When the foundation is placed immediately beneath the lowest part of the super structure, it is termed as shallow foundation. The object of this type of foundation is to distribute the structural loads over a wide horizontal area at shallow depth below the ground level.
The various types of foundations which can be included under shallow foundation are:-
1. Spread Footings
In case of spread footings, the base of the member transmitting load to the soil is made wider so as to distribute the load over wider area. All types of foundation can be covered under the term spread. However, from design and construction point of view they can be treated separately.
2. Grillage Foundation
When heavy structural loads from columns, piers or stanchions are required to be transferred to a soil of low bearing capacity, grillage foundation is often found to be lighter and more economical. This avoids deep excavation and provides necessary area at the base to reduce the intensity of pressure within safe bearing capacity of soil. Depending upon the material used in construction grillage.
3. Eccentrically Loaded Footings
As far as practicable, the foundation, (it may be for a wall or column), should be so shaped and proportioned that the center of gravity of the imposed loads coincide withe center gravity of the supporting area of base. However, when walls or columns are to be placed close to property lines, that required supporting areas of the base cannot be placed concentrically with the imposed loads without overlapping the property lines.
4. Combined Footings
A combined footing is so proportioned that the center of gravity of the supporting area is in line with the center gravity of the two column loads. A combined footing may be rectangular or trapezoidal in shape. Rectangular shape is only possible where loading condition is such that either the two columns are equally loaded or the interior column carries greater load. On the other hand, in case of trapezoidal footing, no such condition is applicable.
5. Mat or Raft Foundation
In made-up ground, soft clay or marshy site having low value of bearing capacity, heavy concentrated structural loads are generally supported by providing raft foundation. Also the structure is liable to subsidence on account of its being located in mining area or due to uncertain behaviour of its sub-soil water condition, raft foundation should be preferred. It provides an economical solution to difficult site conditions, while pile foundation cannot be used advantageously and independent column footing becomes impracticable.
Raft foundation consists of thick reinforced concrete slab covering the entire area of bottom of the structure like a floor. The slab is reinforced with bars running at right angles to each other both near bottom and top face of the slab. Sometimes it is necessary to carry the excessive column load by an arrangement of inverted main beams and secondary beams, cast monolithically with the raft slab.