MEASUREMENT OF DEPTH TO WATER TABLE IN A HEAVY CLAY SOIL

Abstract

Water tables in a heavy clay soil were observed using: a 1.2 m deep perforated pipe, 2 series of shallow, unlined boreholes of depths of 30 and 50 cm below the surface and 1 series of piezometers to 30 cm. All holes had a diameter of 2 cm, as required in clay soils. Pressure potentials were measured in duplicate by transducer tensiometry at 10 cm depth intervals. Under nature conditions, different water levels were observed in the unlined boreholes at the shallow depths, will pressure potentials in surrounding soil were negative. Only the level in the deep hole corresponded with the level of zero pressure potential, as obtained by tensiometry. Occurrence of water in the shallow holes, which initially seemed to suggest the presence of a shallow water table, was explained by applying artificial rain containing chloride as a tracer. An estimated 10% of this rain was absorbed by the upper 20 cm of the soil; .apprx. 10% was found within 10 min in the shallow boreholes (not in the piezometers); and the remainder moved rapidly downward in the soil along vertically continuously larger voids. The water table at the 60 cm depth showed a rapid temporary rise. The shallow boreholes form artificial cavities in a 3-dimensional system of interconnected, larger natural voids, which conduct the water. These voids may intercept and feed these cavities through their vertical walls. Drainage through their bottom is usually slow, allowing the presence of water during several days, even though the surrounding soil is unsaturated. Water tables should, therefore, not be observed by shallow, unlined boreholes, but by installing tensiometers or piezometers. Deep, unlined boreholes can be used when none of the soil horizons had a low hydraulic conductivity.