Entrapped air effects on dipole flow test in sand tank experiments: Hydraulic conductivity and head distribution

The dipole flow test (DFT) was applied to investigate the effects of entrapped air on quasi-saturated hydraulic conductivity (K) in a cylindrical aquifer model (sand tank). The sand tank was equipped with 58 spatially distributed piezometers and screened well to accommodate the dipole probe (DP). The experiments included tests with and without pre-saturation of the sand medium with CO2, a technique intended to minimize the impacts of entrapped air on K. A mathematical model was developed and applied to interpret the piezometer drawdown data. Estimation of K from DP chamber drawdowns produced good results when the medium was pre-saturated with CO2. However, without CO2 pre-saturation, the effect of entrapped air was apparent from the nonlinear drawdown responses to the pumping rate. In general, the quasi-saturated K was approximately 50% of the fully saturated K.