The unconsolidated sediments that permit substantial infiltration are sands, silts, and gravels. Several experiments on the mechanics of infiltration with sands and glass beads involve one or the other of the following: a bulk mass of liquid originating at the surface of the medium in a stream ranging from a thin thread to a sizable flow; a sheet of liquid laid entirely across the surface of the medium; and, finally, drops of liquid similar to raindrops falling on the surface. All result in essentially the same picture, that of a drop of liquid falling in a capillary tube. The water moves as a bulk mass. There may, be some film flow locally, but it is relatively insignificant. During the process of infiltration each element of a sand, silt, or gravel extracts enough water to satisfy its capillary requirements, to the extent that they are not satisfied by water already present from previous infiltrations. In gravel capillarity is usually negligible. The retained water per unit mass of dry soil is known as specific retention and may also be called field capacity. The balance of the infiltrating water proceeds downward until it either is totally absorbed by the sand or reaches the water table as recharge. The equations of capillarity are reviewed from a thermodynamic standpoint and are given in both differential and integral form. It is concluded that infiltration is a downward movement of water in bulk form, under ordinary hydraulic laws subject only to the reduced or otherwise altered pressures caused by capillarity within the bulk mass of liquid. Film flow appears to be confined to local flow to contact points, which situation is compatible with the condition that the free, energy Fs, of capillary surfaces in a sand, given by Fs = ∫ σ dO, where σ is the surface tension of water and dO is an element of the capillary surface, must be at a minimum. The results when applied to the infiltration process agree with field data. (Key words: Hydraulics; infiltration in sediments; groundwater)
Water Resources Research – Wiley
Published: Jun 1, 1967
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