Foundation is the structural components of any structure which bears the load of the structure safely and transfer or distribute it suitably to the ground soil strata without exceeding the bearing capacity of the soil. The Difference between Shallow and Deep foundation is described below in detail.
Thus for the design or adaptation of any foundation, the maximum probable load of superstructure, area occupied by structure, conditions of the soil strata (i.e. Bearing capacity, groundwater table, types of soil like cohesive or non-cohesive, etc must be tested and analyzed at first.
Depending upon the depth and mechanisms of the load transfer to ground strata, they are broadly divided into two types, which are further divided into subtypes. The major types are,
- Shallow foundation
- Deep foundation
1. Shallow foundation
Shallow foundations are that foundation in which load is transmitted to soil underlying immediately below the substructure. That means the load is directly transferred by the foot of the foundation only.
They are generally used if the subsoil near the ground surface has adequate strength. However, they are monolithic to the entire structure they are a basic component or part of the structure to withstand the entire structure.
Even if the deep foundation is constructed due to inadequacy in bearing capacity, the shallow foundation(general raft over pile) is still constructed over the top of the deep foundation to transfer the load of the entire structure evenly and safely.
The basic criteria needed to be fulfil to make the shallow foundation is given by,
Where,D id depth of the foundation foot from the ground level
B is width of foundation
Types of shallow foundation
- Wall foundation or strip footing or continuous footing: These are thicker base area base wall constructed below the plinth level along with the ground soil.
- Individual footing or spread footing: These are the large base area foot constructed along the centerline of the column of structure, to a certain depth rested at the soil.
- Strap footing or cantilever footing: These are simply individual footing jointed together by tie beam to form the monolithic single substructure
- Combined footing: When the two or more columns are near two each other, the feet of a two-column structure are constructed as a single foot, then it’s called combined footing. Based on their shape it can be either rectangular footing or trapezoidal footing.
- Mat foundation or raft foundation: These are the thick reinforced slab constructed along the area covered by substructure in a certain depth below ground level.
Advantage of shallow foundation
- These foundations are easy to construct without the need for modern and complex machinery. Moreover, it requires less excavation. Thus cost during construction is less than that deep foundation.
- They can be constructed with help of simply skilled manpower with general thumb rules and basic design. Simple supervision and testing are enough to build such a foundation.
- Very advantageous if, bearing capacity of the soil is very high. No extra techniques to improve the bearing capacity of the soil is needed to be done.
- Advantages to small load-bearing structures like residential houses, simple commercial buildings.
- The settlement is reduced in compressive soil.
Disadvantages of shallow foundation
- The shallow foundation is not appropriate to construct over the soil having less bearing capacity.
- They are structural disadvantages if the groundwater table is high. Moreover, there will be a lot of expenditure on the dewatering process.
- They are not applicable to irregular and unstable soil structures as there is a high probability of differential settlement.
- They cannot withstand high load superstructure and eccentric loads.
- They are not suitable for the bank of rivers.
2. Deep foundation
In deep foundation, the load is transferred gradually from the ground surface to the bottom of the foundation which means that both lateral and bottom od the entire foundation is used as load transfer member to ground strata.
These foundations are used when the soil underlying the substructure doesn’t have the adequate bearing capacity, and thus higher bearing capacity of the soil is only obtained at the intense depth.
They are also used for the construction of massive superstructures that have an intense load, unstable soil with a higher groundwater table (like in bridge construction ), etc.
The basic criteria require to make the deep foundation is given by,
D/B > 1
Where,D id depth of the foundation foot from the ground level
B is width of foundation
Types of deep foundation
- Pile foundation: Piles are the long elongated slender (generally cylindrical) structure that is embedded deep inside the ground level until desire bearing capacity to. Transfer load safely is achieved. Their lateral surface plays more role in load transfer rather than the bottom area, and this action mainly occurs in frictional piles. However, they can also be rested on the stable bed strata to transfer load through the base and such action generally occurs in driven piles. The piles are sometimes also used to stabilize the soil strata. The numbers of piles are needed to be embedded on the ground due to the small cross-sec area and then their heads are covered with a single monolithic slab or raft as pile cap. And above them, superstructures are erected.
- Well foundation: In a good foundation, the big diameter holes are excavated removing the low bearing capacity soil. And then are lined with concrete or masonry with the bulk hard (generally concrete) strata at the base to make stable. The lined holed are then infill with the concrete or scrap of metals and graded soil mix. Then the filled hole (well) is covered with a good cap which is generally a raft foundation for the construction of the superstructure.
- Shaft foundation (horizontal load-bearing piles ): Piles are sometimes needed to withstand lateral forces like wind pressure, earth pressure, earthquake, water pressure, etc. And thus piles are even needed to be constructed at an inclination or simply vertical to withstand such lateral loads. They are usually of thicker diameter and short length compare to piles.
Advantages of deep foundation
- They can be even constructed in a high water table area. The dewatering process is comparatively less as only small areas with greater depth are needed to be excavated.
- The load is transferred throughout the foundation elements to the soil holding substructure rather than only at bottom of the foundation.
- They can be used in soil having less bearing capacity. Also, they are used to modify the bearing capacity of soil sometimes.
- They can withstand large superstructure loads. So the structure which has a probability of high load over them can be safely designed with decreasing risk of failure.
- They help to decrease unequal settlement and also are used to withstand eccentric loading.
- Sheet piles are even used to decrease the permeability of the water and decrease the water pressure in the soil where the substructure is made.
- They can be even design to resist uplift pressure.
Disadvantages of deep foundation
- They are very complex and difficult to construct. Moreover required a high degree of supervision, precision in work. The soil should be properly analyzed and inspected first by a geotechnical-engineer.
- Require deep boring machines, drillers, cranes for installing such a foundation. The labor should be very skilled and well known about such a process.
- They are very risky to construct, so very precautionary measures should be taken before each step of construction.
- Can alter the surrounding properties of soil and groundwater movement.
Difference between Shallow and deep foundation
|Description||Shallow foundation||Deep foundation|
Load bearing is generally at base of the footing i.e. End bearing mechanism.
Load bearing is due to lateral friction as well as end-bearing too.
|Transfer of load mechanism||Loads are immediately transfer to the below the ground level of substructure. They are prefer when bearing capacity of soil is very high||Loads are gradually transfer throughout the substructure soil strata. They are prefer when bearing capacity of soil is too low.|
|Uses||Generally used for lesser load per unit area like simple residential and commercial buildings Is used when bearing capacity of upper strata of soil is high. Use for uniform and stable soil.||Used when higher(larger) load per area are most probable in entire life time of structure like Bridges, sky scrapper, flyover etc Is applied id the bearing capacity of upper layer of soil are extremely low. Used for unstable soil like clay which have extremely high expansion and smelly property, and unstable soil like loose cohesion less sandy loam|
|Cost||They are comparatively cheap and easily constructed with simple tools and equipment and analysis of soil.||They are very costlier process, as they require very skill manpower and supervision, advanced boring equipment, lifting equipment, and details report of soil.|
|Ground water table||The ground water table for such foundation should be below the foundation level.||When the ground water level is high and are near to surface od ground such as river side, then such foundation must be installed.|
|Perception||They are monolithically constructed with the superstructure, thus the foundation are treated as single unit of structure.||They might or might not be monolithically constructed. They are considered or treated aa soil improving methods for building superstructure more than entire superstructure units.|
I hope this article on “Difference between Shallow and Deep foundation” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha