Hidden Depths – The Science Behind the Broad Street Sinkhole

“The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down what seemed to be a very deep well.” – so reads one of the many literary quotes shared by Oxford’s libraries on Twitter following the spontaneous appearance of a hole in the tarmac by the Bodleian library. It’s not surprising that this saga has captured people’s imaginations – when what may have been weeks of erosion culminates in such rapid subsistence of a main road, it seems only natural to imagine that, like Alice, we too could be in danger of being swallowed up, and probably never know anything about it.

So what causes sinkholes, and why do they appear so suddenly?

The variety that rattled motorists and tour bus companies in Oxford this week is called a cover-collapse sinkhole. These form when a space or cavern forms under the ground. The soil that makes up the roof of this cavern, called the overburden, falls under gravity into the space by a process called “spalling”, leaving only a thin crust of soil or tarmac to lie in wait for a hapless vehicle or person to wander over it.

Burst water pipes can produce an accelerated version of the natural process that usually makes these sinkholes. The water flowing past at high pressure disrupts the rock and soil around the pipe, but also carries debris away from the site, aiding the formation of a vacuum. This produced tragic consequences in Guatemala City back in 2007, when a 330 foot sinkhole opened up due to heavy rain and ruptured sewer lines, swallowing homes and killing at least three people.

But sinkholes can also form sites of natural beauty – the famous Mexican cenotes came about when soft limestone bedrock collapsed, exposing the underlying groundwater. The resulting caves host a range of marine life, as well as many bizarre, blind and pigment-less cave-dwelling species.

This little natural disaster here in Oxford has quite rightly provoked interest and curiosity, perhaps best summed up by an English Faculty tweet: “Wherever he saw a hole he always wanted to know the depth of it. To him this was important.” – Jules Verne, Journey to the Centre of the Earth.