There’s been so many sinkholes in Auckland lately. Why?

By and is republished with permission

Sinkholes seem to be making headlines every couple weeks in our biggest city of late.

As recently as Wednesday, more than a dozen Auckland beaches were closed after a sewer pipe burst in Parnell, causing a sinkhole.

In August, a sinkhole closed a key road in Auckland’s Ōtāhuhu. And in July, yep, you guessed it, another sinkhole in College Hill, caused by a burst storm pipe.

That storm pipe is worth focusing on, because it carries storm water, and there’s been a lot of that in Auckland this year.

Rain, rain, go away

Speaking to RNZ’s Morning Report about the College Hill sinkhole (which, two months on, is still very much there and causing traffic woes for those entering Auckland’s CBD from Ponsonby), Auckland Council Healthy Waters strategy head Andrew Chin warned more sinkholes would likely open up.

“[That’s] not a surprise at all. The system’s under a massive amount of stress, the ground is absolutely saturated so there’s a lot of ground movement, so where the infrastructure’s in a fragile state you’ll start to see these problems cropping up.”

And that’s if the water even makes it into the storm water system.

While grass fields absorb the water straight into the ground, and much of the rain on roads gets shuffled off into gutters, drains and pipes, all it takes for a raindrop to cause havoc is a little crack in the road.

Crews are still working to repair a burst sewer pipe in Parnell that's caused a large sinkhole.
Crews are still working to repair a burst sewer pipe in Parnell that’s caused a large sinkhole. (Source: Supplied)

But regardless of how the water gets underground, once it’s there, it’s a problem.

That’s because rainwater skews slightly acidic on the pH scale, coming in around 5.6. It’s naturally a bit on the acidic side, but carbon dioxide and atmospheric gases add to that acidity.

Once it’s under the surface, the gases that came with it dissolve with the water, and suddenly you have carbonic acid eating away at the soil under the ground. A weak acid, sure, but an acid nonetheless.

The collapse

Beyond this, it doesn’t take a whole lot else to get the sinkhole going. More rainwater means more acid, which means a faster sinkhole.

Alternatively, more rainwater means fuller pipes, which means a greater likelihood of a pipe bursting.

Either way, the ground gets eaten away, until eventually the newly-formed cave can’t support the weight, and in tumbles the surface.

Then congratulations, you have a new sinkhole.

Remember those corrosive atmospheric gases we talked about earlier?

They’re greenhouse gases, and the more of those we pump into the atmosphere, the more corrosive that water becomes when it dissolves with those gases.

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