Shear strength and compressibility characteristics of reconstituted clays

Abstract

Collecting undisturbed soil sample is essential to determine the engineering properties like shear strength and compressibility of cohesive soil. However, it is always challenging to collect soil samples in undisturbed condition from a significant depth below the existing ground surface. Reconstituted samples can be prepared using disturbed samples collected from the similar depth and could provide an insight on the shear strength and compressibility behavior of cohesive soil. Limited research have been carried out on preparation of reconstituted soil cake in the laboratory that could simulate better in-situ condition. The purpose of this study is to address this particular issue. A modified oedometer mould was designed and fabricated to prepare the reconstituted soil cakes. Soil slurries were prepared mixing natural clay powder thoroughly with desired proportions of sand or bentonite clay (as an admixture) and water (near to liquid limit). A number of soil specimens (soil cakes) with varying liquid limits was prepared. The soil slurry was placed in the oedometer mould and consolidation pressure near to effective stresses similar to field condition was applied in stages. The soil cakes thus prepared were used as test specimens for one dimensional consolidation and direct shear tests. Strength and compressibility parameters were thus determined. The shear strength and consolidation parameters obtained from these test results were compared to those of undisturbed samples as well as established relationships from the literature. The results were in good agreement. The approach proposed in this study can be used to assess the shear strength and compressibility parameters of insitu clay soil. For assessing effect of degree of saturation, unconfined compression tests have been conducted for changed degree of saturations (70, 80, 85, 90, 95 and 100%). The undrained shear strength of soil increases with decreasing saturation for soil having liquid limit above 40%. Soil having liquid limit below 25%, effect of saturation on undrained shear strength was insignificant. For investigation of thixotropic aging, test specimens were prepared and preserved in a desiccator without any moisture loss. Direct shear, unconfined compression and consolidation tests have been conducted at 1, 7, 14, 21, 28, 35 and 42 days after the preparation of specimens as per ASTM test standards. The thixotropic aging of soil was found to be 28 to 35 days for soil having liquid limit above 40%.