Volume 24, Issue 2 (June 2001)
Extracting Piezometric Level and Hydraulic Conductivity from Tests in Driven Flush-Joint Casings
Permeability tests in borehole casings must satisfy many conditions to give reliable results. A test can be done only in a driven flush-joint casing. When a casing is rotated, its contact against the adjacent soil does not provide a seal good enough to assess the local k-value. Only water injection can be used (either falling-head or constant-head) because water extraction creates upward forces that destabilize the soil and induce either soil heave or clogging. Other conditions are explained in the paper. Interpretation methods differ mainly in their assumptions about solid matrix deformability during the test. The paper describes an interpretation method based on the equation of mass conservation: it leads to a graph of downward water velocity in the casing. This graph provides the error made in the quick field estimate of piezometric level (PL) for a tested zone. Several examples are provided, including tests that produce hydraulic separation between soil and casing. The PL obtained with this graph was always similar to that given by a monitoring well installed at the same level after borehole completion. From analysis of many tests at the same site, information can be obtained on natural water seepage conditions using a variation of PL versus depth.