The performance of a foundation is based on interface between the loadings from the superstructure and the supporting ground or strata. The nature and conditions of each of these varies; so, the selection of appropriate foundation becomes necessary for these variations depending on circumstances.
Selection of Foundation based on Ground Conditions:
The ground or soil condition is necessary for determining the type of suitable foundation. The soil on which the industrial, commercial or residential building rests may be stable, level and of uniform composition, but in some situations it may be otherwise.
Following are criteria for selecting suitable foundation based on soil conditions:
Where soil close to the surface is capable of supporting structure loads, shallow foundations can be provided. Where the ground close to surface is not capable of supporting structural loads, hard strata is searched for, and in some cases, it may be very deep, like in case of multi-storey buildings, where loads are very high. So, deep foundations are suitable for such cases. Field up ground have low bearing capacity, so deep foundation is required at that place, whereas uniform stable ground needs relatively shallow foundation. Level of the ground also affects foundation selection. If the ground is not levelled, and has gradient then step foundation may be preferred.
Selection of foundation based on Loads from the superstructure:
The loading condition i.e. type and magnitude of loads, depends on the form and type of building to be constructed. In case of low rise building with large span, the extent of loading is relatively modest, so shallow foundation is preferred in this case. While high-rise building with short span has high loads. Therefore, deep foundation is required in such cases. Deep foundation is provided because ground at greater depth are highly compacted.
Foundation is the part of the structure which transmits the loads acting on the structure and the self-weight of the structure, safely to the ground/subsoil.
The first and foremost consideration while designing the foundation is the type of superstructure and the number for floors to be constructed.
In case of framed structure multi-storey building, where loads are concentrated at the point of application, the use of pads and piles are common. Where, loads of the buildings are uniformly distributed, like from masonry claddings, the piles are not needed.
TYPES OF FOUNDATIONS
The terms shallow and deep foundation refers to the depth of soil in which the foundation is made. Shallow foundations can be made in depths of as little as 3ft (1m), while deep foundations can be made at depths of 60 - 200ft (20 - 65m). Shallow foundations are used for small, light buildings, while deep ones are for large, heavy buildings.
SHALLOW FOUNDATIONS
Shallow foundations are also called spread footings or open footings. The 'open' refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing. During the early stages of work, the entire footing is visible to the eye, and is therefore called an open foundation. The idea is that each footing takes the concentrated load of the column and spreads it out over a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of the soil.
There are several kinds of shallow footings: individual footings, strip footings and raft foundations.
In cold climates, shallow foundations must be protected from freezing. This is because water in the soil around the foundation can freeze and expand, thereby damaging the foundation. These foundations should be built below the frost line, which is the level in the ground above which freezing occurs. If they cannot be built below the frost line, they should be protected by insulation: normally a little heat from the building will permeate into the soil and prevent freezing.
individual footings

Individual footings awaiting concreting of the footing column.
Individual footings are one of the most simple and common types of foundations. These are used when the load of the building is carried by columns. Usually, each column will have its own footing. The footing is just a square or rectangular pad of concrete on which the column sits. To get a very rough idea of the size of the footing, the engineer will take the total load on the column and divide it by the safe bearing capacity (SBC) of the soil. For example, if a column has a vertical load of 10T, and the SBC of the soil is 10T/m2, then the area of the footing will be 1m2. In practice, the designer will look at many other factors before preparing a construction design for the footing.

Individual footings connected by a plinth beam. Note that the footings have been cast on top of beds of plain cement concrete (PCC), which has been done to create a level, firm base for the footing.
Individual footings are usually connected by a plinth beam, a horizontal beam that is built at ground or below ground level.
strip footings
Strip footings are commonly found in load-bearing masonry construction, and act as a long strip that supports the weight of an entire wall. These are used where the building loads are carried by entire walls rather than isolated columns, such as in older buildings made of masonry.
Raft or mat foundations

Raft Foundations, also called Mat Foundations, are most often used when basements are to be constructed. In a raft, the entire basement floor slab acts as the foundation; the weight of the building is spread evenly over the entire footprint of the building. It is called a raft because the building is like a vessel that 'floats' in a sea of soil.
Mat Foundations are used where the soil is week, and therefore building loads have to be spread over a large area, or where columns are closely spaced, which means that if individual footings were used, they would touch each other.
Read our page on raft foundations.
DEEP FOUNDATIONS
pile foundations
A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground so that structures can be supported on top of it.
Pile foundations are used in the following situations:
When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer. When a building has a very heavy and concentrated loads, such as in a high rise structures. Pile foundations are capable of taking such higher loads than spread footings.
There are two types of pile foundations, each of which works in its own way.
End Bearing Piles
In end bearing piles, the bottom end of the pile rests on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts like a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load therefore bypasses the weak layer and is safely transferred to the strong layer.
Friction Piles
Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil.
To visualise how this works, imagine you are pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream. Once you have pushed it in, it is strong enough to support some load. The greater the embeddedment depth in the ice cream, the more load it can support. This is very similar to how a friction pile works. In a friction pile, the amount of load a pile can support is directly proportionate to its length.
In practice, however, each pile resists load by a combination of end bearing and friction.
The type of super structure and their classification
The type of super structure can be classified into 2 types:
Load Bearing Structure Framed and Load Bearing Structures:
Although load-bearing structures are very rare now, they are particularly useful for small structures like sheds, garages, etc. They are used by low rise residential houses also (G+1 floor construction). The foundation for load bearing structure can be as shown in the drawing below. The foundation runs all along the load bearing walls in this type of structure.

For low rise framed structures the most common type of foundation used is isolated RCC pad footings with grade beams, made of RCC (reinforced cement concrete).
But with the increase in the number of floors in the structure, the load to be carried by the foundation system increases and consequently the area of the foundation needs to be increased. After the proper calculation of loads, the type of foundation system suitable for the building (superstructure) is decided. In multi-storey buildings the loads are concentrated at the point of application, use of pad and pile foundations are common. Where the loads are uniformly distributed, piles are not needed.
The next consideration is the soil type and its SBC (safe bearing capacity). The soil type indicates if the soil is prone to settlement and uplift, and the SBC gives the load bearing capacity of the soil. A soil investigation prior to construction gives the requisite characteristics of the soil, particularly about the depths of the different layers of soil, called soil profile, by drilling bore holes in the site at suitable intervals. If the soil close to the surface is capable of supporting structural loads, then shallow foundations are used, else deep foundations need to be adopted for the structure.
The soil type also gives the depth of the water table. The data from the soil investigation gives enough basis for the selection of the foundation type.
In any case, particular attention shall be paid to:
Groundwater level Underground water courses Old drains, wells, & pits Old foundations Presence of sulphates and other aggressive/injurious chemicals in the groundwater and soil
Also Read in details : Types of Foundation used in Building construction
Suitability of foundation on the basis of soil type
Clayey Soils (Laterite Soils and Black Cotton Soil have high clay content):
This is the most abundant type of soil in India (on the basis of percentage share of total area). It has a high moisture retention capacity; hence, it is prone to expansion and uplift which results in uneven settlement and cracks in foundation, plinth and grade beams and on-grade floor slabs.
Best Practices in Clayey Soils (Laterite Soils and Black Cotton Soil)
Collect rainwater (and runoff) and either store it or dispose it in drains via proper drainage structures. This reduces the amount of water infiltrating the soil, and causing expansion. If the depth of the water table is quite low, which means the water table is closer to the ground surface. It is advisable to extend the depth of the foundation, below the zone prone to moisture content fluctuation.If the depth of the black cotton soil at a given site is only 1 to 1.5 m, the entire black cotton soil above the hard bed may be completely removed and the foundation may be laid on the hard bed below. The swelling of soil in direct contact with the foundation material causes maximum damage. Hence it is necessary to prevent the direct contact of black cotton soil with masonry work below ground level. These can be achieved by making wider trenches for foundation and filling spaces on the either side of the foundation masonry with sand or moo rum. One and two storey buildings and residences are particularly vulnerable to damage because they are not heavy enough to suppress the uneven settlement, as compared to multi-storeyed buildings. The structural loads should be balanced properly so that the load on the foundation is as uniform as possible. This should be given due importance while designing the layout/plan of the building. Construction in black cotton soil should be undertaken during dry season.

Loamy soil
Loam soil is the ideal soil for construction: typically it’s a combination of sand, silt and clay. It is dark in color and soft, dry and crumbly to the touch. Loamy soil is great for supporting foundations because of its consistently balanced properties, especially how it maintains water at a balanced rate.
Recommended Foundation Type:
Loamy soil is suitable for construction of isolated pad footings. The area and depth is again governed by bearing capacity, depth of groundwater table, and depth of load bearing stratum. Isolated pad footings are provided under columns and are joined/tied together by connecting beams. If the building has a single storey then continuous strip foundation can be used but isolated RCC pad foundations are preferable provided they are economical.

Desert soils are basically of sandy texture. This type of soil has poor clay content and also lacks in moisture content.
Recommended Foundation Type:
In desert soils, multi-storeyed buildings usually have pile foundations, especially driven piles or augured cast in-situ piles. In case of loose sand extending to a great depth, raft foundation maybe appropriate as spread footings may settle excessively.

Silty soil can be smooth to the touch and retains water longer because of its smaller particles. However, because of its tendency to retain moisture it is cold and drains poorly. This causes the silty soil to expand, pushing against a foundation and weakening it, making it not ideal for support.
Recommended Foundation Type:
If ‘firm silt’ or ‘firm silty clay’ deposit extending to great depth is available, then RCC spread and isolated pad footings are appropriate. The depth of the foundation should be greater than the zone of erosion and the zone of swelling and shrinkage.

Mountain soils and forest soils are most likely to have boulders at various depths, so pile foundations are not suitable. Isolated RCC pad foundations under stilts are usually preferred for buildings on sloping grounds. For residential buildings with one or two floors, Isolated RCC pad foundations are suitable. For single storeyed buildings RCC strip foundations are suitable.
Careful site selection is important in hilly, mountainous and forest regions/areas. No residential buildings should be constructed on slopes steeper than 25 degrees.
Tips for Foundation Selection:
Pile foundations are recommended for high rise buildings and whenever uplift is expected. If the allowable bearing capacity is available only at a greater depth, the foundation can be rested at a higher level for economic consideration and the difference in the level between the base of the foundation and the depth at which the allowable bearing capacity occurs can be filled with either –Concrete of allowable compressive strength not less than the allowable bearing pressure in compressible fill material for example sand, gravel, in which case the width of the fill should be more than the width of the foundation.
The design and choice of the foundation system is only as good as its execution in the field, hence the choice of foundation should be made keeping in mind the ease with which it can be executed/constructed at the site. The construction of different types of foundation requires labour of different skill set and different level of skills.
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