Different types of deep foundations are needed to transmit loads from a structure through weak compressible soils or fill into deeper, stronger, and less compressible soils or rocks, or for other functional reasons.
The main aim is to bypass the soft or weak soil strata down into a depth where the strong strata are located.
All the different types of deep foundations are built too far below the finished ground surface for their base carrying capacity to be impacted by surface circumstances.
If inappropriate soils are available near the surface, a deep foundation can be employed to shift the weight to a deeper, more capable stratum deep below into the ground.
Deep Foundations – Definition
A deep foundation, in construction, is a type of foundation that is buried so deep into the ground surface and distributes the load of the structure to the ground at a deeper level farther than a shallow foundation.
The construction procedure of building deep foundations and footings is more complicated and costly than that of shallow foundations.
Different types of Deep foundations
Here are different types of deep foundations:
- Pier foundation (caissons)
- Buoyancy rafts (also known as hollow box foundations)
- Pile foundations
- Shaft foundations
Now, let us look at the above-mentioned one after the other.
When we talk about different types of deep foundations, we cannot do without mentioning a basement.
A basement is a portion of a structure that lies below ground level, either partially or totally. The terms ‘basement’ and ‘cellar’ are frequently used interchangeably and can refer to any type of below-ground area.
The terms ‘basement’ and ‘cellar’ are frequently used interchangeably and can refer to any type of below-ground area.
Generally, basements are often designated as a special space for a building’s utility, e.g. where you can find the fuse, water heater, electric panel, circuit breaker, air-conditioning system, etc., or space for vehicle parking space (not garage)
These areas can be used as a residence, storage facilities, vehicular parking space, and other functions.
Sometimes, when basements are to be constructed or excavated trench shoring is employed
2. Pier Foundation
You’ve probably seen a house or two constructed on stilts if you’ve ever visited a fishing village/ beach area. The majority of these houses are built on a pier, or pier-and-beam, foundation, which is similar to a wood deck.
Pier foundations can also be classified as one of the different types of deep foundations but are usually shallower in depth than pile foundations. When the top layers consist of decomposed rock overlying strata of sound rocks, a pier foundation is preferable to a pile foundation because It becomes difficult to drive bearing piles into the decomposed rock in this situation.
A pier foundation may be easily built in stiff clays that give significant resistance to the driving of a bearing pile.
The methodology of construction is the major difference between a pile foundation and a pier foundation. Pier foundations just transfer the load through the bearing, whereas pile foundations transfer the load through friction and bearing. another major difference is that a pile is installed by driving while a pier by excavating.
Types of pier foundation
There are two types of pier foundation
i.) Concrete or brick/ masonry pier foundation
ii.) Drilled caissons
i.) Concrete or brick/ masonry pier foundation
Brick masonry or concrete foundation piers may be utilized in excavated pits when the depth of the hard stratum is less than 5 meters below ground level.
The depth of the hard bed determines the size and spacing of the pier, and the composition of the soil determines the weight of the load coming from the superstructure.
Concrete is used to construct the concrete pier, which can be precast or cast in situ.
A precast concrete pier is made at a factory and then driven into the ground at the desired position.
A cast steel pier is placed at the bottom of the precast concrete pier, which is normally reinforced with steel wires of around 30mm to 50mm in cross-section and up to 20m in length.
ii.) Drilled caissons
Drilled caissons are also called Cylindrical foundations because they are mainly in a cylindrical shape.
Drilled caissons are usually achieved by mechanical drilling means.
They can be subdivided into three:
- Concrete caisson with the enlarged bottom.
In this type of drilled caisson, a cap is given at the top, which is at ground level. Above it, the cap brickwork is completed. A pier is built underneath this cap, which is followed by the larger bottom, also known as the bell. This bell has a 60-degree slant at the bottom.
- Caissons of steel pipe with concrete filled in the pipe
In this type of drilled caisson, a cap is also given at the top, just like the concrete caisson with an enlarged bottom which is at ground level. A steel shell is formed beneath this cap at both ends. This steel shell is the exterior section. This is where the Concrete is poured within the steel casing.
- Caissons with concrete and steel core in steel pipe.
This type’s arrangement is similar to that of steel pipe caissons with concrete filled in the steel pipe. The only difference is that a rod is attached in the middle area of the steel core, giving the framework additional stability as the weight-bearing capacity is increased owing to the steel core.
Pier foundation Advantages:
i.) When it comes to design, it has a broad range of options. We may use a variety of materials to improve the visual appeal while staying inside our budget.
ii.) By under-reaming the bottom, the bearing capacity may be increased.
iii.) Pier foundations save time and money since they do not require substantial excavation or a huge portion of concrete
3. Buoyancy rafts (also known as hollow box foundations)
Buoyancy rafts a.k.a hollow box foundation is one of the types of deep foundations being used in the construction of a foundation on soft and poor soils.
Hollow box foundations are also referred to as floating foundations.
The buoyancy raft operates on the same concept as a floating structure, with the raft’s support mostly obtained from the displacement of soil or waste by the volume of a huge hollow foundation.
It’s meant to remove enough overburden so that the superstructure load may be delivered to the earth with little or no increase in the initial tension on the sub-strata before excavation and construction.
The planned building’s basement may be formed from the bottom slab, which can be coupled with the ground slab and retaining walls to make the raft. It can also take the shape of a cellular structure.
Under the different types of deep foundations, this one is peculiar because they adopt the principle of a ship floating in the water, where the displaced water equals the weight of the ship.
This type of deep foundation is more environmentally safe than a pile foundation for places with very weak soils requiring significant depths.
Hollow box foundations are used majorly for oil refinery structures.
4.) Pile foundations
A pile is a long, square, or cylinder-shaped structure built of a strong material like concrete.
Piles are driven into the earth to provide stable types of deep foundations for structures that are erected on top of them.
Piles are used to transmit loads from buildings to hard strata, rocks, or high-bearing-capacity soil.
The piles provide structural stability by being firmly planted in the ground. Because pile foundations are embedded in the ground, they are more resistant to flood and erosion.
Because pile foundations bear such a heavy load, they must be properly designed. A qualified engineer would first examine the soil in which the piles are positioned or buried to ensure that it is not overburdened beyond its bearing capacity.
Each pile has an impact zone on the soil it surrounds. It is indeed important to position the piles far enough apart so that the weights are dispersed equally throughout the bulb of soil that bears them, rather than concentrated in a few spots.
Why and when should you use pile foundations?
- When there is a lot of water in the earth (water table is high)
- Where the superstructure is subjected to heavy and non-uniform loads.
- When the soil is compressible at a shallow depth, then pile foundation is necessary.
- When there is a risk of scouring owing to its location near a riverbed or the coastline, for example.
- When the structure is near a canal or deep drainage system.
- When soil excavation to the necessary depth is not feasible owing to unfavorable soil conditions. Due to a significant input of seepage, it becomes difficult to maintain the foundation trenches dry by pumping or any other means.
Classes of piles
Piles are designed, categorized, and classified into 2 main groups.
A. By their functions
These are further subdivided into the following:
i.) End bearing piles – Until a firm stratum is reached, bearing piles are driven into the earth with construction equipment. Bearing piles are the pillars that support the building by resting on a hard stratum. Bearing piles allow vertical loads to be transferred to the hard stratum underneath the structure.
ii.) Friction piles – When the engineer examines and carries out necessary tests and it is confirmed that the soil is soft and there are no firm strata available, friction piles are used. The surfaces of these piles have been roughened to improve the surface area and frictional resistance. They are based on frictional resistance between their outer surface and the soil with which they come into contact.
Friction piles need not sit on strata that are too hard.
iii.) Batter piles – When there is need to resist inclined loads then batter piles work best.
iv.) Guide piles – Guide piles are used mostly in the building of cofferdams, which provide strong foundations for under-water constructions.
B. By the material they are made up of
Below are the different types of deep foundation piles available based on their material composition:
i.) Steel Piles – These piles are usually I-sections. It has a tiny cross-sectional area, making it easy to drive in the dirt. These piles can be used as bearing piles, however, they cannot be utilized as friction piles since the structural load will force them to sink in the earth.
Vibration hammers are used to drive steel piles to a certain required depth or point of resistance. To support your structure, you can use a whole range of driven piles, from tiny diameter, tube piles to huge diameter steel caissons. During installation, driven piles obtain geotechnical capacity by displacing dirt around the shaft and compacting the soils near the toe of the pile.
The Steel piles can be open or sealed at both ends or can be I-sectioned.
Construction equipment with a high frequency oscillation hammer or a percussive hammer will be used to drive the piles into the ground.
ii.) Timber/ wooden piles – Timber piles are the oldest of all the piles which are still in use currently. In the past, wooden piles were used to support structures in regions where the soil was poor or weak. Even though, Jetties are still built with wooden piles till today. Long Trees with extraordinarily straight trunks are required for this because two tree trunks cannot be joined, therefore, the pile length is restricted to the length of a single tree trunk.
The diameter of the timber pile ranges from 200 – 500mm and the length is calculated by multiplying its diameter by 20. For instance, if a trunk diameter is 300mm, then, the length, “L”, of the trunk should be (300 x 20 = 6000mm).
Wooden piles have a high maintenance cost because they are wood and can be destroyed by fungus or white ants if they come into touch with water.
Precast displacement piles are often erected using the drive method or, less commonly, the vibratory method. We may get a reasonably high pile load capacity due to the natural convergence of pile shafts. They are a highly cost-effective, efficient, and a safe foundation option for both temporary and permanent constructions when properly placed.
A number of historical structures built on timber piles that have been in use for thousands of years may attest to this.
For your information, if you didn’t know already, Venice in Italy is a city set on wooden piles spanning the entire sea which it is set upon.
Pressure-impregnated timber piles are used to support permanent constructions.
iii.) Concrete Piles – Concrete piles are created using concrete.
These piles come in different sizes from 300 – 500 mm in diameter. These piles must be a minimum of 20 meters in length and a maximum of 30 meters in length.
Underwater or coastal constructions such as bridges, oil rigs, and helipads on water, naval airports (on sea) are supported by concrete piles, which are typical structural foundation elements.
Underwater structure loading is made up of two parts: vertical structural loads and lateral wave loads. The combination of these two loading components has a big influence on how the pile behaves and how the stresses are distributed. Furthermore, when the pile is subjected to a minor structural force vs a huge structural load, the pile will behave differently.
iv.) Composite piles – The term composite pile refers to piles that are made up of more than one material. For example, if concrete and wood are used to construct a pile, it can be called a composite pile. In regions where the water table is high, these piles are utilized. Because concrete and wood are both effective water absorbers, these piles are employed in such situations.
Types of deep foundation construction process (installation of piles)
There are two major types of pile construction methods
A. Displacement (driven piles)
Instead of removing the material around the pile shaft, driven (or displaced) piles are pushed, jacked, vibrated, or screwed into the earth, displacing the earth material outwards as well as downwards.
Driven piles may increase the density of loose soil. This is suitable for offshore applications. They are also stable in soft compressing soils.
Driven piles are divided into two categories:
These two categories depend on the need for installation.
The factors that can influence this need includes, but are not limited to:
a.) The depth at which you plan to dig
b.) The angle or inclination at which your piles will be driven after calculations from the civil engineer
c.) Environmental concerns that may have an influence on inhabitants, vegetation, or animals in area
d.) The type of material used to create your piles
i.) Driven in situ/ cast-in-place
These are formed either with a permanent concrete or steel casing or with an interim concrete or steel casing and then driven into the ground.
i.) Drive a steel tube into the ground.
ii.) Remove all sand/ debris from inside a thin-walled steel tube
iii.) after the steel is clean of sand/ earth, a welded steel reinforcing cage is lowered into the tube
iv.) finally, Pour wet specified concrete mixture into the tube to form the pile.
The casing i.e the thin-walled steel tube is just a formwork to bring out the shape of the pile and also hold the soil back in case it is so loose that it would cave back in after boring. After the casting is accomplished, it has no structural role to perform. it becomes useless but if the soil is very cohesive, (which means that if you drill a hole in the ground about 300 x 15000 mm wide, the soil will keep the shape of the hole rather than collapsing into it and blocking it, then there will be no need to leave a casing in the ground as this can save you a lot of costs since you can use the same case for numerous bores.
ii.) Prefabricated/ Precast driven piles
These can be made of concrete, steel or wood, but constructed off-site.
Prefabricated piles are hammered or rammed into the earth with construction equipment called pile driver, after being cast on-site and allowed to properly cure.
The pile driver keeps the pile absolutely straight before driving it into the earth by each ram (blow). Each blow is delivered by raising a large weight and releasing it on top of the pile, which is briefly protected from disintegration by a pile cap. The pile driver has two purposes: it operates as a crane, lifting the pile from a horizontal position on the ground and rotating it into the proper straight/ vertical position, and it hammers the pile into the earth.
Piles should be pounded into the earth until they refuse to go further into the ground.
B.) Replacement (bored piles)
Bored (or replacement) piles remove excess material during boring to create a hole in the ground where the pile is put in place. Bored piles are usually poured in situ.
Bored piles are more common in urban locations because they produce less vibration. Sometimes, when boring/ drilling takes place simultaneously with the concrete being poured in place it is called continuous flight auger (CFA)
Engineers frequently combine a few piles and cover them with a pile cap. A pile cap is a thick concrete cap that extends over a small set of piles and serves as a foundation for the construction of a column. This column’s burden is subsequently spread among all of the piles in the group.
The advantages of pile foundation
- The pile’s length, width, and size can be pre-determined based on the site’s requirements.
- This approach is simple and only takes a little amount of material and work.
- It’s possible to install it in quite lengthy lengths.
- Precast piles helps in shortening the time it takes to complete a project.
- Piles can be used in areas where drilling and holes are not permitted.
- It is possible to put it in a very large area.
- The use of piles in a site make the work quite nice and tidy.
The disadvantages of pile foundation:
- Driving the piles requires the use of heavy machinery.
- In salt water, marine borers can damage pilings.
- When piles are driven, vibrations occur, which have an impact on nearby structures therefore, underpinning might be needed for nearby structures.
- Driving across stones and pebbles may quickly ruin a pile.
- It’s tough to predict the exact length needed to be drilled ahead of time. Therefore there is always a lot of wastage
5. Shaft foundation:
This can also be labeled as one of the different types of deep foundations. It is formed inside a deep excavation and is supported by an in-place lining before being filled with concrete or other prefabricated load-bearing components.