Building on Clay Soils
Clay soils can be subject to dramatic changes in volume as their moisture content varies. Clay soils contain very fine mineral particles, silt and sand. The ratio of each of these determines how the clay will shrink and expand. It is usual to excavate foundations to a depth where the moisture content of the clay remains stable.
British Standard 8004 recommends a minimum depth of one metre for foundations, but if the clay is very soft you may need a special foundation specified by an engineer.
Any foundation excavated in a clay soil should be concreted as soon as possible after it is dug. Even left over night it can dry out or get wet, in which case it will need to be deepened a further 50 to 100mm. Foundations built over a dried out trench bottom may suffer heave when the clay takes up moisture and swells; if built over a wet trench bottom, the clay may swell which will cause settlement later when it compresses under load.
If your conservatory is being built close to trees and shrubs, you should also read the following page.