Stress History Effects on Dynamic Modulus of Clay

Remolded kaolinite and bentonite triaxial specimens were subjected to isotropic consolidation pressures of 10 psi to 100 psi and overconsolidation ratios of 1 through 10. Steady-state torsional vibrations with amplitudes of 0.00015, 0.0003, and 0.0006 radians were applied to the top of each specimen, and the dynamic shear modulus was determined by resonant-column techniques. Shear moduli decreased less than 5% with increasing amplitude for the range of amplitudes used. Stress history effects were much more significant. For both soils, the shear modulus increased with decreasing void ratio and increasing effective pressure. For normally consolidated conditions, the shear modulus increased approximately with the 1/2 and 2/3 power of effective pressure for kaolinite and bentonite, respectively. For overconsolidated kaolinite, regression analysis indicated the shear modulus depends on both void ratio and effective pressure. For overconsolidated bentonite, the shear modulus is predominantly influenced by void ratio. For each soil, the shear modulus for both normal and overconsolidation states can be predicted from one equation.