Compacted Clay: Difficulties Obtaining Good Laboratory Permeability Tests

Testing the permeability of compacted clay is not easy. The hydraulic conductivity, k(S-r), depends upon porosity nc and degree of saturation Src after compaction, and the values reached during the permeability test, n(cf) and S-rf. The four values are needed to predict k(S-r) 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(S-r< 100 %) may be confused with its saturated value, k(sat), 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(S-r) values, which can then be used with statistics to predict the full- or large-scale hydraulic conductivity and leakage of a liner or cover.