Mechanics of sampling disturbances in clay soils.

Abstract

This research provides an insight into the quality of a soil sample during the penetration of a soil sampler. The investigation of the mechanical disturbances in a clay soil is conducted by using an updated Lagrangian finite element formulation with the 2nd Piola-Kirchhoff stress rate (the Truesdell stress increment) to account for the large deformation behavior near the sampling tube. The penetration of the sampler is simulated by spliting a group of nodes ahead of the penetration route up to a sufficient depth and applying incremental deformation to match the geometric configuration of the sampling tube. Consolidation effect is included to account for the rate of penetration. Thin-layer elements are added into the inside wall of the sampling tube to model the soil-sampler interface. The modified Cam-clay model is used to simulate the behavior of the soils. An experimental study was conducted to study the variations of stresses and pore water pressures at the soil-sampler interface. The numerical results show that (1) soil samples are subjected to three distinct stages of vertical strain history, compression-extension-recompression. The first stage of compression, in particular, causes irrecoverable changes in the virgin soil properties of the soil; (2) the undrained shear strength of a disturbed sample reconsolidated to the in situ stress condition is larger than the virgin soil for normally consolidated soils but it is smaller for over consolidated soils; (3) the sampling disturbances due to friction at the soil-sampler interface increase as the sampler penetrates the soil. As a result, long samples will be seriously degraded; (4) the increase of the rate of penetration can reduce the degree of disturbances; (5) the piston sampler induces much larger disturbances than the open-drive sampler.