Sinkholes – Terra Firma not so firm?

What are sinkholes?

Sinkholes are large holes appearing (often suddenly) in the ground which may vary in width and depth.  The consequences of such occurrences can be far reaching and devastating to both life and property.

What causes sinkholes to appear?

They occur all over the world and the U.K. is no exception. In the U.K. there are several different causes.

Generally, the underlying cause is rock which dissolves leaving a void in to which the surface land collapses. The speed or likelihood of a sinkhole appearing will depend on the type of underlying rock and surface rock or material.

Where the cause is not by the dissolving of rock, washing away or erosion of weak pieces of rock by pressure of water can cause voids to occur and hence sinkholes to appear. Also, there are numerous man-made reasons.

What precipitates their occurrence?

The events which trigger the occurrence of sinkholes are:

  • the gradual dissolution of rock in most cases;
  • heavy rain or flooding of rivers;
  • leaking or bursting of water pipes;
  • building works (by changing the drainage within a certain area and putting pressure on the underground water levels);
  • changes to the water table as a result of prolonged climate change; or
  • mining (which can cause voids in itself or change the water levels to create voids).

Examples of Sinkholes

Some examples have hit the headlines in the last few years, a number of which are in Kent as follows:-

Example one

February 2014 – A 5 metre deep hole suddenly appeared in the middle of the M2 motorway near the village of Erriottwood, near Faversham and resulted in partial closure of the motorway for about 9 days.  It apparently required about 40 tonnes of shingle to fill the hole.

Cause:  There were two possible causes:-

  1. an old chalk mine beneath leaving a void in to which the surface layers collapsed; or
  2. the natural dissolving of the chalk leaving a void into which the surface layers collapsed.

Example two

March 2015 – Properties including both homes and shops in Lawrence Square, Northfleet, Gravesend, Kent had to be evacuated after a sinkhole appeared causing great safety concerns.

Cause:  The sinkhole is believed to have been partially man-made. In the past there was chalk mining in the vicinity for over 200 years and many of those workings have gone unrecorded.  It is likely that the rainfall at the time triggered the collapse and creation of the sinkhole with the surface layers collapsing into the old voids left by the historical mining.

Example three

July 2017 – Part of the car park of the Primark store in Westwood Cross collapsed into a sinkhole.

Cause It was partially man-made as there was a collapse of the surface layers above underground tunnels believed to be created by the British Army during the First World War for training exercises (and probably used during both world wars).

Example four

October 2017 – At the junction of Millmead Road and Irvine Road in Margate a sinkhole of 5 metres deep and 1.5 metres wide suddenly appeared.

Cause: Unknown.

What am I supposed to do if this happens?

The simple answer to this is “nothing”, if like the poor man in Florida who, on the 1st March 2013, was asleep in his bed when he and the entire floor of his bedroom disappeared forever!

Otherwise, if you survive, there are some very practical tips:-

Sudden appearance of Sinkholes:

  • Cordon off the area.
  • Notify the landowner and the emergency services.
  • Notify the utility providers.
  • Contact your local authority.

If the development of the sinkhole is gradual and not sudden:

  • It should be monitored by experts and advice and assistance can be sought from your local authority and experts such as the British Geological Survey (details on- line).

What prudent steps can I take?

If you are buying or leasing land or buildings, you should carry out a desktop environmental survey which will look into the known history and geology of the site.  Obviously there are some things that are not recorded but if it is recorded then such a search will reveal the same.  It will identify ground stability hazard risks and other matters and should be carried out as part of your due diligence conveyancing process.

If you are a builder or developer you should, as part of your planned development, give consideration as to whether to build a Sustainable Drainage System (SuDs) which replicates natural drainage and reduces flooding and build-up of water.

Conclusion

There is a useful map on-line showing the U.K. subsidence hotspots between 2013 and 2017 (see www.geobear.co.uk) and the British Geological Survey website (see www.bgs.ac.uk) has some very useful information and guidance.

It seems likely that the incidence of sinkholes is set to increase substantially in the future given climate change (with sporadic heavy flooding events) and increased population (making heavy demands on land use).  As a consequence, we should all make checks on the ground beneath our property.