Regional subsidence effects on seismic soil-structure interaction in soft clay

Abstract

Regional subsidence effects on dynamic soil properties and ground layering deformation are often ignored in practice, when dealing with seismic soil-structure interaction analyses. Nevertheless, these effects can substantially change the frequency content and spectral accelerations in both free field and in the soil-structure system. Pore pressure variations over the project economic life are due to both regional subsidence as well as dissipation of excess pore pressure caused by the structure weight. These variations lead to changes in effective stresses, which in turn, modify the dynamic properties such as shear wave velocity distribution and modulus degradation and damping curves, as well as soil layer thickness and shape. These changes can be substantial in highly compressible very soft clay, such as that found in Mexico City valley. This paper presents a numerical study on the seismic response of a conventional five-story building supported by a compensated box foundation built in soft clay, considering these effects. Three-dimensional finite difference models were developed with the software FLAC3D. Initially, the evolution of effective stresses with pore pressure was established based on in-situ piezometer measurements of an instrumented site, and laboratory data. Then, changes in dynamic properties were taken into account based on the results gathered from series of resonant column tests conducted for several effective consolidation stresses, and a PS suspension logging test. The static behavior of the soil-structure system was assessed. For the cases studied herein, the complex interplay between soil nonlinearities, which lead to fundamental period elongation of the soil deposit, Tp, and the overall tendency of ground consolidation to shorten it, controls the variations in the spectral ordinates depending on how close Tp is of the predominant period of the excitation.