Borehole variable-head permeability tests in compacted clay liners and covers

The performance of clayey liners and covers is assessed by several methods including borehole variable-head permeability (k) tests. Different interpretations of the end-of-casing and Lefranc tests are discussed. In Quebec, most tests have been interpreted using the velocity-graph method. Its equations are presented and differences are illustrated by examples. Numerical simulations confirm the validity of the velocity-graph method. It eliminates a frequent error in the k value resulting from a poor evaluation of the piezometric level for the test. Using an incorrect interpretation method may yield a k value up to 50 times lower than the actual value. Smearing the clayey surface in the injection zone may be a source of uncertainty in the k value. A numerical analysis shows that a 1 or 2 cm smeared zone may change the test k value by over one order of magnitude. It is also shown that it is illusory to try to extract the anisotropy in hydraulic conductivity of the compacted clay.