Compacted Clay: Difficulties Obtaining Good Laboratory Permeability Tests

Testing the permeability of compacted clay is not easy. The hydraulic conductivity, k(Sr), depends upon porosity nc and degree of saturation Src after compaction, and the values reached during the permeability test, ncf and Srf. The four values are needed to predict k(Sr) using a dual-porosity model with two parameters, a and b. However, many published test reports do not give these four values. The tested clay is often unsaturated, and the measured k(Sr < 100 %) may be confused with its saturated value, ksat, whereas it may be one to three orders of magnitude lower than ksat. This, in turn, may lead a designer to poorly predict the total leakage of a lined cell or lagoon. For fully documented test data, parameter a is between 0.001 and 0.1 and parameter b is between 2.7 and 3.3 (around 3 for a perfect cubic law). Once the values of a and b have been found with correctly performed and fully documented tests, a local k(Sr) value can be predicted at each place the field density and Src have been assessed. This yields many predicted local k(Sr) values, which can then be used with statistics to predict the full- or large-scale hydraulic conductivity and leakage of a liner or cover.