Effect of Relative Compaction on Dynamic Behaviour of Low Plasticity Cohesive Soil

Relative density is commonly used to quantify the looseness or denseness of the in situ soil. However, its determination is not possible in the case of cohesive soils. Hence, relative compaction is used as a governing parameter to represent the different states of natural in situ conditions in cohesive soils. Relative compaction is defined as the ratio of field dry density to the maximum dry density determined by the standard Proctor test. The amount and rate of strength and stiffness degradation of soil under dynamic loading depend largely on their compaction state. The aim of the current study is to evaluate the effect of relative compaction on the dynamic behaviour of low plasticity cohesive soil collected from Chobari Dam, Gujarat. A series of two-way displacement-controlled cyclic triaxial tests and 1D consolidation (oedometer) tests were performed on the specimens of Chobari Dam soil. The specimens were prepared at different relative compactions of 65, 70, 75, 80, 85, 90 and 100%. The hysteresis behaviour of Chobari Dam soil was evaluated based on the variation in shear modulus (G), damping ratio (D) and cyclic stiffness degradation index (δ). Shear modulus was found to increase with the increase in relative compaction, while cyclic stiffness degradation was observed to decrease as relative compaction decreased to 80%. The variation in cyclic stiffness degradation was observed to be insignificant for relative compaction below 80%. Rate of degradation of damping ratio was observed to be reduced with the decrease in relative compaction.