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Pile Foundation

PILE FOUNDATION:

Pile Foundation

In situation where soil at shallow depth is poor, in-order to transmit load safely, the depth of foundation has to be increased till suitable soil strata is met. In view of increased depth such foundation are called Deep Foundation, Well Foundation, Pile Foundation, and Pier Foundation are Deep Foundation.
Pile is a small dia. shaft which can be driven or installed into the ground whereas Pier and well Foundation are large dia shaft, constructed by excavation and sunk to the required depth.

CLASSIFICATION OF PILE:

  1. Material and Composition
  2. Mode of Transfer of Load
  3. Method of Installation
  4. Function or Action
  5. Displacement of Soil

1) Based On Material and Composition:

  • Timber Piles
  • Steel Piles
  • Concrete Piles
  • Composition of Piles: These may be either of concrete and timber or concrete and steel. Composite piles are rarely used in practice as it is difficult to provide joint between two different materials.

2) Based On Mode of Transfer of Load:

  • End Bearing Piles
  • Friction Piles
  • Combined End Bearing and Friction Piles

3) Based On Method of Installation:

  • Driven Piles
  • Bored and Cast in Situ Piles
  • Driven and Cast in Situ Piles
  • Jack Piles
  • Screw Piles

4) Based On Function or Action:

  • Load Bearing Piles
  • Tension or Uplift Piles
  • Compaction Piles
  • Anchor Piles
  • Fender Piles
  • Batter Piles

5) Based On Displacement of Soil:

It has been observed that the best way of classification is on the basis of effect of installation on the soil such as:
  • Displacement Pile
  • Non-Displacement Pile

PILE LOAD CAPACITY IN COMPRESSION:

  • For satisfactory performance of pile foundation are:
  1. Safety Against Shear Failure
  2. Safety Against Settlement
  • Load Capacity of Pile can be estimated by:
  1. Static pile load formula
  2. Pile load test
  3. Pile driving formula
  4. Correlation with penetration data

#SINGLE PILE LOAD CAPACITY

(1) Static Pile Load Capacity:
Calculation of Ultimate Load Qu is given as
Qu=(Qpu+Qf)
where,
Qu - Ultimate load
Qpu - Ultimate point load

Case 1: If Qpu >>> Qf, Pile is called Point Bearing Pile
Case 2: If Qpu <<< Qf, Pile is called Friction Pile

Since General Equation for Point Bearing Resistance,
qpu = CNc + (Sigma)Nq + 0.5BrNr
where,
B - Width of diameter of pile
Sigma- Gama * Hieght
Nc, Nq, Nr = bearing Capacity Factors
C = Unit Cohesion
r = Effective Unit weight of Soil

In Deep Foundation 
qpu = CNc + (Sigma)Nq

Hence Ultimate Point Load is Given is:
Qpu = qpu.Ab
where,
qpu = Unit Point Bearing Resistance
Ab = Cross sectional area of pile

Hence Ultimate Skin Friction is given as:
Qf = Fs.As
where,
Fs = Unit Skin Friction Resistance
As = Surface Area of Pile in Contact with soil

Therefore, the Ultimate Load Capacity Qu, can be expressed as sum of:
Qu = qpu.Ab + Fs.As


PILE IN GRANULAR SOIL (SAND & GRAND):

DRIVEN PILE:

For Point Bearing Resistance
Sandy Soil:

It has been observed that the value of effective stress increase up to a certain depth of sand, after which the increase in value is insignificant, after certain depth in its assumed to be constant. Hence in case of sand the value of sigma is taken corresponding to depth.
  • 15 * Diameter of Pile for Loose to Medium Sand
  • 20 * Diameter of Pile for Dense Sand
Sand Soil

F.O.S. =  2.5 for Qf
Sand Soil

For Skin Friction
Granular Soil:

As discussed above the ultimate frictional resistance Qf is given as :
Qf = Fs.As
Granular Soil

BORED CAST IN SITU PILE:

Due to boring condition, the value of (K) is very small, hence load carrying capacity is very small as compared to driven pile.
Rest of calculations will be done as Driven Piles.
Point Bearing Resistance of Bored Cast In Situ Piles = 0.5 * Point Bearing Resistance of Driven Pile
F.O.S. = 2.5
Pile Foundation

PILES IN CLAY

Piles in Clay generally carry most of the load by virtue of Skin Friction.
In case of Clay's
qpu = Cub.Nc & fs = ( alpha.Cu )

Qu = (Cubase*Nc).As + (alpha.Cu).As
where,
alpha = Adhesion Factor
Cu = Undrained Cohesion till the Length of Pile
Cub = Undrained Cohesion at the base of Pile
Nc = 9, as per Skempton for deep foundation

Alpha depends on the density of soil as:
Alpha = 1 for Very Loose Clay
Alpha = 0.3 for Very Stiff Clay
F.O.S. = 2.5 

That All For This DailyInfo. Take Care.

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