London Clay is a very high or extremely high shrinkage clay and the most likely clay material in the London area to give rise to subsidence or heave. It is normally classified when tested as extremely or very high shrinkage clay. In London Clay areas, or Head deposits made from London Clay, subsidence is normally by clay shrinkage, and that is caused normally by root action.
A Head deposit is a reconstituted deposit, reconstitution probably being caused by an ice age. In London, it is usually London Clay that is the material involved. Head deposits frequently contain organic material, the organic content deriving from the crushing of the trees and vegetation by the ice. Where a drain leaks in a Head Deposit area, a heave can result. Building on an organic clay can lead to tertiary settlement lasting more than a century.
Woolwich and Reading Beds
The material constituting Woolwich and Reading Beds varies but the clay component is normally analysed as a high shrinkage clay, and thus is less susceptible to causing subsidence than is London Clay.
Thanet Sand is a typical sand, normally fairly strong and well-graded, but in the course of time can be affected by the action of water. In this type of material, where subsidence occurs, it is usually due to a concentration of water washing away some of the soil beneath the foundations.
Upper Chalk normally gives a good foundation material for houses but in some instances one can have a high decomposition in the top layers of the chalk and in some instances there can be an overlying shallow clay deposit above the chalk. One also has to be careful also of such effects as swallow holes, caused by leaking drains and similar dissolving the chalk. Swallow holes can remain undetected when a house is built and thereafter give rise to subsidence.
Alluvial Deposits, having been deposited by water, are susceptible to the action of water. Thus subsidence in such areas can be caused by leaking drains, or leaking water supply pipes.
River Terraces are usually more granular in content than alluvium and, having been laid down by the action of a river as their name implies, are usually located at the bottom of a valley or on its side. In such an area, the type of material to be expected can not usually be defined until the material has been sampled.