砂柱排水真空阻滞效应的试验和动力学解释

含低渗透盖层的强透水层在排水过程中可能产生包气带真空并阻滞排水。这种真空阻滞效应具有什么动力学特征，是岩土渗流领域有待解决的新课题。以细砂作为粗砂的盖层，进行了双层结构砂柱的排水试验，观测到了包气带真空度先快速增大，然后缓慢减小的过程，排水速率显著低于无细砂覆盖的情况。细砂盖层厚度越大，真空阻滞效应越强烈。基于水-气渗流理论提出了解释试验现象的水流方程和气流方程，并得到了排水早期和后期真空度变化的近似解析公式，说明真空度峰值随盖层厚度呈非线性增加趋势。对试验结果进行参数分析，发现盖层透气性在排水后期明显大于早期，反映了盖层含水率对透气性的影响。

Vacuum decay effect in drainage of sand columns:Experiments and dynamic analysis

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.