Monotonic, Cyclic, and Postcyclic Responses of an Alluvial Plastic Silt Deposit

Abstract

This study presents a comprehensive investigation into the monotonic, cyclic, and postcyclic response of a medium stiff, plastic, lightly-overconsolidated, alluvial clayey silt deposit. The laboratory investigation is performed on natural, intact, and reconstituted specimens derived from high-quality samples to establish the role of the stress history and soil fabric on cyclic resistance, excess pore pressure generation, and postcyclic behavior. The results of cone penetration tests (CPTs), vane shear tests (VSTs), and downhole and crosshole geophysical tests are used to compare the monotonic, stress-controlled direct simple shear (DSS) test data and interpret differences in the cyclic response of natural and reconstituted specimens. Constant-volume, stress-controlled cyclic tests indicate that the naturally-overconsolidated (OC), artificially normally-consolidated (NC), and reconstituted OC silt exhibit the cyclic mobility-type failure mechanism. The cyclic resistance of the intact, natural silt was determined to be sensitive to overconsolidation, with greater pore pressure generation potential observed in the NC silt for the same loading conditions. Despite significantly denser conditions, the reconstituted OC silt specimens exhibited a cyclic resistance approximately 44% lower than the natural, intact OC specimens at the cyclic resistance ratio corresponding to 15 cycles of loading. The magnitudes of postcyclic volumetric strain in the natural, intact specimens were independent of the overconsolidation ratio (OCR) and depended on the maximum excess pore pressure generated during cyclic shear. Strain-controlled cyclic DSS tests were used to identify the cyclic threshold shear strain, γtp, to generate residual excess pore pressure, which is approximately 0.012% in the naturally OC silt, and the cyclic degradation indices for various shear strain amplitudes. The various stress- and strain-controlled cyclic response of this natural silt deposit are compared to similar plastic soils reported in the literature to provide a well-characterized soil deposit for reference by geotechnical practitioners.