Geotechnical Engineering

DHARM

692 GEOTECHNICAL ENGINEERING

Check for block failure:

Frictional resistance of the block

= 2.1 × 4 × 6 × 30

= 1512 kN

Safe load with η = 2.5 is

1512

25.^ ≈ 605 kN
Hence, the safe load may be taken as 400 kN for the group, although the factor of safety
falls short of 2.5 slightly.

Example 16.7: A 16-pile group has to be arranged in the form of a square in soft clay with
uniform spacing. Neglecting end-bearing, determine the optimum value of the spacing of the
piles in terms of the pile diameter, assuming a shear mobilisation factor of 0.6.

At the optimum spacing, efficiency of the pile group is unity.

Let d and s be the diameter and spacing of the piles. Let L be their length.

Width of the block for a 16-pile square group,

B = 3s + d

Group capacity for block failure

= 4L(3s + d) × c

where c is the unit cohesion of the soil.

Group capacity based on individual pile failure

= n[(0.6c)(πdL)]

= 16 × 0.6 πd Lc

Equating these two,

4 Lc(3s + d) = 16 × 0.6 πd Lc

12 s + 4d = 9.6 πd

s =

(. )96 4

12

π−

d = 2.18d

∴ The optimum spacing is about 2.2 d.

Example 16.8: A square pile group of 9 piles passes through a recently filled up material of 4.5

m depth. The diameter of the pile is 30 cm and pile spacing is 90 cm centre to centre. If the

unconfined compression strength of the cohesive material is 60 kN/m^2 and unit weight is 15

kN/m^3 , compute the negative skin friction of the pile group.

Individual piles:

Cohesion =

1

2

× 60 = 30 kN/m^2

Negative skin friction

Qng = n × PDn c

= 9 × π × 0.3 × 4.5 × 30 = 1145 kN