A Role of Pore Air in Infiltration Process

Abstract

This paper deals with an investigation on the mechanism of confined infiltration under a ponding condition at the surface of the sand layer, especially the mechanism of exchange of the penetrating water and the air in such a layer. After carrying out experiments of ponded infiltration with a constant water depth into a homogeneous and air-dried sand layer in a lucite cylinder with a bottom plate, a theoretical analysis is undertaken by considering such an infiltration as continuous and simultaneous flows of water and air in average by referring to results of a supplementary experiment of the exchange of liquid and air in a U-shaped tube. And then, the analytical results are compared with the experimental ones. The results disclose the following: 1) The moisture profile is formed by quasi-saturated zone (QSZ) and unsaturated zone (USZ). The QSZ develops in the neighbourhood of the sand surface, and as time goes on, it declines in development and approaches a certain depth asymptotically. The USZ continues to develop accompaning by the downward movement of the wetting front. 2) Although, strictly speaking, the infiltration process is discontinuous because of an intermittent escape of pore-air from the sand surface, such a process can be dealt with, in average as continuous. The movement of water and air obeys the law of Darcy's type in the QSZ and the generalized Darcy's law in the USZ. 3) The QSZ which is developing through a formation stage has a significant effect on the air escape. Its resistance becomes at least several times as large as one in the USZ. The escape condition is determined at the lower end of the QSZ by the thickness of such a zone, the water entry value and the ponding depth. 4) In stage where the QSZ is being formed and has developed enough, characteristic behaviour of water and air appear.