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

Areas used for examples are in south London. However, the same types of problem may occur where or close to where you live or want to live. For instance problems with chalk can occur in the South Downs area near Brighton or in the North Downs area near Luton. In clay areas, as found in many areas in and near Croydon, subsidence occurs due to clay shrinkage. Such shrinkage is normally caused by two actions: evaporation of water from the soil due to the warm weather and take-up of water by tree roots.

In clay areas, if one part of your house is close to a tree then it will undergo greater downward movement than the other part. If the effect is large your house may start to crack. When a single tree is involved, the cracking is normally diagonal, but when many trees are involved the cracking pattern is more complicated. Wandle and areas west of Wandle give rise to the sources of the river Wandle which flows on to Wandsworth. Areas of underlying chalk exist in the hilly parts of South Croydon, Purley, or close to the road running west from Wandle through Carshalton to Sutton.

Subsidence can be induced by the chalk on which such areas are built, dissolving. Such "solution features" are either caused by excess water building up in one area or by the flow of underground water through zones of chalk which are weaker than others. So, if your rainwater gullies leak and you are in a chalk area, you risk finding the chalk has softened and started to dissolve. The pressure on the "putty chalk" so formed, will increase and the part of your house close to the gully will start to subside.

Underground water flow can also cause movement to occur on the line of the flow of water. I have dealt with a case of damage on a line between the back left corner of a house and the front right corner of the house. Under the church and graveyard area near Beddington Park, there are said to be underground tunnels dug, said to have been dug possibly to aid escape from the church. More likely, these underground tunnels were formed by the passage of underground water and grave digging has added to the disturbance.

In the West Croydon area one encounters a type of soil that was deposited by rivers or inland lakes which existed there in the past. Having been deposited by water action, such soil is readily disturbed by any new water action. Subsidence in this area is usually due to too much water rather than too little and sometimes if minor cured just by repairing the drains. Another form of subsidence seen recently in Reading, Norwich and Edinburgh for example, arises from the effects of past quarrying. Usually a useful layer of ground was mined, leaving the soil on top to be supported by pillars of unmined soil. Timber props were often placed, but they may rot. As the rate of removal of the layer was about 90%, any disturbance to the remaining "pillars" can prove disastrous to housing built on the surface above the voids.

This problem has not been seen in Croydon, at least so far. However, just north of the Addiscombe Road, quarrying of gravel took place in days gone by; the areas excavated were reinstated in a way that now causes subsidence of some houses built there. Loosely filled sites often gives rise to subsidence due to "collapse compression". Excess water will lubricate the soil in such an area and combined with the overall weight of the soil and possibly with other effects such as decay, can cause collapse. Some areas of Merton experience this type of subsidence and our new weather patterns may help to promote it.