Types of Foundations
Based on the position with respect to ground level, Footings are classified into two types;
1. Shallow Foundations
2. Deep Foundations
Shallow Foundations are provided when adequate SBC is available at relatively short depth below ground level. Here, the ratio of Df / B < 1, where Df is the depth of footing and B is the width of footing. Deep Foundations are provided when adequate SBC is available at large depth below ground level. Here the ratio of Df / B >= 1.
1 Types of Shallow Foundations
The different types of shallow foundations are as follows:
• Isolated Footing
• Combined footing
• Strap Footing
• Strip Footing
• Mat/Raft Foundation
• Wall footing
Some of the popular types of shallo w foundations are briefly discussed below.
a) Isolated Column Footing
These are independent footings wh ich are provided for each column. This type of footing is chosen when
• SBC is generally high
• Columns are far apart
• Loads on footings are less
The isolated footings can have diffe rent shapes in plan. Generally it depends on the shape of column cross section Some of the popular s hapes of footings are;
The isolated footings essentially co nsists of bottom slab. These bottom Slabs can be ei ther flat, stepped or sloping in nature. The bottom of the slab is reinforced with steel mesh to r esist the two internal forces namely bending moment and shear force.
The sketch of a typical isolated foot ing is shown in Fig. 1.
b) Combined Column Footing
These are common footings which support the loads from 2 or more columns. Combined footings are provided when
• SBC is generally less
• Columns are closely spaced
• Footings are heavily loaded
In the above situations, the area required to provide isolated footings for the colu mns generally overlap. Hence, it is advantageous to provide single combined footing. In some case s the columns are located on or close to property line. In such cases footings cannot be extende d on one side. Here, the footings of exterior and i nterior columns are connected by the combined foo ting.
Combined footings essentially consist of a common slab for the columns it is supportin g. These slabs are generally rectangular in plan. Sometimes they can also be trapezoidal in plan (refer Fig. 2). Combined footings can also have a connecting beam and a slab arrangement, which is similar to an inverted T - beam slab.
c) Strap Footing
An alternate way of providing com bined footing located close to property line is the s trap footing. In strap footing, independent slabs below columns are provided which are then connec ted by a strap beam. The strap beam does not re main in contact with the soil and does not transfer a ny pressure to the soil. Generally it is used to com bine the footing of the outer column to the adjace nt one so that the footing does not extend in the adjoining property. A typical strap footing is shown in Fig. 3.
d) Strip Footing
Strip footing is a continuous footing provided under columns or walls. A typical strip footing for columns is shown in Fig. 4.
e) Mat Foundation
Mat foundation covers the whole plan area of structure. The detailing is similar to two way reinforced solid floor slabs or flat slabs. It is a combined footing that covers the entir e area beneath
a structure and supports all the wa lls and columns. It is normally provided when
• Soil pressure is low
• Loads are very heavy
• Spread footings cover > 50% area
A typical mat foundation is shown in Fig. 5.
2 Types of Deep Foundations
Deep foundations are provided wh en adequate SBC is available at large depth below GL. There are different types of deep foundations. Some of the common types of deep foundations are listed below.
• Pile Foundation
• Pier Foundation