Influence of Reconsolidation Procedure on Small Strain Shear Modulus and Undrained Shear Behavior of Silts Subjected to Tube Sampling Disturbance
Don J. DeGroot
This thesis presents results of a laboratory testing program that studied the effects of laboratory simulated tube sampling disturbance on the undrained shear strength behavior of reconstituted low plasticity silts at overconsolidation ratios of 1.0 and 3.6. The three test soils consisted of two different mixtures of kaolin clay and silica silt and a reconstituted natural Dedham silt. Triaxial tests with bender elements were conducted on the reconstituted samples using the Ideal Sampling Approach (ISA) followed by post-ISA reconsolidation and undrained shear. The specimens were subjected to ± 1% and ± 3% ISA axial strain cycles. Changes in shear wave velocity and small strain shear modulus during simulation of tube sampling was used to develop a better understanding of the effects of sample disturbance on the undrained stress-strain-strength behavior of silts.
Laboratory simulated tube sampling disturbance changed the undrained shear behavior of the low plasticity silts from contractive to dilative. The reconstituted silt samples experienced significant loss in effective stress due to ISA disturbance which decreased with an increase in overconsolidation ratio and plasticity. The normalized undrained shear behavior of the tested reconstituted silt samples did not show any dependency on the consolidation stress level, however, an increase in the consolidation stress level increased the tendency for the low plasticity silts to exhibit dilative behavior. The effect of Recompression and SHANSEP consolidation procedures on recovering the undisturbed behavior was found to be dependent on the plasticity of the soil, to some extend on the pre-ISA consolidation stress, level of ISA disturbance experienced by the specimen and the overconsolidation ratio.
The measured shear wave velocity and small strain shear modulus at various stress states showed significant reduction during ISA disturbance which could be used as an indicator of sample disturbance in similar types of silts. However, after reconsolidating the specimens back to the initial effective stress state the shear wave velocity and small strain shear modulus values were mostly recovered indicating little to no influence of sample disturbance. The amount of reduction in shear wave velocity and small strain shear modulus was found to be dependent on plasticity of soil, level of disturbance and overconsolidation ratio.