Laboratory investigation of permeability measurement in clay using outflow from unsupported cavities

Abstract

Specimens of reconstituted clay were contained under vertical pressure in a 250 mm diameter cylindrical test cell fitted with a peripheral drain. Using a self-boring device, a 35 mm diameter cavity was formed in the centre of each specimen, and outflow permeability tests were performed to simulate possible in situ test procedures. Either the cavity extended over the whole specimen height and radial flow conditions prevailed, or the cavity was shorter and a steel lining was progressively withdrawn to induce both vertical and radial flow. Generally, the cavities were observed to degrade and partially fill with softened material, yet the measured permeabilities were higher than expected on the basis of other information. Pore water pressure measurements suggested that hydraulic fracturing was taking place around the cavities, even though applied pressure heads were very small. Fractured material is then thought to have fallen into the cavities. In specimens containing silt layers, the cavities did not degrade so much, and more satisfactory permeability measurements proved possible. The results suggest that the assumption of a stable cavity, necessarily made when interpreting some in situ tests, may not be justified.

A constant-head pumping test method using direct-push equipment for in situ hydraulic conductivity measurements

“…Köber et al (2009) conducted field experiments with different DP tools to investigate the possibilities of combining the tools in various ways to produce enhanced DP investigations, and showed that such combined approaches can identify flow regimes for aquifers with a complex structure. In addition, studies of constant-headbased test approaches that use self-boring pressuremeters have been conducted (Hird & Srisakthivel, 2005;Ratnam et al, 2005).…”

Laboratory investigation of pore pressure dissipation in clay around permeable piles

“…The benefits of vertical drains have been identified for embankment on soft clay deposit both experimentally (Pothiraksanon et al 2010) and numerically (Shen et al 2005). However, Hird and Srisakthivel (2005) pointed out that hydraulic fracturing could occur around vertical cavities at relative small pressure heads, where the drains were degraded by partially softened fill material and the permeability was lowered. Similarly, Weber et al (2010) reported the degradation of consolidation for stone columns.…”