Session 17 – 18 PILE FOUNDATIONS Course: S0484/Foundation Engineering Year: 2007 Version: 1/0.
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Transcript of Session 17 – 18 PILE FOUNDATIONS Course: S0484/Foundation Engineering Year: 2007 Version: 1/0.
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Session 17 – 18 PILE FOUNDATIONS
Course : S0484/Foundation Engineering
Year : 2007
Version : 1/0
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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
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INTRODUCTION
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TYPES OF PILE FOUNDATION
STEEL PILE
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TYPES OF PILE FOUNDATION
CONCRETE PILE
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TYPES OF PILE FOUNDATION
CONCRETE PILE
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TYPES OF PILE FOUNDATION
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TYPES OF PILE FOUNDATION
WOODEN PILE
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TYPES OF PILE FOUNDATION
COMPOSITE PILE
COMBINATION OF:
- STEEL AND CONCRETE
- WOODEN AND CONCRETE
- ETC
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PILE CATEGORIES
Classification of pile with respect to load transmission and functional behaviour: 1. END BEARING PILES
These piles transfer their load on to a firm stratum located 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
2. FRICTION PILES
Carrying capacity is derived mainly from the adhesion or friction of the soil
in contact with the shaft of the pile
3. 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.
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PILE CATEGORIES
END BEARING PILE
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PILE CATEGORIES
FRICTION PILE
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PILE CATEGORIES
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.
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PILE CATEGORIES
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.
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PILE CATEGORIES
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DETERMINATION OF PILE LENGTH
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BEARING CAPACITY OF PILE
Two components of pile bearing capacity:
1. Point bearing capacity (QP)
2. Friction bearing capacity (QS)
SPU QQQ
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BEARING CAPACITY OF PILE
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POINT BEARING CAPACITY
SQUARE FOUNDATIONqu = 1,3.c.Nc + q.Nq + 0,4..B.N
CIRCULAR FOUNDATIONqu = 1,3.c.Nc + q.Nq + 0,3..B.N
For Shallow Foundation- TERZAGHI
- GENERAL EQUATION
idsqiqdqscicdcsu FFFNBFFFNqqFFFNccq ......5,0........
Deep Foundationqu = qP = c.Nc* + q.Nq* + .D.N*
Where D is pile diameter, the 3rd part of equation is neglected due to its small contribution
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
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POINT BEARING CAPACITYMEYERHOFF
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)
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POINT BEARING CAPACITYMEYERHOFF
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PILE FOUNDATION AT MULTIPLE SAND LAYER (c = 0)
QP = Ap .qP
dlblldl
llP qD
Lqqqq
10
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)
POINT BEARING CAPACITYMEYERHOF
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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
POINT BEARING CAPACITYMEYERHOF
PILE FOUNDATION AT SATURATED CLAY LAYER (c 0)
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'3
21' q
Koo
• 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
oKNq
N
NqNc
21
*3*
cot1**
POINT BEARING CAPACITYVESIC
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POINT BEARING CAPACITYVESIC
r
rrr I
II
1
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
tan'tan'12 qc
G
qc
EI s
s
sr
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POINT BEARING CAPACITYVESIC
12
1ln3
4*
rrINc
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:
Type of soil Ir
Sand 70 – 150
Silt and clay (drained) 50 – 100
Clay (undrained) 100 – 200
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POINT BEARING CAPACITYJANBU
QP = Ap (c.Nc* + q’.Nq*)
cot1**
.tan1tan* tan'22
2
NqNc
eNq
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POINT BEARING CAPACITYBORED PILE
QP = . Ap . Nc . Cp
Where: = correction factor = 0.8 for D ≤ 1m = 0.75 for D > 1mAp = section area of pilecp = undrained cohesion at pile pointNc = bearing capacity factor (Nc = 9)
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FRICTION RESISTANCE
fLpQs ..
Where:p = pile perimeterL = incremental pile length over which p and f are taken constantf = unit friction resistance at any depth z
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FRICTION RESISTANCE
SAND
fLpQs ..
tan'.. vKf
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)
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FRICTION RESISTANCE
CLAY
Three of the presently accepted procedures are:
1. methodThis 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.
2. method (Tomlinson)
3. method
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FRICTION RESISTANCE
CLAY - METHOD
avs fLpQ ..
uvav cf 2'
Where:v’= mean effective vertical stress for the entire embedment lengthcu = mean undrained shear strength ( = 0)
VALID ONLY FOR ONE LAYER OF HOMOGEN CLAY
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FRICTION RESISTANCE
CLAY - METHOD
L
LcLcc uuu
..... 22,11,
FOR LAYERED SOIL
L
AAAv
...' 321
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FRICTION RESISTANCE
CLAY - METHOD
fLpQs ..
ucf .
For cu 50 kN/m2 = 1
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FRICTION RESISTANCE
CLAY - METHOD
fLpQs ..
'. vf 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)
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FRICTION RESISTANCEBORED PILE
LpcQ us 45.0
Where:
cu = mean undrained shear strengthp = pile perimeterL = incremental pile length over which p is taken constant
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ULTIMATE AND ALLOWABLE BEARING CAPACITY
SPU QQQ
FS
QQ Uall
5.13SP
all
QQQ
FS= 2.5 - 4
DRIVEN PILE
BORED PILE
2U
all
5.2U
all
QQ D < 2 m and with expanded at pile point
no expanded at pile point
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EXAMPLE
A pile with 50 cm diameter is penetrated into clay soil as shown in the following figure:
GWL5 m
5 m
20 m
NC clay = 18 kN/m3
cu = 30 kN/m2
R = 30o
OC clay (OCR = 2) = 19.6 kN/m3
cu = 100 kN/m2
R = 30o
Determine:
1. End bearing of pile
2. Friction resistance by , , and methods
3. Allowable bearing capacity of pile (use FS = 4)