‘Sinkhole’ in the middle of the M2 | 11 February 2014

The ‘sinkhole’ that formed in the middle of the M2 on 11 February 2014 is situated about 350 metres to the south-east of the village of Erriottwood.

This whole area is underlain by chalk bedrock belonging to the Seaford Chalk Formation.

There are two possible causes of the hole: natural dissolution of the chalk or chalk mining. In some situations the two interact.

There is some evidence of local mining in this area and sinkholes are also common.

Natural dissolution of the chalk?

Chalk can dissolve naturally over a long period of time due to the infiltration of weakly acid water.

The source of the acid is mainly from carbon dioxide in atmosphere and from vegetation producing carbonic acid (locally there is also acid derived from the breakdown of iron sulphide that can produce a weak sulphuric acid).

The process of dissolving the calcium carbonate of the chalk has left behind residual deposits of insoluble materials that form cappings to the local hills mapped by geologists as ‘clay with flints’; there are also older re-worked deposits included with these.

During the ice ages some of these deposits have moved by freeze-thaw (solifluction) forming areas that geologists call ‘head’. In places, the chalk has continued to dissolve beneath the capping of clay with flints (or head) forming cavities bridged by the overlying materials that can later collapse forming sinkholes.

Shallow chalk mining?

In the past, chalk was mined for agricultural or building lime. Where the chalk mine is shallow, the mine roof may collapse allowing the overlying clay with flints or head to temporarily bridge the opening before finally collapsing into a sinkhole.

The trigger for a sinkholes collapsing is commonly water ingress due to heavy rainfall or burst/leaking pipes.

The water washes the clay, from the clay with flints, down into the underlying natural or man-made cavity weakening the bridge of capping material that collapses forming the sinkhole; it is common for this to happen adjacent to roads where water run-off is concentrated.

Further information about sinkholes on chalk.

Pictures of the sinkhole are available from the BBC | M2 sinkhole causes severe disruption.

The area around Erriottwood is underlain by chalk bedrock belonging to the Seaford Chalk Formation. There are two possible causes of the sinkhole, natural dissolution of the chalk or chalk mining, in some situations the two interact

Contact

For more information contact Dr Tony Cooper (BGS Honorary Research Associate) or Dr Vanessa Banks