Types of shallow foundations – Foundations are broadly classified into shallow foundations and deep foundations. This article is a total overview of shallow foundations and the types of Shallow foundations.
What is a shallow foundation ?
Shallow foundations transfer the load laterally to the soil. It is also called stripped foundations. The depth of a shallow foundation is less than its width. Shallow foundations are adopted when the load acting on a structure is reasonable and has a competent soil layer capable of negotiating the loads available at a shallow depth or shorter depth.
A shallow foundation is placed on the surface of the ground. The depth of a shallow foundation can range anywhere between 1 meter to 3.5 meters and sometimes more. The width of the shallow foundation is greater than the depth.
Types of shallow foundations
There are different types of shallow foundations adopted as per site conditions and design requirements.
Shallow foundation – Spread footing or isolated footing
The spread footing is one of the most commonly used types of shallow foundations. They are also called isolated footing or individual footings. Spread footings are further classified into simple spread footing, sloped spread footing, and stepped spread footing based on the shape of the footing.
- Simple spread footing
- Sloped spread footing
- Stepped spread footing
Simple spread footing
This is a common type of spread footing. Simple spread footing consists of a base footing with a single column over it. This type of foundation is used for structures with reasonable loads and bearing capacities.
Sloped spread footing
In this type of foundation the footing is sloped as shown in the figure. The footing carries a single column. The cross section of these types of footings are trapezoidal.
Stepped spread footing
When the loads are high steps are provided in the footings as shown in the figure.
Types of shallow foundations- Strip footing
Strip footings are also called wall footings. They are used for providing load-bearing brick/stone/RCC walls over the footings. Strip footings run continuously throughout the wall area of a building. These types of footings are also used when the spacing between the columns is very less and the footings overlap each other.
Types of shallow foundations – Combined footing
Combined footing consists of two or more columns over a single footing. These types of footings are adopted when the distance between the two individual footings are very less and overlap each other. A combined footing is also provided in areas where further excavation is not possible due to any flushing with the boundary. A combined footing is classified as rectangular combined footings and trapezoidal combined footings.
Strap footings are also called cantilever footings consisting of two individual footings connected through a beam strap. The beam strap is designed as a rigid structure. These types of foundations are economical than combined footings.
Mat or Raft foundations
Raft foundation – One of the most commonly used types of foundation in construction is a continuous slab resting on the soil and covering the total area of the proposed structure. There are different types of raft foundations based on their applications. The selection of the type of raft foundation depends on a lot of factors like bearing capacity, loads, site conditions, etc.
Raft foundation/Mat foundation is a solid slab placed at a designed depth spreading over the entire area of the structure. Raft foundations consist of columns and shear walls for transferring loads coming on the structure to the ground. These types of foundations are mainly used when the bearing capacity of the soil is low and becomes difficult for individual footings to negotiate the loads. The raft foundation helps to transfer the entire load of the structure to a larger area.
Also read : RAFT FOUNDATION – TYPES & ADVANTAGES
Also read : PILE FOUNDATIONS – TYPES & ADVANTAGES
Types of shallow foundations – Suitability
Shallow foundations are very easy to construct and do not require highly skilled manpower and professional supervision. These foundations can even be done with the help of medium-skilled workers. A shallow foundation is very economical when compared with a deep foundation. Shallow foundations are end bearing type foundations that transfer loads to the end of the foundation.
Shallow foundations are considered as the most preferred option when the safe bearing capacity of the soil is reasonable and the structural loads are within the permissible limits.