Site-Specific Dynamic Behavior of Cohesive Soils-A Case Study from Northeastern India

Seismic prone regions undergoing major infrastructural progress require site-specific or region-specific dynamic soil characteristics for efficient earthquake resistant design of structures. Dynamic soil characteristics are traditionally represented using the strain-dependent dynamic properties (low strain shear modulus-Gmax; modulus reduction variation-G/Gmax; and damping ratio-D variation with shear strain-γ) over wide strain range. This study investigates the dynamic response of two soft cohesive soils sampled from the northeastern part of India, a highly active seismic region. Three advanced element testing techniques (bender element, torsional resonant column, and dynamic simple shear) have been adopted to assess the cyclic response at various testing conditions, including confining pressure, cyclic strain amplitude, loading cycles. The experimental data obtained from independent testing are combined together to have a comprehensive soil behavior over a wide range of shear strains (0.0001% to 5%). The experimentally obtained data have also been compared with traditionally adopted curves and found that a significant variation is noted in the dynamic response of tested soils. It is concluded from this study that mere utilization of empirical data is not feasible for dynamic analysis in the case of soft cohesive soils.