The foundation is an important part of a structure that sits below the ground level and provides support to the entire structure. It distributes the entire weight of the superstructure to the rock or soil below it.
The Shallow foundations are generally used when the soil near the ground surface has sufficient bearing capacity to support the structure.
When the soil near the ground surface is not capable of supporting a structure, deep foundations are required to transfer the load to deeper strata.
The pile foundation is one of the many types of deep foundations. In this article, let’s explore the different types of pile foundations used in the construction industry.
What is a pile foundation?
A pile is a slender structural member made of materials like steel, concrete, or wood.
A pile can be either driven into the soil or it can be cast in the site by excavating a hole and filling it with reinforcement and concrete.
Pile foundations can be used under the following conditions:
- When the soil just below the ground surface is very weak to support the load transmitted by the structure.
- When the superstructure load is so heavy
- Used in places where there are expansive soils, such as black cotton soil, which creates unnecessary pressure on the pile through swelling or shrinking
- Used in places where the structure is subjected to any horizontal or uplift forces in addition to the vertical loads.
Types of pile foundation
Pile foundation can be classified according to
- The material used
- The mode of load transfer
- Method of Construction
- Displacement of soil
The types of piles in these classifications are briefly discussed below.
1) Classification according to the material used
There are four types of piles according to the material used.
a) Steel piles
Generally, these steel piles are either in the form of thick pipes or rolled steel sections. These piles are driven into the ground with their ends open or closed and are provided with a driving point or shoe at the lower end.
Epoxy coatings are applied to reduce the corrosion of steel piles. By taking corrosion into account, an additional thickness of the steel section is usually recommended.
b) Concrete piles
Reinforced Cement Concrete is used in the production of concrete piles. These piles are either prepared in the factory or cast at the site.
The piles which are prepared in the factory are called precast piles. The piles which are cast at the site are called cast-in-situ piles.
Precast piles can also be used. These piles are prestressed using high-strength steel cables. A cast-in-situ pile is constructed by making a hole in the ground to the desired depth and then filling it with concrete.
A cast-in-situ pile may be cased or uncased. A cased pile is constructed by driving a steel casing into the ground to the desired depth and filling it with concrete. An uncased pile is constructed by driving the case into the ground and gradually withdrawing the casing as the fresh concrete is filled.
c) Timber piles
Timber piles are made from tree trunks after necessary trimming. The timber used for making these piles should be straight, sound, and free from defects.
A driving shoe is provided at the lower end to prevent damage during driving. To avoid damage to the top of the pile, a metal cap is provided.
Timber piles that are below the groundwater table generally have a longer life span. The timber piles that are above the groundwater table level can be attacked by insects, hence these piles should be painted with preservatives such as creosote oils to have a longer life span.
d) Composite piles
A composite pile is made up of two materials. The lower portion of the pile may be concrete and the upper portion may consist of steel.
It comes in handy at places where there is a water table level. The lower portion of the pile may have timber below the water table level and the upper portion may be concrete or vice versa. Thus, by using composite piles, we can leverage the benefits of both piles.
However, it is very difficult to provide a proper joint between two dissimilar materials, hence composite piles are rarely used in practice.
2) Classification based on the mode of transfer of loads
Based on how the pile is transferring its load to the soil, the piles can be classified into 3 types.
a) End-bearing piles
End-bearing piles are used when a hard stratum or a bedrock is located within a reasonable depth from ground level.
These piles transmit their loads through their bottom tips. Hence, these piles act as columns and transmit the load through a weak stratum to the firm stratum below.
The ultimate load-bearing capacity of the pile depends upon the bearing capacity of the bedrock or hard stratum. End-bearing piles are also known as point-bearing piles.
b) Friction piles
Friction piles are used when a hard stratum or a bedrock doesn’t exist at a reasonable depth from ground level.
These piles transfer their loads through skin friction between the embedded surface of the pile and the surrounding soil. Hence these friction piles do not reach the hard stratum.
Friction piles are also known as floating piles, as these piles do not reach the hard stratum.
c) Combined end bearing and friction piles
These piles transfer loads by the combination of end bearing at the bottom of the pile and friction along the surface of the pile.
3) Classification based on the method of construction
Based on the method of construction, the pile may be classified into the following categories.
a) Driven piles
These piles are driven into the soil by applying heavy hammer blows on their tops. A metal cap is provided at the top of the pile to avoid damage. It is most commonly used to support buildings, tanks, towers, bridges, etc. Driven piles are also known as displacement piles.
b) Driven and cast-in-situ piles
These piles are formed by driving a steel casing with a closed-end into the soil. The casing is later filled with reinforced concrete. The casing may or may not be withdrawn.
c) Bored piles
These piles are constructed by digging a hole into the ground and then a reinforcement cage is placed into that hole, followed by filling it with concrete. These piles are also known as replacement piles since the soil in that area is replaced by a concrete pile.
d) Screw piles
These piles have a helix spiral around their surface, which allows us to screw these piles into the soil. The process is similar to screwing a nail into the wood. Screw piles have several helices depending upon the nature of the load and soil conditions.
4) Classification based on function
The piles can be classified into the following types, depending on their uses.
a) Load-bearing piles
These piles are used to transfer the load of the superstructure to a suitable stratum below the ground level by either end bearing or friction or by both. These piles are commonly used in heavy and tall buildings, bridges, towers, etc.
b) Compaction piles
These piles are driven into loose granular soils to increase their relative densities. The bearing capacity of the soil is increased due to the densification caused by the vibrations.
c) Tension piles
These piles are designed to carry the tensile forces. These piles are used to anchor down structures that are subjected to hydrostatic uplift forces or overturning forces.
d) Sheet piles
Sheet piles are generally made of steel or timber. However, sometimes Reinforced cement concrete should also be used.
These sheets have interlocking edges and are driven into the ground to retain water or earth. The interlocked sheet piles will form a continuous temporary or permanent wall.
e) Fender piles
Fender piles are larger vertical members which are driven into the ground near the waterfront structures such as ports or harbors to prevent the structure from the impact of ships and vessels.
f) Anchor piles
These piles are typically driven deep into the ground and secured with concrete or other materials to create a stable foundation and to provide resistance against horizontal pull or uplift forces. These piles are also used to provide anchorage for anchored sheet piles.
5) Classification based on the displacement of soil
Based on the volume of soil displaced during installation, the piles can be categorized into the following categories.
a) Displacement piles
All driven piles are displacement piles because the soil is displaced laterally during installation. The lateral displacement may cause heaving of the surrounding ground.
Precast concrete piles and closed-end piles are called high-displacement piles and the steel H-piles are called low-displacement piles.
b) Non-displacement piles
Bored piles are non-displacement piles. As the soil is removed when the hole is drilled, there is no displacement of soil during installation. The installation of these piles caused very little change in the surrounding soils.
Hope you understand everything you need to know about the types of pile foundations used in the construction industry. If you find this article helpful, let us know in the comment section.