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Subsidence

Subsidence refers to the movement of the ground beneath a property, expanding or contracting, to produce visible signs of movement in the structure of the house above ground, such as widening cracks and doors and windows that fail to open and close properly.

The solution is usually to underpin the property and thereby stabilise the foundations and stop the movement. Subsidence is associated with the sinking of ground; where the ground rises this is known as heave.

According to the Royal Institute of Chartered Surveyors there are two principal causes of subsidence.

• Houses built on clay may suffer subsidence if the clay beneath the house dries out. The drying process might either be caused by a prolongued dry spell or the presence of trees nearby, their roots absorbing water from the soil. The drying clay soil shrinks and contracts, exerting tremendous forces on the foundations. Increasingly hot and dry summers are thought to have contributed to a recent rise in subsidence claims.
• A cracked or broken drainpipe, mains water leak, or underground water course beneath a house is liable to wash away soil from the foundations causing movement in the structure above. Houses built on soil with a high sand or gravel content will be more severely impacted by this type of subsidence.

Possible indications of an underlying subsidence problem include, most obviously, new or widening cracks in interior plaster and outside brickwork, but also doors and windows jamming and bulging wallpaper where there are no obvious signs of a problem with damp. Early diagnosis of subsidence can greatly assist in its rectification. Therefore contact your house insurer as soon as possible once a problem is suspected. Every house insurance policy will have its own set of exclusions and exemptions with regard to subsidence; check the small print carefully. In claims for subsidence, there is usually quite a substantial excess that the policyholder has to pay.

Ascertaining whether a problem with subsidence exists can take some while. If there are visible cracks these may need to be surveyed and measured over a period of time in order to demonstrate whether there is movement.

It is important to differentiate subsidence from settlement. Settlement is the natural process of underlying ground 'settling' to accommodate the weight of a new building. Different types of ground settle at different rates. Buildings constructed on rock, gravel or sand settle soon after construction is complete. It can, however, take many years for new buildings to settle on clay-based, silt-type or peat soils. Once the process of settlement has run its course it should not recur, unless the underlying soil balance is disturbed, for example by mining or tunneling underneath or by building on top of the old foundations. Of course, the way different parts of a building settle over time may also vary if the soil-types beneath it vary.

Once subsidence has been diagnosed it is by no means always the case that underpinning work needs to be carried out. If underpinning is found to be necessary, its execution is quite protracted. The usual method of underpinning increases the depth of the existing foundations to a stable stratum by excavating the soil beneath and replacing it with concrete. Other types of underpinning include piles (the insertion of hefty column-type foundations) and pressure grouting (liquid cement to consolidate the ground). The underpinning work has to be carried out in stages to avoid any risk of damage or collapse. The works will require building regulations approval and the preparation of a detailed structural design. Usually, a test hole will be dug next to the old foundations and a surveyor and/or structural engineer will report on the problem before any major works are undertaken.

If, through surveys, measurements and monitoring, it can be ascertained that the subsidence problem has effectively run its course and that the ground beneath has stopped its movement, it may be concluded that repair and redecoration of the property is sufficient, albeit that such repairs may need to be repeated from time to time. Hence cracks can be chiselled out and filled, cracked bricks replaced, walls replastered and doors and windows eased. Other actions which can be taken include pruning the roots of nearby trees and pollarding the branches to reduce their water intake. Tree removal might also be considered where Tree Preservation Orders are not in force. Also, clearing and repairing blocked and broken drains as soon as practicable when found to be leaking.

Where a house has been underpinned it can be straightforward to arrange underpinned property insurance. Homes that have been affected by subsidence and subsequently been subject to remedial work and/or underpinning can still be insured with policies that will maintain cover for subsidence, landslip and heave.

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