Field variable-head test in low-permeability materials: Assessing the effects of trapped gas pocket and cavity expansion

Abstract

Trapped gas and cavity expansion may affect the data of a variable-head test in a monitoring well. In a low-permeability material, some gas may be trapped in the top of the filter pack. Presence of a gas pocket may be alleged when the initial change in water level in the pipe is less than the theoretical value due to the slug volume. A variable-head test with trapped gas is described by three equations to be solved simultaneously. The gas pocket contraction leads to an underestimation of the hydraulic conductivity, k, especially for small pipes used to test natural or compacted clay. It must be considered, however, that the difference between the theoretical and measured changes in initial water level is due to the sum of trapped gas contraction plus cavity expansion (falling-head) or trapped gas expansion plus cavity contraction (rising head). This difference cannot be used to assess the volume of trapped gas unless an independent mechanical test has established the material deformability. For example, pressuremeter tests can be done in cavities that are prepared with the same care as those used for the installation of piezometers. A simple test that consists of adding or removing known volumes of water in the pipe of the piezometer is described. Field data are provided along with the method to handle them and extract the correct k value. To avoid gas entrapment in the filter pack, a small diameter plastic tube can be installed and used as a gas vent.