Subsidence

On a purely statistical basis, the coming summer is projected to be hot. This means that areas of South London built on clay are likely to suffer subsidence, especially areas built on London Clay. Subsidence also seems to be worse when a dry summer follows a wet winter. However, subsidence has a variety of causes. Whilst often thought of as occurring in late summer/early autumn, it can also occur after heavy and/or prolonged rain but for different reasons.

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

Insurers define subsidence in words such as “downward movement on which the structure stands”. When filing an insurance claim for subsidence it is important to bear this in mind. One has to make the distinction between subsidence and settlement. Settlement occurs when the soil is compressed by additional weight such as the construction of a new house. One tries to design foundations to keep the pressure on the soil fairly uniform, so that settlement, which is fairly inevitable, is uniform and the house owner doesn’t know it is occurring.

If settlement is not uniform however, there may be cracking. However, if settlement does occur, it happens more rapidly shortly after the structure has been built and is usually therefore covered by any NHBC warranty. However, one must be careful when building a new extension. The old part of the house has undergone whatever settlement it is going to undergo but the new part will inevitably undergo some settlement of its own unless built on rock. The designer should ensure that the pressure on the foundations of a new extension is controlled to minimise this effect. Moreover the joint with the original house should be carefully designed with the possibility of settlement in mind.

Piled foundations significantly reduce settlement but usually cost more. However, if the only way of removing excavated soil arising from dug foundations is to carry it through your house, then piling may be a better solution on both scores as no or only a little excavation is then required.

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Where does subsidence occur?

Areas used for examples in this website 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. Our principal 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 in the chalky area of Beddington. 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, it can be 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.

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How can I deal with subsidence?

To cure your subsidence, you obviously need someone to first find out the cause. A chartered engineer experienced in such matters will obtain the necessary information about the soil and match it with the cracking patterns in your walls. Having formed a hypothesis about what is wrong, he/she will then arrange tests to check. Such tests may be as simple as drain testing or root analysis. With clay soil, it is also important to drill boreholes to obtain moisture profiles for the soil where the worst subsidence is located, at locations of suspected subsidence and also at a place away from any trees (but all boreholes will be located in areas of the same type of geology).

This helps the engineer to see if underpinning is needed and if so, how deep and how great an area is required. If chalk solution features are suspected, dynamic probes may be used. Where chalk voids as described above are found, these can be filled by pumping in grout. If subsidence seems to be caused by a leaking drain, the drain should obviously be repaired but underpinning may be needed, to ensure there is no other problem causing slight disturbance and allowing the subsidence to continue.

In most cases one tries to undertake underpinning or whatever other action is decided upon, before repairing any cracks, unless piles need to be driven next to an unsafe wall, which obviously must first be repaired to avoid it collapsing. Masonry repairs can be undertaken in several ways including injection with epoxy resin, local rebuilding of the wall, drilling holes for stainless steel bars with serrated surfaces and embedding stainless steel bars into the mortar courses.

Around the serrated surface of the bars, one places epoxy resin. Where the cracking is bad, more than one of these methods may be combined. To have your subsidence problems efficiently dealt with, we recommend you to engage the services of a chartered civil or structural engineer who has had formal training in the subject of soil mechanics and who has practical experience of analysing soil and designing foundations.

There is a very old method of piling known as screw piling. This method has been re-activated and modernised by the construction industry, but the building industry has not yet caught up. We can design screw piles.

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Choosing a house with subsidence in mind

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. 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 cost of claims occurring in that postcode or nearby postcode.

This totally ignores the reality that the risk depends mainly on the type of soil. Heavy clays like London Clay subside due to tree roots causing shrinkage, whilst subsidence in an area of River Terraces can be caused by soil being washed away by too much water. 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 chalk areas, there is the risk of solution features. These are formed by gases in water dissolving the chalk to form either a hole or a putty chalk area below ground level. Piling is usually the only realistic cure.

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