Seismic Site Response Analysis Considering Partially Saturated Soil Conditions

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The seismic response of partially saturated soils differs from that of dry or water saturated soil deposits. Yet, the available site response analysis methods ignore the influence of partial saturation in the soil. This research will study the effects of partial saturation on the seismic site response analysis and how different suction (degree of saturation) profiles affect the seismic response of a soil layer. Suction due to partial saturation increases the effective stresses acting on the soil, altering the shear modulus, shear wave velocity, nonlinear deformation response, and damping. As a result, the distribution of suction with depth is expected to affect the propagation of seismic waves and the resulting ground accelerations. This, in turn, influences the seismic demand imposed on the soil and surface structures. Although the shear wave velocity employed in current site response analysis methods accounts for partial saturation of the soil, it only reflects the degree of saturation at the time of shear wave velocity measurement. Hence, the seasonal fluctuation of the water table and its impact on degree of saturation may alter the site response. The current state of practice commonly relies on procedures that include dynamic material properties of either water saturated or dry soils as the most conservative scenarios. This might be an appropriate assumption for problems dealing with soils' strength and deformation. However, stiffer partially saturated soils due to inter-particle suction forces result in a higher site natural frequency and lower damping, which may adversely affect the site response. In this study, the steady state-infiltration through an unsaturated soil layer inside a geotechnical centrifuge will be implemented to control the soil layer?s degree of saturation. Cyclic loads with different amplitudes and frequencies, and earthquake motions with different intensities will be applied to the soil layers with various suction profiles. These profiles are controlled by the infiltration rate and the centrifugal acceleration. The experimental data will be compared with the numerically estimated site responses incorporating effective stress-based, suction-dependent dynamic material properties. This research will advance the fundamental knowledge of unsaturated soil dynamics and seismic performance of geotechnical systems by incorporating the effects of seasonal fluctuation of degree of saturation on site response. The work will assist in better understanding of fundamental mechanism of wave propagation through an unsaturated soil layer. This project will be a substantial step towards more sustainable and safer seismic designs of buildings and infrastructure. The potential findings of the project will help in assessing the performance of current approaches in evaluating the seismic site response and provide practical recommendations to consider the partial saturation in current seismic site response analysis methods. The graduate student development will prepare students to lead the profession through advanced technical training in geotechnical earthquake engineering, water infiltration, and physical modeling, as well as mentoring opportunities. The importance of earthquake engineering and seismic hazards will be introduced to college Civil Engineering and K-12 students through undergraduate research, courses, and outreach programs.