Foundation Engineering

Foundation engineering is the utilization of soil mechanics and rock mechanics in the plan of foundation elements of structure. Foundations are usually notable as the bases that are situated beneath the ground level and they move the structural load to the ground. The foundation is the initial stage of a building construction. The foundation of a building varies with its nature of soil, environmental condition and the type of a building. The selection of types of foundation in construction considers the soil properties underlying the area, position of water table, tolerance of the structure and the type of structure and loading conditions.

Here we have discussed about the basic requirements of foundation and types of foundation in construction i.e. types of shallow foundation and types of deep foundation.

Basic requirements of Foundation

Some of the basic requirements of foundation are:

• 1. The foundation must be properly located considering any future influences which would adversely affect its performance particularly for footings and mats or rafts.
• 2. A foundation must be safe against shear strength failure or soil rapture.
• 3. The settlement of a foundation especially the differential settlement must be within permissible limits.
• 4. In general the foundation should be placed at a depth where soil stratum is adequate from point of view of bearing capacity and settlement.

So, these are the mostly followed basic requirements of foundation that are also used for identifying the types of foundation in construction site.

Types of Foundation in Construction

There are generally two types of foundation in construction:

1. Shallow Foundation

2. Deep Foundation or Pile Foundation

Shallow Foundation

A shallow foundation, as per Terzaghi, is one whose width is more prominent than its depth. Shallow foundations are found just beneath the most minimal piece of the wall or a column which they support. Footings are primary individuals, made of block work, workmanship or concrete that are utilized to send the heap of column or wall to such an extent that the heap is disseminated over an enormous region. In fact, a footing is an enlargement of the base of the column or wall it supports. The most regularly used types of foundation in construction is the shallow foundation.

Types of Shallow Foundation

There are generally 5 types of shallow foundation:

1. Strip Footing

To bear the load of walls, a strip footing is provided. A strip footing is also provided for a row of columns which are so closely spaced at their spread footings overlap or nearly touch each other. In such a case, it is more conservative to give a strip footing than to give various spread footings in a single line. A strip footing is otherwise called as continuous footing.

2. Spread Footing or Isolated Footing

A spread footing or isolated footing is given to help an individual column. Spread footings are generally of circular, square or rectangular chunks of shape of a uniform thickness. Sometimes it is ventured or haunched to spread the load over an enormous region. A spread footing is also known as pad footing.

3. Combined Footing

A combined footing supports two columns. It is used when the two columns are so near one another that their individual footings would overlap. A combined footing is likewise given when the property line is so near one column that a spread footing would be unconventionally stacked when kept altogether inside the property line. By joining it with that of an inside column, the load is equitably circulated. A combined footing might be rectangular or trapezoidal in arrangement.

4. Strap Footing or Cantilever Footing

A strap footing or a cantilever footing comprises of two isolated footings associated with a primary strap or a lever. The strap associates the two footings with the end goal that they act as one unit. The strap basically goes about as an associating beam and doesn’t take any soil response. The strap is planned as an unbending beam. The individual footings are intended to such an extent that their joined line of activity goes through the resultant of the absolute burden.

A strap footing is more practical than a combined footing when the suitable soil pressure is moderately high and the distance between the columns is enormous.

5. Mat Foundation or Raft Foundation

A mat foundation or a raft foundation is an enormous piece supporting various columns and walls under the whole construction or the huge piece of the design. A mat foundation is required when the suitable soil pressure is low or where the columns and walls are near the point that singular footings would cover or almost contact one another.

Mat foundations are useful in reducing the differential settlements or non-homogeneous soils or where there is a large variation in the loads on individual columns. The design is limited to the determination of the depth of footing, soil pressure, shear force and bending moments.

Deep Foundation or Pile Foundation

At the point when the soil at or close to the ground surface isn’t fit for supporting a structure, deep foundations are needed to move the loads to the deeper layers. Deep foundations are along these lines utilized when surface soil is unsatisfactory for shallow foundation, and a firm layer is profound to the point that it can’t be reached financially by shallow foundations. Deep foundations are the types of foundation in construction that is not regularly followed unless there is necessity in a site.

A deep foundation is by and large significantly more costly than a shallow foundation. It should be embraced just when a shallow foundation isn’t feasible. In specific circumstances, a completely repaid floating raft might be more practical than a deep foundation. In some cases, the soil is improved by various methods to make it suitable for a shallow foundation.

Types of Deep Foundation

There are generally 6 types of deep foundation:

1. Pile Foundation

Pile foundations are used when the strata at or just below the ground surface is highly compressible and very weak to support the loads transmitted by the structure. This type of foundations are required for the transmission of the structural loads through deep water to a firm stratum. Piles are required when the soil conditions are to such an extent that a wash out, erosion or scour of soil may happen from under a shallow foundation.

Pile foundations are used to resist the horizontal forces in addition to support the vertical loads in earth retaining structures and tall structures that are subjected to horizontal forces due to wind and earthquake. Collapsible soils, such as loess have a breakdown of structure joined by an unexpected reduction in void ratio when there is an expansion in water content.

Piles are utilized to move the heap past the zone of possible moisture changes in such soils. In the event of expansive soils, for example, black cotton soil, which swell or shrink as the water content changes, piles are utilized to move the load underneath the active zone.

2. Caissons

A caisson is a sort of foundation of the state of the hollow prismatic box, which is worked over the ground level and afterward sunk to the necessary depth as a solitary unit. It is a watertight chamber utilized for establishing foundations submerged as in rivers, lakes, harbors, etc. The caissons are of three types:

• Open Caissons: Open caissons are of hollow chambers, open both at the top and the bottom. The lower part of the caisson has a bleeding edge. The caisson is sunk into place by eliminating the soil from within the shaft until the bearing layer is reached. Well foundations are special type of open caissons used in India.
• Pneumatic Caissons: Pneumatic caissons are closed at the top but open at the bottom. A pneumatic caisson has a working camber at its bottom in which compressed air is maintained at the required pressure to prevent entry of water into the chamber. So, these type of excavations are done in dry.
• Floating Caissons: Floating caissons are open at the top but closed at the bottom. These caissons are developed ashore and afterward shipped to the site and floated to where these are to be finally installed. These are sunk at that spot by filling them with sand, ballast, water or concrete to an evened out bearing surface.

3. Cylinders

These foundations are placed when there is required to place only a single cylindrical unit.

4. Basement Foundations

These are hollow or empty bases intended to give working or extra room in the subterranean level. The foundational layout is administered by the utilitarian prerequisites instead of from contemplations of the most proficient technique for opposing outer earth and hydrostatic pressing factors. They are generally constructed in open excavation places.

5. Buoyancy Rafts

Buoyancy rafts are hollow or empty foundations intended to give a light or semi-light base underneath which the net loading on the soil is diminished to the ideal low intensity. Buoyancy rafts can be constructed in sunk caissons or open excavations and they are more expensive then the general form of foundations.

6. Drilled Shaft Foundations

A drilled shaft foundation is a large diameter concrete cylinder built in the ground. For construction of a drilled shaft foundation, an enormous distance across opening is drilled in the ground and thusly loaded up with concrete. The difference between a drilled shaft foundation and a bored foundation is basically the size. Generally, bored foundations are of diameter less than or equal to 0.6 meter. The shafts of size larger than 0.6 meter are generally designated as drilled shaft foundation. A drilled shaft foundation is a type of deep foundation constructed to transfer heavy axial or lateral loads to a deep stratum below the ground surface.

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