Stress Integration Approach for Soil Damping Measurements in Torsional Testing

Determination of strain associated with shear modulus and damping from hysteresis loops generated on torque-rotation plane during torsional test is complicated. This is due to nonuniform stress-strain variation occurring linearly with radius in a soil specimen in torsion. A conventional equivalent radius approach proposed by Chen and Stokoe (1979) is adequate for evaluating strain associated with shear modulus at low to intermediate strain levels. The approach is less accurate for damping measurement, as well as measurements at high strain. Stress integration developed by Sasanakul and Bay (2008) is used to account for the nonuniform stress-strain effect more precisely. The approach involving integration of an assumed soil stress-strain model is used to generate a plot of equivalent radius ratio versus strains based on shear modulus and damping separately. This paper presents the development of equivalent radius ratio curves used for damping measurement. The stress integration approach is applied for hyperbolic, modified hyperbolic, and Ramberg-Osgood models. Variation of equivalent radius ratio with normalized rotation for each model is presented. The results illustrate the usefulness of the stress integration approach and suggest that using a single value of equivalent radius ratio to calculate strains is not appropriate.