Mechanisms of downhill creep in expansive soils

Abstract

A detailed survey of two tracts of houses on sloping terrain underlain by expansible clay-rich soils indicates that expansive soils are especially susceptible to downhill creep. This poses an additional threat on top of the already well recognized problem of expansive soil damage to light structures. The sites are located in Waco on the South Bosque shale and in San Antonio on the Houston Black clay. Increases in the plasticity index, slope and age all favour increasing damage in Waco and the creep motions preferentially induce cracking on the sides of the houses parallel to the slope. Little dependence on age or slope at the San Antonio site indicates downhill creep is not an important factor there. A study of volume changes vs. soil water potential verifies that a difference exists in mechanical behavior between the soils of the two sites. Both soils exhibit a sudden increase in water content at a low soil water potential as the potential is decreased from -15 bars. However, only the Waco soil exhibits a corresponding rapid increase in volume. Creep tests were performed on undistrubed samples of the natural soils over a range of soil water potentials and on a suite of artificial soils of increasing montmorillonite content. The Waco soil shows a rapid increase in creep rate over the range of potential that produces the sudden volume increase whereas the San Antonio soil retains a relatively high stiffness to a very low potential. ...