Types of Foundations
In this article, broad guidelines for fixing the dimensions of foundation are given and different types of foundations are explained.
Dimensions of Foundation
Guidelines for minimum dimensions are given below:
(a) Depth of Foundation: For all types of foundations minimum depth required is calculated using Rankine’s Formula:
where p = safe bearing capacity of soil
w = unit weight of soil
ϕ = angle of repose of soil.
However in any case it is not less than 0.9 m. Finding safe bearing of the soil is an expert’s job, and it is found after conducting tests in field or in Laboratories. However general values for common soils are listed in Table 1.
(b) Width of Foundation: Width of wall foundations or size of column footing is determined by first calculating the expected load and then dividing that with SBC.
Thus, Width of wall foundation = Load per unit length of wall/S.B.C. of soil
Area of column footing = Load carried by column/S.B.C. of soil
Conventional Spread Footings
These types of foundations are commonly used for walls and masonry columns. These foundations are built after opening the trenches to required depth. Such footings are economical up to a maximum depth of 3 m. As these foundations are suitable depth, they are grouped under shallow foundations. Figure 1 shows a conventional spread footing for a wall and Fig. 2 shows it for a masonry column.
Before building these footing trenches are opened to required depth and the soil is rammed well. Then a plain concrete of mix 1:4:8 is provided. Its thickness varies from 150 to 200 mm. Over this bed, stone masonry footing is built. It is built in courses each course projecting 50 to 75 mm from the top course and height of each course being 150 to 200 mm. In case of wall footing the projections are only one direction while in case of columns, they are in both directions. The projection of bed concrete from the lowest course of foundation masonry is usually 150 mm.
There are mainly two types of R.C.C. footings:
1. One Way Reinforced Footing: These footings are for the walls. In these footings main reinforcements are in the transverse direction of wall. In longitudinal directions there will be only nominal reinforcement.
2. Two Way Reinforced Footings: For columns two way reinforced footings are provided.
The following types of the footings are common:
(i) Isolated Column Footings: If separate footings are provided for each column, it is called isolated column footing. Figure 3 shows a typical isolated column footing. The size of footing is based on the area required to distribute the load of the columns safely over the soil. These footings are provided over a 100 to 150 mm bed concrete. Required reinforcements and thickness of footing are found by the design engineers. Thickness may be uniform or varying.
(ii) Combined Footings: Common footings may be provided for two columns. This type of footing is necessary when a column is very close to the boundary of the property and hence there is no scope to project footing much beyond the column face.
Figure 4 shows a typical combined footing. The footing is to be designed for transferring loads from both columns safely to the soil. The two columns may or may not be connected by a strap beam.
(iii) Continuous Footings: If a footing is common to more than two columns in a row, it is called continuous footing. This type of footing is necessary, if the columns in a row are closer or if SBC of soil is low. Figure 5 shows this type of footing.
(iv) Mat Footing/Raft Footing: If the load on the column is quite high (Multistory columns) or when the SBC of soil is low, the sizes of isolated columns may work out to be to such an extent that they overlap each other. In such situation a common footing may be provided to several columns as shown in Fig. 6. Such footings are known as raft footings.
If the beams are provided in both directions over the footing slab for connecting columns, the raft foundations may be called as grid foundation also. The added advantage of such footing is, settlement is uniform and hence unnecessary stresses are not produced.
High rise buildings are built with steel columns encased in concrete. Such columns carry very heavy load and hence they need special foundations to spread the load to a larger area of soil. Grillage foundation is one such special foundation. It consists of one tier or more tiers of I-sections steel beams.
Figure 7 shows a typical two tier grillage foundation. Top tier consists of less number but large size steel section while lower tier consists of larger number but smaller size steel sections. Column load is transferred to the top tier through a base plate.
The grillage beams are unpainted and are encased in concrete with minimum cover of 100 mm beyond the edges of steel sections. A minimum clear space of 75 mm should be maintained between the flanges of adjacent grillage beams so that concreting can be made properly. To maintain spacing, pipe separators are used.
Inverted arch foundations are provided in the places where the SBC of the soil is very poor and the load of the structure is through walls. In such cases inverted arches are constructed between the walls.
End walls should be sufficiently thick and strong to withstand the outward horizontal thrust due to arch action. The outer walls may be provided with buttress walls to strengthen them. Figure 8 shows a typical inverted arch footing.
Foundations in Black Cotton Soil
Black cotton soil swells during rainy season and cracks in summer due to shrinkage. These shrinkage cracks are 100 mm to 150 mm wide and 0.5 m to 2 m deep. Swelling creates upwards pressure on the structure and shrinkage creates downward pull. It results into cracks in foundations wall and roof. Hence foundations in black cotton soil need special care.
In case black cotton soil is only to a depth of 1.0 m and 2.0 m it is economical to remove entire black cotton soil from the site and build the foundation on red soil.
Apart from this black cotton soil should be removed from the sides of the foundation and filled with sand and gravel.
In case the depth of black cotton soil is more, the following type of foundation may be provided:
1. Strip or Pad Foundation: Strip foundations are for walls while pad foundations are for columns. In these foundations the attempt is to keep black cotton soil from foundation by interposing layers of sand and gravel. These foundations should be constructed during dry season. Suitable plinth protection should be made around external walls with its slops away from the wall, so that moisture do not penetrate the foundation during rainy season. Figure 9 shows such foundations.
2. Pier Foundation with Arches: A pier is a vertical column of relatively larger cross-section than piles. For walls carrying heavy loads, piers are dug at regular intervals and filled with plain concrete. These piers are taken up to good bearing strata. Then the piers are connected by concrete or masonry arch. Over these arches regular masonry is built. Figure 10 shows a typical pier foundation with arches.
3. Under Reamed File Foundations: Under reamed piles are bored and then concreted at the sites. Their length may vary from 3 to 6 m. They are provided with reams and reinforcement. The pile spacing varies from 2 to 4 m. The top of piles are provided with capping beams over which walls are built.