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22 failure surface and is used in conjunction with maximum 3.2 Slopes and Embankments ground accelerations. Other analytical approaches search for a critical active pressure zone defined by a bi-linear The dominant theme in the literature on the topic of eval- failure surface. uating the seismic stability or performance of slopes and · External stability is addressed in most guidelines by assum- embankments was the use of either pseudo-static or the New- ing the M-O method for determining the earthquake- mark sliding block methods of analysis. Whereas dynamic induced active earth pressures in the fill behind the rein- response analyses (particularly of large earth structures forced soil mass. To evaluate the potential for sliding, the such as dams) using computer programs such as FLAC were AASHTO LRFD Bridge Design Specifications assume only finding increasing use, for routine seismic design of slopes 50 percent of the earthquake active pressure acts in con- and embankments related to highways, the pseudo-static junction with the reinforced soil mass inertial load on the method has found wide acceptance, while the use of New- assumption that the two components would not be in phase, mark sliding block deformation method was gaining favor, which is questionable and requires further evaluation. In particularly where pseudo-static methods resulted in low addition, the limitations and problems with the use of the factors of safety. Often results of the deformation analysis M-O equations for external stability assessments are simi- indicated that the amount of deformation for a slope or lar to those previously described for conventional semi- embankment was tolerable, say less than 1 to 2 feet, even gravity retaining walls, and along with performance criteria when the factor of safety from the pseudo-static analysis is based on allowable wall displacements, can be addressed in less than 1.0. a similar manner to approaches described for semi-gravity walls. 3.2.1 Seismic Considerations for Soil Slopes As discussed in the next chapter, studies related to wall A number of considerations relative to the seismic analysis height/stiffness and ground motion dependent seismic of slopes and embankments are summarized below. coefficients for design, along with improved approaches for evaluation of internal and external seismic stability, are · As described in both the MCEER (2006) Seismic Retro- clearly needed. fitting Manual for Highway Structures and the SCEC (2002) Guidelines for Analyzing and Mitigating Landslide Hazards in California, recommended practice for the analysis of seismic 3.1.3 Soil Nail Walls slope or embankment performance is a displacement-based Soil nail walls act in a similar manner to MSE walls, but analysis using a Newmark sliding block approach. This are typically a ground reinforcement technique used for cut approach also was adopted by the NCHRP 12-49 Project for slopes as opposed to fill slopes in the case of MSE walls. As evaluating liquefaction-induced lateral spread displacement described in an FHWA Geotechnical Engineering Circular at bridge approach fills or slopes. No. 7 Soil Nail Walls (FHWA, 2003), soil nail walls have per- · Newmark displacements provide an index of probable seis- formed remarkably well during strong earthquakes, with no mic slope performance. As a general guideline, a Newmark sign of distress or permanent deflection. displacement of less than 4 inches often is considered to rep- Choukeir et al. (1997) note a seismic design methodology resent a "stable" slope, whereas more than 12 inches is con- similar to that previously described for MSE walls. Caltrans sidered unstable from a serviceability standpoint. Several have developed a computer program SNAIL for the design design charts correlating Newmark displacement with the of soil nail walls based on a limit equilibrium approach ratio of yield acceleration (defined as the acceleration using a two-wedge or bilinear failure surface for both inter- required to bring the factor of safety 1.0) to the peak acceler- nal and external stability considerations, including the spec- ation exist. The approach identified in Chapter 4 involved ification of horizontal and vertical seismic coefficients. The review of the existing data for the purpose of developing computer program GOLDNAIL also is widely used in prac- improved design charts applicable to nationwide seismic tice during the design of soil nails. This software also can be hazard conditions--with different charts produced for WUS used to evaluate the performance of anchored walls by versus CEUS sites. replacing the nail with a tendon having a specified strength · As previously discussed for retaining wall design, studies and pullout capacity described in the literature suggest that displacement-based As the design issues for MSE and soil nail walls are gener- analyses are very sensitive to the frequency and amplitude ally similar, analysis methods for development were also characteristics of earthquake acceleration time histories somewhat similar, with potential applications of the SNAIL and to earthquake duration, together with the earthquake and GOLDNAIL programs also requiring review. response characteristics of higher walls, slopes, or embank-