Pressure selections for triaxial permeability testing

It is a common occurrence that geotechnical engineers, geologists and scientists request triaxial permeability testing from Construction Material Testing (CMT) laboratories without specifying the required pressures or detailing the application. This is generally as a result of a lack of understanding of the test procedures and how the pressure selection during the various stages affect the overall test result reported. Some specifiers may also nominate pressures which provide a result which potentially indicates a soil compliance under those test conditions, yet if it were tested under conditions reflective of the engineering application, would in fact fail the required specification. There are three stages of the test which require different pressure selections;

1. Saturation pressures based on soil type and plasticity;
2. Consolidation pressures to set the specimen equivalent to the insitu condition;
3. Running the test at the desired confining stress or the mean effective stress using pressures relevant to the soil type.