35 0.5 0.2 50.76 1.78 75.58 11.63 49.64 9.93 3.72 3.28 0.00 166.39 89.79 9.04 36 0.3 0.2 47 1.58 85.15 12.57 52.21 10.44 3.83 3.17 0.00 149.07 80.44 7.70 37 0.1 0.2 43.24 1.38 97.49 13.51 55.55 11.11 3.93 3.07 0.00 132.55 71.52 6.44 TABLE 3-2 Design Example 1—tabulated results of FSpullout at each reinforcement level Angular distortion = δtotal / span length = 0.1225 m / 24 m = 0.0051 Tolerable angular distortion for simple span = 0.005 Angular distortion = 0.0051 ≈ 0.005 (OK) Design summary: The configuration for the trial design is shown in Figure 3-7. Abutment configuration: Load-bearing wall height, H1 7.5 m Back wall height, H2 2.2 m Facing modular concrete blocks (200 mm x 200 mm x 400 mm) Front batter 1/35 Sill type integrated sill Sill width 1.5 m Sill clear distance 0.3 m Embedment 200 mm (one facing block height) Reinforcement: Minimum stiffness at =1.0 percent, T@=1.0 percent = 12.0 kN/m Minimum ultimate strength, Tult = 65.8 kN/m Length in load-bearing wall = 7.0 m

Length of top three layers in load-bearing wall = 7.5 m Vertical spacing in load-bearing wall = 0.2 m Length in back wall = 5.75 m Vertical spacing in back wall = 0.3 m Design Example 2: A GRS Abutment with an Isolated Sill and a Short Lower Wall Step 1: Establish abutment geometry, external loads and trial design parameters Wall heights and external loads: Total abutment height, H 3.0 m Load-bearing wall height, H1 2.4 m Back wall height, H2 0.6 m Traffic surcharge, q 9.4 kN/m2 Bridge vertical dead load, DL 35 kN/m Bridge vertical live load, LL 40 kN/m Bridge horizontal load, F2 1.75 kN/m Span length 10 m (simple span) Approach slab is not used Trial design parameters: Sill width, B 0.6 m Clear distance, d 0.3 m Sill type isolated sill Facing modular concrete blocks Facing block size 200 mm x 200 mm x 400 mm Batter of facing 1/35 (6 mm setback for each block) Reinforcement spacing 0.2 m Note: As the batter of 1/35 corresponds to an angle of 1.6°, less than 8°, the abutment wall is to be designed as a vertical wall, and the coefficient of earth pressure is to fol- low the general Rankine case. The configuration of the initial trial design for the GRS abutment is shown in Figure 3-8. 0.2 m (Embedment) 7 m 1.5 m Sill 1 Front batter 7. 5 m (L oa d-b ea rin g w all ) 35 2. 2 m (B ac k w all ) 0.3 m = 65.8 kN/m = 12 kN/m Reinforcement: T ult @T ε =1% 0.2 m Approach Slab 4.25 m 1.5 m 0.3 m Compressible Layer Figure 3-7. Design Example 1—configuration of the trial design. 121

122 Step 2: Establish soil properties Reinforced fill: The selected fill satisfies the following criteria: 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 (0.075 mm) sieve; PI ≤ 6. The friction angle of the fill = 37°, as determined by one set of the standard direct shear test on the portion finer than 2 mm (No. 10) sieve, using a sample com- pacted to 95 percent of AASHTO T-99, Methods C or D, at optimum moisture content. φtest = 37°, γrf = 20 kN/m3, Ka(rf) = tan2 (45° − φrf/2) = 0.26 Note: The “design friction angle” is taken as one degree lower than φtest, i.e., φdesign = φrf = 36° (see The Recommended Design Method Step 2). Retained earth: φre = 30°, γre = 18 kN/m3, Ka(re) = tan2 (45° − φre/2) = 0.33 Foundation soil: φfs = 30°, γfs = 20.0 kN/m3, qaf = 300 kN/m2 Step 3: Establish design requirements External stability design requirements: – Sliding ≥ 1.5 – Eccentricity ≤ l/6 – Sill pressure ≤ allowable bearing of the reinforced fill qallow = 345 kPa (as determined in Step 4 below) – Average contact pressure at the foundation level ≤ allowable bearing pressure of the foundation soil, qaf = 300 kPa V1 DL LL B' = B - 2e' 2e' B = 0.6 m d = 0.3 m t = 0.3 m ΣVa Traffic Surcharge: q = 9.4 kN/m2 H 1 = 2 .4 m (L oa d-B ea rin g W all ) Reinforced Fill Soil Unit Wt. = 20 kN/m3 φrf = 36° K a(rf) = 0.26 F3 F4 V4 Retained Earth Soil Unit Wt. = 18 kN/m = 0.33 a(re)K 3 L - 2e 2e L = 2.10 m Foundation Soil (Preliminary Reinforcement Length) H 2 = 0 .6 m (B ac k W all ) H ' = T ot al A bu tm en t H t. = 3. 0 m Fq F1 V5 C A Vq φre = 30° Figure 3-8. Design Example 2—configuration of the abutment.

Internal stability design requirements: – Factor of safety against pullout FSpullout ≥ 1.5 – Facing connection strength is OK with reinforce- ment spacing = 0.2 m (see The Recommended Design Method Step 3) Step 4: Determine allowable bearing pressure of reinforced fill Determine the allowable bearing pressure of the rein- forced fill qallow with the following conditions: φdesign = φrf = 36° Reinforcement spacing = 0.2 m (uniform spacing with no truncation) Isolated sill, sill width = 0.6 m (1) From Table 3-1, for φ = 36° and reinforcement spacing = 0.2 m, allowable bearing pressure = 200 kPa. (2) From Figure 3-1, the correction factor for a sill width of 0.6 m is 2.3; thus the corrected allowable bearing pressure = 200 kPa × 2.3 = 460 kPa. (3) Reduction factor of 0.75 applies for an isolated sill. Thus, qallow = 0.75 × 460 kPa = 345 kPa. Step 5: Establish trial reinforcement length Select a preliminary reinforcement length = 0.7 * total abutment height L = 0.7 × H = 0.7 × 3.0 m = 2.1 m Step 6: Evaluate stability of footing/sill The preliminary sill configuration and forces acting on the sill are shown in Figure 3-8. The dimensions of the sill are B 0.6 m d 0.3 m H2 0.6 m t 0.3 m With unit weight of concrete, γconcrete = 23.6 kN/m3, the fol- lowing forces acting on the sill are determined: V1 = (B × t)  γconcrete V1= (0.6 m × 0.3 m)  23.6 kN/m3 = 4.25 kN/m DL = 35 kN/m (from Step 1) LL = 40 kN/m (from Step 1) Fq and F1 are included in this design example conserva- tively. Fq = Ka(rf)  q  H2 Fq = 0.26  9.4 kN/m2  0.6 m = 1.47 kN/m F1 = 1/2  Ka(rf)  γrf  H22 F1 = 1/2  (0.26)  20 kN/m3  (0.6 m)2 = 0.94 kN/m F2 = 1.75 kN/m (from Step 1) Check factor of safety against sliding: ΣVa = sum of vertical forces acting on the sill ΣVa = V1 + DL + LL ΣVa = 4.25 kN/m + 35 kN/m + 40 kN/m = 79.25 kN/m ΣFa = sum of horizontal forces acting on the sill ΣFa = Fq + F1 + F2 ΣFa = 1.47 kN/m + 0.94 kN/m + 1.75 kN/m = 4.16 kN/m FSsliding = (79.25 kN/m − 40 kN/m)  tan 36°/ 4.16 kN/m = 6.85 > 1.5 (OK) Check eccentricity requirement: ΣMOA = sum of overturning moments about point A ΣMOA = Fq  (H2/2) + F1  (H2/3) + F2  (t) ΣMOA = 1.47 kN/m  (0.6 m/2) + 0.94 kN/m  (0.6 m/3) + 1.75 kN/m  (0.3 m) = 1.15 kN/m  m ΣMRA = sum of resisting moments about point A ΣMRA = V1  (B/2) + (DL + LL)  (B/2) ΣMRA = 4.25 kN/m  (0.6 m/2) + (35 kN/m + 40 kN/m)  (0.6 m/2) = 23.78 kN/m  m e = eccentricity at the base of the sill e = (0.6 m/2) − (23.78 kN/m  m − 1.15 kN/m  m)/79.25 kN/m = 0.01 m B/6 = 0.6 m/6 = 0.10 m e < B/6 (OK) Check allowable bearing pressure of the reinforced fill: psill = applied pressure from the sill p Va B esill = − Σ 2 ' e B M M Va RA OA = − − 2 Σ Σ Σ FS Va LL Fasliding rf = −( )Σ Σ tan φ 123

124 psill = 79.25 kN/m/[0.6 m − (2  0.01 m)] = 136.64 kN/m2 psill = 136.64 kPa < qallow = 345 kPa (OK) Step 7: Check external stability of reinforced fill with the preliminary reinforcement length established in Step 5 The forces needed to evaluate the external stability of the abutment are shown in Figure 3-8. These forces are cal- culated as follows: V4 = (L × H1)  γrf V4 = (2.1 m × 2.4 m)  20 kN/m3 = 100.8 kN/m V5 = [(L − d − B) × H2]  γrf V5 = [(2.1 m − 0.3 m − 0.6 m) × 0.6 m]  20 kN/m3 = 14.4 kN/m Vq = (L − d − B)  q Vq = (2.1 m − 0.3 m − 0.6 m)  9.4 kN/m2 = 11.28 kN/m F3 = [Ka(re)  (q + γre  H2)]  H1 F3 = [0.33  (9.4 kN/m2 + 18 kN/m3  0.6 m)]  2.4 m = 16.0 kN/m F4 = 1/2  Ka(re)  γre  H12 F4 = 1/2  (0.33)  18 kN/m3  (2.4 m)2 = 17.11 kN/m ΣVa = 79.25 kN/m (from Step 6) ΣFa = 4.16 kN/m (from Step 6) I1 is the influence depth caused by the horizontal forces in the back wall (see Figure 3-9): I1 = (d + B − 2  e)  tan (45° + φrf/2) I1 = [0.3 m + 0.6 m − 2  (0.01 m)]  tan (45° + 36°/2) = 1.73 m Check factor of safety against sliding for the reinforced volume: ΣV = sum of vertical forces acting on the foundation soil 1 2 3 4 5 6 7 8 9 10 11 z 2 = 0 .6 m l 1 = 1 .7 3 m D = 2.08 m1 z 45 + φrf /2 2e' = 0.02 m B' = 0.58 m L = 2.40 m (Second Trial) d = 0.3 m Li = 0.66 m (at no. 7) Le = 1.69 mLa = 0.71 m (at no. 7) (at no. 7) Failure surface Influence line (2V:1H slope) Figure 3-9. Design Example 2 - Notations of the quantities for internal stability evaluation.

ΣV = V4 + V5 + Vq + ΣVa ΣV = 100.8 kN/m + 14.4 kN/m + 11.28 kN/m + 79.25 kN/m = 205.73 kN/m ΣF = sum of horizontal forces acting on the foundation soil ΣF = F3 + F4 + ΣFa ΣF = 16.0 kN/m + 17.11 kN/m + 4.16 kN/m = 37.27 kN/m FSsliding = (205.73 kN/m − 40 kN/m − 11.28 kN/m)  tan (30°)/37.27 kN/m = 2.39 > 1.5 (OK) Check eccentricity requirement for the reinforced volume: ΣMO = sum of overturning moments about point C ΣMO = F3  (H1/2) + F4  (H1/3) + ΣFa  (H1 − I1/3) ΣMO = 16.0 kN/m  (2.4 m/2) + 17.11 kN/m  (2.4 m/3) + 4.16 kN/m  [2.4 m − (1.73 m/3)] = 40.47 kN/m  m ΣMR = sum of resisting moments about point C ΣMR = V4  (L/2) + (V5 + Vq)  [(L − d − B)/2 + (d + B)] + (ΣMRA + ΣVa  d) ΣMR = 100.8 kN/m  (2.1 m/2) + (14.4 kN/m + 11.28 kN/m)  [(2.1 m − 0.3 m − 0.6 m)/2 + (0.3 m + 0.6 m)] + [23.78 kN/m  m + 79.25 kN/m  (0.3 m)] = 191.92 kN/m  m MS = moment about point C caused by traffic surcharge MS = Vq  [(L − d − B)/2 + (d + B)] MS = 11.28 kN/m  [(2.1 m − 0.3 m − 0.6 m)/2 + (0.3 m + 0.6 m)] = 16.92 kN/m  m e = eccentricity at the base of the reinforced volume e = (2.1 m/2) − [(191.92 kN/m  m − 16.92 kN/m  m) − 40.47 kN/m  m]/(205.73 kN/m − 11.28 kN/m) = 0.36 m L/6 = 2.1 m/6 = 0.35 m e > L/6 (NG) As a second trial, the length of the reinforcement was increased to 2.4 m to ensure the eccentricity require- ment. The eccentricity requirement of the reinforced e L M M M V Vq R S O = − −( ) − −2 Σ Σ Σ FS V LL Vq Fsliding fs = − −( )Σ Σ tan φ volume is checked again with the new reinforcement length of 2.4 m as follows: V4 = (L × H1)  γrf V4 = (2.4 m × 2.4 m)  20 kN/m3 = 115.2 kN/m V5 = [(L − d − B) × H2]  γrf V5 = [(2.4 m − 0.3 m − 0.6 m) × 0.6 m]  20 kN/m3 = 18.0 kN/m Vq = (L − d − B)  q Vq = (2.4 m − 0.3 m − 0.6 m)  9.4 kN/m2 = 14.10 kN/m ΣV = sum of vertical forces acting on the foundation soil ΣV = V4 + V5 + Vq + ΣVa ΣV = 115.2 kN/m + 18.0 kN/m + 14.10 kN/m + 79.25 kN/m = 226.55 kN/m ΣMR = sum of resisting moments about point C ΣMR = V4  (L/2) + (V5 + Vq)  [(L − d − B)/2 + (d + B)] + (ΣMRA + ΣVa  d) ΣMR = 115.2 kN/m  (2.4 m/2) + (18.0 kN/m + 14.10 kN/m)  [(2.4 m − 0.3 m − 0.6 m)/2 + (0.3 m + 0.6 m)] + [23.78 kN/m  m + 79.25 kN/m  (0.3 m)] = 238.76 kN/m  m MS = moment about point C caused by traffic sur- charge MS = Vq  [(L − d − B)/2 + (d + B)] MS = 14.10 kN/m  [(2.4 m − 0.3 m − 0.6 m)/2 + (0.3 m + 0.6 m)] = 23.27 kN/m  m e = eccentricity at the base of the reinforced volume e = (2.4 m/2) − [(238.76 kN/m  m − 23.27 kN/m  m) − 40.47 kN/m  m]/(226.55 kN/m − 14.10 kN/m) = 0.38 m L/6 = 2.4 m/6 = 0.4 m e < L/6 (OK) Check allowable bearing pressure of the foundation soil: Calculate the “influence length” D1 at the foundation level and compare with the effective reinforcement length, L. D1 = d + B + H1/2 = d + (B − 2e) + H1/2 D1 = 0.3 m + [0.6 m − 2  (0.01 m)] + 2.4 m/2 = 2.08 m e L M M M V Vq R S O = − −( ) − −2 Σ Σ Σ 125

126 L = L − 2e L = 2.4 m − 2  (0.38 m) = 1.64 m Because D1 at the foundation level is greater than L (L = L−2e), the contact pressure on the foundation level, pcontact, is calculated as follows: pcontact = 226.55 kN/m/[2.4 m − 2  (0.38 m)] = 138.14 kN/m2 qaf = 300 kN/m2 (from Step 2) pcontact = 138.14 kN/m2  qaf = 300 kN/m2 (OK) Step 8: Evaluate internal stability at each reinforcement level With geosynthetic reinforcement, the coefficient of lateral earth pressure is constant throughout the entire wall height, per Section 4.3b, NHI manual. The internal stability is evaluated by checking the rein- forcement pullout failure. Check reinforcement pullout failure: Pr = pullout resistance Pr = F*  α  (σv  Le)  C  Rc F* = pullout resistance factor F* = 2/3  tan φrf F* = 2/3  tan(36°) = 0.48 α = a scale effect correction factor ranging from 0.6 to 1.0 for geosynthetic reinforcement; for geotextile, α is defaulted to 0.6, per Section 3.3b, NHI manual. (σv  Le) = normal force at the soil-reinforcement inter- face at depth z (excluding traffic surcharge) (σv  Le) = (σvs  Le) + (Δσv  Li) Le = length of embedment in the resistant zone behind the failure surface at depth z Le = L − La La = length of embedment in the active zone at depth z La = (H1 − z) tan (45° − φrf/2) Li = length of embedment within the influence area inside the resistant zone; this length can be measured directly from the design drawing. p V L econtact = − Σ 2 C = reinforcement effective unit perimeter; C = 2 for strips, grids, and sheets Rc = coverage ratio; Rc = 1.0 for 100 percent coverage of reinforcement σh = horizontal pressure at depth z σh = Ka(rf)  (σvs + Δσv + q) + Δσh σvs = vertical soil pressure at depth z σvs = (γrf  H2) + (γrf  z) Δσv = distributed vertical pressure from sill Δσv = ΣVa / D D = effective width of applied load at depth z For z ≤ z2: D = (B − 2e) + z For z > z2: D = d + (B − 2e) + z/2 z2 = 2  d Δσh = supplement horizontal pressure at depth z For z ≤ I1: Δσh = 2  ΣFa  (I1 − z)/(I12) For z > I1: Δσh = 0 Tmax = maximum tensile force in the reinforcement at depth z Tmax = σh  s s = vertical reinforcement spacing FSpullout = factor of safety against reinforcement pullout FSpullout = Pr / Tmax Let depth z be measured from the top of the load-bearing wall. Reinforcement no. 7 at z = 1.0 m (see Figure 3-9) would serve as an example for determining the FSpullout. σvs = (γrf  H2) + (γrf  z) σvs = (20 kN/m3  0.6 m) + (20 kN/m3  1.0 m) = 32.0 kN/m2 z2 = 2  d = 2  (0.3 m) = 0.6 m  z = 1.0 m > z2 = 0.6 m, D = d + (B − 2e) + z/2 D = 0.3 m + [0.6 m − 2  (0.01 m)] + 1.0 m/2 = 1.38 m Δσv = ΣVa / D Δσv = 79.25 kN/m/ 1.38 m = 57.43 kN/m2 z = 1.0 m  I1 = 1.73 m,  Δσh = 2  ΣFa  (I1 − z)/(I12) Δσh = 2  (4.16 kN/m)  (1.73 m − 1.0 m)/(1.73 m)2 = 2.03 kN/m2 σh = Ka(rf)  (σvs + Δσv + q) + Δσh

No. Depth s vs D v h h Tmax La Le Li ( v ⋅ Le) Pr FSpullout z (m) (m) (kN/m2) (m) (kN/m2) (kN/m2) (kN/m2) (kN/m) (m) (m) (m) (kN/m) (kN/m) 1 2.2 0.2 56.0 1.97 40.21 0.00 27.42 5.48 0.10 2.30 1.87 203.84 118.48 21.61 2 2.0 0.2 52.0 1.87 42.36 0.00 26.94 5.39 0.20 2.20 1.67 184.82 107.42 19.94 3 1.8 0.2 48.0 1.77 44.75 0.00 26.52 5.30 0.31 2.09 1.47 166.09 96.54 18.20 4 1.6 0.2 44.0 1.67 47.43 0.31 26.49 5.30 0.41 1.99 1.26 147.58 85.78 16.19 5 1.4 0.2 40.0 1.57 50.45 0.88 26.80 5.36 0.51 1.89 1.06 129.16 75.07 14.01 6 1.2 0.2 36.0 1.47 53.88 1.45 27.22 5.44 0.61 1.79 0.86 110.70 64.34 11.82 7 1.0 0.2 32.0 1.37 57.81 2.01 27.77 5.55 0.71 1.69 0.66 91.99 53.47 9.63 8 0.8 0.2 28.0 1.27 62.35 2.58 28.48 5.70 0.82 1.58 0.46 72.79 42.31 7.43 9 0.6 0.2 24.0 1.17 67.68 3.15 29.39 5.88 0.92 1.48 0.25 52.77 30.67 5.22 10 0.4 0.2 20.0 0.97 81.62 3.72 32.54 6.51 1.02 1.38 0.05 31.85 18.52 2.84 11 0.2 0.2 16.0 0.77 102.79 4.29 37.57 7.51 1.12 1.28 0.00 20.46 11.89 1.58 σ Δσ Δσ σ σ TABLE 3-3 Design Example 2—tabulated results of FSpullout at each reinforcement level σh = 0.26  (32.0 kN/m2 + 57.43 kN/m2 + 9.4 kN/m2) + 2.03 kN/m2 = 27.73 kN/m2 Tmax = σh  s Tmax = 27.73 kN/m2  (0.2 m) = 5.55 kN/m La = (H1 − z) tan (45° − φrf/2) La = (2.4 m − 1.0 m) tan (45° − 36°/2) = 0.71 m Le = L − La Le = 2.4 m − 0.71 m = 1.69 m Calculate the normal force at z = 1.0 m: (σv  Le) = (σvs  Le) + (Δσv  Li) (σv  Le) = (32.0 kN/m2  1.69 m) + (57.43 kN/m2  0.66 m) = 91.98 kN/m Pr = F*  α  (σv  Le)  C  Rc Pr = (0.48)  (0.6)  91.98 kN/m  (2)  (1) = 52.98 kN/m FSpullout = Pr / Tmax FSpullout = 52.98 kN/m/ 5.55 kN/m = 9.55 FSpullout = 9.55 > 1.5 (OK) The values of FSpullout for all the reinforcements in the load-bearing wall are summarized in Table 3-3. Step 9: Determine the required reinforcement stiffness and strength The minimum “working” reinforcement stiffness is deter- mined as T@=1.0 percent ≥ σh(max)  s The minimum ultimate reinforcement strength is deter- mined as Tult ≥ Fs  T@=1.0 percent The recommended combined safety factor is Fs = 5.5 for reinforcement ≤ 0.2 m, and Fs = 3.5 for reinforcement spacing of 0.4 m. From Table 3-3, σh(max) is 37.57 kN/m2, which occurs at reinforcement no. 11 with depth z = 0.2 m and s = 0.2 m. T@=1.0 percent = σh(max)  s T@=1.0 percent = 37.57 kN/m2  (0.2 m) = 7.51 kN/m The uniform reinforcement spacing is 0.2 m, hence Fs = 5.5. Tult = Fs  T@=1.0 percent Tult = 5.5  (7.51 kN/m) = 41.31 kN/m A reinforcement with minimum “working” stiffness, T@=1.0 percent = 7.5 kN/m and minimum ultimate strength (per ASTM D4595), Tult = 41.3 kN/m is required. Step 10: Design of back/upper wall Reinforced fill: Same as that of the load-bearing wall Reinforcement: Same as that of the load-bearing wall Reinforcement length: 1.3 m (without an approach slab, the back wall reinforcement length is to be flush with the rein- forcement in the load-bearing wall) Reinforcement layout: Vertical spacing = 0.2 m 127

128 Step 11: Check angular distortion between abutments δabutment = abutment settlement δabutment = 1.5 percent  H1 δabutment = 0.015  (2.4 m) = 0.036 m δfoundation = foundation settlement (as determined by con- ventional settlement computation methods) Egδfoundation = 0.01 m δtotal = total settlement δtotal = δabutment + δfoundation δtotal = 0.036 m + 0.01 m = 0.046 m Angular distortion = δtotal / span length = 0.046 m / 10 m = 0.0046 Tolerable angular distortion for simple span = 0.005 Angular distortion = 0.0046 < 0.005 (OK) Design summary: The configuration for the trial design is shown in Figure 3-10. Abutment configuration: Load-bearing wall height, H1 2.4 m Back wall height, H2 0.6 m Facing modular concrete blocks (200 mm x 200 mm x 400 mm) Front batter 1/35 Sill type isolated sill Sill width 0.6 m Sill clear distance 0.3 m Embedment 200 mm (one facing block height) Reinforcement: Minimum stiffness at  =1.0 percent, T@=1.0 percent = 7.5 kN/m Minimum ultimate strength, Tult = 41.3 kN/m Length in load-bearing wall = 2.4 m Vertical spacing in load-bearing wall = 0.2 m Length in back wall = 1.3 m Vertical spacing in back wall = 0.2 m Figure 3-10. Design Example 2—configuration of the trial design. 1 35 Front batter 0.6 m0.3 m 0. 3 m 2. 4 m 0.2 m (Embedment) 2.40 m (L oa d-b ea rin g w all ) Sill 0.2 m Reinforcement: T@ ε = 1% = 7.5 kN/m = 41.3 kN/mT ult 0. 6 m 1.3 m 0.2 m

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Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Report 556: Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing, presents the findings of research undertaken to develop a rational design method and construction guidelines for using geosynthetic-reinforced soil (GRS) systems in bridge abutments. The report includes two appendixes. A third appendix, "Verification of the Analytical Model, " is available as NCHRP Web-Only Document 81.

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