Clarification of capillary rise in dry sand

The extent of capillary rise into initially dry granular soil, that is exposed at the surface and subjected to wetting at the base, is not easy to predict. Simple rising head column experiments were performed using pure water in medium-grained poorly graded and unconsolidated sand to shed light on the extent to which water rises in initially dry soil and its distribution. Rising head experiments have been conducted in initially dry sand to explore these phenomena in columns instrumented with Time Domain Reflectometry (TDR) probes, the data from which are compared with water contents ultimately found gravimetrically from sample slices. Water was found to reach much higher than theoretical estimates based on an equivalent capillary tube, suggesting that capillary rise is responsible for fluid transport well into the zone of unsaturated soil above the saturated capillary zone. This remained the case even when the sand was subjected to strong rates of evaporation at its surface. This study challenges the conventional understanding of capillary rise in unsaturated soil and has particular relevance in desert soils subject to rapid evaporation.