Intact, Disturbed, and Reconstituted Undrained Shear Behavior of Low-Plasticity Natural Silt

Abstract

This paper presents a laboratory investigation of undrained triaxial shear behavior of a natural low-plasticity silt from Halden, Norway in the intact, disturbed, and reconstituted states. Sherbrooke block sample and reconstituted specimens were subjected to simulated tube sampling in a triaxial stress path cell system prior to reconsolidation and undrained shear to assess the effects of disturbance on undrained shear behavior, undrained shear strength, and effective stress friction angle. Shear stress and pore-pressure development were evaluated relative to that measured for the undisturbed reference state, i.e., that measured on specimens from the intact block sample. Specimens trimmed from fixed-piston tube samples collected from the field site were tested for comparative purposes. Collectively, the results demonstrated that neither the volumetric method of evaluating sample quality for clays nor shear wave velocity tracked sample disturbance well for this low-plasticity silt. Relative to the reference intact block sample tests, simulated tube sampling resulted in an increasingly pronounced dilative-type behavior during postdisturbance undrained shear and a general increase in undrained shear strength. Specimens from the block sample that were subjected to simulated tube sample disturbance had similar stress–strain behavior as that from conventional anisotropically consolidated triaxial compression tests conducted on specimens from the tube samples, suggesting that significant alteration of the intact soil state occurred during tube sampling. Practical suggestions for selection of undrained shear strength for intact low-plasticity silts that exhibit dilative behavior such as the Halden silt are proposed.