Residents near the ‘sinkhole’ that opened up in St Albans recently face up to a year out of their homes after ground investigations showed a potential danger zone up to three times larger than the existing collapse.
Results from the geotechnology microgravity survey, commissioned by Hertfordshire County Council and carried out by Wales-based Geo Technology, showed a large area next to the collapse which could itself be at risk of falling in.
A 12m-diameter, 7m deep ‘sinkhole’ appeared on Thursday 1 October in the residential area of Fontmell Close in the Hertfordshire town.
The Geo Technology survey showed that the collapse occurred within the boundary of a backfilled clay pit that had been excavated in the 19th century for brickmaking then backfilled with waste from the area.
The county council said a chalk excavation in the bedrock beneath the clay pit was the most likely explanation for the cause of the collapse. Although there is no documented evidence of chalk mining, the council said there was a limekiln in the area which was evidence of chalk use.
It added that a number of anomalies had been identified, one of which appeared to have characteristics similar to that which resulted in the sinkhole.
Initial modelling had suggested this area of low mass or gravity under the ground could be larger than the existing collapse, possibly by up to three times, said the council.
Further investigations and potentially other works need to be carried out, which could take between six months and a year. Those homes already vacant since the existing collapse will have to stay away for that period.
The council said it was working with householders and their insurers and loss adjusters on the best way forward.
“Since we received the report we have met with residents to discuss its findings and to help them understand the contents of the report,” said Hertfordshire County Council deputy director of environment Rob Smith.
“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.”
The existing hole was filled with 535m3 of foamed concrete over a period of four days and the council has said that there had since been very little movement outside of the hole. It said that the concrete plug had seen a settlement of 5-8mm, which was expected due to the weight of the concrete over the top of the loosened ground.