“Subsidence is a multimillion-pound problem as far as insurers are concerned,” says Matthew Tarling of the Association of British Insurers. Last year, the insurance industry paid out more than £375m in subsidence claims, a massive 200% hike on 1994’s figure of £125m. Much of this money went straight into the pockets of building contractors and specialist underpinning contractors that rebuild and underpin damaged homes.
But things could be about to change for these specialists. Ron Packman, the principal of structural engineer Packman Lucus, has developed and patented a rehydration system set to slash the cost of rectifying subsidence by eliminating the need for rebuilding and, in some instances, removing the need for underpinning.
Subsidence is caused by the ground beneath properties shrinking as it dries out. About half of UK houses are built on clay, and, when combined with nearby trees – which suck water from the ground – and an ever warmer and drier climate, this can easily lead to damage. As differential settlement occurs, cracks forming in the building’s structure can often be so severe that parts of it become unstable and have to be demolished and rebuilt.
Packman’s innovative system works by carefully feeding water into the ground at precise points around the building, causing the sunken ground to swell and return to its original height. Repairs can then be carried out to the building’s structure to prevent the problem recurring.
The idea of rehydration is not new. Where Packman’s system differs from previous, unsuccessful systems is through the precise control of where and how much water is injected into the ground, dictating which areas will swell and by how much. By careful application of water, the system can be used to shuffle the damaged part of the building back to its original position and so minimise the cost of repair work.
How it works in practice
Bristol-based consultant Subsidence Management has successfully tested the system on six buildings. The latest is a three-storey terraced house in north-west London.
“The whole terrace is built on clay,” says Subsidence Management director Maciek Kawecki, “but a cherry tree in the road outside one house has led to subsidence, causing its two-storey bay to pull away from the front.” Massive 32 mm cracks have formed at the top of the bay. “The damage was so bad that normally we would have recommended that the bay be demolished and reconstructed on new foundations,” explains Kawecki. However, he managed to convince the insurance company that the problem could be corrected far less expensively using the new rehydration system.
First, Kawecki took soil samples from around the front of the house to assess the influence of the tree’s roots and ascertain where water would need to be added. Then, three 150 mm diameter, 2 m deep boreholes were drilled around the bay close to the footings: two on the right-hand side, where more water was needed, and one on the left. The boreholes were lined with a plastic pipe collar and filled with pea-gravel. A copper pipe connected to the water main via a metering system links all three boreholes, while a dribble valve regulates the amount of water fed to each borehole.
The position of the boreholes is critical. The damaged bay had not only started to pull away from the house into the street, but had also started to lean to the side where the dehydration was at its worst.
Kawecki carefully started the rehydration process. His aim was to correct the sideways lean of the bay, then the forward movement. The two boreholes on the sunken right-hand side of the bay were targeted first. Over a period of weeks, the ground on this side was fed with water through the boreholes, which were kept topped up using the dribble valves. As it rehydrated, the ground began to swell and slowly righted the bay. Kawecki monitored the success of the rehydration process by using plastic gauges fitted over the cracks to measure movement.
Once the sideways lean had been corrected, all three boreholes were used to feed water into the ground to apply swelling pressure across the whole front of the house. This carefully shuffled the bay back to its original position and closed the movement cracks.
Applications for historic buildings
For this house, once the subsidence had been rectified, the boreholes were capped off and the damage to the structure made good. The council refuses to remove the tree, so the bay will have to be underpinned to prevent recurrence. Kawecki says that, had the tree been removed, the system could have been retained as an alternative to underpinning “by maintaining moisture levels in the ground at a constant level”.
The rehydration process took just over three months to complete but Kawecki is pleased with the final result. “If we’d demolished the bay and rebuilt it, the total cost to the insurers would have come close to £25 000, if you include the cost of accommodating the building’s occupants elsewhere during the rebuilding works. With this system, the total bill is closer to £7000.”
Kawecki says the system has other applications, particularly for historic buildings, and can also be used as a preventive measure. “It’s a form of risk management,” he adds. To ensure the system works for all types of subsidence, a kit of parts is available that includes an impermeable membrane to limit evaporation from the ground, and a system of pipes that radiate out from the borehole to ensure that moisture can still be fed into the ground for rehydration, even in the most impermeable of clays.
So, will Kawecki be using the system in all subsidence cases? “It depends on the cost of the installation and the cost of the repairs,” he says. One thing is for sure, however. A warmer climate and an ageing housing stock mean subsidence will remain a problem for years to come.