The influence of soil suction on the collapse settlement of different soils in South Africa

Abstract

Partially saturated soils are often dense with a high bearing capacity and will subsequently only suffer small amounts of compression under normal foundation loads. However, when wetted under load many such soils undergo a marked and sudden increase in settlement, the phenomenon which is known as collapse settlement. Prodigious development have taken place on potentially collapsible soils in South Africa, especially on the Berea Red Sands, the granitic soils of the Highveld, residual Basement Granite soils in the Lowveld (markedly the Witrivier, Tzaneen and Bushbuck Ridge areas) as well as recently on the Kalahari Aeolian Sands in the Lephalale area. Even though levels of development have been intense in such areas, the subject of collapsible soils has not received much attention in South Africa in recent years, with very little being published on the subject since Schwartz’s state of the art paper on collapsible soils in 1985. Soil suction can be considered one of the most important parameters describing the stress state at different moisture conditions in an unsaturated soil. Generally, porous materials have the ability to attract and retain water. This ability is described as suction, and can thus be seen as the attraction the soil exerts on the moisture. The collapse process in partly saturated soils is best considered in terms of two separate components of effective stress; the applied stress and the suction. During this research the collapse phenomenon in South African soils was investigated by focussing on the collapse mechanism of dry or partially saturated collapsible soils during the incremental increase in soil moisture content under constant load. Samples were collected from both typically collapsible residual and collapsible transported soils in an effort to relate the collapse behaviour of the material to its geological origin. The change in suction pressure with change in moisture content for the same materials was monitored and related back to the collapse process. Subsequently the influence of the applied stress and suction pressures on the collapse behaviour could be compared for each material.(hb) Copyright