Heavy rainfall has contributed to sinkholes appearing in the UK at the fastest rate since 1987, with developers potentially facing huge costs from hidden threats below the surface.
In the last month, huge craters have appeared on the M2 (forcing its closure) and in housing estates in areas as far apart as Yorkshire and Hemel Hempstead, where they have threatened to swallow up entire homes.
Sinkholes are more likely to occur in the North-east, but the South of England has experienced a spate of incidents this month (see box).
British Geological Survey hydrogeologist Vanessa Banks says the last time she can recall a similar frequency in the number of sinkholes appearing was in 1987.
“It is definitely an unusual and higher-than-average activity level, but it is not that we never get any”
Clive Edmonds, Peter Brett Associates
She says unusual weather patterns and “incredible amount of rainfall” can destabilise soil and initiate the collapse of normally stable cavities, causing a sinkhole.
Construction and development are also potential triggers, she says, as modifying surface drainage or changing the loads imposed on the ground can result in sinkholes appearing.
A house was left on the verge of collapse in Ripon, north Yorkshire, earlier this month after a sinkhole caused cracks in the ground, forcing three homes to be evacuated.
The incident occurred the week after a Ripon Property Developments’ proposal for 75 new homes on the nearby site of the former auction mart in Ripon was refused by members of the Harrogate Borough Council planning committee due to concerns about the stability of the land.
Harrogate Borough Council has a system of guidelines and controls to manage development in Ripon, where gypsum deposits roughly follow the path of the River Ure.
It has three planning zones: Zone A, where there is no gypsum; Zone B, where there is gypsum at depth; and Zone C, where gypsum is present at a shallow depth and susceptible to dissolution.
In Zone C, where much of the city is built, there are full planning constraints. To build within Zone C, developers must first complete a ground stability report, a site appraisal by a geotechnical expert and a programme of ground investigation.
Developments also need reinforced foundation designs to prevent sudden collapse. The Ripon bypass, for example, has a plastic reinforcing net within its embankments.
So what do developers have to do to convince local authorities, whose resources and technical expertise are often stretched, to grant consent for schemes in regions prone to sinkholes?
Ripon mayor Mick Stanley says the local authority needs developers to provide clear evidence of ground stability by carrying out proper site investigations.
He says historic and geological research must be carried out to find the most suitable sites, pointing to Ripon Cathedral, which has been on the same site since the 7th century, and the market on the same site since the 12th century.
Clive Edmonds, an engineering geologist and partner at Peter Brett Associates, explains that naturally occurring sinkholes appear when there is a gap in a dissoluble rock, such as chalk, gypsum or salt.
The gap in the rock, called a solution pipe, is often shaped like an ice cream cone, is filled with weakly compacted soil and can be formed over tens of thousands of years.
Large amounts of water, such as prolonged heavy rain or a burst water pipe, can move or wash the soil out of the gap causing the surface to collapse.
Consequently not all buildings or roads in towns where there are holes are vulnerable because they will not all be built above a solution pipe.
Holes tend to be more common in areas of dissoluble rock with clay, sand or gravel above.
Clive Edmonds, an engineering geologist and partner at Peter Brett Associates, says: “In the last couple of years you might have had six per year. Since July 2013 we have had increasing numbers to deal with.
“It is definitely an unusual and higher-than-average activity level, but it is not that we never get any.”
Land with the potential for holes can be filled or supported to prevent them occurring. Dr Edmonds says man-made cavities can be bulk infilled and natural cavities can be treated using compaction grouting with viscous mortar grout, which pushes together the particles in the loose soil.
He says well-known developers have been caught out in the past by setting aside money for contamination issues with land but overlooking, or having to spend huge sums on rectifying, ground instability.
“You might have put £100,000 aside for contamination and it [ended up costing] £500,000 or £750,000 to sort out the ground instability issue,” he says, adding that it could lead to “reputational issues” for those developers caught out.
Dr Edmonds is working on a 10 m-wide sinkhole that opened up in Oatridge Gardens, Hemel Hempstead.
His team have filled the hole, 10 m wide by 5 to 6 m deep, with foamed concrete. He says: “We didn’t want to add weight to the ground but it gives passive support to the side of the building because at the moment the corner is sitting over fresh air.”
Companies such as Atkins, Arup and Peter Brett Associates have in the past carried out work worth tens of millions under the Land Stabilisation Programme, a fund which dealt with non-coal mine-related land instability but has been closed for several years.
Land Stabilisation Programme axed
In 2000, front walls of two properties in Field Road, Reading, collapsed. The homes, some which date from the 1800s, were built on a manmade hill above old clay pits mined for the town’s brick and tile-making industry.
One of the holes was 7 to 8 m deep and 10 m across. “We found maybe in excess of 20 mines,” Mr Edmonds says. “Most were small-scale, but some were quite large, more than 150 m across. A lot of houses were threatened by instability.”
Peter Brett Associates were brought in by Reading Borough Council to investigate and stabilise the site with funding from the Land Stabilisation Programme, a government fund for dealing with non-coal mine-related land instability.
The work on the site cost £12m and was completed in 2012. The fund was run by English Partnerships, which is now part of the Homes and Communities Agency, and ended several years ago. It paid for other schemes carried out by Atkins and Arup.
David Shilston, technical director for engineering geology at Atkins and president of the Geological Society of London, says planners can search databases to check whether the site of a planning application could be vulnerable to holes.
Buyers can carry out a search or survey before purchasing or get advice from engineers.
“We have the technology to resolve many of those problems,” he says. “The key thing that triggers them is entry of water into the ground.” Mr Shilston says an important issue is management of surface water drainage.
“It is important to avoid the most subsidence-prone areas and to investigate then design developments to cope with potential problems”
Vanessa Banks, British Geological Survey
Dr Banks says hazards associated with sinkholes should be mitigated by appropriate planning, good site investigation (with geophysics and boreholes), appropriate design and proper maintenance of infrastructure such as drains and services.
“It is important to avoid the most subsidence-prone areas and to investigate then design developments to cope with potential problems,” she says, adding that records are kept of areas where mining has taken place and ground may be weaker as a result.
She says the risk of sinkholes affecting property and infrastructure could be mitigated by avoiding building in certain places, such as over former mining sites or former brickworks.
As for planning for extreme and unusual weather? Little can be done, she warns, so prepare for the worst and don’t expect sinkholes to stop occuring anytime soon.