Types and Uses of Foundation in Construction
Foundation is one of the most essential substructures on which a building stands. It is beneath the ground soil and the ground on which it rests is called foundation bed. Foundation transfers the load from the structure constructed on it to the soil. Generally, the foundation is provided to:
- distribute the weight of the structure over a large area of soil;
- avoid unequal settlement;
- prevent the lateral movement of the structure; and
- increase structural stability.
Categories and Types of Foundation
Foundation is broadly divided into two categories: shallow foundation and deep foundation. The foundation to be chosen depends largely on the soil profile, as well as the size and load of the structure to be constructed.
Shallow foundations are also known as spread footings. The depth of the foundation is less than its width. The depth is usually 6ft from the lowest finished floor. Shallow foundations are basically used in cases where the sufficient bearing capacity of the soil is shallow and when the foundation material does not result in undue settlement. They are mostly used for low-rise building construction. There about five types of shallow foundation.
- Individual footings
- Strip footings
- Combined footings
- Raft or mat footings
- Strap or Cantilever footings
This is also called isolated footings and is the most widely used foundation. Individual footing is used to support a single column and is mostly suitable when columns are not closely spaced, when the loads on the footings are less and the safe bearing capacity of the soil is usually high at low depth. This type of footing is economical compared to others because the requirement of the shuttering material is less. Also, it requires no skilled labour and is its simple shape makes it time-saving. The sub-types of individual footing include stepped footing which are provided with steps in foundation sides. The second is simple spread footing where only foundation base is constructed and followed by a column. The third type is called slopped footing which has a foundation base and is also followed by a column.
This is also known as wall footing. It is a continuous footing constructed at the load bearing masonry construction where walls will be acted as load bearing structure. The footing is constructed continually under the walls to support the load developed on the walls. It is used to distribute loads of structural or non- structural load-bearing walls to the ground in such a way that the load-bearing limit of the soil isn't outperformed. It runs along the direction of the wall.
The width of the strip foundation is often 2-3 times the width of the wall. The materials used in the constructed include brick, stone and reinforced concrete.
The combined footing is constructed when two or more columns are close enough and their isolated footings overlap each other. It is a combination of isolated footings, but their structural design differs.
The shape of this footing is a rectangle and is used when loads from the structure is carried by the columns.
Raft footing is usually used in the construction of basements. The entire basement floor slab acts as the foundation while the weight of the building is spread evenly over the footprint of the building. Raft footing is used when the soil is week so that the building loads are spread over a large are. It is also used where columns are closely spaced. It is designed as a single mat of all the load-bearing elements of the structure to prevent differential settlement of individual footing. Raft footing is economical when one-half of the structure is covered with individual footings and wall footings are provided.
This is similar to combined footings as it is chosen for reasons identical to those of combined footings. In cantilever footing, the foundation under the columns is built individually and connected by a strap beam. Usually, the exterior footing is connected by a strap beam with interior footing if the edge of the footing cannot be extended beyond the property line.
Deep foundations are deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions. They are usually 3m depths below finished ground level and can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface. The sub-types are discussed below.
- Pile foundation
- Drilled Shaft or Caissons foundation
Pile foundation is made of concrete, timber, or steel. It is like a small-diameter column which is driven into or cast in the ground and is mainly used in bridge construction. Pile is used when the soil below the foundation does not have the enough sufficient bearing capacity to carry the load of building into deep soil up to hard strata. Pile foundation is used to transmit loads to the lower level of the ground by the combination of friction pile and end-bearing pile at the pile point or base. There are end bearing pile, friction pile, settlement reducing pile, tension piles, laterally-loaded piles and piles in fill.
This is similar to pile foundations but is known as high capacity cast-in-situ foundation. Caisson foundation resists loads from structure through shaft resistance, toe resistance and/or combination of both of these. It is constructed with an auger. Caisson foundation is capable of transferring column loads larger than pile foundations and is used where the depth of hard strata below ground level is located within 10m to 100m.
Caisson foundation is not suitable when deep deposits of soft clays and loose, water-bearing granular soils exist. It is also not suitable for soils where caving formations are difficult to stabilize, soils made up of boulders, artesian aquifer exists.