October 1, 2015

expert reaction to the St Albans sinkhole

A 20 m diameter sinkhole has opened up in St Albans, leading to the evacuation of several homes.

Peter Hobbs, Engineering Geologist at the British Geological Survey (BGS), said:

“Sinkholes are caused by dissolution of rock such as chalk.  Alternatively, they can be caused by collapse into former mine workings (for flint within the chalk).  Changes in the water table may affect the stability of underground cavities.

“Recently, the weather in this part of the country has been relatively dry and water tables may have lowered.  The strata at the site consist of about 4 metres of sands, silts and gravels overlying chalk.  It isn’t clear at present which of these factors could be the cause.  Sinkholes in this chalk formation are not uncommon.”

Prof. Simon Redfern, Professor of Mineral Physics at the University of Cambridge, said:

“St Albans sits on the Upper Chalk, soft rocks formed from the fossilised shelly remains of marine creatures living in a sea that covered south east England millions of years ago. Today, these soft chalky rocks can develop subterranean channels and chambers, in just the same way as the limestone caves of the Peak District and elsewhere, and are relatively easily dissolved by water. When an underground cavern develops it may grow to a size that its roof collapses, and at the surface it suddenly appears as a sinkhole.

“It seems likely that today’s sinkhole in St Albans has formed from just this sort of process. It demonstrates that the firm rocky foundations that we expect to rely upon may not always be so steadfast, and that dynamic processes at and near the Earth’s surface are constantly reshaping our planet.”

Declared interests

None declared