Subsidence

Introduction

Subsidence is a hidden risk that can affect almost every home. The ground beneath us is constantly shifting due to natural and man-made influences. As well as the natural geology of an area, mining, tree planting, water abstraction, and tree removal are all causes of everyday subsidence.

After the hot, dry summer of 2003, there was a sharp increase in calls to insurance companies about potential subsidence as the lack of rain dried out the clay soil on which many homes are built. Future predictions of global warming point to more of the same, especially in the south of England, with further lowering of water tables, which could make matters worse.

In the 1970s there was only £5m of subsidence claims due primarily to the lack of information available for homebuyers and homeowners. Total claims are now pushing past the £400M level.

Subsidence can sound worrying, but is a problem that can usually be remedied, provided action is taken in time.

Homecheck uses British Geological Survey geological data to help estimate the risk to domestic properties of potentially damaging subsidence related to natural geological hazards within a particular postcode. This includes a detailed assessment of the rock types in the postcode and their engineering characteristics, and the possibility that they may cause ground movement.

Please note that our assessment of subsidence risk is just for guidance. It does not take into account factors such as the presence of trees, the age of a building, the nature of its foundations, or the presence of any man-made activity in the area (such as landfills, excavations and drains, or mining) which can all contribute to the risk.

Natural subsidence hazard data is supplied to Homecheck by the British Geological Survey and reproduced under licence.

• If you are moving home, make sure you get all the relevant information before you buy. Your solicitor will help you do this by asking the relevant questions during the conveyancing process
• Protect yourself by taking adequate buildings and contents insurance cover to protect your house and belongings in the event of your property being affected by subsidence
• You can get a guideline risk rating from homecheck.co.uk You can also purchase a Homecheck Professional Environmental Report, which provides information on subsidence hazard as well as comprehensive data on other potential environmental hazards including historic and current land use that could cause contamination, landfill, radon, flood risk, hazardous substance and coal mining
• Look out for the signs of subsidence, and be aware of factors such as trees and building activity in your area, that can effect subsidence

The signs and causes of Subsidence

• The most obvious sign in brick or stone built houses is when cracks or bulges appear in the walls
• Other signs include doors and windows that have previously opened easily beginning to stick and ripples in wallpaper not caused by damp
• Bear in mind that most houses experience some movement, which can lead to superficial cracks, without ever suffering serious subsidence. However, if you become concerned, get professional advice immediately.
• Natural hazards can cause subsidence including ground movement as a consequence of swelling clays, unstable slopes, ground dissolution, cavities and compression. Such hazards relate directly to the 'below ground geology' and are inherent in the rocks themselves. However, it is possible that they may be exaggerated or subdued by the interaction of the other 'above ground factors', including vegetation (e.g. trees with large roots) availablity of water and various human activities such as building. The impact of such hazards also depends on the type, condition and construction of the buildings themselves
• There are well known geological issues in different parts of the UK. In the South East of England, for example, shrinkable clay sub soil is common. These soils can shrink during hot dry summers causing a building above to settle or drop downwards
• Subsidence can also be caused by shallow mining activity. Shallow mining is defined as workings within 40 metres of the ground surface. In general, shallow mining workings have a greater potential for generating severe ground movement at the surface than deeper workings. Although shallow mining can cause the ground movement, it will not necessarily cause building movement. This depends on the type, condition and construction of the building and other influencing factors particular to the site.
• Subsidence is a problem that can be resolved, provided proper investigation is carried out and steps taken to remedy problems before excessive damage is caused. In extreme cases of subsidence the complete demolition and rebuilding of the property would be required although fortunately this is relatively rare. If a property is in an area with a high risk of subsidence it may be necessary to get further advice from a structural engineer

Case Studies

The hidden dangers of subsidence

A couple from north-west London faced a property nightmare when their dream home turned out to have a serious subsidence problem.

The vendor of the three-bedroom garden flat did not disclose the environmental problem affecting the property. As the purchasers also failed to obtain an environmental report, they did not discover the subsidence until just before exchange of contracts.

Although the couple managed to withdraw from the sale, they had not only suffered substantial aggravation, but also had to pay £3000 in legal and surveyor's fees.

The couple could have avoided their expensive house purchase disaster by asking their lawyer to order a Homecheck Professional Ground Stability or Environmental report.

A Homecheck Environmental Report would have highlighted the risk of subsidence, as well as a range of other hazards such as nearby landfill sites, flood plains, or contaminated land and is available on this website for £41.95. Simply enter your house number and postcode details on the front page of this website to begin the ordering process.

5 May 2004 - Sunday Telegraph

Woman cheats death as her patio collapses into quarry

A woman today told how she cheated death by inches when her patio suddenly collapsed falling into a 40-foot cavern. Sue Astill was left peering over the edge of a huge hole created when 8ft of garden slipped into a quarry beneath it. Engineers immediately ordered Mrs Astill off the property for fear that the house could slide down the steep bank. Mrs Astill noticed cracks appearing in her patio and asked her brother-in-law to have a look. "We were standing by the edge of the patio about a foot from the house and the next thing it just went. It was really eerie. The ground in front of us just disappeared before our eyes." A large section of her garden is now lying in water at the bottom of Clint Hill Quarry, last mined for granite in the 1920s.

Subsidence Facts

The most visible ways of finding out if a property is subsiding are to recognise any new or expanding cracks in plasterwork or outside brickwork, doors or windows sticking for no reason, and rippling wallpaper that is not caused by damp.

Discovering a property has subsidence is not the worst situation; it is more difficult trying to insure a home with a history of being affected by subsidence and slippage.

Between the mid 1970s and today insurance claims for subsidence have risen from £5m to over £400m.

(Source: UK Online Home Insurance)

Following the widespread heat wave of 2003, insurers estimate a pay out of more than £375m in claims.

FAQs

What is subsidence?

Natural hazards can cause subsidence including ground movement as a consequence of swelling clays, unstable slopes, ground dissolution, cavities and compression. Such hazards relate directly to the 'below ground geology' and are inherent in the rocks themselves. However, it is possible that they may be exaggerated or subdued by the interaction of other 'above ground' factors, including vegetation, availability of water and human activities. The impact of such hazards also depends on the type, condition and construction of the buildings. Subsidence is a common problem in properties in the South East of England, where buildings are constructed on shrinkable clay subsoil. These soils suffer from the disadvantage of shrinking during hot dry summers and thereby sink or reduce drastically in volume and causing a building above to settle or drop downwards.

Subsidence can also be caused by shallow mining activity with shallow mining defined as workings within 40 metres of the ground surface. In general, shallow mine workings have a greater potential for generating severe ground movement at the surface than deeper workings. Although shallow mining can cause the ground movement, it will not necessarily cause building movement. This depends on the type, condition and construction of the building and other influencing factors particular to the site.

Subsidence is a problem that can be resolved, provided proper investigation is carried out and steps are taken to remedy problems before excessive damage is caused. In extreme cases of subsidence the complete demolition and rebuilding of the property would be required. If a property is in an area with a high risk of subsidence it may be necessary to get further advice from a structural engineer.

I am in a high-risk area but have never suffered from subsidence

Being in a high-risk area doesn't mean your home or business will definitely be affected by subsidence ' many other factors come into the equation. The information provided is a guide and should prompt people who live in a high-risk area to be aware and to find out more.

Where does Homecheck get its information?

Homecheck uses British Geological Survey data to help estimate the risk to domestic properties of potentially damaging subsidence related to natural geological hazards within a particular postcode. This includes a detailed assessment of the rock types in the postcode, their engineering characteristics, and the possibility that they may cause ground movement.

Will this increase my insurance premium?

Insurers have known about these risks for many years ' this information is not new. Rates are calculated over the long term and where subsidence is most expected. Properties in the highest risk areas may need to be reassessed ' premiums may increase in future; but premium increases are not solely a result of subsidence.

The Association of British Insurers (ABI) recommends that people go through a broker to get the best deal. Most buildings and contents insurance policies provide subsidence cover but it is important to check your policy. Check out the ABI's website for more information at www.abi.org.uk

Nobody told me my house was at risk when I bought it

Landmark Information Group has been providing comprehensive environmental risk information for over ten years. The Homecheck Environmental Report includes a property specific assessment of subsidence risk for just £41.95 and is available by entering your details on the front page of this website. Alternatevly, please contact Homecheck on 0844 844 9966.

[Source: Homecheck]