Experiments with a Miniature Piezocone in Thinly Layered Soil

A piezocone, with a cross-sectional area of 100 mm2 and a pore pressure filter either at its apex or at its shoulder, was driven at the standard rate of 20 mm/s into samples of thinly layered soil. The samples consisted of alternating layers of normally consolidated kaolin clay about 18 mm thick and layers of fine sand or silt about 2 mm thick enclosed in a 252-mm-diameter rigid-walled cell. The performance of the piezocone was evaluated with respect to the detection of the more permeable layers during penetration and estimation of the coefficient of consolidation from pore pressure dissipation tests in a clay layer at the end of penetration. Control tests were performed on 2 clay samples without any sand or silt layers. Changes in the pore pressure response enabled the more permeable layers within the layered samples to be easily detected, but their spacing was too close for individual layers to be detected using the cone resistance profile. Generally, the detection obtained from pore pressures was better with the apex filter than with the shoulder filter. The permeable layers had a measurable influence on dissipation test results, but their influence on the derived values of the coefficient of consolidation was never large by comparison with their effect on the overall consolidation behavior of the soil.