Session 17 – 18 PILE FOUNDATIONS Course: S0484/Foundation Engineering Year: 2007 Version: 1/0

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

TYPES OF PILE FOUNDATION

STEEL PILE


CONCRETE PILE


CONCRETE PILE



WOODEN PILE


COMPOSITE PILE

COMBINATION OF:

- STEEL AND CONCRETE

- WOODEN AND CONCRETE

- ETC

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.

PILE CATEGORIES

END BEARING PILE

PILE CATEGORIES

FRICTION PILE

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.

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.

PILE CATEGORIES

DETERMINATION OF PILE LENGTH

BEARING CAPACITY OF PILE

Two components of pile bearing capacity:

1. Point bearing capacity (QP)

2. Friction bearing capacity (QS)

SPU QQQ

BEARING CAPACITY OF PILE

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

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)

POINT BEARING CAPACITYMEYERHOFF

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

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)

'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


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


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

POINT BEARING CAPACITYJANBU

QP = Ap (c.Nc* + q’.Nq*)

cot1**

.tan1tan* tan'22

2

NqNc

eNq

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)

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

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)

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

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

FRICTION RESISTANCE

CLAY - METHOD

L

LcLcc uuu

..... 22,11,

FOR LAYERED SOIL

L

AAAv

...' 321

FRICTION RESISTANCE

CLAY - METHOD

fLpQs ..

ucf .

For cu 50 kN/m2 = 1

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)

FRICTION RESISTANCEBORED PILE

LpcQ us 45.0

Where:

cu = mean undrained shear strengthp = pile perimeterL = incremental pile length over which p is taken constant

ULTIMATE AND ALLOWABLE BEARING CAPACITY

SPU QQQ

FS

QQ Uall

5.13SP

all

QQQ

FS= 2.5 - 4

DRIVEN PILE

BORED PILE

2U

all

QQ

5.2U

all

QQ D < 2 m and with expanded at pile point

no expanded at pile point

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)

Documents

Session 17 – 18 PILE FOUNDATIONS Course: S0484/Foundation Engineering Year: 2007 Version: 1/0