6th January 2016
In early 2015 at Lawrence Square, Northfleet, Gravesend, Kent – an extended period of subsidence culminating with the appearance of a sinkhole has forced several shops to close and evacuation of 7 sets of council tenants for fears of further subsidence.
Gravesend, like much of Kent, rests upon the Seaford Chalk Formation – a soft, white and soluble rock which has been mined extensively in the region for over 2000 years for the purpose of agriculture and construction. Many of these workings are unrecorded, meaning that voids in the subsurface where mineral extraction has occurred are scattered all over the County. The subsidence of Lawrence Square is most likely the result of a collapse of one of these voids following extensive rainfall allowing surface layers to sink into the empty space with all that lies on top (i.e. shops and flats).
The recent changes in rainfall patterns, witnessed as periods of unusually heavy rainfall and storms, has accelerated the rate of sinkhole occurrences in the last few years. Increased dissolution of these soluble minerals following heavy rainfall acts to weaken arches and pillars upholding these voids promoting their failure. Nine other sinkholes and deneholes (man-made chalk mines and caverns) have appeared in Kent since the start of 2014 including the 15ft deep denehole which opened up beneath the M2 forcing its partial closure for 9 days. Others include the opening of a denehole beneath Rainham Mark Grammar School, Gillingham, and a 10ft deep hole swallowing two sheds in Swanley.