Pile foundation

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25 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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Pile foundations

Capacity of Single Pile


Using Theory (c,
φ
)


Using SPT value


Using SCPT Value


Using Dynamic Formula


Pile Load Test


Static Formula

In
-
situ Penetration Tests

Dynamic Formula
-

PRINCIPLE





Energy

Workdone

Wh

Q
u
s

s
Wh
Q
u

W
-

weight of the driving hammer

h
-

height of fall of hammer

Wh

-

energy of hammer blow

Q
-

ultimate resistance to penetration

S
-

pile penetration under one hammer blow

Q
u
s

-

resisting energy of the pile

DYNAMIC FORMULA


Hileys

Formula


Engineering News



Energy used
by pile to
move down

Energy losses

Energy Input

Hiley’s

Formula


The energy loss E
1

due to the elastic compressions of the pile cap, pile
material and the soil surrounding the pile



The energy loss E
2

due to the interaction of the pile hammer system


Hileys

Formula
-

Energy losses


The energy loss E
1

due to the elastic compressions of the pile cap, pile
material and the soil surrounding the pile




c
1

= elastic compression of the pile cap


c
2

= elastic compression of the pile


c
3

= elastic compression of the soil.



C
Q
c
c
c
Q
E
u
u




3
2
1
1
2
1
Hileys

Formula
-

Energy Losses


The energy loss E
2

due to the interaction of the pile hammer system





W
p

= weight of pile


C
r

= coefficient of restitution

p
r
p
W
W
C
WhW
E



2
2
1
Hileys

Formula










where,










η
h



Efficiency of the hammer

R
R
C
C
s
Wh
Q
r
h
u





1
1
2

W
W
R
p

Hileys

Formula

Type of soil

c
3

Hard Soil

0

Resilient Soil

0.2

Elastic compression c
1

of cap and pile head

Elastic compression c
2
of pile

Elastic compression c
3

of soil

Pile Material

Range of Driving
Stress kg/cm
2

Range of c
1

Precast concrete pile with
packing inside

cap

30
-
150

0.12
-
0.50

Timber pile without cap

30
-
150

0.05
-
0.20

Steel H
-
pile

30
-
150

0.04
-
0.16

Hileys

Formula

Hammer Type

η
h

Drop hammer

1.00

Single acting

0.75
-
0.85

Double acting

0.85

Diesel

1.00

Efficiency of pile hammer

Material


C
r

Wood pile

0.25

Compact wood cushion of steel pile

0.32

Cast iron hammer on concrete pile
without cap

0.40

Cast

iron hammer on steel pile without
cushion

0.55

Coefficient of restitution C
r

Engineering News Formula






W
-

weight of hammer in kg

H
-

height of fall of hammer in cm

s
-

final penetration in cm per blow (set)

C
-

empirical constant



C
s
Wh
Q
u


6
The
set

is taken as the
average
penetration per blow for the last 5
blows of a drop hammer or 20 blows of
a steam hammer

C = 2.5 cm for a drop hammer

C = 0.25 cm for single acting hammer

Problem


A 40 x 40 cm reinforced concrete pile 20 m long is driven
through loose sand and then into dense gravel to a final set of 3
mm/blow, using a 30
kN

single
-
acting hammer with a stroke of
1.5 m.


Determine the ultimate driving resistance of the pile if it is fitted
with a helmet, plastic dolly and 50 mm packing on the top of the
pile. The weight of the helmet and dolly is 4
kN.

The other details
are: weight of pile = 74
kN
; weight of hammer = 30
kN
; pile
hammer efficiency
η
h

= 0.80 and coefficient of restitution Cr =
0.40.


Use the
Hiley

formula. The sum of the elastic compression C is
C
= c
1

+c
2

+c
3
= 19.6 mm.

PILE LOAD TEST

Pile Load Test


Load tests may be carried out on a
working pile
or a
test pile


Pile load tests on a
single pile
or
group of piles


For the determination of


Vertical load
bearing capacity


Uplift load
capacity


Lateral load
capacity


Load test may be of two types


Continuous

load

test.


Cyclic load
test.

Vertical Pile Load Test Assembly

Load


Settlement Curves

Determination of
Qu

from Load
-
Settlement Curve


Qu
, can be determined as the abscissa of the point where the
curved part
of the load
-
settlement curve
changes to a falling
straight line


Qu

is the abscissa of the
point of intersection
of
the initial and final
tangents
of the load
-
settlement curve


Qa

is
50 percent of the ultimate load
at which the
total settlement
amounts to
one
-
tenth of the diameter of the pile

for uniform
diameter piles.


Qa

is sometimes taken as equal to
two
-
thirds of the load
which
causes a
total settlement of 12 mm


Qa

is sometimes taken as equal to
two
-
thirds of the load
which
causes a
net (plastic) settlement of 6 mm

Recap
-

Capacity of Single Pile


Using Theory (c,
φ
)


Using SPT value


Using SCPT Value


Using Dynamic Formula


Pile Load Test


Static Formula

In
-
situ Penetration Tests

PILE GROUPS

Some Examples

Multistoried Building Resting on Piles

Some Examples

Piles Used to Resist Uplift Forces

Some Examples

Piles used to Resist lateral Loads

Pressure isobars of single
pile

Pressure Isobars of Group of piles with

piles placed farther apart

Pressure Isobars of Group of piles
closely spaced

Typical
Arrangement of
Piles in Groups

Minimum Spacing between Piles


Stipulated in building codes


For straight uniform diameter piles
-

2 to 6 d


For friction piles


3d


For end bearing piles


passing through
relatively compressible strata
, the spacing of
piles
shall not be less than 2.5d


For end bearing piles passing through compressible strata and
resting in stiff clay
-

3.5d



For compaction piles
-

2d
.

Pile Group Efficiency









u
gu
g
Q
Q

CAPACITY OF PILE GROUP


Feld’s

Rule


Converse
-
Labarre

Formula





Block failure criteria



FELD'S RULE


Reduces the capacity of each pile by 1/16 for each adjacent
pile

CONVERSE
-
LABARRE

FORMULA











mn
n
m
m
n
g
90
1
1
1







m = number of columns of piles in a group,

n = number of rows,

θ

= tan
-
1
( d/s) in degrees,

d = diameter of pile,

s = spacing of piles center to center.



u
g
gu
Q
Q

PILE GROUP


Driven piles


Bored piles



Pile group in sandy soil


Pile group in clayey soil


Block Failure


c = cohesive strength of clay beneath the pile group,

L = length of pile,

P
g
= perimeter of pile group,

A
g
= sectional area of group,

N
c

= bearing capacity factor which may be assumed as
9 for deep foundations.

Recap



Capacity of single pile


Capacity of pile group