Soil type is undoubtedly one of the most underestimated features in a self build project, and can have a dramatic impact if unacknowledged. The purpose of foundations is to carry a weight over a surface. Weak types of soil or rock means low load-bearing capacity and therefore will require a specific type of foundation.
If you’re unsure of the type of soil on the plot on which you are planning to build, your local authority will be able to advise.
It’s important to be aware of the various types of soil in order to best plan your housing foundations.
Chalk soils can be erosive so it’s important to be aware of hollows or caves. Strip foundations are best used on chalk ground. These are strips of concrete which support load-bearing walls. Wide strip foundations are used to spread over larger areas and reinforced with steel to increase its load-bearing capacity.
Deep strip foundations are placed at a lower level in a deeper trench within the soil which has suitable bearing capacity.
Generally, due to the expansion and shrinkage of clay, it is necessary to excavate foundations to a depth where the clay is of consistent moisture. Compressible layers are used prior to concreting the foundations to line the trench.
Certain types of rock such as granite, limestone and shale have high load bearing capacity. Uneven rock surfaces may need to be leveled off and topsoil may require drainage.
Sand and gravel
As long as the ground has good load-bearing capacity, strip foundations can be used for dry gravel and sand subsoils.
Although sand usually holds together well, sheet piling is often used to preserve the ground prior to concrete being poured.
Peat is a weak subsoil and therefore reinforced raft foundations are likely to be required in this case. Raft foundations allow a structure to float on top of the soil. It requires large, wide strips which spread over a wide bearing area.