Subsidence may be caused by:
• London clay
• Leaking drains
This part of the south east has a thick layer of marine ‘London’ clay laid down 60-50 million years ago when this area lay under a warm tropical sea. A layer consisting of a mix of clays, silts and sands – Claygate Beds – overlies the London clay, with a top layer of sand deposited by the enormous Bagshot river that ran from west to east across the whole of Wessex 50 million years ago. Hampstead is on the top and southerly slopes of one of London’s sandy hills. These hills remained after glacial melt water from the last ice ages washed away much of the surrounding areas down to the London clay. The top of Hampstead’s hill consists of this Bagshot sand through which water passes easily. The next Claygate Beds layer provides a semi-permeable barrier that checks the water and sends it out as springs at the boundary zone, for example those on the meadow below Kenwood House. These springs provide the water for the many surface wells around Hampstead and the rivers of north London – the Fleet, Tyburn and Westbourne rivers.
Claygate Beds allow a small amount of water to pass through, but the next layer, London clay, is much less permeable. Water does not pass through it, except via sand partings left behind by older streams and springs, so some further springs will still be found at this boundary zone.
Building on Clay
The Georgians and Victorians built many terraces throughout London. They knew of clay’s characteristic of swelling when wet in winter and shrinking in summer. This change in volume, roughly 7%, means an individual house, provided it has uniform foundations, should move up and down uniformly, avoiding subsidence, with the plasticity of lime mortar absorbing any slight movement. Terraces stretch over a wider area, so the Georgians and Victorians, in their wisdom, generally floated them on the surface of clay with small shallow fixings, so that the whole terrace went up and down with the seasons without showing signs of subsidence. Lathe and plaster internal walls, within their framework of wooden skirting board / wainscot and ceiling mouldings, keep movement isolated and help prevent cracking of the exposed walls.
Add an extension however, with different foundations and rigid cement mortar, and differential subsidence can occur. Digging out a basement too can cause the following:
• Fixings into the deeper denser clay mean that part of the building moves less than the rest, resulting in differential subsidence. The modern basement will also be made completely water impermeable, damming up any water previously passing through sand partings, causing lakes to form underground. This occurred behind the Royal Free Hospital and was responsible for the major subsidence of St Stephen’s Church; vibration from the piling speeding up the slide down the hill, lubricated by the water.
If the effect of a basement is to constrain flowing ground water into narrower gaps, perhaps under neighbouring buildings, this can cause sand to wash away. Drains or foundations lose their support, or cause the surrounding clay to swell considerably more than the usual seasonal change, attracting tree roots. Trees do not contribute to subsidence without the primary role of water acting on clay first. Since they will be part of the solution and give us great pleasure, taking them out is not only an immediate waste of money, it means much more needs to be spent later, when the true cause is discovered. A lack of understanding of the true action of water on clay and trees, and an insufficiently strong legal protection for trees in situations where subsidence is occurring in the presence of trees, are all contributing to the gradual denuding of Hampstead’s magnificent canopy cover.
(The London Tree Officers Association has been alerted to the effect of leaking drains by their work with the engineer Jeremy Johnson, and are gathering data to investigate this further. Jeremy Johnson has provided us with an article that explains how leaking drains cause subsidence. We also believe that geology can have a similar effect in Hampstead when water in springs and sand partings is diverted and starts causing problems. Diverted water can also mean diversion away from trees.)
Our advice when subsidence cracking becomes evident is first to consider whether it is of real concern. Insurance companies report that they do not consider 40% of claims – those claims made for cracks of a millimetre or less. Householders should consider this, since the mention of subsidence increases premiums. The application of a filler and a new coat of paint may be sufficient and much less costly.
If the cracks are significant and not known to have occurred before, then the source of leaking water or the cause of excess water should be sought.
This may be a down pipe whose joints have come apart, or a soil pipe underground going to or from a manhole. Nearby basement or foundation work will have altered ground pressures which may reduce the support for such pipes causing them to crack or joints to come loose. Trees will expand their trunks and the large roots just below the trunk over time. This can push on pipes placed very close to tree trunks, or trees that have been planted over them. It makes sense to move the pipes out of harms way, and preferably not to plant trees over pipes in the first place. It is a fallacy that the small fibrous tree roots sometimes found inside drainage pipes are able to break pipes open. Roots don’t know the water is there until the pipe is already cracked. In Hampstead this is much more likely to be caused by the local hydrogeology with its several boundary zones and zones of relatively unstable land on slopes, or the effect of basement digging on ground pressures. Thames Water may be able to tell you of local drain pipes that have burst or are leaking. These should be rapidly mended to stop more sand being washed away or clay expansion alleviated as soon as possible. Trees aid in re-balancing the situation.
f you have signs of subsidence and heave in your road, with pot holes and an undulating surface, it may be that adjoining basement work or a burst water main preceded it. Looking down pot holes can reveal the existence of quite large cavities that have formed under roads. Interference in the ground water can contribute to both subsidence (slow wash-out of material) and heave (clay swelling). Remember too that swimming pools can be dug into the floor of a basement so they make the basement – and thus the block to ground water flow – several meters deeper. Swimming pools built into clay on a slope are also notorious for eventually cracking. Slow leaks of pool water will cause clay expansion/heave and have been known to lubricate the movement of buildings down clay slopes. Basement boundary walls on slopes have to take the weight of the hill above them if there are local slip surfaces in the clay made by glacial movement during the last ice age. If local vibration from machinery and diverted ground water is present, this will shake and lubricate this slippage.
These are just some of the reasons for concern for basements built in hilly Hampstead with its springs and geological boundary zones.
Eric Robinson has written an explanation of the local geology.
See also Jeremy Johnson’s article “Leaking Drains: the real cause”.