What is pile foundation?
A pile is a slender structural member made of steel, concrete, or wood. A pile is either driven into the soil or formed in situ by excavating a hole and filling it with concrete.
A pile is a relatively small diameter shaft. It is a deep foundation where the depth is greater than the width (Df >B). When the upper ground stratum at the site is weak or soft or loose or of a swelling type, the load from the supper structure is to be transferred to deeper firm strata. The structural loads in such conditions may be transferred to deeper firm strata by means of piles.
Types of piles | Types of pile foundation
The piles can be classified according to:-
1) Material used
2) Function of the pile
3) Method of construction
4) The displacement of soil
5) The Uses
1) Based on material used
a) Steel piles
Steel piles are generally either in the form of thick pipes or rolled steel. H-sections. H-section piles can be driven up to 36 m in length and can carry a load up to 170 tons whereas screw piles can be driven up to a length of 24 m and carry a load up to 250 tons.
b) Concrete piles
Concrete piles are generally modes of RCC and they are either precast or cast in-situ piles. Connect piles can be constructed up to a length of 27m and can withstand load up to 100 tons.
Concrete piles advantages and disadvantages
|S.N||Concrete piles Advantages||Concrete piles Disadvantages|
|1.||It can be long-lasting as there is no fear of rusting concrete.||Even concrete does not rust, but reinforcement in pile can be rusted and make the pile weak.|
|2.||Fresh Concrete is flowable so it fills all the gaps below the pile hole.||There is a chance of altering the shape of the pile below the ground while casting the concrete into the hole of the piles.|
|3.||Concrete piles can be constructed at site.||It is brittle ad heavy in nature so, pre-prepared piles is difficult and hard to set into hole.|
|4.||It can be designed according to the load on the structure economically.||It can not be reused like steel after the completion of the project period.|
c) Timber piles
Timber piles are made from tree trunks after proper trimming. Timber piles are used at water logged area or below water table after treating them chemically. Timber piles can be driven to a depth of 20 m and can carry up to 60 tons.
d) Composite piles
Composite pile are made of two or more materials. These may be made of either concrete and timber or concrete and steel.
2) Based on function of pile
a) End bearing/Base bearing pile
End bearing piles are used to transfer load through the pile tip to a suitable bearing stratums, passing soft soil or water.
b) Friction pile:-
Friction piles are used to transfer loads to a depth in a frictional material by means of skin friction along the surface area of the piles.
c) Combined base bearing and friction piles
These piles transfer loads by a combination of end bearing at bottom of the pile and friction along the surface of pile and friction along the surface of pile shaft.
3) Based on the method of construction
Driven pile/Precast piles
Cast-in situ pile
Driven and cast-in-situ pile
4) The displacement of soil
Small displacement soil
Large displacement soil
5) Based on Uses
Load bearing pile
Sheet pile Anchor piles
Uses of Pile for Foundation
The uses of piles for the foundation in construction are given below.
a) To carry vertical compressive loads
Bearing piles are used to support vertical loads from the foundations of buildings and bridges. Point-bearing piles and friction piles are normally used for resisting vertical loads.
b) To resist uplift load
Tension piles are used to resist upward forces in structures subjected to uplift, such as buildings with basements below groundwater level.
c) To resist horizontal or inclined loads
Laterally loaded piles support horizontal or inclined forces such as foundations of retaining walls, bridges, dams, etc.
What is a piling?
Piling is the process of deeping piles below the soil using different technical planning, design, and required equipment in construction known as piling.
Piling is not an easy task as it covers different steps and different methods depending upon the soil condition and the capacity of the structure. For more details on how piling is done, read the article below.
Construction of deep foundation – Piles
There is a great question, How to install pile foundations for different structures? Don’t worry we will see with an easy diagram. So, the various steps to construct the pile for a deep foundation are given below.
1) Driven and case in place piles
2) Bores and cast in situ piles
Further, read the article below in detail.
1) Driven and case in place piles
The procedure for constructing driven and cast-in-place piles are as follows
a) Forming plug
Steel casing is then driven into the ground for a small depth after cleaning the casing, concrete is placed at its bottom and rammed to form a plug.
b) Driving tube
The casing is then driven by a ram falling through a certain height inside the casing.
c) Forming base
When the casing is driven to the required depth, the casing is held and the concrete plug is forced out of the casing. Further ramming of the expelled concrete is done to form a bulb.
d) Forming shaft
The casing is then raised while the successive charge of concrete is rammed in place to form a rough surface of the pile.
2) Bores and cast in situ piles
The procedures for constructing bored and cast in situ pile is as follows:
a) Making the hole
The pile is constructed by making a hole in the ground by means of hand-operated Augur. The under-reamer is then lowered into the cleaned hole.
b) Forming bulb
The under–reamer is pressed down and rotated. Due to rotary action, the soil is cut and falls into a bucket. The process is done till an enlarged end is formed.
c) Reinforcement in position
The reinforcement case in then lowered as shown in the figure.
d) Completed pile
After the reinforcement case is lowered, concreting is done. The following diagram shows the sequence of operations.
How to select suitable types of piles for construction?
The selection of the pile types is usually made from estimation based on the soil conditions and the magnitude of the load. The factors that govern the selection of piles are given below.
- Length of the pile in relation to the load and types of soil
- Character of structure
- Availability of materials
- Types of loading
- Factors causing deterioration
- Ease of maintenance
- The estimated cost of types of piles
- Availability of funds
Negative Skin Friction in piles (NSF)
Negative skin friction or down drag is a phenomenon that occurs when a soil layer surrounding a portion of the pile shaft settles more than the pile. It is a downward drag force exerted on a pile by the soil surrounding it.
Such a force can exist under the following conditions
- If a fill of clay is placed over a granular soil layer in which a pile is driven, the fill will gradually consolidate. The consolidation process will exert a downward drag force on pile as shown in figure (a) during the period of consolidation.
- If a fill of granular soil is placed over a layer of soft clay as shown in figure (b).
- Lowering of the water table will increase the vertical effective stress on the soil at any depth, which will induce consolidation settlement in clay. If the pile is located in the clay layer, it will be subjected to downward drag.
Effect of negative skin friction
It reduces the load carrying capacity of the pile (Qa’=Qa-QNSF). Further, it adds an extra imposed load on the pile.
1)) For individual piles (clay)
a) For cohesive soil (clay)
b) For cohesionless soil (sand)
2) For pile group
a) For cohesion less soil (sand)
b) For cohesive soil (clay)
Remedial measures of negative skin friction
- Construct a pile of small cross-sections.
- Provide space between pile and casing while driving, filled with viscous materials, and withdraw the casing.
- Coat the surface of the pre-cast pile with bitumen before driving.
How to design pile foundation?
Q. Design a friction pile group to carry a load of 25 KN including the weight of the pile cap for a site where the soil is uniform clay to a depth of 20 meter underlying by rock. Average unconfined compressive strength of the clay is 100 KN/m2. Take factor of safety 3 and Nc as 9.
Average unconfined compressive strength (qu) = 100 KN/m2
Let, length of pile = 10 meter
Diameter of pile = 0.3 meter
Number of pile = n
Adhesive factors (Alpha) = 0.4
1) When piles are acting individually;
For friction pile, neglecting the base bearing term,
Allowable load (Qa) = Qu/F = 188.5/3 = 62.83 KN.
Total load to be carried = 235 KN
Therefore, Number of piles required = 235/62.83 = 3.74, Nearly = 4
Use 4 piles in square pattern, 2 in each row spacing = 3 x d = 3 x 0.3 = 0.9 meter
and, Size of square = 0.9 + 0.3 = 1.2 meter
2) When piles are acting in a group.
Allowable load (Qa) = Qu(g)/F = 960/3 = 320 KN > 235 KN
This shows that the piles as individual ones are more susceptible to failure. Hence, use 4 piles, each 0.3-meter diameter and 10 meters in length.
Q) In a pile group, 25 piles are arranged in a uniform pattern in a soft clayey soil with equal spacing in all directions. Determine the optimum value of the spacing. Neglect the end bearing effect and take the adhesion factor 0.7. Assume that each pile is circular section and take average undrained cohesion of 18 N/m2.
Let us consider, x be the c/c spacing between piles and d be the diameter of piles. 25 piles are to be arranged in a uniform pattern, which means that there would be 5 piles in each row and there would be 5 rows. the square so formed will be of size B xB.
Where, B = 4 x Spacing + 2 x (1/2) pile diameter
i.e B = (4x +d)
Now, load carrying capacity when group fails as block.
But, according to the question, we can neglect the end-bearing effect.
Load carrying capacity when piles fail individually.
Qu(g) = n x Qu(s)
Equating (1) and (2) for optimum spacing of piles, we get,
55 x Df x d x C = 2.8 x C x Df (4x+d)
or, (4x + d) = 19.64 x d
or, 4 x = 18.64 x d
or, x = 4.66 d
Hence, optimum spacing is 4.66 times the diameter of piles.
Screw pile foundation | Helical pile foundation
Screw pile foundations are those whose piles have screw-like structures at the base of the pile. Piles are made up of steel which is rotated by the mechanical machine to be berries under the ground.
The screw below the pile helps to remove soil below piles and helps to go through the ground layers.
Installation of Screw pile foundation
Batter piles are those which are made inclined by some angle with horizontal. Batter piles are used when there is high lateral stress exerted on the structure.
These types of piles are pre-constructed piles of reinforced concrete that are buried under the ground with the help of digging holes with the help of equipment.
How many piles are needed for a house?
The total no. of piles needed for a house depends upon the load on the house and the self-load of the materials used to construct a house. Generally for the small house, we have to provide Piller at the center and at all corners of the house.
But if our house is more than 155 square feet then, we have to provide piles in the mid of the corners also.
Piles vs piers
Conclusion for pile foundation
A Pile foundation should be used where the soil below the foundation is loose. Pile foundation should be designed carefully because a small careless may cause unequal settlement and damage whole the structure.