What is meant by pile foundation?
A pile is a slender structural member having a very small area of cross-section relative to its length. It is a deep foundation where depth is greater than width. It is used when a shallow foundation can’t support the structure.
Uses of the pile (necessity of pile foundation.)
- Transfer load to strong and or less compressive strata.
- Transfer loads in weak soil by friction between the pile and the soil.
- Compact loose granular soil.
- Provide foundation and inclined forces from bridge abutment and retaining walls.
- Carry uplift forces.
- Stiffen soils beneath machine foundation to control both the amplitude vibration and natural frequency of the system.
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Types of piles:
- Based on function
(i) Bearing piles
(ii) friction piles. (iii) combined bearing and friction piles.
- Based on pile materials
- timber piles (ii) concrete piles (iii) steel piles
(c) Based on the method of installation
- Precast pile (ii) cast-in situ pile (iii) screw pile (Iv) steel section (v) bored and cast in situ piles.
Precast Concrete Piles:
Precast concrete piles are manufactured in the factory and then driven into the ground. A bore is dug into the ground by inserting a steel shell. If the shell is left in place, it is called a shell pile. If the shell is removed it is called a shell-less pile. The pre-cast concrete piles may be tapered or parallel-sided. They come in a variety of shapes, including square, octagonal, and circular. Read in detail Here [Precast Concrete Piles]
Design of piles:
The design of piles includes determination of section, longitudinal reinforcement, lateral reinforcement.
Precast concrete piles can be round, square or octagonal in section. The minimum size of the precast pile is 250 mm square and the maximum size is less than 600 mm.
Based on the length of the piles, the following cross-sectional dimensional can be adopted: –
- Upto 10 m length – 250 mm ( square)
- 10 to 12 m length – 300 mm ( square)
- 12 to 15 m length – 350 mm ( square)
- 15 to 18 m length – 400 mmm ( square)
- More than 18 m – 450 mm ( square)
2. Longitudinal reinforcement:
According to IRC 78, the area of longitudinal reinforcement should not be less than the following stipulation:-
(i) For piles with a length less than 30 times the least lateral width = 1.25%
(ii) For piles with a length 30 to 40 times the least width = 1.50%
(iii) For piles with a length greater than 40 times the least width = 2%
(c) Lateral reinforcement:
- They supplement the resisting driving stresses so are of great importance. These should be in the form of loops, spirals or links.
- The minimum diameter should not be less than 6 mm.
- For a distance of about 3 ties of the least width or diameter from the each end of the pile, the volume of lateral reinforcement should not be less than 0.6 % of the gross volume.
- In the body of the pile the lateral reinforcement shall not be 0.2 % of the gross volume.
1. End bearing pile: Used to transfer load through water or soft soil to a suitable bearing stratum)
2. Friction pile: Used to transfer load through the friction ground perimeter
3. Compaction pile: Used to compact loose granular soil and thus increasing bearing capacity of pile
4. Tension pile: To protect the structure through uplift pressure
5. Anchor pile: To provide an anchor against the horizontal pool
6. Feeder or Dolphin pile: Used to protect the structure in the water against impact caused by ships or other floating objects
7. Better pile: This pile is used to resist inclined force
Ex:- Simplex, Franki, Vibru, Mc-Arthurs, Precast, Sheet pile.
Under the following circumstances pile foundations are used:
(a) When the hard soil is encountered at great depths and the provision of spread foundation economical.
(b) When the provision of raft and grillage foundation is expensive.
(c) When concentrated heavy loads are to be taken by the foundation.
(d) When the scouring depth in the river bed is very much deep.
(e) When the topsoil is compressible.
Suitability of pile foundation
- Grillage & Raft foundation not possible
- Transfer heavy Live Load & Dead Load
- Seasonal variation of Ground Water Table
- In marine structure
SELECTION OF THE TYPE OF PILES
Following factors influence the choice of the type of piles:
(b) Length of pile required
(c) Availability of constructional materials
(d) Depth of water table
(e) Time available
(f) Possibility of damage to the pile while driving.
(g) Resistance of pile to the hard stratum at the time of driving.
(h) Presence of acids, salts, etc. in the water of soil which may affect the material of the pile.
(i) Whether the pile is to be taken underwater or it will be above water etc.
Fig.2. shows a pier constructed on a group of piles or piers pile foundation.
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For driving the pile to the required depth following equipment is needed.
1. Pile frame
3. Pile hammer
4. Winches etc.
- Pile frame.
It is a steel structure of height varying from 10 m to 25 m. At its bottom, it has a platform for supporting engines and tools required to drive the pile and space for standing the driver, etc. The intermediate platforms provide space for workers to stand while driving the pile.
- Pile hammer.
It is a tool that gives blows at the top of the piles for driving them into the ground.
Hammer may be of the following types:
(a) Drop hammers
(b) Single acting steam hammers
(c) Double acting steam hammers
(d) Diesel hammers
(e) Differential acting steam hammers
- Drop hammers.
These are simple weights varying from 1.0 to 40 tones in weight. These are lifted by manual labor and allowed to fall from a height of 15 m to 6.0 m directly on the pile, which drives the pile into the ground.
- Single acting steam hammers.
These hammers are raised by means of steam or compressed air and allowed to fall on the top of the pile under gravitational force. Such hammers can give up to 60 blows per minute.
- Double acting steam hammers.
In these hammers lifting and dropping both is done by means of steam or compressed air. The weight of the hammer is about 500 kg. but due to dropping by steam and gravitational force, the effective weight of the hammer comes out to be about 3000 kg. These hammers give 100 to 200 blows per minute. Nowadays such hammers are most commonly used.
- Diesel hammer.
It is a small, lightweight, self-contained, and self-acting type hammer, It requires a small diesel engine to operate. These are mobile and can be taken from one place to another easily.
- Differential acting steam hammers.
This type of hammer has the advantages of both single-acting and double-acting steam hammers. It possesses the height of fall and weight of a single hammer and number of blows that I double-acting hammer
(f) Vibrating hammer.
These hammers produce heavy vibrations, which are transmitted to hammers are used for driving sheet piles. They drive more than 40 m sheet pile per minute.
3. Leads : These are used for guiding the hammer and piles while driving.
4.Winches : These are used for lifting and lowering piles etc.
BEARING POWER OF PILES
The bearing power of the pile is the static longitudinal load that can be supported safely. It can be determined as follows:
1. By dynamic pile driving formula:
3. By actual loading
2. By static formula
1. By dynamic formula
(a) Engineering News formula
R= (16.65 WH)/ (S+ 0,254 P/W)
(ii) Engineering News formula for steam hammer.
R= ( 16.65 WH)/ (S+2.54)
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(iii) Engineering News formula for drop hammer.
R= (16.65 WH)/ ( S+2.54)
R= Allowable safe load on the pile in kg. or (tonnes)
W=Weight of striking part of the hammer in kg
H= Height of fall of hammer in metres
P= Weight of pile driven in kg, or tonnes
S=Penetration of pile per below in cm (usually average of last five blows).
2. Static formula
R = Af + ap.
A= Surface area of pile in m²
f= Frictional resistance of soil at pile surface in kg/m²
a= Cross-sectional area of piles at the bottom in m²
p= Bearing capacity of soil at pile end (bottom) in kg/m²
R =Safe allowable load on pile in kg.
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