With a rapid rise in the number of solution feature and chalk mine collapses last winter, the next few months look set to be busy for the remedial grouting sector.
Ground collapses in chalk made the national news earlier this year following the opening up of a hole in the central reservation of the M2 in Kent and a car being “swallowed” by a collapse in Walter’s Ash, Buckinghamshire.
Once the attention of the national media was caught, there followed almost daily reports in February of new incidents, but was there really a rise in the number of events or just a greater awareness of them.
“Normally there might be an average of one collapse a month on the chalk outcrop in the UK, but in February alone this year we were working on 13 new cases,” says Peter Brett Associates partner Clive Edmonds, who specialises in the remediation of chalk collapses.
“The heavy rainfall that we experienced during the winter months certainly seems to have been a factor in the increased rate,” he adds.
“In 2011 and 2012 there was nothing out of the ordinary in the rate of collapses but the heavy rainfall over the summer of 2012 and at the end of last year and start of this one seems to coincide with an increase in the number of events reported.”
The capping is rarely something that was designed and it is probably more luck than judgement that they haven’t failed before now
Improvement in the weather is unlikely to halt the trend, as the mechanism of failure changes as groundwater drops, according to Edmonds.
“The failures we saw in the early part of the year were a result of the roofing strata weakening, softening and increasing in weight as water drains through it,” he says.
“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.”
Edmonds is keen to point out that many of the recent collapses have been as a result of the failure of crown holes where the capping material over historic mine workings has given way, rather than solution features.
This trend towards more problems being associated with mining activity is something that Bam Ritchies contracts manager Andrew O’Donovan has also observed.
“I believe the trend we’re seeing at the moment is not just down to the heavy rain - although that is a trigger - but is also down to the age of these mine workings,” he explains.
“The capping is rarely something that was designed, and it is probably more luck than judgement that they haven’t failed before now.”
Despite these changes in incident rate, it is surprising that Edmonds reports that the insurance industry and home buyers are not responding by taking a more proactive approach to identifying the risk.
“Checks can be done at the home-buying survey stage, but I think this data is not being interpreted sufficiently for the risk to be understood,” he says.
Nonetheless, O’Donovan reports that many of the remedial grouting schemes recently undertaken by Ritchies has been on work to protect properties at risk from known mine workings rather than being emergency-led repairs.
This suggests that once a collapse has occurred there is impetus in a local area to consider which other properties may be at risk.
It is the Homes and Communities Agency that is footing the bill for much of this planned work, rather than insurers or homeowners, but its funds are not limitless.
If the increased incidence rate is due to the aging mine workings, it could mean the rise in collapses in early 2014 is just the start of a growing trend.