That Sudden Sinking Feeling

Here is an issue which doesn’t get a great deal of coverage in the text books or feature highly in typical soil mechanics texts.  South Africa has a number of areas which are subject to collapsing soils, and from my point of view, living here in KwaZulu Natal, we have a fine body of collapsible material in the Berea Sands which extend for considerable distances along the eastern seaboard of the province.  But collapsible soils are by no means limited to the Berea Sands, and occur within many aeolian (wind blown) deposits, of which the Berea Sands are an example, as well as weathered granitic and quartzitic materials.  The latter two examples occur extensively in Gauteng, Northern Province and Mpumalanga, and to a lesser extent the Cape.

Collapsible soils are known to comprise a mixture of coarser sand grains in a matrix of finer material, with intermolecular, electrostatic, capillary and chemical bonds assisting in holding the soil mass together.  They generally have a low density, are highly voided and the soil fabric may well collapse if they are loaded and allowed to take up moisture.  Which is exactly what often happens on a site when construction starts.  Earthworks upsets the natural ground conditions, there is increased run off due to the vegetation being stripped, water ponds on badly drained platforms, and then sewer and water lines are installed, both of which have the potential of leaking water into the soils.  Or perhaps a septic tank and French drain system is installed which, by its very nature, is designed to put water into the ground.  Once the soil is wetted up, collapse may well be imminent with, of course, associated damage to the structure.

In short then, great care must be taken in identifying problem soils on site, and once identified, preventing the collapse described above.  Various methods are available for determining if soils are potentially collapsible, beginning with identifying the geology, in situ observation of voids in the soil,  cone penetrometer tests, density tests, and more comprehensively but perhaps not more conclusively, collapse potential and one dimensional triaxial tests.  If problem soils are identified on a site, then the necessary precautions need to be taken to prevent damage occurring, which include drainage precautions, removing and recompacting the offending soil horizons below the structure, attending to drainage or perhaps piling through the collapsible soils if the building is particularly sensitive.