What causes sinkholes is water working on dolomite. Over a very long time, water combined with carbon dioxide dissolves the rock and leaves openings under a cover of soil; water coming from above, out of a leaking pipe, badly drained stormwater or heavy rain, then washes that soil down into the openings until the ground over them gives way. Pumping out groundwater, as gold mining on the West Rand once did, can set sinkholes off as well.
For a plumber in Krugersdorp this is not an abstract question. A dolomite belt runs through the middle of Mogale City, and the first trigger on the Council for Geoscience’s list is the one plumbers deal with: leaking pipes. Meanwhile a sinkhole on the R563 is still growing nearly 20 months after it first appeared, TimesLIVE reported on 15 October 2026. This guide takes the causes one at a time, shows where the dolomite actually is, and lists the signs a homeowner should watch for.
What causes sinkholes: rock that dissolves and soil that moves
Dolomite is a carbonate rock, and the Department of Water Affairs’ dolomite guideline explains that it “can dissolve in the presence of water combined with carbon dioxide”. The process is slow and natural. Over millions of years it opens fissures, cavities and caves. Mogale City’s own planning framework says the dolomite belt “gave rise to the majestic caves that attract many visitors each year”, the caves of the Cradle of Humankind.
A cavity on its own is not a sinkhole. What gives way is the cover above it: soil and weathered rock spanning the opening like an arch. The Council for Geoscience defines a sinkhole as the “collapse of an arch or dome which spans an air filled void”, set off by water getting in, from “leaking services or ponding water”, or by a lowering of the groundwater table. The result is generally circular, steep-sided and deep, and up to tens of metres across.
Water is what moves the soil. Water soaking down from the surface carries fine soil with it into the openings below, and the arch thins from underneath. The guideline puts it plainly: “water leaks at the surface (from a broken pipe for instance) can erode the soil cover into cavities in the dolomite bedrock which can then result in a sinkhole appearing on surface.”
There is a slower version too. Dolines, or compaction subsidences, are “fairly shallow, enclosed depressions”, which the guideline links mainly to the artificial lowering of the groundwater level. Instead of a hole, the ground sags.
The three triggers the Council for Geoscience lists
In its slides on the dolomite hazard, published by the Parliamentary Monitoring Group in February 2013, the Council for Geoscience named three “triggering mechanisms”: leaking water-bearing services, poor surface water management, and artificial lowering of the groundwater table. All three come down to water.
Leaking water-bearing services
These are the water and sewer pipes: the municipal mains in the street and the pipes on every property. A burst is obvious and gets fixed. The leak that matters more is the quiet one, a joint weeping into the soil under a lawn or a slab, because the water goes down instead of coming up and can run for a long time before anyone notices. A cracked sewer pipe or a gully that keeps overflowing puts water into the ground in the same way. The same slides note that as development spread, the growing density of services became a “known cause of sinkholes if they leak”.
Poor surface water management
This is rain and stormwater that is allowed to collect and soak in: downpipes emptying against a wall, paving that falls towards the house, a low corner of the garden where water stands after every storm. The CGS definition names “ponding water” beside leaking services for a reason. On dolomite land the minimum precautions adopted by the NHBRC ask for roof water to be discharged at least 1.5 metres from the walls, and for the ground against the house to fall away from it, so that water drains off instead of standing.
Lowering the groundwater table
Groundwater matters to the ground’s stability. The dolomite guideline says that keeping the original groundwater conditions in an aquifer “can therefore be critical to dolomite stability”, and that letting the water table move up and down outside its natural range dissolves more rock and raises the risk. Geologist Chulumanco Koti told The Star in October 2025 that “the West Rand has experienced sinkholes caused by groundwater dewatering to support the gold mining industry”. A household plumber has nothing to do with this trigger, but it explains a good deal of the area’s history.
What causes sinkholes in roads? The R563 and the R374
Two provincial roads in Mogale City have had sinkholes since early 2025. On the R563, the Hekpoort road, one first appeared in February 2025, according to The Star. In June 2025 the ward councillor, Gwen Blitz, told TimesLIVE that it had caused more than ten accidents, and blamed the dolomite under the road and “heavy rains”. On 15 October 2026 TimesLIVE reported that the sinkhole “continues to grow nearly 20 months after it first appeared”, and that the Gauteng roads department had admitted there was no budget to repair it.
The second, on the R374 near the Cradle Valley Hotel between Kloofzicht and Glenburn Lodge, appeared in March 2025 and was still unrepaired when The Star reported on it in October 2025. The Star described the ground in the area as “alternating layers of chert and dolomite”, and quoted Koti on how such sinkholes form: “Karst sinkholes form when the underlying carbonate rock dissolves over time, creating voids beneath the surface.” Then: “Rainwater seeping through cracks accelerates this process.”
None of these reports blames a leaking pipe. The causes they give are the dolomite and the rain: the same story of water on dolomite, at the scale of a road.
Where is the dolomite in Mogale City?
Not under all of Krugersdorp. Mogale City’s Spatial Development Framework 2022 to 2027 calls “the band of dolomite running through the area” the municipality’s key geological feature, and says the belt runs “in a south-western or north-eastern direction”, cutting the municipality into two halves. It names wards 27, 30 and 33 as the ones affected, and notes that “a significant portion of the Cradle of Humankind World Heritage Site is underlain by a dolomitic band”. Ward 27 lies directly north-west of Krugersdorp, and Tarlton is in ward 30.
The same framework says “the existing urban areas are on quartzite that is generally suitable for urban development.” So many town properties are not on dolomite, but no published list says which erven are. The municipality’s Land Use Scheme settles it property by property: its Dolomitic Overlay Zone applies SANS 1936, the national standard for developing dolomite land, to erven “affected by dolomitic conditions”. The municipality can confirm whether your erf is one of them.
The rock does make itself known. The dolomite guideline illustrates the hazard with a photograph captioned “Large sinkhole in the dolomite rocks near Krugersdorp.”
Being off the dolomite does not make water in the ground harmless. The framework notes that “the granite in Muldersdrift is significant since it might be prone to collapsible soils”, and a 2015 study of such soils by Brink and Van Rooy in the Journal of African Earth Sciences found that “when wetted under load many such soils undergo a marked and sudden increase in settlement”. On any ground, a leak that runs under paving or a driveway carries soil away with it.
What are the first signs of a sinkhole?
Often there are few. The Council for Geoscience warns that sinkholes “can occur with little warning”, and that “cracks in walls or settlement in the ground are often the early warning signs.” Around a house, that means watching for two kinds of change: the ground moving, and water going where it should not.
- New cracks in walls, especially ones that widen, or doors and windows that start to stick in their frames.
- A patch of lawn or paving that sinks, or a slab that sounds hollow underfoot.
- Water standing after rain where it never used to, or against a wall.
- A damp patch that never dries, in the garden or at the foot of a wall.
- A water meter that keeps turning with every tap closed, or a bill that jumps for no clear reason.
- A gully or drain that keeps overflowing onto the ground.
The first two are about the ground; the rest are about water. A small, slow leak may never make a wet patch at all when the water drains straight down, which is why the meter is the most useful of these checks. With every tap closed and nothing filling, the meter should stand still.
How do you fix a sinkhole? Start with the water
Not with a spade and a load of soil. Repairing a sinkhole is engineering work, and the first step in any repair is to stop the water that is feeding it. Until someone qualified has looked at the ground:
- Keep people, pets and cars away from any hole or sunken patch.
- Stop the water. If a pipe on your property could be leaking, close the valve at the water meter. That shuts off everything on your side.
- Report what is not yours. A hole in a road or pavement, or a leak on the street side of the meter, should be reported to Mogale City.
- Photograph and date the cracks, so whoever assesses the ground can see whether they are growing.
- Find out about your erf. If you do not know whether it is in the Dolomitic Overlay Zone, ask the municipality, and tell anyone who works on the ground or the pipes.
Where the plumber’s part starts
Of the three triggers, leaking services is the one a homeowner controls, and only on their own side of the meter. Finding a leak that has never surfaced is a job of testing rather than digging: a meter check, then a pressure test on the installation, then the pipework split into sections until the leak has a location. The page on leak detection in Krugersdorp sets out how that works, including the one-hour pressure test that shows a repair has held.
If you have noticed any of the signs above, or the meter will not stand still, use the booking form below. It goes to the plumber, who phones you back to hear what is happening before anyone drives out.