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Session 17 – 18 PILE FOUNDATIONS

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Session 17 – 18 PILE FOUNDATIONS

Course: S0484/Foundation Engineering

Year: 2007

Version: 1/0

Topic:

- Types of pile foundation
- Point bearing capacity of single pile
- Friction bearing capacity of single pile
- Allowable bearing capacity of single pile

STEEL PILE

CONCRETE PILE

CONCRETE PILE

WOODEN PILE

- COMPOSITE PILE
- COMBINATION OF:
- STEEL AND CONCRETE
- WOODEN AND CONCRETE
- ETC

Classification of pile with respect to load transmission and functional behaviour:

- END BEARING PILES
These piles transfer their load on to a firm stratumlocated at a considerable depth below the base of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile

- FRICTION PILES
Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile

- COMPACTION PILES
These piles transmit most of their load to the soil through skin friction. This process of driving such piles close to each other in groups greatly reduces the porosity and compressibility of the soil within and around the groups.

END BEARING PILE

FRICTION PILE

Classification of pile with respect to effect on the soil

- Driven Pile
Driven piles are considered to be displacement piles. In the process of driving the pile into the ground, soil is moved radially as the pile shaft enters the ground. There may also be a component of movement of the soil in the vertical direction.

Classification of pile with respect to effect on the soil

- Bored Pile
Bored piles(Replacement piles) are generally considered to be non-displacement piles a void is formed by boring or excavation before piles is produced.

There are three non-displacement methods: bored cast- in - place piles, particularly pre-formed piles and grout or concrete intruded piles.

Two components of pile bearing capacity:

- Point bearing capacity (QP)
- Friction bearing capacity (QS)

For Shallow Foundation

- TERZAGHI

SQUARE FOUNDATION

qu = 1,3.c.Nc + q.Nq + 0,4..B.N

CIRCULAR FOUNDATION

qu = 1,3.c.Nc + q.Nq + 0,3..B.N

- GENERAL EQUATION

Deep Foundation

Where D is pile diameter, the 3rd part of equation is neglected due to its small contribution

qu = qP = c.Nc* + q.Nq* + .D.N*

qu = qP = c.Nc* + q’.Nq* ; QP = Ap .qp = Ap (c.Nc* + q’.Nq*)

Nc* & Nq* : bearing capacity factor by Meyerhoff, Vesic and Janbu

Ap : section area of pile

PILE FOUNDATION AT UNIFORM SAND LAYER (c = 0)

QP = Ap .qP = Ap.q’.Nq* Ap.ql

ql = 50 . Nq* . tan (kN/m2)

Base on the value of N-SPT :

qP = 40NL/D 400N (kN/m2)

Where:

N = the average value of N-SPT near the pile point (about 10D above and 4D below the pile point)

PILE FOUNDATION AT MULTIPLE SAND LAYER (c = 0)

QP = Ap .qP

Where:

ql(l) : point bearing at loose sand layer (use loose sand parameter)

ql(d) : point bearing at dense sand layer (use dense sand parameter)

Lb = depth of penetration pile on dense sand layer

ql(l) = ql(d) = 50 . Nq* . tan (kN/m2)

PILE FOUNDATION AT SATURATED CLAY LAYER (c 0)

QP = Ap (c.Nc* + q’.Nq*)

For saturated clay ( = 0), from the curve we get:

Nq* = 0.0

Nc* = 9.0

and

QP = 9 . cu . Ap

- BASE ON THEORY OF VOID/SPACE EXPANSION
- PARAMETER DESIGN IS EFFECTIVE CONDITION

QP = Ap .qP = Ap (c.Nc* + o’.N*)

WHERE:

o’ = effective stress of soil at pile point

Ko = soil lateral coefficient at rest = 1 – sin

Nc*, N* = bearing capacity factors

According to Vesic’s theory

N* = f (Irr)

where

Irr = Reduced rigidity index for the soil

Ir = Rigidity index

Es = Modulus of elasticity of soil

s = Poisson’s ratio of soil

Gs = Shear modulus of soil

= Average volumetric strain in the plastic zone below the pile point

For condition of no volume change (dense sand or saturated clay):

= 0 Ir = Irr

For undrained conditon, = 0

The value of Ir could be estimated from laboratory tests i.e.: consolidation and triaxial

Initial estimation for several type of soil as follow:

QP = Ap (c.Nc* + q’.Nq*)

QP = . Ap . Nc . Cp

Where:

= correction factor

= 0.8 for D ≤ 1m

= 0.75 for D > 1m

Ap = section area of pile

cp = undrained cohesion at pile point

Nc = bearing capacity factor (Nc = 9)

Where:

p = pile perimeter

L = incremental pile length over which p and f are taken constant

f = unit friction resistance at any depth z

- Where:
- K = effective earth coefficient
- = Ko = 1 – sin (bored pile)
- = Ko to 1.4Ko (low displacement driven pile)
- = Ko to 1.8Ko (high displacement driven pile)
- v’ = effective vertical stress at the depth under consideration
- = soil-pile friction angle
= (0.5 – 0.8)

FRICTION RESISTANCECLAY

- Three of the presently accepted procedures are:
- method
- This method was proposed by Vijayvergiya and Focht (1972), based on the assumption that the displacement of soil caused by pile driving results in a passive lateral pressure at any depth.
- method (Tomlinson)
- method

Where:

v’= mean effective vertical stress

for the entire embedment length

cu = mean undrained shear strength ( = 0)

VALID ONLY FOR ONE LAYER OF HOMOGEN CLAY

FOR LAYERED SOIL

For cu 50 kN/m2

= 1

Where:

v’= vertical effective stress

= K.tanR

R = drained friction angle of remolded clay

K = earth pressure coefficient at rest

= 1 – sin R (for normally consolidated clays)

= (1 – sin R) . OCR (for overconsolidated clays)

Where:

cu = mean undrained shear strength

p = pile perimeter

L = incremental pile length over which p is taken constant

DRIVEN PILE

FS= 2.5 - 4

BORED PILE

D < 2 m and with expanded at pile point

no expanded at pile point

A pile with 50 cm diameter is penetrated into clay soil as shown in the following figure:

NC clay

= 18 kN/m3

cu = 30 kN/m2

R = 30o

5 m

5 m

20 m

GWL

OC clay (OCR = 2)

= 19.6 kN/m3

cu = 100 kN/m2

R = 30o

- Determine:
- End bearing of pile
- Friction resistance by , , and methods
- Allowable bearing capacity of pile (use FS = 4)