Volume 29, Issue 6 (November 2006)
Interpreting Variable-head Tests Performed in Open Boreholes or Monitoring Wells with Several Screens
Variable-head permeability tests can be performed in open boreholes or monitoring wells having either a long screen or several screened zones. In all cases, a test involves several layers composed of fractured rocks or pervious soils. In the hole or the pipe, the static water level represents some mean value of the hydraulic heads within the aquifer layers, a complex pattern of water circulation having been established within the open hole or the screened aquifers. This paper provides first the equation relating the static water level to the hydraulic and geometric parameters of the aquifers, and then the extended theory for variable-head tests under such conditions. According to theory and numerical analyses, such tests should provide straight semi-log head responses and velocity graphs. In real conditions, however, as shown by a series of field tests performed in a monitoring well equipped with three screens, broken line velocity graphs may be obtained. Such piecewise linear lines indicate that the product “hydraulic conductivity × shape factor” varies during the test within one or several aquifer layers. These variations can be related to internal erosion, which happens close to a screen when the gradient and seepage forces change direction during testing. This is confirmed by numerical modeling.