There are several types of deep foundations that can be used depending on the soil conditions and load requirements. These include basement foundations, buoyancy raft or hollow box foundations, well/caisson foundations, pier foundations, drilled shaft foundations, and pile foundations. Each type has advantages and disadvantages related to cost, construction difficulty, and suitability for different soil and loading conditions. Common examples of deep foundations used include caissons for bridge piers, drilled shafts for structures with large axial and lateral loads, and piles beneath structures with high groundwater or compressible soils. The type of deep foundation selected depends on the project needs and subsurface environment.

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Deep Foundation:
A deep foundation is a type of foundation that transfers building loads to the earth farther
down from the surface than a shallow foundation does to a subsurface layer or a range of
depth
Generally constructed depth over 3m.
Purpose:
Deep foundation is used for the following purposes:
• When the foundation is supposed to carry loads at depth.
• If the soil beneath structure to be constructed is weak compressible soil.
• When the soil is non-suitable, Deep foundations are used to transfer the load to a
deeper strata containing strong soil or rock.
Types of Deep Foundation:
Following are the types generally used in construction;
1. Basement Foundation.
2. Buoyancy Rafts Or Hollow Box Foundations.
3. Well Foundation Or Caissons.
4. Pier Foundation.
5. Drilled Shaft Foundation.
6. Pile Foundations.
1. Basement Foundation:
These are constructed when some space below ground
level is required. These substructures provide working
place or storage space for various purpose.
Designing method depends on the purpose of structure
rather than resistance to external earth (Pressure of
nearby soil) and hydrostatic pressure.
If they’re built in correct way, they will hold up better
in longevity than the others types of foundation.
Advantages:
• More, Cheap Square Footage.
• Seasonal Living Space i.e., warm in winter and cool in summer.
• Easy-Access for Repairs.

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• Storm Protection.
Disadvantages:
• Increased Cost.
• Potential Flooding.
Note: The best scenario to prevent potential flooding is ensuring a natural path
for drainage.
• Lack of Natural Light.
Use:
Used in Basements.
2. Buoyancy Rafts Foundation:
This type of foundation is constructed over soft
and weak soils. When loaded, it acts as a
floating (buoyant) structure and designed in a
way that it reduces the intensity of load over
the soil.
It is a type of Floating foundation.
Also known as Hollow Box Foundations,
Compensated Foundations or Deep Cellular
Rafts.
These are called Compensated foundations
because the soil below the foundation is
removed during the construction process. The weight of superstructure is equal to weight of

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soil and water removed. Like this, the removed weight is compensated by the weight of the
superstructure.
Design Considerations for Buoyancy Raft:
The general design involves following step by step procedures:
1. Depth of the excavation is determined initially, from the plan.
2. Center of gravity to remove overburden that should be equal to sustain the structural
buoyancy.
3. Basement design decided should be compared with the opinions and needs of client.
4. Water pressure calculation for checking the flotation.
5. Finally design details for external walls, floors and separating walls is prepared. The
main design concern is earth pressure, moments and the shear forces. Consideration
of special flotation design is must.
Advantages:
• Elevation of a structure protects it from high water levels.
• Soil settlement chances are reduced due to the fact that total weight of structure
is equal to the removed soil and water.
Note that slight increase in loading will not bring any extreme change in settlement limit.
Disadvantages:
• Due to larger depths of weak soils, excavation also increases, hence
increasing the project cost.
• Catastrophic Movement can occurs if excavation beyond the critical depth of
stability is done.
• Earth pressure from surrounding soft soils may cause settlement to the
support of foundations.
• Space is wasted.
• There are possibilities of leakage of gasses into these hollow cells. As these
situations are unaware, they remain unventilated. This would cause
unexpected explosions, even if there are small chances of ignition anywhere
around.
Use:
1. Areas of low-altitude often face high floods which results in collapsing of houses. For
increasing the house elevation, buoyant foundation is useful.

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3. Well Foundation OR Caissons:
Well Caissons or Caissons are box like or circular or
rectangular structures which are sunk from the surface of
either land or water to desired/required depth.
From inside, they are hollow and can be filled with sand
and plugged at the bottom.
The have larger diameter than Pile foundation.
Purpose/Suitability:
• Deep sandy or soft soils.
• Heavy structures on low bearing capacity soils.
Advantages:
• Economical.
• Environmental friendly.
• Can be reached to great depths.
• Minimum handling equipment is required.
Disadvantages:
• Expert engineers are required to design construction.

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• Workers may experience giddiness, breaking of eardrums, bursting of blood vessels
in the nose or ears and end up in paralytic death which is known as caisson disease.
• Help of divers can be required.
• Not good for polluted sites.
• Construction process is difficult.
Use:
• Pump house.
• Bridge piers.
• Abutment in rivers and lakes.
4. Pier Foundation:
A pier foundation is a collection of large
diameter cylindrical columns to support the
superstructure and transfer large super-imposed
loads to the firm strata below. It stood several
feet above the ground.
Also known as “Post foundation”.
Advantages:
• Less labor and materials required.
• Minimal disruption to soil environment.
• A lot of variety in designs.
• Saves money and time.
• As diameter of shafts is greater,
inspection is possible.
• Design can be changed by Engineers is desired.

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Use:
Bridges.
5. Drilled Shaft foundations:
Drilled shafts are also called drilled piers, caissons or bored
piles.
These are used to support structures having large axial and
lateral loads.
These foundations are constructed by drilling a
borehole within a deep excavation then concrete or some
other pre-fabricated load-bearing units is placed in it.
Length and size can be changed easily.
Shaft foundations can be constructed near existing structures
and under low overhead conditions due to which they can be used in numerous seismic
retrofit projects.
It can be difficult to install them in soils with boulders, soft soil, loose sand, and sand under
water.
Construction Steps:
• Excavation of piers.
• Providing supports.
• Concreting.

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Working:
Drilled shafts are typically designed and constructed to support axial forces through a
combination of side friction and end bearing resistance. They range in diameter from 24” to
120” and can be installed to depths in excess of 300 feet.
Types of Drilled Shafts:
Use:
Drilled shafts are an economic solution that can be installed in a variety of different ground
conditions to accommodate large axial, lateral and overturning forces.
6. Pile Foundation:
Piles are relatively long, slender members that
transmit foundation loads through soil strata
of low bearing capacity to deeper soil or rock
strata having a high bearing capacity.
It is a type of foundation that is supported on
piles. It consists of a base called, pile cap
(RCC slab) support at bottoms by piles.
The load to the soil in contact is transferred
by piles either by friction only or by friction
in combination with bearing at their ends.
Purpose/Suitability:
• When soil is very soft and solid base is not available at a reasonable depth.

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• When grillage and raft foundations prove to be very expensive.
• High building with heavy concentrated loads.
• Construction of a building along river bed or sea shore.
• Can also be used to help resist uplift, overturning, and lateral forces.
Type of Pile Foundation:
Some of the main types are mentioned below;
I. End bearing Piles.
II. Friction Piles.
III. Settlement Reducing Pile.
I. End bearing piles:
Piles which terminate in hard,
relatively impenetrable
material such as rock or very
dense sand and gravel.
Most of carrying capacity is
acquired from the resistance of
the stratum below pile at its toe
Fig. (a).

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II. Friction Pile:
Most of carrying capacity is acquired from
the by skin friction or adhesion.
When piles do not reach an impenetrable
stratum at a reasonable depth, but are
driven for some distance into a penetrable
soil. Their carrying capacity is derived
partly from end bearing and partly from
skin friction between the embedded surface
of the soil and the surrounding soil Fig. (b).
III. Settlement Reducing Piles:
Beneath the central part of a raft foundation, these piles
are incorporated in order to reduce differential
settlement to an acceptable level.
Such piles act as a reinforcement beneath the soil below
the raft and prevent dishing of the raft in the center.

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Uses Of Pile Foundation:
• High groundwater beneath the structure.
• High and non-uniform load of superstructure.
• Presence of highly compressible soil at shallow depth.
• Location of the structure near the river bed or sea shore..
• Passage of canal or deep drainage systems near the structure.