A foundation in construction is the lowest part of the structure constructed in direct contact with the soil to distribute the load from a column or wall over a large area of the soil. The function of the foundation in construction is to transfer loads from the structure to the soil safely. The size of the foundation depends on the loads from the structure and the bearing capacity of the soil. Foundations are generally constructed in brickwork, masonry, or concrete under the base of a wall or columns.
There are broadly two types of foundations in construction, called shallow foundations and deep foundations. A shallow foundation is constructed at the maximum depth of 1.5m below the earth’s surface and transfers the load to hard strata present at a shallow depth. A deep foundation is constructed at a depth greater than 1.5m below the ground surface.
Shallow foundations are generally used for small residential buildings or small structures when the soil at shallow depth has sufficient bearing capacity. While deep foundations are used for tall buildings like a skyscraper or when a building is constructed on very weak soil. Deep foundations are also used for small buildings if there is a plan for the vertical extension of them in the future.
Read More: Types of Foundations in Construction with Their Uses
Function and Purpose of Foundation
The functions and purposes of foundations for all load-carrying structures are:
- To provide the stability to the structure. A stronger foundation provides more stability to the structure.
- To provide a proper surface for the development of the substructure at a proper level and over a firm bed.
- To avoid the lateral movements of the structure.
- Transferring the structural load to the earth and distributing them uniformly helps in the prevention of differential settlement of the building.
- To distribute the load from the structure over a large base area so that the pressure is within the allowable bearing capacity of the soil.
- To prevent the effect of soil movement on the structure.
- To prevent the scouring and the undermining issues.
Depth of Foundation
The depth of foundation depends on the following factors:
- Bearing capacity of the soil.
- Shrinkage and swelling in clayey soils due to seasonal changes causing soil movements.
- Depth of frost penetration in case of fine sand and silt.
- Possibility of future excavation works nearby
- Depth of groundwater table
- Removal of topsoil and variations in ground level. So, the minimum depth of the foundation should not be less than 50cm.
The recommended depth of foundation is from 1m to 1.5m from the ground level for small constructions.
Width of Foundation
The width of the foundation should be as per the drawings based on the structural design.
The minimum widths for the light loaded buildings such as houses, flats, and school buildings, with not more than two stories:
- 75 cm for one brick thick wall.
- 1m for one and a half brick wall.
Construction of Foundation
The procedure for the construction of foundations starts with marking the layout for excavation and the centerline of the foundation as per the construction drawing. Then the earth is excavated to the required depth as per the drawing or till the hard surface is reached. The surface of the excavated trench is leveled by using plain cement concrete (PCC) to get a strong base for the foundation construction.
Then a reinforcement cage is placed inside the excavated trench as per the reinforcement design, or the cage is directly made over the PCC surface for the construction of concrete foundations. This reinforcement cage provides strength to the foundation for the loading conditions. These reinforcements are connected with the superstructure above. Then formwork is placed to cover the length and width of the foundation to provide shape to the placed concrete. Concrete is poured into the foundation, compacted and the construction of the foundation is done.
A foundation can be constructed with stones masonry, and brick masonry other than concrete. Stones and bricks are generally used for load-bearing structures and for especially for walls. The selection of these materials also depends on the structure design loads and the type of soil.
The foundation for each structure is designed such that:
- The soil below the foundation structure does not undergo shear failure
- The settlement of the foundation is within the limit
- The pressure on the soil does not exceed the allowable bearing capacity of the soil.
Procedure for Construction of Foundation
The processes executed in the foundation works are given below:
- Marking the layout and centerline of the foundation on the ground surface with the length and width as per the drawing, with allowances for the movement and working.
- Excavation of earthwork in trenches.
- Laying of plain cement concrete to level the excavated trench surface.
- Laying of the reinforcement as per the design and drawing in case of concrete foundation construction. For brickwork or stone masonry foundations, the work can be started from the PCC surface.
- For a concrete foundation, the formwork is used to provide the shape of the foundation.
- Then concrete of the specified mix is poured into the formwork and it is well compacted.
- Once the foundation is set, the formwork is removed and the foundation is cured for a minimum of 14 days.
- The concreting for the columns begins.
- Once the columns are constructed to the ground level, the trench is refilled with the soil and the construction of the concrete foundation is complete.
- For the brickwork or stone masonry foundations, once it is completed above the ground level, up to the plinth level, the soil is refilled into the trenches.
