Geotechnical Engineering

DHARM

PILE FOUNDATIONS 691

Qg = c′. Nc. Ag + c. Pg. L

Here, cohesion c = c′ = 45 kN/m^2

Nc is taken as 9.

L = 9 m B = 2s + d = 2 × 0.9 + 0.3 = 2.1 m

Pg = 4B = 8.4 m

Ag = B^2 = 2.1^2 m^2 = 4.41 m^2

Substituting,

Qg = 45 × 9 × 4.41 + 45 × 8.4 × 9

= 5,186 kN.

Individual pile failure:

Qg = n[qeb. Ab + fs. As]

= n[cNcAb + α. c. As]

= 9 45 9×× × 4 032 +075 45× ×× ×03 9

L

NM

O

QP

π .. π.

= 2,835 kN

In this case, individual pile failure governs the design. Allowable load on the pile group

=

2 835

25

,

.

≈ 1,130 kN.

Example 16.6: Design a square pile group to carry 400 kN in clay with an unconfined com-
pression strength of 60 kN/m^2. The piles are 30 cm diameter and 6 m long. Adhesion factor
may be taken as 0.6.

Cohesion, c =

60

2

= 30 kN/m^2

Frictional resistance of one pile

= α. c. πd. L

= 0.6 × 30 × π × 0.3 × 6 = 101.8 kN

Safe load per pile =

101.8

3

≈ 34 kN (with a factor of safety of 3)

Approximate number of piles required =

400

34

≈ 12

Let us try a square 16-pile group with centre-to-centre spacing of 60 cm.

Efficiency ηg = 1 –

φ

90

11

°

L −+ −

NM

O

QP

mn nm

mn

()( )

φ = tan–1(d/s) = tan–1 (d/2d) = 26.565

= 1 –

26 565

90

4343

44

. ×+×
×

L

N

M

O

Q

P = 0.56

Safe load on the pile group = 0.56 × 16 × 34 ≈ 305 kN with a factor of safety of 3.

It will be 400 kN with η =

305 3

400

×

= 2.3