INFLUENCE OF SEISMIC POREWATER PRESSURE INCREASE, SOIL SOFTENING, AND LIQUEFACTION ON SITE RESPONSE ANALYSIS AND BUILDING CODE DESIGN SPECTRA
Description
Seismic site response analysis is commonly used to predict ground response due to local soil effects. Advancements in the knowledge of shear-induced excess pore pressure generation has led to the development and implementation of pore pressure response models for site response analysis with effective stress consideration. This research chiefly regards the development of a new approach to calculate 1D site response analysis using nonlinear effective stress approach. The present research studies the impact of PWP buildup, soil softening and potential liquefaction on site response. The study is conducted in three stages: (1) Initially, a Porewater Pressure generation models are tested and the best performing are incorporated into an existing constitutive model to compute stress-strain response during shaking using effective stresses and solve dynamic one-dimensional (1 – D) site response problems. (2) A parametric study of influential variables involved in site response calculations is performed, and (3) An examination of history cases are used to know the behavior of nonlinear, effective stress site response analysis. In the first stage, this study focuses on the evaluation of the performance of simplified nonlinear effective stress constitutive models commonly used in one-dimensional (1D) site response analysis for assessment of porewater generation and liquefaction potential in soft soil deposits at the element level. Initially, it was evaluated the performance of several porewater pressure generation models using a database of 116 cyclic triaxial compression and cyclic simple shear tests. The two better-performing porewater pressure generation models are combined with an existing simplified hyperbolic constitutive model to develop a quasi-coupled model termed MRDF+u (modulus reduction and damping curve fit using a reduction factor and including porewater pressure generation, u). Then, it was evaluated this quasi-coupled constitutive model using select cyclic triaxial compression and cyclic direct simple shear tests and protocols for selecting PWP model input parameters are proposed using empirical correlations. Despite the inability of the coupled model to capture dilation, the new coupled model reasonably captures the cyclic behavior observed in the tests and may be used to perform effective stress-based 1D site response analysis.