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

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

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100 TABLE 3-1 Recommended allowable bearing pressures of a GRS abutment, with an integrated sill (sill width = 1.5 m), on a competent foundation Design Friction Angle of Fill1,2 = 34 = 35 = 36 = 37 = 38 = 39 = 40 Reinforcement Spacing 180 kPa 190 kPa 200 kPa 220 kPa 235 kPa 255 kPa 280 kPa = 0.2 m (8 in.) (26 psi) (27.5 psi) (29 psi) (32 psi) (34 psi) (37 psi) (40.5 psi) Reinforcement Spacing 125 kPa 140 kPa 155 kPa 175 kPa 195 kPa 215 kPa 240 kPa = 0.4 m (16 in.) (18 psi) (20 psi) (22.5 psi) (25 psi) (28 psi) (31 psi) (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. (1) Use Table 3-1 to determine the allowable bearing correction factor. A minimum sill width of 0.6 m is pressure under the following condition: (a) an recommended. "integrated sill" configuration, (b) sill width = (3) If an "isolated sill" is used, a reduction factor of 1.5 m, (c) a sufficiently strong reinforcement, and 0.75 should be applied to the corrected bearing (d) a competent foundation. pressure determined in Step (2). "Isolated sill" (2) Use Figure 3-1 to determine a correction factor for refers to an isolated footing separated from the the selected sill width. The allowable bearing pres- upper wall of the abutment; whereas an "integrated sure for the selected sill width is equal to the allow- sill" refers to a sill integrated with the upper wall able pressure determined in Step (1) multiplied by the as an integrated structure. Correction Factor vs. Sill Width 2.5 2 Correction Factor 1.5 1 0.5 0 0.6 1 1.5 2 2.5 3 3.5 4 4.5 Sill Width (m) Figure 3-1. Relationship between sill width and the correction factor.

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

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

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

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104 The minimum required reinforcement stiffness in the other words, the reinforcement must have a minimum direction perpendicular to the wall face, T@ =1.0 percent, "working" stiffness of at least 11.3 kN/m (or 65 lb/in.). should be determined by In addition, the minimum ultimate tensile strength, Tult Fs * T@ =1.0 percent = 5.5 (11.3) = 62.1 kN/m (or 357 T@ =1.0 percent h (max) * s lb/in.). 10. General Revision: If the heights of the load-bearing where h (max) is the maximum lateral stress in the rein- walls at the two ends of a bridge differ significantly, forced fill and s is the vertical reinforcement spacing. the angular distortion between the abutments may For non-uniform reinforcement spacing, s = (1/2 dis- exceed 0.005 (a limiting value recommended by the tance to reinforcement layer above) + (1/2 distance to NHI manual for a single-span bridge); therefore, it is reinforcement layer below). The lateral stress in the a good practice to preload or even prestress the load- reinforced fill, h, can be calculated as h = Ka (Z bearing abutment walls. The proper magnitude of pre- + v) + h, as suggested by the NHI Manual. loading or prestressing and the reduction in differential The minimum value of the ultimate reinforcement settlement caused by preloading of a reinforced soil strength in the direction perpendicular to the abutment mass may be evaluated by a procedure recommended wall face, Tult, should be determined by imposing a by Ketchart and Wu (2001 and 2002). Preloading typ- combined safety factor on T@ =1.0 percent to ensure satis- ically reduces the vertical deformation of a reinforced factory long-term performance, to ensure sufficient soil mass by twofold to sixfold, depending on the field ductility of the abutment, and to account for various placement density, and the lateral deformation by uncertainties, i.e., about threefold, as evidenced by limited case histories (see Chapter 2). Tult Fs * T@ =1.0 percent Basis for the Revision: The benefits to be gained The recommended combined safety factor is Fs = 5.5 by preloading and/or prestressing a GRS bridge- for reinforcement spacing 0.2 m, and Fs = 3.5 for supporting structure have been demonstrated in in- reinforcement spacing of 0.4 m. The combined safety service bridge abutments (e.g., Black Hawk bridge factor only applies to the backfill material and place- abutment), in full-scale experiments of bridge sup- ment conditions specified in Recommended Construc- porting structures (e.g., FHWA Turner-Fairbank bridge tion Guidelines in this chapter. pier), and in GRS abutment walls constructed by the Japan Railway (e.g., Tatsuoka et al., 1997; Uchimura Basis of the Refinement/Revision: The maximum et al., 1998). Extensive research on the subject has reinforcement strains measured in GRS walls, piers, been conducted by Tatsuoka et al. (1997) and Ketchart and abutments under service loads typically are on the and Wu (2001). order of 0.1 percent to 2.0 percent; however, the ulti- mate strength of geosynthetic reinforcements typically 11. General Refinement: The NHI manual does not occurs at a strain over 10 percent. Geosynthetic rein- address the design of the back wall (the upper wall). forcements of a similar strength can have rather differ- The back wall should be designed in a similar manner ent load-deformation relationships. In design, it will be as the load-bearing wall. In most cases, the same fill, prudent to specify the resistance required at the work- same reinforcement, and same fill placement condi- ing load to ensure satisfactory performance under the tions as those of the load-bearing wall should be used, in-service condition. In addition, a minimum value of although the default reinforcement spacing in the the ultimate reinforcement strength is also needed to approach fill can be increased somewhat (e.g., from ensure adequate ductility and satisfactory long-term 0.2 m to 0.3 or 0.4 m). The length of all the layers of performance and to account for uncertainties. The rec- reinforcement (at least in the top three layers, if there ommended combined safety factors are derived from is a significant space constraint) should be about the cumulative long-term reduction factors for GRS 1.5 m beyond the end of the approach slab to produce mass (see Wu, 2001) in conjunction with an overall a "smoother" surface subsidence profile over the uncertainty factor of 2.5. entire design life of the abutment. As an example, for a 10-m-high abutment (a 7.5-m- Basis for the Refinement: Experiences from actual high lower wall plus a 2.5-m-high upper wall) with construction of GRS walls and bridge-supporting = 34, the maximum vertical stress is about 200 kPa, structures. and the maximum lateral stress, h (max) = 56.6 kPa. For reinforcement spacing of 0.2 m, the minimum required 12. General Refinement: If there is no significant space tensile stiffness at 1 percent strain, T@ =1.0 percent h (max) constraint, it is recommended that the reinforcement * s = 56.6 kPa * 0.2 m = 11.3 kN/m (or 65 lb/in.). In length of the top three layers (all the layers, if there is

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

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106 Figure 3-3. Details of reinforcement layout near the top of the load-bearing wall. Step 1: Establish abutment geometry and external the abutment; whereas "integrated sill" refers to a loads and trial design parameters sill integrated with the upper wall as an integrated structure). Establish abutment geometry and loads (see Figure 3-4): Facing type (dry-stacked concrete modular blocks, Total abutment height, H (the sum of lower wall timber, natural rocks, wrapped geosynthetics, or height and upper wall height) gabions) and facing block size (for concrete modular Load-bearing wall (lower wall) height, H1, as mea- block facing). sured from the base of the embedment to the top of Batter of facing (a minimum front batter of 1/35 the load-bearing wall to 1/40 is recommended for segmental wall facing Back wall (upper wall) height, H2 to provide improved appearance and greater flexi- Traffic surcharge, q bility in construction. A typical minimum setback Bridge vertical dead load, DL of 5 to 6 mm between successive courses of facing Bridge vertical live load, LL blocks is recommended for blocks with height = Bridge horizontal load 200 mm). Bridge span and type (simple or continuous span) Reinforcement spacing (the default value for reinforce- Length of approach slab ment spacing is 0.2 m). For wrapped-faced geotextile The embedment of a GRS abutment wall need only be walls, temporary walls, or walls where facing may be a nominal depth (e.g., one block height). If the founda- added in the future, a reinforcement spacing of 0.15 m tion contains frost-susceptible soils, they should be is recommended. Reinforcement spacing greater than excavated to at least the maximum frost penetration line 0.4 m is not recommended under any circumstances. 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. Step 2: Establish soil properties Establish trial design parameters: Check to ensure that the selected fill satisfies the fol- Sill width, B (a minimum sill width of 0.6 m is lowing criteria: 100 percent passing 100 mm (4 in.) recommended). sieve, 0-60 percent passing No. 40 (0.425 mm) sieve, Clear distance between the back face of the facing and 0-15 percent passing No. 200 (0.075 mm) sieve; and and the front edge of the sill, d (the recommended plasticity index (PI) 6. clear distance is 0.3 m). Sill type (integrated sill or isolated sill). "Isolated Establish reinforced fill parameters: sill" refers to a sill separated from the upper wall of Wet unit weight of the reinforced fill.

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107 Vq Traffic Surcharge: q = 9.4 kN/m2 LL V3 H2 = 2.20 m Retained Earth (Back Wall) DL Fq Soil Unit Wt. = 18.8 kN/m3 re = 30 F2 F1 V5 V2 K a(re) = 0.33 V1 H' = Total Abutment Height = 9.70 m d B z Reinforced Fill Soil Unit Wt. = 18.8 kN/m3 (Load-Bearing Wall) rf = 34 H1 = 7.50 m K a(rf) = 0.28 F3 V4 F4 C L - 2e 2e Foundation Soil L = 7.00 m Figure 3-4. Design Example 1--configuration of the abutment. The "design friction angle" of the reinforced fill, Wet unit weight of the retained earth design, is taken as 1 degree lower than the friction Coefficient of active earth pressure of the retained angle obtained from tests, design = test - 1, where earth test is determined by one set of the standard direct shear tests on portion finer than 2 mm (No. 10) Establish foundation soil parameters: sieve, using a sample compacted to 95 percent of Friction angle of the foundation soil AASHTO T-99, Methods C or D, at the optimum Wet unit weight of the foundation soil moisture content. Allowable bearing pressure of the foundation soil, qaf If multiple direct shear tests are performed, the smallest friction angle should be used in design. Step 3: Establish design requirements For instance, if two sets of tests are performed-- both showing a friction angle of 35--the "design Establish external stability design requirements: friction angle" will be 35. On the other hand, if a Factor of safety against reinforced fill base sliding 1.5 single set of tests shows that a soil has a friction Eccentricity L/6 (L = length of reinforcement at angle of 35, then the "design friction angle" will base of the reinforced zone) be taken as 34. Average sill pressure allowable bearing pressure of the reinforced fill, qallow Establish retained earth parameters: Average contact pressure at the foundation level Friction angle of the retained earth allowable bearing pressure of the foundation soil, qaf

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

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

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110 Influence line (2V:1H slope) d = 0.3 m 2e' = 0.22 m Failure surface B' = 1.28 m 0.6 m 37 z2 36 35 34 l1 = 2.97 m z 33 32 31 30 29 La = 2.66 m (at no. 25) Le = 4.34 m (at no. 25) 28 27 26 25 Li = 0.17 m (at no. 25) 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 45 + rf /2 3 2 1 D1 = 5.33 m Figure 3-6. Design Example 1 - Notations of the quantities for internal stability evaluation. throughout the entire wall height. The internal stability defined as the tensile resistance at the working strain. It is evaluated by checking the factor of safety against rein- is recommended that a tensile strain of 1.0 percent be forcement pullout failure at each reinforcement level. taken as the reference working strain, and the resistance The factor of safety against pullout failure, FSpullout, at at strain = 1.0 percent be used for specification of the any given reinforcement level, is equal to pullout resis- required reinforcement stiffness. tance at the reinforcement level divided by Tmax. Tmax is the The minimum required reinforcement stiffness in the maximum reinforcement tensile force at the reinforce- direction perpendicular to the wall face, T@=1.0 percent, is to ment level where the pullout safety factor is being evalu- be determined as ated. Tmax is calculated as the product of the average active lateral earth pressure at the reinforcement level multiplied T@=1.0 percent h(max) s by the reinforcement vertical spacing. The pullout resis- tance, on the other hand, arises from the frictional resis- where h(max) is the maximum lateral stress in the rein- tance at soil-reinforcement interface along the portion of forced fill, and s is the vertical reinforcement spacing. For reinforcement lies beyond the potential failure plane. The non-uniform reinforcement spacing, s = (1/2 distance to potential failure plane is taken as the active Rankine fail- reinforcement layer above) + (1/2 distance to reinforce- ure surface with a uniform vertical surcharge. FSpullout at ment layer below). all reinforcement levels should be 1.5. 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 Step 9: Determine the required reinforcement combined safety factor on T@=1.0 percent as stiffness and strength Tult Fs T@ =1.0 percent Both a minimum value of ultimate tensile stiffness and a minimum value of tensile strength of the geosynthetic The combined safety factor is applied to ensure satis- reinforcement should be specified to ensure sufficient ten- factory long-term performance, to provide sufficient sile resistance at the service load and a sufficient safety ductility of the abutment, and to account for various margin against rupture failure. The tensile stiffness is uncertainties. The recommended combined safety factor