Those Dreaded Expansive Soils

It is often the small things in life which are important. And they are often overlooked. Take for instance heaving soils. The American Society of Civil Engineers estimates that a quarter of all homes in the United States have some damage caused by expansive soils and in a typical year in the United States they cause a greater financial loss to property owners than earthquakes, floods, hurricanes and tornadoes combined.

This is a sobering statistic and it certainly doesn’t get the dramatic coverage that say Hurricane Katrina received. Admittedly there wasn’t the same loss of life and human suffering, but having said that the costs and problems associated with damage to structures brings its fair share of heartache. South Africa is no less susceptible to heaving soils and it was de Bruyn, Collins and Williams in 1956, consolidated by van der Merwe in 1964, who did much of our pioneering work on heaving clays in the Highveld. Heaves in excess of 23 cm over a 5 year period have been recorded and it takes no imagination to realise that any structure founded on this material will be irreparably damaged if no precautions are taken during construction.

However many people are totally unaware of the effects of heaving soils and this is probably due to the slow rate at which the damage occurs. The effect is insidious and cannot be attributed to a single event – the damage then being ascribed to poor construction methods. Clays have an inherent ability to expand as they take up water into their crystal lattice. Similarly they are able to contract as they become desiccated. Most of you will have seen desiccation cracks in dry river beds, where layers of mud have dried out to form a crazy-paved surface with the underlying sand clearly visible through the cracks. These contractions are due to loss of water from the clay. Perhaps one of the most radical clays around, Sodium Montmorillonite – commercially known as Bentonite –can expand up to twelve times its volume when wetted up. Imagine a cubic centimetre of this material, not much larger than a sugar cube, swelling to 12 times that size. The addition of water causes the sodium ions to hydrate, generating a negative charge on the Bentonite plate. Since like electric charges repel each other the platelets are moved apart causing heave. This is an extreme example, but montmorillonite clays in general have a propensity to heave with associated damage to any structure founded thereon.

The National Home Builders Registration Council has acknowledged the dangers of expansive soils and set out regulations on how best to mitigate these effects. The amount of heave which may take place is a function of both the activity of the clay and the thickness of the clay deposit. The cumulative heave of say three metres of low expansive clay will be greater than that of say one metre of highly expansive material.

In this light then it is important that the geology of any site is understood, that an assessment of the underlying soil moisture condition is made, that the soil structure is examined for fissuring and slickensiding and that the buildings in the vicinity are inspected to determine if they have been subject to heave damage. Foundation indicator tests will determine the clay content and Atterberg Limits of the material, from which an indirect assessment of the mineralogy and the heave potential can be made. Only then can a realistic assessment be made of the founding conditions of the site, and based on this the correct foundation recommendations be given. It always astounds me however that more money will be spent on the Italian tiles for a house than on a decent foundation assessment. Any unforeseen heave will put paid to any fancy tiles, finishes and perhaps the entire structure.