Engineers at Hertfordshire County Council have warned that a cavity three times larger than an original sinkhole could open up on a street in St Albans.
The local authority has published results of a survey of the roadway and footpath at the site of the sinkhole in St Albans, which saw families evacuated from their homes earlier this month. The 12-metre diameter and seven metre deep sinkhole appeared in the early hours of 1 October.
A geotechnology microgravity survey of the road and footpath in Fontmell Close and Bridle Close has shown that the collapse occurred within the boundary of a backfilled clay pit that had been excavated in the 19th century for brickmaking and backfilled with waste from the area.
It added: “Chalk excavation in the bedrock beneath the clay pit is the most likely explanation for the cause of the collapse. Although there is no documented evidence of chalk mining, the presence of a limekiln is evidence of chalk use.
“A number of anomalies have been identified, one of which appears to have characteristics similar to that which resulted in the sinkhole. Initial modelling suggests this could be larger than the existing collapse, possibly by up to three times. Further investigation will be necessary.”
The council has yet to decide what further work will be carried out, but it is working with householders and their insurers and loss adjusters on the best way forward.
Rob Smith, deputy director of environment at Hertfordshire County Council, said: “Since we received the report we have met with residents to discuss its findings and to help them understand the contents of the report.
“We appreciate this is upsetting news and are continuing to work together to determine the next steps. The safety of residents remains our priority.
“We are keen to enable people in the area to be able to come and go from their homes more easily and are looking to create a more permanent temporary access road.”
Photo credit: Hertfordshire County Council’s Fire and Rescue Service.