Shallow Foundations support structures at a shallow depth below the ground surface or at a shallow depth below the deepest basement of a building. There are three main types of shallow foundations:
- Spread Footings
Square or rectangular shaped blocks of reinforced concrete that typically support a single column or wall of a building. The allowable bearing pressure on the soil determines the size of the spread footing.
- Combined footings
Two or more columns are supported on a reinforced concrete footing. They are often used where one of the columns is located along a property line and it’s not possible to extend the spread footing beyond the edge of the building.
- Mat foundations
This foundation extends below the entire building and distributes the load from multiple columns and walls of the structure to the soil below. May be used for an entire building or a portion of the building. They are sometimes used to resist water pressures when groundwater extends above the level of the floor of the lowest basement of a building. The selection of the type of foundation to be used for an individual structure is typically a function of the expected loads and the allowable bearing pressures. Typically, a mat foundation is only considered when the area of the spread footings or combined footings exceeds about 50 percent of the gross area of the building.
The design of a spread footing is relatively simple compared to the rigorous analysis necessary for the design of a mat foundation. Mat analyses require estimates of the elastic modulus of the subgrade soil to determine the distribution of loading to the subgrade, bending moments and shear for design of the mat foundation. Mat foundation design usually involves use of one of a number of computer programs.