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OCR for page 94
94 also relates to the angle of the nail. Most nails are angled at Results of the work completed for retaining walls includes 10 to 20 degrees to the horizontal in contrast to the horizon- charts showing the effects of cohesion within the soil on the tal orientation of the reinforcement within the MSE wall. This seismic earth pressure coefficients that were developed. These would likely stiffen the soil nail wall relative to the MSE wall, effects can result in a 50 percent reduction in the seismic active all other conditions being equal. From a design standpoint, it earth pressure; however, it may be difficult in some cases to also is not clear if seismic forces are adequately modeled by confidently rely on this benefit. In view of current uncertain- the pseudo-static approach currently taken. These issues need ties, the designer needs to consider the implications of over- to be further evaluated during independent research efforts. estimating the effects of cohesion on the seismic active and Many nail walls will be located in areas where there is a co- passive earth pressures. hesive content to the soil into which the nails are installed. Two wall types were considered in detail during this study: For these sites the effects of cohesion on the determination of (1) semi-gravity walls and (2) MSE walls. seismic earth pressure coefficients, as discussed in Section 7.3, should be considered. The proposed approach for gravity walls uses either the M-O seismic active earth pressure equation, the charts in Figures 7-11 and 7-12, or the generalized limit equilibrium 7.9.3.2 Seismic Design Methodology method to determine seismic active forces. These forces are Based on material presented in the previous paragraphs, used to conduct bearing, overturning, and sliding stability the recommended design methodology is summarized by the checks. A key question that still exists for this type of wall following steps: is whether inertial forces from the soil above the heel of a semi-rigid gravity wall (for example, Figure 7-10 in this 1. Establish an initial wall design using the computer pro- report) is defined by the entire soil mass times the seismic gram SNAIL or GOLDNAIL for static loading, using ap- coefficient or some lesser value. The MSE design methodology includes a critical review of propriate load and resistance factors. This establishes wall dimensions and weights. the existing AASHTO guidance, including internal stabil- 2. Establish the site peak ground acceleration coefficient (kmax) ity, and then identifies a step-by-step approach for evalu- and spectral acceleration S1 at 1 second from the 1,000-year ating stability. Reference is made to the need to change ex- maps adopted by AASHTO (including appropriate site soil isting software to handle this approach. Questions also still modification factors). exist on the distribution of stresses within the reinforce- ment strips during seismic loading. 3. Determine the corresponding PGV from correlation equations between S1 and PGV (provided in Chapter 5). Three other wall types were considered to lesser extents: 4. Modify kmax to account for wall height effects as described nongravity cantilever walls, anchored walls, and soil nail walls. in Section 7.6. Use the modified kmax in the SNAIL or The design approach for each of these walls also used the re- GOLDNAIL program. If the wall can tolerate displace- sults of work presented in previous sections and chapters. ments, use the SNAIL or GOLDNAIL program to estimate the yield acceleration, ky. Use the yield acceleration to esti- For nongravity cantilever walls, the M-O method is believed mate displacements following the procedures in Chapter 5. to be an appropriate method to determine seismic active pressures as long as there is flexibility in the wall and the Note that both computer programs also provide an evalu- soil behind the wall is primarily cohesionless. Otherwise, ation of global stability, and therefore, it is not necessary to charts in Figures 7-11 and 7-12 or a generalized limit equi- perform an independent global stability analysis with a limit librium method can be used to estimate the seismic active equilibrium program such as SLIDE. earth pressure. The seismic coefficient used for design can be reduced by a factor of 0.5 as long as 1 to 2 inches of 7.10 Conclusions average permanent deformation at the excavation level are acceptable. A structural engineer should make this evalua- This chapter summarizes the approach being recommended tion. Checks on wall deflections also should be made to for the seismic design of retaining walls. Force-based methods confirm that the basic assumptions associated with wall using the M-O equations and a more generalized displacement- displacement are being met. Seismic passive pressures based approach were evaluated. The methodologies intro- should be determined using a log spiral approach, such as duce new height-dependent seismic coefficients, as discussed suggested by Shamsabadi et al. (2007). in Chapters 5 and 6 and further refined in Section 7.5 for In the case of the anchored wall, either a limit equilibrium these analyses. procedure or a displacement based procedure suggested by

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95 Whitman can be used. Seismic active earth pressures for zone and whether the current models adequately account the limit equilibrium approach can be estimated using the for these distributions. Additional research is still required M-O equation, charts in Figures 7-11 and 7-12, or the gen- to evaluate these questions. eralized limit equilibrium approach. Soils must be homo- geneous and cohesionless if using the M-O equation while In a number of areas it was apparent that significant defi- the generalized limit equilibrium method can accept com- ciencies exist with current design methodologies. These de- binations of soil conditions. The seismic coefficient for ficiencies reflect the complexity of the overall soil-structure these analyses can be reduced by 50 percent as long as 1 to interaction problem that occurs during seismic loading. The 2 inches of average permanent movement are acceptable nature of these deficiencies is such that for several of the wall and as long as anchor tendons and grouted zones are not types (for example, MSE, anchored, and soil nail) independent overstressed. The Whitman displacement-based approach research efforts involving specific model and prototype testing accounts for changing anchor tendon forces during seismic will be required to fully understand the mechanisms involved loading and appears to represent the fundamental mecha- in seismic loading. nisms that occur during seismic response of this wall type. While there is considerable work to be done, past expe- However, the additional effort to make these evaluations rience also suggests that many of these wall types have per- may not be warranted in areas where seismicity is low, and formed well during relatively high seismic loading, despite the normal performance and proof testing of the anchors having either no provisions for seismic design or a very sim- provides sufficient reserve capacity. ple analysis. In most cases this good performance occurred Soil nail walls can be treated as semi-gravity walls from an when walls were flexible or exhibited considerable ductility. external stability standpoint. In most cases seismic coeffi- More problems were observed for rigid gravity walls and non- cients can be reduced by 0.5 since this type of wall can usu- gravity cantilever walls, often because of the lack of seismic ally tolerate several inches of permanent movement. For design for these walls. The methodologies suggested in this internal stability there are still questions on the distribu- chapter should help improve the seismic performance of tion of seismic forces to the nails within the reinforced these walls in the future.