Types of Foundation and Methods of Construction of Foundation
Foundations provided immediately beneath the lowest part of the structure, near to the ground level are known as shallow foundations. Such foundations are mostly placed on the first hard and firm strata available below the ground level.
Shallow foundations are further classified into the following types:
1) Spread Footing Foundations :
Foundations constructed by increasing the are at the base of the structure by means of offsets, are called spread footing foundations. In such foundations, spread is given under the base of a wall or a column by providing offsets. This spread is known as footing and the foundation itself is called spread footing.
Broadly speaking, all types of shallow foundations can be referred to as spread footing foundations. However ,from design and construction point of view, they have been designated separately. The various types of spread footing foundations are ;
a) Wall footings:
This is a common and the simplest type of spread footing foundation; It consists of a number of courses of bricks, the lowest being usually twice the thickness of wall above. In this type of foundation, the base width of the wall is increased by providing 5cm (one-fourth of brick length) offsets on either side of the wall. The depth of each course is usually 10 cm
In some cases, however, the bottom course is made 20cm deep. In the case of footing for store walls, the size of offset is slightly more than that of the brick wall footing.
Method of Construction of Wall Footing:
For constructing such a footing, a bed of lean cement concrete (1:8:16) is first laid over the entire length of the wall. The thickness of this lean concrete bed is usually kept 15cm and its width is kept 20 to 30 cm more than that of the bottom course.
In no case, the depth of the concrete bed should be less than its projection beyond the wall base. For foundations over firm soil or compacted ground, the concrete bed below the brick wall footing may be dispensed with. After laying the concrete for constructing the concrete bed, it should be properly compacted and cured before laying the base course of the wall.
This is the cheapest type of spread footing foundations and is largely used for walls of ordinary buildings.
b) Masonry Pillar Footing:
Isolated footings are used to support the individual pillars and columns constructed in brick or stone masonry. They are stepped down in the wall footing after providing 15cm offset of foundation concrete on all the sides.
c) Concrete Column Footing:
These are either stepped type, slate type or slope type, having projections in the vae concrete. To support heavy loads, reinforcement is also provided at the base. The reinforcement provided is in the form of steel bars and is placed in both directions. Concrete column footings may be either isolated footings or combined footings. Isolated footings are used to support the individual columns, whereas combined footings are used where projections of different columns are not possible on all the sides, due to limited space.
The combined footings are generally rectangular in shape when both columns are of same sections and trapezoidal in shape when the columns are of different sections.
2) Grillage Foundation :
The foundation which consists of one or two tiers of wooden or rolled steel section with space filled up with concrete is known as Grillage foundation. This is so called because the bed constructed in this type of foundation is called grillage. This type of construction avoids deep excavation and provides the necessary area at the base of the structure to reduce the intensity of pressure within the safe bearing capacity of the soil.
The object of this type of foundation is to spread the load over a large horizontal area at the base of a structure.
Depending upon the material used in construction, grillage foundations are further classified into two types.
Steel Grillage Foundation
Method of construction of steel grillage foundation:
For construction of the timber grillage foundation,
Suitability of steel grillage foundation:
Steel grillage foundations are useful for structures like columns, piers, stanchions subjected to heavy concentrated loads and hence are employed for foundations of the buildings such as theaters, factories, town, halls etc. Timber grillage foundations re usually provided for timber columns subjected to heavy concentrated loads.
3) RAFT FOUNDATION :
The foundation consisting of a thick R.C.C slab covering the whole area of a mat is known as raft foundation.
Method of construction of Raft Foundation:
This type of foundation is useful for public buildings, office buildings, school buildings, residential quarters etc, where the ground conditions are very poor and bearing power of the soil is so low that individual spread footing cannot be provided.
4) STEPPED FOUNDATION :
For constructing this type of foundation, excavation is done into steps having short length and uniform thickness and the masonry work is done on the horizontal bed of concrete thus prepared.
If there is any possibility of slipping of the structure bodily, R.C.C piles can be driven along its base concrete on the sloping side.
5) INVERTED ARCH FOUNDATION :
The foundation consisting of inverted arches between the piers is known as inverted arch foundation. In this type of foundation, the load from the piers is transferred to the soil by constructing arches in inverted position at their at their base. The rise of the inverted arches is about one – fifth to one – tenth of the span and they are usually build in half-brick rings. The position of arches may be either along the row of piers or across the row of piers in both the directions depending upon the nature of soil and the type of land to be taken by them.
METHOD OF CONSTRUCTION :
For constructing this type this type of foundation;
This of foundation is not commonly used for buildings, but it is quite suitable for other structures like bridges, reservoirs, tanks, supports for drainage lines etc.
The foundation constructed sufficiently below ground level with some artificial arrangements such as piles, wells etc, at their base are called deep foundations. Deep foundation are further classified into the following types ;
1) PILE FOUNDATION :
A foundation ( spread footing or grillage ) supported on piles is called a pile foundation. A pile foundation usually consists of a base of spread footing or grillage supported by piles at their bottom. Piles distribute the load of structure to the soil in contact either by friction alone or by friction combined with bearing at their ends.
This type of foundation is suitable under the following situations ;