Session 17 – 18 PILE FOUNDATIONS. Course: S0484/Foundation Engineering Year: 2007 Version: 1/0. PILE FOUNDATIONS. Topic: Types of pile foundation Point bearing capacity of single pile Friction bearing capacity of single pile Allowable bearing capacity of single pile. INTRODUCTION.
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Session 17 – 18 PILE FOUNDATIONS
Course: S0484/Foundation Engineering
Year: 2007
Version: 1/0
Topic:
STEEL PILE
CONCRETE PILE
CONCRETE PILE
WOODEN PILE
Classification of pile with respect to load transmission and functional behaviour:
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
Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile
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 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 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:
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
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
= (0.5 – 0.8)
FRICTION RESISTANCECLAY
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