A numerical study of the composite behavior of permanent and temporary shaft liners subjected to strong ground shaking

The seismic performance of a proposed 37-meter-diameter by 52-meter-deep permanent shaft installed within a “temporary” diaphragm wall-supported excavation for a soft-over-stiff soil siteis evaluated via 3D finite element analyses. To this end, results are presented from two sensitivity analyses performed as part of the Central Bayside System Improvement Project in San Francisco, California, USA. The variables considered include the stiffness of the diaphragm wall (to account for possible degradation over time) and the effect of tying (or not tying) the permanent structure to the diaphragm wall (e.g. to resist hydrostatic uplift). The findings indicate there can be significant effects from each of these variables on the demands in the permanent shaft, which are typically not apparent from simplified closed-form solutions. It is concluded that consideration of site-specific conditions is important to the evaluation of composite shaft behaviorand that ignoring the presence of the diaphragm wall is not necessarily conservative.