GEORGE NEWS - Sinkholes are not common in the area as they are mostly formed near dolomitic or limestone-type rocks, in the Free State, Northern Cape and Gauteng areas, where these rocks are common, says Dr Esmé Spicer, geologist / geotechnical consultant and director of Rock Hounds.
Spicer was approached by George Herald for comment after a large hole appeared in Heriot Drive, Denvar Park, on Saturday 13 March, due to a main water pipe burst. Spicer could not comment about the specific incident because there is an ongoing investigation into the incident.
She did however give some insight into the forming of sink holes and the geology of the George area.
"Groundwater can occur in fractures and gravel approximately 20-50m above the granite plutons, well below civil structures. Active weathering of the dolomitic rocks by mine waste water seepage and regular acid rainfall, caused by pollution, increase the speed with which the sinkholes form significantly in these areas."
George is located predominantly on granite and weathered granite. "Layers of clay and koffieklip (ferricrete) are present approximately 1 to 2m below the surface, above the weathered granite rock. Kaaimans and Table Mountain sandstone group sedimentary rocks are located closer to the foot of the mountains and passes," she said.
Part of the large sinkhole that appeared due to a main water pipe burst in Heriot Drive, Genevafontein. Photo: Eugene Gunning.
Contributing factors
Weathered granite experiencing active water movement over time, such as pipe leakages, can cause fine clay particles to be washed away around larger particles, forming minor cavities that connect over time to form larger cavities, said Spicer. This can result in a sinkhole slowly forming over time in an area that is not normally prone to sinkhole formation, eventually causing the collapse of structures.
Heavy rains can also cause softer material to be washed out from under road structures, along rivers, high clay-rich areas such as near Hartenbos, or in softer dune areas.
Along the coast, wave action is another factor that can create major cavities that can cause collapse of rock structures. The final culprit is biological weathering, caused by roots or animals that dig into structures or small cracks and causing collapse over time.
"Naturally occurring soil material has inherent variability, which is important to recognise and manage over time," said Spicer. "The purpose of geotechnical investigations is to describe soil strata (layers), material properties, factors influencing stability and bedrock levels and to assist in the safe design for civil engineering projects."
Advice for homeowners
She said property owners should keep an eye open for regular pipe bursts and washing out of large quantities of fine soil material. Large cavities in or under weight-bearing structures should be isolated and fixed.
Her advice to homeowners is do regular maintenance around their house, keeping storm water drains open, keeping pipes and gutters healthy (open) and keeping roots and branches away from drainage systems. In this regard, water is the enemy. It should be channelled along dedicated systems away from buildings and structures. Proper support is required for steep slopes and coastal properties.
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