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
Heave is often said to be the reverse of subsidence and this is probably a fairly simple way to introduce it. Heave, as known with domestic structures, arises when a clay is able to absorb more water than it had hitherto and thereby expand. There is a different type of heave than mentioned above that arises when one excavates an over-consolidated clay such a London Clay.
Here the reduction in pressure on London Clay gives rise to a heave as the clay tries to expand vertically to be restored back to what it was prior to being compressed in the past. If one were designing an underpass or office with a basement the prior investigation would need to be designed to predict the extent of this type of heave.
How heave occurs | How can I deal with heave?
Settlement should be distinguished from subsidence. Settlement is downward movement of that part of a site which is loaded by the weight of the structure or other material. Settlement takes place over a period of time depending upon the type of material involved. E.g.: a motorway embankment built on rather boggy ground will undergo fairly considerable settlement even at the bottom of the embankment as the boggy ground is compressed by the new weight of material. The top of the embankment undergoes even greater settlement because of both the movement at the bottom and consolidation within the embankment itself.
A house built on well- graded sandy material will undergo little settlement, whereas one built upon a weak clay will undergo settlement over a long period of time, perhaps twenty-odd years. If peat is involved there is no knowing how long the settlement will take and in such circumstances, one is better off piling the foundations down to a depth below the influence of the peat.
What are the effects of settlement? | How can I deal with settlement?
Recently there has been an increase in landslips due, it is believed, to high rainfall last winter. A hot summer is projected, and its effect could be to open up the clay shrinkage cracks deeper and wider than usual. Thus, during subsequent winter rain, especially taking into account our apparent new rainfall pattern, the enhanced hydrostatic pressure of water in these cracks, will again promote more landslip.
What is landslip? | Where does landslip occur? | How can I deal with landslip?