Introduction

Seismic performance evaluation of soil-structure systems built on very soft high plasticity clays is a complex problem, especially when expected changes in effective stresses during the economic life of the structure due to dissipation of excess pore pressure caused by the building gravity loads and regional subsidence are very large. These changes in effective stresses lead, in turn, to large settlements. This is particularly important in urban areas located in highly compressible clays, such as Mexico City, where the settlement rate of regional subsidence reaches about 10 cm/year in average, but can go as large as 35 cm/year in some areas. Thus, it is common to have ground settlements ranging from 40 to 90 cm, due to load consolidation, and around several meters due to regional subsidence [1]. These settlements produce changes in both soil profile configuration (i.e. layer thickness and geometry), as well as dynamic properties, such as shear wave velocity distribution with depth and modulus degradation and damping curves. These factors impact the seismic response of the soil-structure system. The effect of dynamic properties changes on the seismic response of sites located in soft clay, has been only marginally studied by other researches [2–۵], finding that the variation of shear wave velocities and modulus degradation and damping curves with effective confining stresses can modify significantly the computed response. Nevertheless, the impact of these variations in the seismic performance of soilstructure systems has not been addressed, neither the effect of changes in the soil profile configuration after several meters of sinking. These effects however, can drastically modify both free field, near field and structural response over time. This paper presents a numerical study of the seismic response of a conventional five-story building supported by a partially compensated box foundation built in highly compressible soft clay, considering these effects. Three-dimensional finite difference models were developed with the software FLAC3D. Initially, the evolution of effective stresses with the pore pressure was established based on in-situ piezometer measurements, and laboratory data. Then, variations in dynamic properties were taken into account based on a series of resonant column tests conducted for several mean effective consolidation stresses and a suspension logging test. The static behavior of the soil-structure system was assessed. The free field model response was calibrated for moderate to strong level of shaking (i.e. return periods of 125 and 250 years, respectively) comparing the fully nonlinear analyses results with equivalent linear analyses carried out with the program SHAKE [6]. Finally, the seismic performance of the soilstructure system was studied to evaluate the impact of the changes on dynamic properties and layering configuration on the seismic response, considering an extreme subduction event associated to a 2475 years return period. Insight was gained regarding the complexity of the interplay of the effective stress history, and static and seismic soil-structure performance during an extreme earthquake.