Experimental modulus degradation and damping curves for sands under pure compressional loading

Soil nonlinearity during earthquake loading can be very important for site response analysis and earthquake hazard assessment. Soil nonlinearity, resulting from the elasto-plastic response of soil subjected to strong earthquake shaking, can be generally represented by strain-dependent modulus degradation and damping curves. While this effect has been quite extensively studied for the shear component of seismic waves, the effect of nonlinearity related to compressional waves is still poorly defined. In this study, curves representing the strain-dependent degradation of the constrained modulus (M) and the associated increased damping, are experimentally developed and presented. Cyclic triaxial tests under constrained lateral deformations were conducted on a sandy soil at different confining stresses. The results revealed that the experimentally measured modulus degradation and damping curves follow the same pattern and trend and have the same amplitude as those of experimental curves reported in literature for the shear modulus (G), as well as analytical curves reported for compressional modulus (M). The experimentally developed M degradation and D curves can be used in vertical site response analysis and be incorporated into seismic hazard analysis and ultimately improve resilience to vertical ground motions. The findings from the present study are limited to clean sand. Effects of fines content and plasticity may require modifications in the test procedure and are hence the topic for future studies.