What Is Pile Foundation?
A Pile Foundation is one type of slender structural member made of steel, concrete, wood, or composite material. A Piles may be cast-in-situ excavating a hole and filling it with concrete or precast concrete member which is driven into the soil. There are different Types of Pile Foundation used in construction.
In case, the strata of good bearing capacity are not available near the ground, the foundation of the structure has to be taken deep with the purpose of attaining a bearing stratum that is suitable in all respects.
The most common forms of deep foundations are :
- Pile foundations
- Cassions or Aell foundations
There are different pile foundation types are used to transfer structural loads to a depth where the soil has the desired capacity to transmit the loads. Piles are somewhat similar to columns in that loads developed at one level are transmitted to a lower level, but Piles obtain lateral support from the soil in which they are embedded so that there is no concern with regard to buckling.
It is in this respect they differ from columns. Piles are relatively long. The Piles may be driven or placed vertically or with a batter. A Pile foundation usually consists of a number of Piles, which together support structure.
Uses of Pile Foundations
Pile foundations are preferred in the following situations
- The load of the superstructure is heavy and its distribution is uneven
- The topsoil has poor bearing capacity.
- The subsoil water is high so that .pumping of water from the open trenches for the shallow foundations is difficult and uneconomical.
- There are large fluctuations in the subsoil water level.
- Where timbering to the trenches is difficult and costly.
- The structure is situated on the seashore or river bed, where there is a danger of scouring action of water.
- Canal or deep drainage lines exit near the foundation.
- The topsoil is of expansive nature.
- Piles are used for the foundation of transmission towers, an off-shore platform that is subjected to uplift forces.
Factors Affecting the Selection of Pile Foundations
The following factors should be considered while selecting types of pile foundation:
- Nature of Structure
- Loading Condition
- Availability of funds
- Availability materials and equipment’s
- Types of soil and its properties
- Groundwater table
- Self-weight of Pile
- Durability of Pile
- Cost of Pile
- Maintenance of Pile
- Length of Pile required
- Numbers of Pile required
- Case study of an adjacent building
- Facilities available for pile driving
- Difficulties in pile driving
Types of Pile Foundations
A. Types of Pile Foundations Based on Their Function or Uses
Based on function piles are classified as,
- End Bearing Pile
- Friction Pile
- Composite Pile
- Tension Pile
- Anchor Pile
- Fender Pile
- Better Pile
- Sheet Pile
1. End Bearing Pile
These types of pile foundations penetrate through the soft soil and their bottom or tips rest on the hard stratum. These piles act as columns. The soft material surrounding the pile provides some support. For end-bearing pile Qu = Qp
2. Friction Pile
When the foundation of structural has loose soil, then the friction piles are extended to a depth that frictional resistance developed at the sides of the piles equals the load coming on the piles. A friction pile is generally utilized where hard foundation strata available at greater depth.
3. Compaction Pile
When piles are driven in loose granular soil with the aim of increasing the bearing capacity of the soil, the pile’s arc termed as compaction piles. These types of pile foundations are themselves do not carry any load.
4. Tension Pile
These types of pile foundation anchor down the structures subjected to uplift due to hydrostatic pressure or due to the overturning moment. It is also called uplift pile.
5. Anchor Pile
Anchor piles used to resist horizontal pull from sheet piling or other pulling forces.
6. Fender Pile
Fender piles is mainly used to protect the seashore structures against impact from ships or other floating objects.
7. Better Pile
They are used to resist large horizontal forces or inclined forces.
8. Sheet Piles
They are used as bulkheads or as an impervious cutoff to reduce seepage and uplift under hydraulic structures.
Sheet Piles used for following purposes,
- To isolate foundation from adjacent soil.
- To prevent the underground movement of water.
- To prevent the transfer of machine vibration to adjacent structures.
- To construct a retaining wall in docks, wharfs, and other marine structures.
- To protect riven banks.
- To retain sides of foundation trenches.
Read More: Difference Between Footing And Foundation
B. Types of Pile Foundation Based on Materials and Composition
1. Precast Concrete Piles
Precast concrete piles arc those which are manufactured in a factory in at a place away from the construction site, and then driven into the ground at the place required. Naturally, these types of pile foundations require heavy pile driving machinery.
Precast Piles may be square, octagonal, or circular in cross-section and may be tapered or Parallel sided longitudinally. Because of driving stresses and handling stresses (i.e. transportation and lifting) the precast concrete piles are usually reinforced.
The size of the pile may vary from 30 cm to 50 cm in a cross-sectional dimension, and up to 20 m or more in length. The reinforcement may consist of longitudinal steel bars of 20 mm to 40 mm in diameter, 4 to 8 nos. with lateral ties of 6 to 10 mm diameter at 100 mm c/c spacing for top and bottom lm length and 300 mm c/c spacing for the middle length. A concrete cover of at least 50 mm is provided. The grade of concrete should be M20
2. Cast -in-Situ concrete Piles
In the cast-in-situ concrete pile, a bore is dug into soil by inserting a casing. This bore is then poured with concrete after placing steel reinforcement if any. The casing may be kept in Position or it may be withdrawn. The piles with casing area known as eased cast-in-situ concrete Piles and those without casing are known as uncased cast-in-situ concrete piles.
1. Raymond Piles Foundation
The Raymond piles are mainly a type of friction pile. It is provided with a uniform taper of I in 30 resulting in shorter piles.
The lengths of Reymond standard piles vary from 6 to 12 m. The diameter of piles varies from 40 to 60 cm at the top and 20 to 30 cm at the bottom. The Pile consists of a thin corrugated shell (casing) closed at the bottom. The steel shell is reinforced with spirally wound hard drawn wire on an 8 cm pitch. It is one of the important types of pile foundations.
The shell is driven into the ground with a collapsible steel mandrel or core in it having the same taper. When the Pile is driven to the desired depth mandrel is mechanically collapsed and withdrawn, leaving the shell inside the ground. The shell is inspected internally by using the light from a mirror or flashlight or drop light. This empty shell is gradually poured with concrete up to the top.
2. Mac Arthur Piles
Mac Arthur is made of corrugated steel shell of a uniform diameter which remains in place, as in Raymond Piles. A heavy steel casing with a core is driven into the ground as shown in fig. These types of pile foundations are used when the desired depth is reached, the core is withdrawn and a corrugated steel shell is placed in the casing: Finally, concrete is placed in the shell, by gradually compacting it, and withdrawing the steel casing:
3. BSP-base driven Piles
These types of pile foundations consist of a helically welded shell of the steel plate and the plug of concrete is placed at the bottom of piles.
Pile driving is done by dropping the hammer on the concrete plug. The casing is driven to the desired depth and then it is filled with concrete.
Read More: 9 Types Of Building Foundation Materials
4. Swage Piles
Swage Piles are used with an advantage in some soils where the driving is very hard or where it is designed to leave the watertight shell, for some time before filling the concrete.
Firstly, a steel shell is placed on a precast concrete plug and a steel core that is not long enough to reach the plug is inserted in the plug.
In the second stage, as the pipe over the plug, until the core reaches the plug, the pipe is swaged out by the taper of the plug thus forming a watertight joint.
In the third step, the pipe is driven up to specific depth. The driving force is practically exerted by the core on the plug and the pipe is pulled down rather than driven.
In the fourth stage, when pile placed at desired depth, the core is removed and poured with concrete.
5. Button bottom Piles
These types of pile foundations are being used where the end bearing area needs to increase. The pile uses a concrete plug, of the shape of the button. This button forms an enlarged hole in the soil during driving.
These types of piles can be used up to lengths of about 23 m and for loads up to 50 tonnes.
In the first stage, a steel pipe with 12 mm thick walls is set on the concrete button. The concrete button diameter is 25mm larger than the pile diameter.
In the second stage, the pipe and button are driven to a specified depth. In the third stage, resting on the button corrugated steel shell is inserted in the pipes
In the fourth stage, the casing is withdrawn leaving the button in place, and the shell is filled with concrete. Reinforcement may be used if necessary.
Uncased cast-in-situ concrete Piles
These types of pile foundation piles do not use casing and hence are cheaper. However, great skills required in their construction. These piles are used only where it is certain that neither soil nor water will fall into the hole, or squeeze into and reduce the size of the hole, and also where adjacent piles will not damage the green concrete. It is essential to have a close installation inspection since no inspection is possible after they are installed.
Following are the cast-in-situ concrete types of pile foundations:
- Simplex Piles
- Franki Piles
- Vibro Piles
- Pedestal Piles
- Pressure Piles
1. Simple Piles
Simplex piles can be driven through soft or hard sois. In this type of pile, a steel tube fitting with a cast iron shoe is driven into the ground up to the desired depth.
Reinforcement, if necessary is put inside the tube, concrete is then poured into the tube, and the tube is slowly withdrawn, without concrete being tamped, leaving behind the cast iron shoe.
In these types of pile foundations, if tamping of concrete is done regular intervals as the tube is withdrawn, we get the simplex tamped pile.
2. Franki Piles
In these types of piles, a plug of dry concrete gravel is formed on the ground surface by a heavy removable pipe shell.
A Drop hammer having weight of about 20 to 35 KN is free dropped on the plug. This results in the formation of a dense plug that penetrates the ground and drags the tube with it on account of friction developed between the tube concrete plug.
When the tube has reduced the desired depth, the tube is held in the position by cables and the hammer is applied to the concrete plug, forcing it down and outward. This results in the enlargement of the base into the mushroom shape.
If required a fresh charge of semi-dry concrete is not to enlarge the bulb.
After that the shaft is manufactured by introducing successive charges of concrete, ramming each in turn, and withdrawing the casing gradually about 300 mm at a time. Corrugations will be formed on the surface of a completed pile.
The pile diameter in Frank piles varies from 50 cm to 60 cm, while the enlarged base may have a diameter of about 90 cm. The pile has a load-carrying capacity of 60 tonnes (660 kN) to 90 tonnes (900 kN).
In the next stage, the shaft is manufacture by inducing successive charge of concrete. ramming each in turn, and withdrawing the casing gradually about 300 mm at a time. Corrugations will be formed on the surface of a completed pile.
3. Vibro Piles
These types of pile foundations are used where the ground is soft, thus offering little frictional resistance to the flow of concrete. The standard and expandable pile can be driven by the Vibro-process.
These piles are formed by driving a steel tube and shoe, filling with concrete, and withdrawing the steel tube.
Standard Vibro piles are made in the size of 45 and 50 cm diameter for loads of 60 to 70 tonnes. They can be formed in the lengths of a 25 m and over.
A Steel tube having cast-iron shoe fitted at the bottom is driven by 2 to 2.5 tonnes hammer operated by steam or compressed air delivering up to 40 blows per minute, with a stroke of about 1.4 m.
4. Pedestal Piles
These types of piles are used where a thin bearing stratum is reached with reasonable depth. The pedestal pile gives the effect of spread footing on this comparatively thin bearing.
The core and casing are driven together into the ground, till the reach the desired level.
The core is taken out and a charge of concrete is placed into the tube. The core is again placed in the casing to rest on the top of poured concrete; pressure is applied on the concrete through the core, and at the same time the casing is withdrawn. The process is repeated until the casing is completely removed.
- H – piles
- Box piles
- Tube piles
1. H – Piles
These types of pile foundations are usually of wide flange sections. They are suitable for trestle type structures in which the piles extend above ground level and also act as columns. Since they have small cross-sectional areas, therefore, they can be easily driven in soils in which it will be difficult to drive ordinary displacement piles. They are used as long piles with high bearing capacity.
2. Box piles
They are rectangular or octagonal in the form filled with concrete. These types of pile foundations are used when it is not possible to drive He-piles into hard strata.
3. Tube piles
In this type, tubes or pipes of steel are driven into the ground. Concrete is filled inside the tube piles. Because of their circular piles are easy to handle and easy to drive in. cross-section, these types of pile foundations are easy to handle and easy to drive in.
These piles are prepared from trunks of trees. They may be circular or square. These types of timber pile have a diameter of 30 to 50 cm with a length not exceeding 20 times its top width.
The Cast iron shoe is provided at the bottom and at the top, a steel plate is fixed. If a group of timber piles is driven, the top of each pile is brought at the same level and then a concrete cap is provided to have a common platform. types of pile foundations have less bearing capacity and are not permanent unless treated.
The hole is then filled with sand and it is well rammed. The sand to used should be moist at the time of placing. The top of sandpile mostly covers with concrete for restricting the ejecting upwards due to lateral pressure.
Sand piles are spaced at 2 to 3 m, usually under the columns of the structure. A load test should be carried out to determine the bearing capacity of a sand pile. A properly constructed sand pile resting on firm strata can take up a load of 100 tonnes/m2 or more.
The length of sandpiles is kept about 12 times its diameter. The diameter of sandpile is dependent upon the load acting on the pile.
C. Classification of Pile Based on Installation Method:
Based on method of installation Piles may be classified as follows
1. Driven Piles
These Piles are generally forced insertion into the soil with a heavy hammer on their tops. Mainly, the precast concrete, timber and steel piles are driven by driving, which may be driven into position either vertically or at an inclination.
2. Driven and Casts-in—Situ Piles
These types of pile foundations are castes by driving a casing with a closed bottom end into the soil. The casing is later filled with concrete. The casing may or may not be withdrawn. If the casing of pile not removed, it is called uncased Pile, and if the casing is not withdrawn, it is called cased Pile.
3. Bored and Cast-in-Situ Piles
These types of pile foundations are formed by excavating a hole into the ground and filling it with concrete.
4. Screw Piles
These types of pile foundations are screwed into the soil.
5. Jacked Piles
These types of pile foundations are jacked into the ground by applying a downward force with the help of a hydraulic jack.
6. Under Reamed Piles
These types of pile foundations are researched and developed by C.B.R.I. to be used as the foundation for black cotton soil, filled up the ground, and other types of soils having poor bearing capacity.
The under-reamed piles generally have a diameter of 20 cm to 50 cm and that of bulb varies from 2 to 3 times the diameter of the pile. The length of under-reamed piles is about 3 m to 8 m. The spacing of piles may vary from 2 m to 4 m. The under-reamed piles can also be used for sandy soils with a high water table.
The load carrying capacity of under reamed piles can be increased by adopting piles or larger diameter or by extending the length of piles or by making more bulbs at the base.
Generally, single under ream pile may have one or more bulb. When two or more bulbs are provided at the base, it is known as multi- bulb under reamed pile. The vertical spacing between two bulbs varies from 1.25 to 1.50 times the diameter of bulb.