Recent heavy rain has caused a five-fold increase in ground collapses as a result of sinkhole and dene holes, according to a statement from the British Geological Survey.
Location of a collapsed dene hole in the middle of the M2 in Kent and another that swallowed a car in Buckinghamshire have raised the profile of the issue in the national media.
However, improvement in the weather is unlikely to halt the trend as the mechanism of failure will change as groundwater drops, according to Chalk solution feature specialist Peter Brett Associates (PBA) partner Clive Edmonds. “The failures we are seeing at the moment are as a result of the roofing strata weakening, softening and increasing in weight as water drains through but as groundwater starts to fall there will be a number of sinkholes and dene holes where the groundwater has been providing hydraulic support and once this is removed there are likely to be new collapses,” he said.
Edmonds reports that PBA normally deals with 10 collapses from solution features or Chalk mines a year but the company has started dealing with more than that number in new cases in the last few months. “We saw an increase in new cases in November and December but the rate of these cases has reached unprecedented levels since Christmas,” he added.
Nonetheless, Edmonds said that solution feature and dene hole collapses tend to be isolated incidents and a collapse at one site does not mean that another nearby will collapse in the immediate future. “The conditions that lead to a collapse is a slow process,” he said. “The problem with gypsum dissolution in Ripon is very different and a collapse in those conditions often results in migration.
“The collapses in Chalk occur in pre-existing features whereas gypsum is very soluble and once groundwater has caused a collapse the water will flow through and start to erode the deposit elsewhere. Ripon has an on-going issue with gypsum dissolution that is well charted.”
The Highways Agency has reported that work to repair the dene hole collapse on the M2 will be completed tomorrow.
“Our investigations are complete and the hole has been stabilised, meaning that traffic can safely use all lanes of the M2,” said Highways Agency asset delivery manager for Kent Simon Duke.
“The work tonight will involve resurfacing lane two of the M2 in both directions following comprehensive geotechnical work. During the overnight work, the M2 will be closed between junctions 5 and 6 in both directions with a clearly signed diversion route in place.”
Although the M2 repairs are nearing completion, the Highways Agency is now investigation some ground movement on the A12 near Lowesoft in Suffolk. The investigation centres around a 5m long, 200mm deep depression in the carriageway that was identified during some maintenance work.