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93 For cases where M-O equations are not appropriate, such the FHWA guidance document to use the same amplifi- as for some combinations of a steep back slope and high site- cation factor used for MSE walls, that is, Am = (1.45 - A)A. adjusted PGA or if the soil behind the wall simply cannot be The basis of using this equation is not given, other than the represented by a homogeneous material, then the generalized FHWA report indicates that performance of the soil nail limit equilibrium methodology should be used to estimate wall is believed to be similar to an MSE wall. the seismic active earth pressure. This pressure can be either The seismic coefficient for design ranges from 0.5 Am to distributed consistent with a static pressure distribution and 0.67 Am. This reduction is based on tolerable slip of 1 to the wall checked for acceptability, or the deformation approach 8 inches with most slip of 2 to 4 inches. The possibility of recommended by Whitman (1990) can be used to evaluate the performing Newmark deformational analysis is noted for forces in the vertical structural members, anchor tendons, and certain soil conditions and high ground accelerations. grouted zone. The M-O equation is used to estimate the seismic active pressure acting on the wall. Reference is made to the angle of the failure plane for seismic loading being different than 7.9.3 Soil Nail Walls static loading. These walls are typically used where an existing slope must Mention is made of the limitations of the M-O procedure be cut to accommodate a roadway widening. The slope is re- for certain combinations of variables, in particular when inforced to create a gravity wall. These walls are constructed the backslope is steeper than 22 degrees and does not cap- from the top down. Each lift of excavation is typically 5 feet ture many of the complexities of the system. in thickness. Nails are installed within each lift. The spacing A detailed design example based on the recommended of the nails is usually about 4 to 5 feet center-to-center in both approach is presented. the vertical and horizontal direction. The nail used to reinforce the slope is high strength, threaded steel bar (60 to 75 ksi). The earlier FHWA report Geotechnical Earthquake Engi- Each bar is grouted in a hole drilled into the soil. The length neering (FHWA, 1998a) also provides some discussion on the of the bar will usually range from 0.7 to 1.0 times the final wall design of soil nail walls. It mentions use of (1) the amplifi- height. Most soil nail walls currently are designed using either cation factor, Am = (1.45 - A)A and (2) for external stability of two computer programs, SNAIL, developed and made avail- using 0.5 times the site-adjusted PGA, as long as the wall can able by Caltrans, and GOLDNAIL, developed and distributed tolerate 10 A (inches displacement) where A is the peak ground by Golder and Associates. These programs establish global and acceleration. This document also references using a seismic internal stability. design coefficient of 0.5A to check seismic bearing capacity stability. Limitations and assumptions for this approach are discussed in Appendix G. Seismic Design Considerations Procedures used to evaluate the external or global stability The seismic design of soil nail walls normally involves deter- of the soil nail wall during seismic loading will be the same mining the appropriate seismic coefficient and then using one as those described previously for evaluating the seismic per- of the two computer programs to check the seismic loading formance of semi-gravity walls and MSE walls. The uncer- case. The AASHTO LRFD Bridge Design Specifications currently tainty with this wall type deals with the internal stability. The does not have any provisions for the design of soil nail walls. computer programs currently used in practice, SNAIL and However, FHWA has a guidance document titled Soil Nail GOLDNAIL, use pseudo-static, limit equilibrium methods Walls (FHWA, 2003) used for soil nail wall design. This doc- to determine stresses in the nail. Checks can be performed to ument has a section on the seismic design of these walls. determine if pullout of the nail, tensile failure, or punching Key points from the seismic discussions are summarized failure at the wall face occur. For the seismic loading case, the below: increased inertial forces are accounted for in the analysis. Similar to the internal stability of MSE walls, the mechanisms Soil nail walls have performed very well during past earth- involved in transferring stresses from the soil to the nails and quakes (for example, 1989 Loma Prieta, 1995 Kobe, and vice versa are complex and not easily represented in a pseudo- 2001 Nisqually earthquakes). Ground accelerations during static, limit equilibrium model. these earthquakes were as high as 0.7g. The good perfor- In principle it would seem that some significant differences mance is attributed to the intrinsic flexibility. These obser- might occur between the seismic response of the soil nail wall vations also have been made for centrifuge tests on model versus the MSE wall. The primary difference is that MSE walls nail walls. are constructed from engineered fill whose properties are well Both horizontal and vertical seismic coefficient can be defined, whereas nail walls are constructed in natural soils used in software such as SNAIL. A suggestion is made in characterized by variable properties. Part of this difference