Foundations are the most crucial part of any structure, whether it is a building, a bridge or a dam. A foundation is a structure that transfers the load of the superstructure to the ground below. There are two types of foundations: shallow and deep. Shallow foundations are used when the soil near the surface has enough bearing capacity to support the load of the structure. On the other hand, deep foundations are used when the soil near the surface has insufficient bearing capacity or when the load of the structure is too heavy. In this article, we will discuss the types of shallow and deep foundations in detail.
Shallow Foundations:
A shallow foundation is a foundation whose depth is equal to or less than its width. Shallow foundations are also called open foundations since they are constructed by digging into the soil. The following are the types of shallow foundations:
Spread Footing: In this type of footing, the base of the structure is gradually widened, so that the mass of the structure can be distributed over a larger area. It provides support to columns or walls. Each column or wall has its own separate footing. Spread footings for columns are square, rectangular, round, etc.
Strap Footing: Two or more separate column footings joined together by beams to form one footing is called cantilever or strap footing. Strap footing is used when the bearing capacity of the soil is low or when the load on the outer column is too high.
Combined Footing: Combined footing provides support to two columns. This footing is mainly done in the following conditions: when two columns are very close and their footings overlap, if the bearing capacity of the soil is low or if the column ends fall on the boundary line and there is no scope for extending the footing beyond the boundary line.
Mat or Raft Foundation: Raft or mat is a combined footing that provides simultaneous support to all the walls and columns of the structure. Raft foundations are used when the bearing capacity of the soil is low, and structures with additional concentrated loads are constructed.
Deep Foundations:
A deep foundation is a foundation whose depth is greater than its width. Deep foundations are not built by digging ditches or pits; instead, they are built by drilling into the soil. The following are the types of deep foundations:
Pile Foundation: Pile foundation is a type of deep foundation that is used when the soil near the surface has insufficient bearing capacity or when the load of the structure is too heavy. Piles are slender structural members of steel, concrete or wood. Piles are driven into the ground or dug into the ground and filled with concrete in place. Pile foundations are used when the soil below ground level is more compressive and weak, unable to support the structure due to low bearing capacity, or if the settlement is uneven due to uneven load on the structure in shallow foundation construction.
Caisson or Well Foundation: Caisson or well foundation is a type of deep foundation that is used in construction in water. Caisson or well foundation is a large hollow cylinder made of steel, concrete or wood. It is sunk into the ground under its own weight or with the help of a dredging pump. Caisson or well foundation is used in the construction of piers, bridges, docks, etc.
Cofferdam: A cofferdam is a temporary structure constructed around a work area to keep water out and facilitate construction activities. It is a watertight enclosure built to enable construction work below the waterline. The cofferdam may be constructed of sheet piles, steel plates, or reinforced concrete. The inside of the cofferdam is dewatered so that workers can carry out construction activities in a dry environment. Cofferdams are used in the following cases-
(a) To construct pier or abutment of a bridge in water or river.
(b) To create dry workspaces for bridge or tunnel construction.
(c) To protect existing structures from the adverse effects of hydraulic forces, such as floods or tidal waves.
Conclusion:
The choice of foundation type depends on several factors such as the soil type, load, climate, and construction cost. Shallow foundations are commonly used when the soil near the surface has a sufficient bearing capacity. They are easier to construct and are cost-effective for small to medium-sized structures. Deep foundations are used when the soil near the surface has a low bearing capacity, and the load is heavy. Pile foundations, caissons, and cofferdams are used to transfer structural loads to deeper layers of soil. The proper design and construction of a foundation are crucial to the safety and durability of any structure.
Shallow foundation and Deep foundation
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