Geotechnical Engineering

DHARM

690 GEOTECHNICAL ENGINEERING

Allowable load on the pile

Qap =

500

325

WH

s

h.

(.)+

for steam hammer,

H being in metres and s in mm.

Wh = 15 kN H = 900 mm = 0.9 m

∴ Qap =

500 15 0 9

3275 25

××

+

.

(. .)

kN = 75 kN.

Example 16.3: A pile is driven in a uniform clay of large depth. The clay has an unconfined

compression strength of 90 kN/m^2. The pile is 30 cm diameter and 6 m long. Determine the

safe frictional resistance of the pile, assuming a factor of safety of 3. Assume the adhesion

factor α = 0.7.

Cohesion of clay =

1

2

× 90 = 45 kN/m^2

Frictional resistance = α. cAs

= 0.7 × 45 × π × 0.3 × 6 kN = 178.13 kN

∴ Safe frictional resistance =

178 13

3

.

≈ 59.3 kN.

Example 16.4: A group of 16 piles of 50 cm diameter is arranged with a centre to centre

spacing of 1.0 m. The piles are 9 m long and are embedded in soft clay with cohesion 30 kN/m^2.

Bearing resistance may be neglected for the piles—Adhesion factor is 0.6. Determine the ulti-

mate load capacity of the pile group.

n = 16 d = 50 cm L = 9 m s = 1.5 m
Width of group, B = (1 × 3 + 0.50) = 3.5 m
For block failure:
Qg = c × Perimeter × Length
= c × 4 B × 2
= 30 × 4 × 3.5 × 9
= 3780 kN
For piles acting individually:
Qg = n(α. cAs)
= 16 × 0.6 × 30 × π × 0.5 × 9
= 4,070 kN
Hence the foundation is governed by block failure and the ultimate load capacity is
3,780 kN.

Example 16.5: A square group of 9 piles was driven into soft clay extending to a large depth.
The diameter and length of the piles were 30 cm and 9 m respectively. If the unconfined
compression strength of the clay is 90 kN/m^2 , and the pile spacing is 90 cm centre to centre,
what is the capacity of the group? Assume a factor of safety of 2.5 and adhesion factor of 0.75.

Block failure:

Since, it is a square group, 3 rows of 3 piles each will be used.