What is subsidence? | Where does subsidence occur? | How can I deal with subsidence? |Choosing a house with subsidence risks in mind | Back to subsidence 

Many factors are involved in choosing a house location such as proximity to schools, ease of commuting, the type of housing available in that area, future perceived value, cost etc. One factor usually ignored is the risk of subsidence. Examples of soil behaviour based on various areas near/in Croydon are quoted as being prone to different effects as examples; similar situations may apply in such areas near where you live or want to live.

Many factors are involved in choosing a house location such as proximity to schools, ease of commuting, the type of housing available in that area, future perceived value, cost etc. One factor usually ignored is the risk of subsidence. Examples of soil behaviour based on various areas near/in Croydon are quoted as being prone to different effects as examples; similar situations may apply in such areas near where you live or want to live. Insurers categorise the country by postcode with different rates of charging for different areas according to how they perceive the risk. Usually they perceive the risk in the number of claims occurring in that postcode or nearby postcode.

This totally ignores the reality that the risk depends mainly on the type of soil. The front of a house may be on London Clay, whilst the back is on River Terraces. The London Clay subsides due to too little water causing shrinkage, whilst River Terraces can be washed away by too much water. Such sites exist in Thornton Heath. In the flatter lower lying areas of West Croydon, one often finds River Terraces. As this material was deposited by fluvial action, it can be further moved by water. Keeping drains intact in such areas is important, whilst proximity of trees is less so. In East Croydon, approximately south of the Addiscombe Road, one finds clay.

Close to the Addiscombe Road it is likely to be London Clay, which whilst excellent for tunnel construction, is also excellent for the subsidence industry. In the Park Hill area, one has some London Clay in the north part, but elsewhere the risk is reduced due to the presence of a different type of clay. In the hilly parts of South Croydon and in Purley, you usually find chalk. BUT, there is often a superficial deposit of material above this chalk. If the superficial deposit is decomposed chalk, it would behave as a clay, and the greater the decomposition, the greater the shrinkage potential.

From Wandle to Sutton, through Carshalton, there is a zone where water runs close to the surface and in some locations, it outcrops and gives rise to the various sources of the River Wandle. The water can dissolve the underlying chalk forming underground flow paths that occasionally collapse. If water is concentrated by humans, it may trickle down to the chalk and form solution features. Such features are usually covered over, so no one may know of the risk, until a circular or similar depression appears in your garden or your hallway. Away from the Wandle - Sutton Road, subsidence in chalk areas is usually caused by leaking drains, leaking water dissolving the chalk.

Topography plays a part in the assessment of risk, especially in clay areas. In the hilly Upper Norwood area, the combination of London Clay soil and the hilly terrain, seems to increase the risk, although the hilly terrain may introduce a phenomenon known as Clay Creep. Houses already underpinned ought to be safer, provided that the underpinning has been designed correctly. Whilst there may be a warranty, this warrant merely claims the work has been done as specified. Houses close to trees of high water demand are more at risk than those close to trees of low water demand. I recently had to redesign an underpinning scheme. Here, even foundations close to an oak tree (with the highest water demand you can find in the UK) had been specified to go to a depth of just 2.50 metres; this was shown reduced on the drawing to 2.0 metres. When checked by excavation, we found foundations to just 1.8 metres with oak tree roots under the 1.8 metre level, and a difference in floor level of 8 inches from close to the oak tree and remote from it.

Another guide as to the potential problems, is to look for cracks. Usually the surveyor nominated by your Mortgagors will call for a report by someone such as myself, if he sees cracking. Usually he/she is right to be concerned. After working out what is wrong, we try to provide a solution, but believe some people will try to put property back on the market. With the law now changed as regards disclosure, a buyer should have his Solicitor ask about past surveys, especially by Chartered Engineers. You cannot legally copy one of our reports, but its existence and the knowledge of the property is still on the market probably without action being taken, is a fair warning. With the change in weather, we may now be encountering, with heavier winter rainfall and hotter summers, the effects of subsidence. may now have greater importance in the selection of where you buy your house.