Development of a novel self-boring permeability measurement technique

Sangkaran Ratnam, Cambridge University
Geotechnical Engineering Group


The measurement of permeability has traditionally been of importance in geotechnics for the prediction of ground movements. Today, permeability measurement has increased prominence with the need to address various environmental issues. Permeability of geomaterials can be measured either in-situ or in the laboratory. The former method is generally believed to yield more representative results. In this thesis, the development of a novel in-situ method of measuring permeability is pursued. The new technique builds upon the capabilities of existing self-boring pressuremeter (SBP) devices. Following self-boring to a specified test depth, the probe is pulled back to leave an unsupported cavity in the ground. Water is then pumped into the cavity and permeated into the soil using a constant flow system located at ground level. The system is separate from the pressuremeter aspects of the device. A sequence of tests including pockets of varying length and a flush bottom, zero length cavity case allow for both horizontal and vertical components of permeability to be determined. SBP tests can also be carried out.

In order to interpret the test, new geometry constants (shape factors), specific to the test cavity shape, were required. A method of computing these constants, using the finite element method (FEM) was developed and a general expression for simple infinite boundary conditions was obtained. Preliminary work for complex finite boundary conditions was also initiated using the FEM. Results from field measurements made using a prototype system are presented. Tests were conducted at three natural soil deposit sites and one engineered barrier test site with varying degrees of success. While the measurements were generally consistent and repeatable, the data at certain sites appear to have been influenced by leakage. Laboratory constant flow testing on high quality samples were also conducted for comparison and to simulate some of the anticipated field conditions highlighted in the literature review.

The technique shows tremendous promise, particularly in soft clay deposits. The new FEM constants were also successfully used to interpret the test data. Further field testing to further explore some of the concerns highlighted during this work are proposed.