Effects of Earthquake Motion and Overburden Thickness on Strain Behavior of Clay and Sandy Soil

Abstract eng:
Effects of earthquakes (EQs) are not limited only to the epicentral region. Depending on the magnitude of the EQ and the frequency content of input motion, the damages can be widespread. The amplitude, duration and frequency content of input motion at a site further changes due to the presence of in-situ soil at the site. Thus, similar to the determination of regional seismic hazard, quantification of local site effect is equally important. Dynamic soil properties which determine the behavior of local soil under EQ loading are not readily available on regional level. Hence, standard dynamic soil properties curves developed for other regions are used for a large number of studies. In the present work, response of two soil columns consisting of clay and sand alone throughout the depth, are analyzed using equivalent linear method using 30 worldwide recorded ground motions. Based on the present analysis, two important conclusions are drawn. It is well established fact that in equivalent linear analysis, the response of soil is governed by one value of strain. First conclusion suggests that understanding equivalent linear response of same soil collectively during various recorded ground motions covering a wide range of ground motion parameters, complete nonlinear soil behavior can be understood. In such case each ground motion will provide dynamic soil properties corresponding to specific level of strain. As a second conclusion, it is found that this strain developed in the soil during each ground motion is a function of peak horizontal acceleration (PHA) of input motion as well as the thickness of overburden. Further, this strain governs the soil behavior during that particular ground motion. Thus, if this value of strain is known, the response of the soil can be determined based on one value of shear modulus and damping ratio avoiding iterative procedure. Based on the above analyses, two empirical correlations are proposed in this work, correlating above value of strain in a soil layer with the PHA of input ground motion as well as overburden thickness above the soil layer. Overburden thickness is used in place of overburden pressure since it is used in available ground response models while determining the value of strains. Knowing the PHA from seismic hazard study and overburden thickness from soil investigation, one value of strain can be estimated based on proposed correlation. The behavior of soil will be governed by this value of strain alone and thus considering complete nonlinear soil behavior may not be needed. It has to be highlighted here that the present work uses two hypothetical soil columns of sand and clay respectively to provide a general idea about the above two conclusions. For case specific correlations however, similar works can be attempted in the future.

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