Dynamic characterisation of soils and seismic analysis of deep foundations

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Site-specific or region-specific strain dependent dynamic soil properties and liquefaction parameters are the primary input parameters required for efficient earthquake resistant design of structures. The present study aims to investigate the dynamic behavior of soils sampled from active seismic regions of India and establish region-specific comprehensive strain dependent design stiffness curves for use in earthquake geotechnical applications. The study also was targeted to demonstrate the application of experimentally obtained dynamic soil properties in seismic ground response analysis studies and seismic analysis of pile foundations. Four soils (two cohesionless soils and two cohesive soils) from Assam state of northeastern India (most active seismic zone in the country) and two cohesionless soil samples from Haryana state (moderate to intense seismic region) of northern India have been collected. A series of laboratory tests have been conducted on the collected soil samples using four independent high quality laboratory testing techniques (bender element tests, resonant column tests, dynamic simple shear tests and cyclic triaxial tests) with varying test parameters. Results obtained from each testing technique are combined and analyzed for obtaining comprehensive dynamic soil properties over wide range of shear strains (0.0001% to 5%) and liquefaction parameters. Region-specific empirical formulations have been proposed based on the experimental results. Test results indicate that the chosen cohesionless soils are prone to severe seismic liquefaction (in their loose condition) even for mild to moderate cyclic loading. Tested cohesive soils indicate the probability of cyclic mobility phenomenon (large strain accumulation, ≥10%) for high intensity cyclic loading. Seismic ground response analyses have been performed employing the established strain dependent dynamic soil properties and liquefaction parameters for four typical sites (three from Assam and one from Haryana). The results from this study point out that the mere utilization of empirical/literature based dynamic soil properties in seismic ground response studies may often underestimate (30 to 40% as observed from the current study) the seismic demands on the structures. Amplification of seismic waves have been observed for low intensity bedrock motions while high intensity bedrock motions attenuated at the surface due to the increased damping at large strains.
Supervisor: A. Murali Krishna