As promised a while ago we can now go a little more in-depth into foundation movement and its causes. Please read our older post What to inspect when buying a home? for foundations overview.
Foundation (soil) movement associated with reactive clays which swells when wet and shrinks when dried out, is a well known fact among most people, however there are other type of soils that can cause equally sufficient movement to cause masonry buildings and their footings to fail.
These are classified into two types, Reactive soils and Non Reactive soils a third one is soil with excessive organic matter in the soil. The latter causes the soils to become spongy and thus has little or no compaction capabilities, if a building is erected on such soil then the organic matter is squeezed together thus producing movement and the bearing of the soil is reduced and foundation failure is imminent.
Reactive soils, as the name suggest, are soils that are usually made of clay or fine silts, these soils then swell when water is introduced and shrink when dried out. Most areas especially in Perth are well identified, like the Darling escarpment and the lowland below, specially designed footings are recommended to be used when building in this clay type soil. Recommended in these areas of course are timber framed homes, as they can tolerate far greater foundation movement without the risk of excessive structural damage. The other alternative of course is the pile your footings into the soil so deep that seasonal changes in moisture content does not affect the pile upon which the house is resting. This method is rather expensive but the best.
Non Reactive soils, are mainly sandy soils either granular or fine and they are mostly very stable but still require even compaction to stop uneven settling which can cause cracking. 7% or more clay material in a sandy soil is regarded as getting towards the reactive spectrum and can cause shrinkage and swelling when wetted.
Settlement is caused when a building that has been erected imposes a load on the soil causing the soil to consolidate. If the footings of the building have been correctly designed, then the load that is imposed by the building will be evenly spread to the footings and the consolidation will be almost negligible.
However if the soil under is of a clay nature then if any moisture is contained in the soil it will then be very slowly squeezed out of the clay and the initial consolidation will be dependent on the depth of the clay vein and the weight of the home, and the area upon which it is built.
In sandy soil the consolidation is relatively quick as the sand granular particles roll into place due to the pressure that is applied by the house footings.
Continued next week:-