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An Experimental Investigation of the Influence of Sampling on the Behavior of Intermediate Soils

Laboratory tests were performed in order to determine the effects of tube sampling disturbance on intermediate soils. Synthetic soils composed of Kaolin clay and silica silt, a reconstituted natural silt, and block sample specimens of a sensitive marine clay were tested. Tube sampling disturbance was simulated in a triaxial stress path system with bender element using the Ideal Sampling Approach prior to reconsolidation and undrained shear. ISA strain cycles of ±0.5, ±1 and ±3% were used to represent progressively greater degrees of disturbance. The influence of overconsolidation ratio (OCR) was explored by testing at OCRs of 1, 1.8, 3.6 and 7.2. Unlike the reference clayey soils, the intermediate soils tested had significant losses in effective stress resulting in densification of the specimens during reconsolidation. This densification changed the undrained shear behavior from contractive to dilative and resulted in underconservative design strengths. Increasing OCR decreased this effect on measured shear strength; however design strengths were still underconservative. SHANSEP recompression partially recovers the undisturbed behavior; however design strengths were still underconservative. Drainage during sample of intermediate soils is possible, however there was little difference in post-disturbance behavior whether the ISA disturbance was conducted drained or. In spite of the dramatic changes in undrained shear behavior and increased strength due to sample disturbance, the intermediate soil specimens experienced relatively small changes in void ratio during reconsolidation and would be rated as good to excellent quality using clay-based sample quality assessment methods, indicating the need for other methods to assess intermediate soils. Bender elements were used to measure shear wave velocity during the disturbance simulations and to test the efficacy of a shear wave velocity stress state framework. The shear wave velocity for the intermediate soils was found to be relatively independent of the density, but rather dependent on the stress state and decreased during ISA disturbance. Reconsolidation resulted in only a small reduction in void ratio but the undrained shear behavior was dramatically different than the undisturbed behavior. The large reduction in shear wave velocity due to sampling disturbance may provide a foundation for creating a sample quality criterion for intermediate soils.
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