The current design practice used by WSDOT for the design of permanent tieback walls is to assume that the static design of a tieback wall retaining clayey soils provides an adequate reserve of strength to prevent failure during seismic loading. This design procedure is based largely on the assumption that the soil and the wall move together during ground shaking and that significant dynamic loads are not produced. For tieback walls retaining sandy soils, it is assumed that dynamic loads are produced. Mononobe-Okabe dynamic soil pressures are added to the static design pressure to account for the dynamic load. The validity of these assumptions and the resultant design practices is evaluated in this study.
A pilot numerical study was conducted on a forty foot high wall with three levels of tiebacks using the program FLUSH. It was found that the wall and the soil tend to move in-phase and only negligible dynamic tie forces are generated. However, the soil above and below the excavation level tends to move out-of-phase, leading to significant dynamic pressures and bending moments in the wall and near the excavation level. It appears that in at least some cases, tieback walls with an adequate static safety factor may suffer significant damage or fail during seismic loading and that the use of Mononobe-Okabe dynamic pressures may be conservative.