Abstract: For high permeability strata underlying a low permeability confining layer, vacuum can occur in the vadose zone during drainage of groundwater and can slow down the drainage. The dynamic characteristic of this vacuum decay effect is still an unresolved subject in the area of seepage in rock and soil. Drainage experiments of double-layer sand column are carried out using fine sand (thickness is 2-7.5 cm) as the confining layer of coarse sand (thickness is 80.5 cm). Vacuum is observed in the vadose zone which increases rapidly up to 25-35 cm H2O and then decreases gradually. As a result, drainage speed is significantly smaller than that without the fine sand layer. The more the thickness of the confining layer, the role of the vacuum decay effect is more essential. In comparison, no vacuum can be observed for a column with uniform coarse sand. Based on seepage theories, simplified flow equations of water and air are proposed to explain the phenomena in the experiments. Approximate analytical solutions are obtained for the early and late periods of the drainage, indicating that the maximum vacuum increases with increasing of the thickness of the confining layer under a nonlinear trend. It is found through parameter identifications that the air-permeability in the late period is significantly higher than that in the early period, which reflects the influence of water-content on confining layer. This study shows the significance of air flow in groundwater drainage from sands if a low permeability soil exits on the top.

Key words: vadose zone, vacuum, fine sand, permeability, dynamics