Types of Shallow Foundations: 4 Types | Soil Engineering

Following are the types of shallow foundations: 1. Spread Footing 2. Combined Footing 3. Raft Foundation 4. Annular Slab or Ring Foundation.

Type # 1. Spread Footing:

Foundation which spreads the load from a wall or column to a greater width is known as spread foundation or footing. The spread footing provided to the walls of a load bearing structure is known as wall footing, continuous footing, or strip footing. Spread footing may also be stepped footing as shown in Fig. 19.2(d) or tapered footing as shown in Fig. 19.2(e).

i. Strip Footings:

Strip footing, shown in Fig. 19.1, is the first and most conventional footing used in the history of civil engineering and may be constructed of stone masonry or concrete. Strip footing constructed of stone masonry usually has a stepped cross section, similar to the one shown in Fig. 19.2(d).

With the development of concrete, however, both load bearing structures and strip footings have become more or less obsolete, except for small lightly loaded residential buildings. It is also known as continuous footing or wall footing.

ii. Isolated Footing:

Spread footing provided to the columns of a framed structure is called isolated footing, column footing, or pad foundation. Square column foundations, shown in Fig. 19.2(a), are the most economical but space restrictions between adjacent columns in a specific direction may warrant rectangular column footings, shown in Fig. 19.2(b). Circular footing, shown in Fig. 19.2(c), is not common and may be used for circular columns as the construction of form work and concreting may be more difficult for them than for square or rectangular footings.

Type # 2. Combined Footing:

Combined footing is used when footings of two adjacent columns are too close or overlap.

i. Rectangular Footing:

Combined footing is most commonly rectangular, as shown in Fig. 19.3(a), for equal column loads.

ii. Trapezoidal Footing:

For unequal column loads, trapezoidal footing, shown in Fig. 19.3(b), may be used to ensure that the centre of gravity (CG) of the column loads coincides with the CG of the foundation in plan.

iii. Strap Footing:

For footings situated near property lines, a strap footing, as shown in Fig. 19.3(c), is used to ensure that the edge of the footing near property line does not extend into the adjacent site. In this case, the footing near the property line (exterior footing) is connected to the footing inside the site (interior footing) through a strap beam. The strap beam transfers the load of the exterior column footing partially to the interior footing through structural action.

Type # 3. Raft Foundation:

Raft foundation, covering the entire area of the loaded structure with a slab, is provided when the total area of all footings is more than 50% of the loaded area. A raft foundation also called as mat foundation is also provided for heavy structures located over highly compressible and weak soils extending to large depth. The plan and section of a raft foundation is shown in Fig. 19.4.

Type # 4. Annular Slab or Ring Foundation:

A ring foundation is sometimes provided for a large water tank with its columns connected through a ring beam and supported over an annular slab, as shown in Fig. 19.5.

If a good bearing stratum of soil or rock is available at a shallow depth, a shallow foundation is the first choice as it is more economical and is always preferred to a deep foundation.

The following are the advantages of shallow foundations when compared to deep foundations:

i Shallow foundations are usually more economical than deep foundations, and are preferred when a soil stra­tum of reasonable strength without excessive compressibility is available at a shallow depth below the ground level (GL).

ii. Construction of shallow foundations is simple as the depth involved in laying the foundation is less.

iii. The equipment required for the construction of shallow foundations is simple and less costly.

iv. Shallow foundations can be constructed in a short time, which would further help in reducing the cost of hiring equipment and labor.

v. The quantum of soil investigation required for shallow foundations is less because the pressure bulb extends to less depth as compared to deep foundations. There is also less uncertainty in the prediction of behavior of shal­low foundations and supporting soil. For deep foundations as the depth of soil involved is more, the uncertainty is also more.

vi Construction of shallow foundations would cause lesser disturbance to geo-surface and hence to the environ­ment and ecology.

The settlement of shallow foundations may be more and requires careful consideration. Otherwise, the structure may undergo damage or failure. In the case of deep foundations, when taken to hard stratum, the settlement of foundation and hence of structure may be less.

A deep foundation is adopted only when a shallow foundation cannot be provided. A deep foundation is pre­ferred when the soil at the surface is weak or highly compressible, and extends to a large depth from the ground surface and the cost of ground improvement together with a shallow foundation is more than that of a deep foundation.