- A structure essentially consists of two parts, namely the super structure which is above the plinth level and the substructure which is below the plinth level.
- The foundation of a building is that part of walls, piers and columns in direct contact with the ground and transmitting loads to the ground.
- Every building needs a foundation of some kind.
- Substructure is otherwise known as the foundation and this forms the base for any structure. Generally about 30% of the total construction cost is spent on the foundation.
- The soil on which the foundation rests is called the “foundation soil”.
Objectives of a foundation
A foundation is provided for the following purposes:
- To distribute the total load coming on the structure on a larger area so as to bring the intensity of load within the safe bearing capacity of soil.
- To support the structures.
- To load the bearing surface at a uniform rate to avoid differential settlement.
- To attain a level and firm bed for building operations.
- To give enough stability to the structures against various disturbing forces, such as wind and rain.
- To prepare a level surface for concreting and masonry work.
- Footings are structural members used to support columns and walls and to transmit and distribute their loads to the soil in such a way that the load bearing capacity of the soil is not exceeded, excessive settlement, differential settlement, or rotation are prevented and adequate safety against overturning or sliding is maintained.
Types of Foundation
1. Shallow Foundation
- Isolated footing
- Combined footing
2. Deep foundation
- Pile foundation
- Pier foundation
Isolated Footing (Single footing, column footing)
- In framed structures where several columns are to be constructed, isolated footings can be adopted.
- The columns involved can be provided with masonry or concrete footing.
- If masonry footing is provided steps are given and the foundation area is thus increased so that the stresses developed at the base in within the limit.
- A combined footing is the one which supports two columns and it may be rectangular or trapezoidal in plan.
- The aim is to get uniform pressure distribution under the footing. For this the center of gravity of the footing area should coincide with the center of gravity of the combined loads of the two columns.
- Combined footings are proportioned in such a way that the centre of gravity of the loads coincides with the centre of gravity of the foundation. Hence these footings have either a trapezoidal or a rectangular shape.
- Pile is an element of construction used as foundation. It may be driven in the ground vertically or with some inclination to transfer the load safely.
- Loads are supported in two ways.
- Friction piles may be made of cast iron, cement concrete, timber, steel, wrought iron and composite materials.
- If the load is supported by resting the pile on every hard stratum, it is called load bearing pile.
- Load bearing piles are steel sheet piles, concrete piles and timber piles.
- A pier and beam foundation, also referred to as a post-and-beam foundation, is a foundation that has a crawl space between the structure and the ground. Pier & beam foundations were standard in homes built before the 1960s with pier and beam construction, the exterior wall of the home is supported by masonry foundation. The interior of the home sits above the floor joist supported by foundation beams, which in turn are supported by concrete piers.
- This type of foundation is mostly used on buildings with post & beam style construction. It is important to have the pier under each supporting wall post. Pole barns may also be built with this type of foundation to protect posts from rotting.
- A pier foundation consists of a cylindrical column of large diameter to support and transfer large super-imposed loads to the firm strata below.
Bearing capacity of soil
- It is defined as the maximum load per unit area which the soil will resist safely without displacement.
- The bearing capacity of the soil can be found by loading the soil, nothing the settlement and by dividing the maximum load by the area on which the load is applied. The maximum load is obtained from the graph between the settlement and load.
- Safe bearing capacity of soil = (ultimate bearing capacity of soil)/(factor of safety).
- On completion of a structure there may be some displacement in the position of the foundation.
- The maximum differential settlement should not exceed 25mm in case of foundations on sandy soil and 40mm in case of foundations on clayey soil.
- The bearing capacity of claimed soils or shrinkable soils can be taken as 50kN/m2 in the absence of the site data.
Requirements of a good foundation
- The foundation should be so located that it is able to resist any unexpected future influence which may adversely affect its performance.
- The foundation should be stable or safe against any possible failure.
- The foundation should not settle or deflect to such an extent that will impair.
- A site investigation is required to determine the chemical and physical properties of the soil.
- Determine the magnitude and distribution of loads from the superstructures.
- Establish the criteria and the tolerance for the total and differential settlements of the structure.
- Determine the most suitable and economic type of foundation.
- Determine the depth of the footings below the ground level and the method of excavation.
Factors affecting design of foundation
- Soil types and ground water table conditions.
- Structural requirements and foundations.
- Site condition and environmental factor.
- The type of construction
- The magnitude of load
- The seismic hazard and vulnerability of site to earthquake.
Material used for foundation
- Waterproofing materials
Protecting foundation against moisture
- Damp Proofing
- Typically, a liquid asphalt applied with a roller or sprayer not an effective barrier for water under pressure.
- But, will prevent ground ‘Moisture’ from migrating through a wall. Typically used in conjunction will drainage pipe.
- Underpinning is the process of strengthening the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons.
- The original foundation is simply not strong or stable enough.
- The usage of the structure has changed.
- The construction of nearby structures necessitates the excavation of soil supporting existing foundations.