WHO’S WHO IN GEOTECHNICAL
Challenges when building on dolomite
Instead of piling, a combined soil and concrete raft approach was used
Found in large areas of Gauteng, dolomite is widely used in the manufacture of roads, concrete and paving materials. But it is a complicated rock to build on.
K
im Timm, structural engineer at infrastructure consulting firm AECOM, explains how and why dolomite needs to be accommodated when designing and constructing foundations, services and the superstructure of a building. “With dolomite, there is often a massive amount of variation in the bedrock. “It will, for example, be just below the surface on one part of the site then fall away to 50 m below ground just a few metres away. This is
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challenging, as it is difficult to apply a uniform approach to foundations,” explains Timm. Therefore, Timm advises that clear and detailed geotechnical information is the best way to be able to plan for surprises that might occur. Money spent on geotechnical investigation frequently leads to money being saved during design and construction. Good geotechnical information minimises the negative consequences of problematic ground conditions and enables projects to run smoother. “As a structural engineer, it is possible to build upon any foundation, as long as you know and understand what is below the ground,” adds Timm. The biggest concern for any structural engineer when building on dolomite is the potential of sinkholes. Dolomite can be soluble. Rainwater and percolating groundwater can gradually dissolve the dolomite over time as it seeps
through joints, fractures and fault zones in the rock. The dissolution of the dolomite gives rise to cave systems and voids in the rock. Soils covering the rock can collapse into these caves or voids, resulting in catastrophic ground movement on the surface such as sinkholes. “This is hugely problematic, especially in built-up areas like Vereeniging and Centurion,” adds Timm.
Exxaro Head Office in Centurion According to Timm, AECOM encountered challenging dolomitic ground conditions at the site of the new Exxaro Head Office in Centurion, Pretoria. “We had to deal with potential sinkholes or pockets on-site before construction commenced. Soil was removed to reach an appropriate foundation level and dolomitic pinnacles within this zone were blasted. Dynamic compaction was then used to increase the density of the underlying material and provide a working platform. This
