Chapter 32: Collapsible Soils

Collapsible soils present significant geotechnical and structural engineering challenges the world over. They can be found in many forms – either naturally occurring or formed through human activities. However, an essential prerequisite is that an open metastable structure develops through various bonding mechanisms. Bonds can be generated via capillary forces (suctions) and/or through cementing materials such as clay or salts. Collapse occurs when net stresses (via loading or saturation) exceed the yield strength of these bonding materials. Collapse is most commonly triggered by inundation through a range of different water sources, although the impact varies with different sources yielding different amounts of collapse. To engineer in and mitigate the effects of collapsible soils, it is essential to recognise their existence, which may not be easy, and to gather vital geologic and geomorphologic information. Collapsibility should be confirmed through direct response to wetting/loading tests using laboratory and field methods. The key challenge faced with collapsible soils is the spatial extent and the degree of wetting that will take place. Care is needed to ensure that appropriate and realistic assessments are undertaken. Ultimately, if treated using one of a suite of the possible improvement techniques available, then the potential for collapse can be eliminated effectively.