Dolomite area map and dolomite risk: what property buyers need to check

Most property risks can be seen: a cracked wall, a damp patch, a stream at the bottom of the garden. Dolomite risk is under the ground and invisible until something gives way. In Gauteng it is a serious consideration, and one that every buyer in an affected area should understand.

A dolomite area map is where the check begins.

What dolomite is and why it matters

Dolomite is a rock that dissolves slowly in slightly acidic water. Over a very long time, groundwater moving through it leaves cavities and voids. The ground above can remain stable for generations and then fail, in one of two ways:

• a sinkhole — a sudden collapse of the surface into a void below;
• a subsidence — a slower, broader settling of the ground.

Both damage buildings and services. Sinkholes can be dangerous.

Large parts of Gauteng are underlain by dolomite. Within the City of Johannesburg it occurs mainly in the south and south-west, while most of the northern suburbs sit on other rock. That is a generalisation, which is exactly why a property-specific check matters.

What triggers sinkholes

The rock has been there for an immense time. What changes is water.

• Leaking pipes — water mains, sewers and stormwater drains. This is the main trigger in built-up areas.
• Poor stormwater management — water ponding against buildings or soaking into the ground.
• Leaking pools and ponds.
• Lowering of the groundwater table, historically through mine dewatering.

This is why development on dolomite comes with strict rules about water: the land can often be used safely, provided water is kept away from the subsurface.

Using a dolomite area map

A dolomite area map shows where dolomitic rock occurs at or near the surface. Used properly, it answers one question: does this property need further investigation?

1. Search the property on checkproperty.co.za and open its report.
2. Look at the site constraints, which include dolomite and soil information for the erf alongside floodlines.
3. If the property is on or near dolomite, treat that as a prompt for the steps below.

A map tells you the geology. It does not tell you the degree of hazard on a particular erf. Dolomite land ranges from low risk, where normal precautions suffice, to high risk, where residential development is not appropriate. Only a site investigation establishes which.

How dolomite land is classified

Development on dolomite in South Africa follows a national standard, which sets out how sites are investigated and what may be built on them.

In outline:

• a dolomite stability investigation is carried out by a competent geo-professional, typically using gravity surveys and boreholes;
• the site is assigned an inherent hazard class reflecting the likelihood and likely size of a sinkhole or subsidence;
• the hazard class, together with the proposed use, determines whether the development is acceptable and what precautionary measures are required.

Lower-hazard land may be suitable for housing at ordinary densities with standard precautions. Higher-hazard land may be restricted to low-density use, commercial use with stringent measures, or no development at all.

The Council for Geoscience plays a role in reviewing dolomite investigations for new development.

What to check before buying on dolomite

For an existing house

• Ask for the dolomite stability report for the township or erf. For newer developments one should exist.
• Ask whether a risk management plan applies, and what it requires of owners.
• Inspect for signs of movement — cracks in walls and paving, doors that no longer close, depressions in the garden, tilting boundary walls.
• Check water management — gutters and downpipes discharging well away from the house, paving sloping away from walls, no ponding.
• Ask about leak history — burst pipes, sewer problems, pool leaks.
• Ask neighbours whether there have been sinkholes or subsidence nearby.
• Speak to an insurer before you sign. Cover for subsidence and sinkhole damage varies, and some policies exclude it.
• Check the bank's position.

For vacant land or a development

• Commission, or obtain, a dolomite stability investigation before the purchase is unconditional.
• Establish the hazard class and what it permits.
• Cost the required foundations and precautionary measures.
• Confirm what the municipality requires for plan approval.

See also buying vacant land: the checks that matter.

Living on dolomite: the precautions

Typical requirements for buildings on dolomite include:

• wet services in flexible, leak-resistant pipes, laid so that leaks are noticed quickly;
• stormwater led away from buildings in sealed channels;
• no soakaways;
• paved or sealed aprons around buildings;
• restrictions on boreholes;
• care with gardens — no heavy watering against walls;
• regular checks of services and prompt repair of leaks.

These are reasonable and manageable. The households that come to grief are generally those that did not know they applied.

Dolomite and development potential

For developers, dolomite affects three things.

Whether you may build. The hazard class may rule out residential use or limit density, whatever the zoning says. Rights under the land use scheme do not override ground conditions.

What it costs. Investigations, engineered foundations and special services add to cost.

How long it takes. The investigation and its review add steps to the programme.

A site assessment that ignores dolomite is incomplete. See development feasibility.

Dolomite is not the only ground risk

Other ground conditions also affect building:

• expansive clays, which swell and shrink with moisture;
• collapsible soils;
• shallow or historic undermining along the old mining belt;
• fill of unknown quality.

The soil information in a property report gives a first indication. A geotechnical investigation gives the answer for a specific site.

Putting dolomite in proportion

A property on dolomite is not automatically a bad purchase. A great many people live safely on dolomitic land. What matters is:

• whether the land has been investigated;
• what the hazard class is;
• whether the required precautions are in place and maintained;
• whether the price reflects the position.

An uninvestigated property on dolomite, with old leaking services and no records, is a different matter from a modern development with a stability report and a managed plan. Further background is in dolomite risk in Johannesburg: what buyers should know.

Frequently asked questions

Does my home insurance cover sinkholes? Not necessarily. Read the policy wording and ask the insurer directly.

Can I drill a borehole on dolomite? It is often restricted. Check the conditions for the area.

Does dolomite show on a title deed? Sometimes conditions of title or township conditions refer to it. See title conditions.

The bottom line

A dolomite area map tells you whether to look further. If a property is on dolomite, find the stability report, understand the hazard class and the precautions, and get an insurer's answer before you commit.

Check a property's site constraints.

This is general information, not geotechnical or engineering advice. Dolomite information is indicative; consult a competent geo-professional for a site-specific assessment.