Cause of sinkholes explained

Engineers assess the damage in New Lynn, Auckland, after yesterdays deluge of rain which caused extensive flooding and damage including a sinkhole opening up. Photo/Brett Phibbs

A sinkhole in the Auckland suburb of New Lynn could be bigger than it looks.

The massive hole in the pavement outside Bike Barn opened up when the area on Great North Road was submerged in chest-high water.

While the road was dried out now, two lanes were still blocked as Fulton-Hogan staff assessed the hole.

One Fulton-Hogan worker said other damage in the area could be connected.

"We're using certain equipment to scan underneath to make sure everything's all right, because we also have breakage down over there and we think it's maybe to do with this."

So what causes sinkholes and where are they found?

Science reporter Jamie Morton put these questions to GNS Science engineering geologist Dr Sally Dellow.

What exactly are sinkholes?

When underground cavities collapse in on themselves and the collapse propagates to the ground surface the resulting feature is a sinkhole.

What are the driving factors that create them?

Natural sinkholes form in limestone country and can readily be found around Waitomo. The Maori name for sinkholes is tomo.

Over thousands of years, water dissolves away the limestone and eventually the overlying rock will collapse and form a sinkhole.

Another type of sinkhole can appear above old mine workings, this happens in Waihi from time to time.

A third type is where underground water erodes away loose sand and silt, creating an underground cavity which then collapses in on itself.

This is the type of sinkhole that has appeared in Auckland in the past few days.

Once they emerge, how are they addressed?

The best policy is to avoid areas prone to sinkholes, so avoiding limestone country and land above old mine workings is a good first step.

Addressing them depends on determining the origin.

If they have formed because of underground erosion of loose sands and silts, then finding the source of the water causing the erosion needs to happen and be addressed - for example, drainage or diversion - and then the sinkhole can be filled in and the ground returned to its pre-sinkhole state if required.

Is there anything about New Zealand soil conditions that make our country more vulnerable to them?

New Zealand is probably not more vulnerable to underground erosion of sands and silts than anywhere else in the world.

Old mine workings, especially in New Zealand where mining is a very recent activity, are easily identified, as is limestone country from geological maps.

Has the hazard of sinkholes become more of a consideration for city planners and if so, why?

Probably not, as the presence of large inflows of water is often the result of broken pipes and mains.

This can in some instances lead on to sinkholes and landslides on sloping ground.

It is probably more of an issue for the owners and operators of pipe networks, mostly councils, or council infrastructure providers and their maintenance plans rather than city planners.

Are certain cities or towns in New Zealand more prone to them: If so, why?

Waihi, because of the old mine workings.

But usually there is a fairly specific combination of events that can be hard to identify in advance. Usually a change such as a broken pipe or heavy rainfall; in underground water flow, causing erosion; or having a flow path that allows sand and silt to be carried away - with the erosion creating a large cavity which the ground above then collapses into.

Is there anything to suggest their occurrence may increase in the future, due to factors such as urban expansion or the impacts of climate change?

Not really, although urban expansion will increase the area in which sinkholes can occur it will not cause more sinkholes, unless the land being developed is particularly vulnerable.

This is difficult to determine before development for the type of sinkholes that occurred in Auckland over the weekend.

Climate change is envisaged to produce more storms and more intense storms so, again, this might see a few more sinkholes appear but being able to make a direct link will be difficult as sinkholes are still relatively rare, compared to, say, landslides.