A foundation is a lower portion of building structure or substructure that transfers its gravity loads to the earth. The design of foundation depends on the total load on the building and the type of sub soil and its bearing capacity.
Foundations are generally broken into two categories mainly, shallow foundations and deep foundations.
Foundation with depth less than 3 meter are shallow foundations. Such foundations are used for structures that do not carry much load. They are also used in case the soil has a low weight bearing capacity.
Types of shallow foundations are –
- Isolated footing or column footing
- Combined footing
- Cantilever or strap footings
- Mat/Raft footings
- Wall Footings
- Raft footing
Foundations with depth greater than 3 meter are deep foundations. They are deep below the finished ground surface such that their base bearing capacity is not affected by surface conditions.
Types of deep foundations –
- Pile foundation
- Pile foundation
Foundation used depending on type of soil –
Rocks such as limestone, granite, sandstone and hard solid chalk have a high bearing capacity. When the soil is rocky with high bearing capacity, then strap foundations work well. The preferred depth of foundation is less than 0.7meter.
Gravel and sand
When the ground has gravel and soil in the sub base, then combined footing and strap foundations work the best. The depth of foundation is less than 1m. Such soils are extremely suitable for constructing foundations and have a good bearing capacity. When the capacity of bearing is more footing or strip foundation is used and when the bearing capacity is comparatively low, combined footing or wide stripped foundation is preferred.
Usually clay has a higher moisture content and when the moisture content becomes stable upto certain depth it becomes suitable for the substructure.Raft foundation is preferred when the clayey soil is compact and hard. Even strap foundation can also be used.
Soft clayey sand is prone to expansion and shrinkage. They have a low bearing capacity and are unsuitable for footing and strap foundation. In such case, Pile foundation is used. Piles are dug deeper until the stable strata is attained and is suitable for construction of substructure.
Peat is one of the poor subsoil for foundation. If a layer of peat is excavated from ground and a stable soil with bearing capacity is found, then strap foundations work well. In case no stable soil is found, then raft foundations are used.