Q&A: The science of sinkholes
A sinkhole in New Lynn could be bigger than it looks. The deep hole in the pavement outside Bike Barn opened up when the area on Great North Rd was submerged in chest-high water. A Fulton-Hogan worker said other damage in the area could be connected. “We’
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.
Has the hazard become more of a consideration for city planners?
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.
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.