The results of a study on the seismic response of permanent tieback walls prepared for the Washington State Department of Transportation (WSDOT) are presented in this paper. The use of permanent walls in highway construction has expanded greatly during the past decade. The WSDOT has been a pioneer in permanent tieback wall construction, especially along interstates I-50 and I-90 in Western Washington. Because of the high seismicity of this area, it is necessary to evaluate the vulnerability of these walls to earthquake loading. It is current WSDOT design practice 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 is based on the assumption that the soil and the wall move together and significant dynamic loads are not produced. For tieback walls retaining sandy soils, Mononobe-Okabe dynamic soil pressures are added to the static design pressure. The validity of these design practices is evaluated in this paper. The results of a literature review clearly show that very little work has been done on the seismic response of tieback walls and no analysis or design procedures have been proposed. A pilot numerical study was undertaken for this research project. A 40-ft-high wall with three levels of tiebacks was analysed using the program flush. For this particular example problem it was found that the wall and the soil tend to move in-phase and that 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 earth pressures on, and bending moments in, the wall near the excavation level. Also, high vertical accelerations are predicted, even though only horizontal accelerations are used as input. The vertical accelerations appear to be caused by rocking of the soil-wall system and lead to high bearing pressures below the wall.
Abstract