Measurement of the overburden component of total potential in swelling field soils

Abstract

Methods are presented for measuring the contribution of the overburden potential, ? = áP, to total potential in swelling field soils. For this purpose the total stress, P, was varied to obtain the load factor, á. In the first method, applicable to shallow topsoil, P was assumed to be zero. This permitted calculation of á from the slope of the unloaded shrinkage curve. In two other methods, the surface load, P(0), was varied in the field above tensiometers, and in the laboratory using undisturbed samples. Values of á were then calculated assuming that loading or unloading did not substantially drain or wet the soil during the measurement period. This assumption appeared reasonable; mean values of á, at constant moisture and void ratios, agreed closely, although site variability was rather high. Within the limits of accuracy of these methods, á did not depend on P. Average values of á, measured at high moisture content on a number of swelling clay soils, were all <0.25.