Geotechnical Engineering

DHARM

PILE FOUNDATIONS 693

Block:

B = 2s + d = 2 × 0.9 + 0.3 = 2.1 m Pg = 4B = 8.4 m

Negative skin friction Ag = B^2 = 4.41 m^2

Qng = CDnPg + γ. DnAg

= 30 × 4.5 × 8.4 + 15 × 4.5 × 4.41 = (1134 + 298)

= 1432 kN

∴ Negative skin friction (the larger of the two values) = 1,432 kN.

Example 16.9: A reinforced cement concrete pile weighing 30 kN (including helmet and dolly)
is driven by a drop hammer weighing 30 kN with an effective fall of 0.9 m. The average pen-
etration per blow is 15 mm. The total temporary elastic compression of the pile, pile cap and
soil may be taken as 18 mm. Coefficient of restitution 0.36. What is the allowable load on the
pile with a factor of safety of 2? Use Hiley’s formula.

Wp = 30 kN Wh = 30 kN R =

W

W

p

h

= 1

Effective fall = ηH = 0.9 m = 900 mm

s = 15 mm C =

1

2

(total elastic compression of pile, pile cap and soil)

Cr = 0.36 =

1

2

× 18 = 9 mm

Qap =

Q WH

sC

CR

R

up h r

2

1

2

1

1

2

=

+

+

+

F

HG

I

KJ

L

N

M

M

O

Q

P

P

()

η.

by Hiley’s formula

=

1

2

30 900 1

15 9

1036 1

11

××^2

+

×

+×

+

L

N

M

O

Q

() P

(. )

()

=

1

2

30

24

900 1 1296

2

..×.

= 317.7 kN

Approximate safe load may be taken as 315 kN.

Summary of Main Points

1. Two general forms of deep foundation are recognised: Pile foundation and pier foundation. Piles
   are long, slender members used to bypass soft strata and transmit loads to firmer strata situ-
   ated below.


2. Piles, other than sheet piles which are commonly used for reducing seepage, derive their capac-
   ity from end-bearing of the tip and skin friction of the surrounding soil against them.
   Piles may be of timber, steel, concrete or composite. Concrete piles may be precast or cast-in-
   place; the former are driven by pile hammers—drop, steam, pneumatic, diesel or vibratory.


3. Pile capacity may be obtained by static analysis—bearing capacity theories such as those of
   Meyerhof and Vesic for deep foundations—or by dynamic analysis.