Water movement in glass bead porous media: 2. Experiments of infiltration and finger flow

Abstract

This paper presents experimental observations of infiltration and finger flow in glass beads. In paper 1 (Lu et al., this issue), we showed that the total surface tensile force is much greater in initially wet profiles than in initially dry profiles. During capillary rise in glass beads, the “jump” process takes place for an initially dry condition, whereas in an initially wet profile not only a jump process but a film thickening associated with film flow characterizes capillary rise. In this paper, infiltration experiments into initially dry glass beads show that the wetting front is relatively saturated and flat compared with the unsaturated and irregular wetting front into an initially wet profile. In the experiments of finger flow, photographs show that the tip of the finger is completely water saturated and that no partially saturated zones exist around the saturated tip. The fingers initiated in a dry zone disappear when they reach an initially wet lower zone even when the packing conditions of the glass beads are identical. Hence the criterion for instability when water is applied at a rate less than the value of the saturated hydraulic conductivity does not apply to an initially wet condition. When a fine layer of glass beads lies on a coarse layer that is initially dry, fingering will take place during infiltration and flow is unstable. If the coarse lower layer is initially wet, finger flow does not develop, and the flow remains stable. More investigations are required to ascertain threshold values of the initial water content causing instability of water movement in porous media.