Seismic Stability of Soil Retaining Walls Situated on Slope

Many soil retaining walls, which were used to stabilize highway embankments constructed on hillside, were severely damaged during the major earthquake (Chi-Chi earthquake, ML = 7.3) on September 21, 1999 in Taiwan. We investigated two typical cases of soil retaining wall damage using survey, soil borings and soil tests. To this end we developed a new pseudo-static method to evaluate the seismic stability of retaining walls situated on slope. Sliding failure along the wall base and bearing capacity failure in the foundation slope were considered in the new pseudo-static method. Results of the analysis showed that seismic stability of the wall against bearing capacity failure may be greatly overestimated when the inertia of soil mass is not taken into account. The analytical results also showed that sliding failure along the wall base occurs prior to the bearing capacity failure of the wall situated on a gentle slope at Site 1. The opposite is true for the wall situated on a steep slope at Site 2. For soil retaining walls constructed on slope, sliding failure of the wall may occur under small input horizontal ground acceleration when the passive resistance in front of the wall is not effectively mobilized. This highlights the importance of improving the strength of backfilled soils in the passive zone when constructing soil retaining walls on slope. The results obtained in the present study also suggest a modification of the current design considerations for soil retaining walls situated on slope.