Soil Conditions and Foundation Stability

The ground beneath a building is rarely uniform, and its behaviour over time has a direct effect on how well a foundation performs. Understanding a few basic soil properties helps explain why certain areas are more prone to foundation problems than others, and why the same building design can behave very differently depending on where it is built.

Bearing capacity

Bearing capacity refers to how much load a soil can carry without excessive settlement. Dense sand, gravel and solid rock generally have high bearing capacity, while soft clay, loose fill and organic soils tend to have lower capacity and may compress significantly under load. Foundations are sized and positioned according to the bearing capacity of the soil they rest on, which is why two buildings of similar size can have very differently sized footings depending on their site.

Expansive clay soils

Some clay soils absorb water and swell, then shrink again as they dry out. This cycle of swelling and shrinking, often linked to seasonal rainfall and to vegetation drawing moisture from the ground, can cause foundations to move slightly up and down or tilt unevenly. Over repeated cycles, this movement is a common cause of cracking in walls and uneven floors, particularly in buildings with shallow foundations that were not designed with sufficient allowance for soil movement.

Managing the effects of expansive clay often involves controlling moisture around a building, for example through proper drainage, correctly positioned downpipes, and care with planting large trees close to foundations, since roots can draw significant moisture from the soil during dry periods.

Dolomitic ground

In parts of South Africa, particularly areas of Gauteng, the underlying geology includes dolomite rock, which can be dissolved slowly by groundwater over long periods, sometimes leading to cavities forming beneath the surface. In some cases this can result in sinkholes or ground subsidence. Because of this risk, building on dolomitic land generally requires specialised geotechnical investigation to assess stability before any foundation design is finalised, and local regulations in affected areas typically set out specific requirements for this type of assessment.

Groundwater and drainage

The level of the water table and how well water drains away from a site both influence foundation performance. Poor drainage can saturate soil near a foundation, increasing the risk of swelling in clay soils or softening in weaker soils generally. Simple measures such as ensuring ground slopes away from a building, maintaining gutters and downpipes, and avoiding water pooling against walls all help protect a foundation over the long term, regardless of soil type.

Fill material

Land that has been artificially built up with fill material, rather than consisting of natural, undisturbed soil, can behave unpredictably unless the fill was properly compacted when it was placed. Buildings constructed on poorly compacted fill may experience uneven settlement over time, which is one reason site history is a relevant consideration during a geotechnical investigation.

Reading the signs on an existing building

Cracks that appear diagonally from the corners of windows and doors, floors that have become noticeably uneven, or doors and windows that no longer close properly can all be signs that soil movement is affecting a foundation. While these signs do not confirm a specific cause on their own, they are a reasonable prompt to have the building assessed by someone qualified to investigate foundation and soil-related issues in more detail.