A shallow foundation is a type of foundation that transfer load to the very near the surface. Shallow foundation are those foundation in which the depth at which foundation is placed is less than the width of the foundation (D<B). This foundation used when the surface soil are sufficiently strong and stiff to support the imposed load. They are generally unsuitable in weak and highly compressible soil.
DIFFERENT TYPES OF SHALLOW FOUNDATIONS
- Isolated footing
- Combined footing
- Strap or Cantilever footing
- Strip footing
- Mat/ Raft footing
1. ISOLATED FOOTING
Spread footing provided to the column of a framed structure is called isolated footing, column footing or pad footing. A isolated footing is circular, square or rectangular slab of uniform thickness. Sometime it is stepped or haunched to spread the load over a large area. Square column footing are the most economical but space restriction between adjacent column in a specific direction may warrant rectangular column footing. Circular footing is not common and may be used for circular column as the construction of form work and concreting may be more difficult for them then for square or rectangular footings.
2. COMBINED FOOTING
A combined footing supports two columns. It is used when the two columns are so closed to each other that their individual footing would overlap. A combined footing is also provided when the property line is so close to one column that a spread footing would be eccentrically loaded when kept entirely with in the property line. By combining it with that of an interior column the load is evenly distributed. A combined footing may be rectangular or trapezoidal in plan.
3. STRAP OR CANTILEVER FOOTING
A strap footing consist of two isolated footing connected with a structural strap or a lever. The strap connects the two footing such that they behave as one unit. The strap is designed as a rigid beam. The individual footings are so designed that their combined line of action passes through the resultant of the total load. A strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and the distance between the column is large.
4. STRIP FOOTING
A strip footing is provided for a load bearing walls. A strip footing is also provided for a row of columns which are so closely spaced that their spread footing overlap or nearly touch each other. In such a case it is more economical to provide a strip footing than to provide a number of spread footings in one line. A strip footing is also known as continuous footing or wall footing. Strip footing is the first and most conventional footing used in the history of civil engineering and may be constructed of stone masonry or concrete.
5. MAT/ RAFT FOOTING
Raft foundation are used to spread a load from a structure over a large area normally the entire area of the structure. They are used when column load or other structural loads are close together and individual pad foundations would interact. A raft foundation normally consist of a concrete slab which extend over the entire loaded area. It may be stiffened by ribs of beams. Raft foundation have the advantage of reducing differential settlement as the concrete slab resist differential movement between loading positions. They are often needed on soft or loose soil with low bearing capacity as they can spread the load over the large area. 