 Lecture – 9
SOIL MECHANICS AND
FOUNDATION ENGINEERING-II
(CE 311)
GROUP ACTION IN PILES
GROUP ACTION IN PILES
 Efficiency of Pile Group (hg)
𝑄𝑔
𝜂𝑔 =
𝑛 × 𝑄1
where Qg = load carrying capacity of pile group
Q1 = load carrying capacity of single pile
 Design of Pile Group
𝑄𝑓𝑔 = 𝑐𝑃𝐿 + A 𝑟𝑏
𝑄𝑓𝑖 = 𝑛(𝑚𝑐𝑃𝐿 + A 𝑟𝑏 )
Group Action
Individual Action
 Settlement of Pile Group
𝜎𝑓
𝐻 𝐶𝑐
𝜌=
𝑙𝑜𝑔10
1 + 𝑒0
𝜎0
PROBLEMS
1.
2.
3.
4.
In a 16 pile group, the pile diameter is 45cm and c/c spacing of the
square group is 1.5m. If c=50kN/m2, determine whether the failure
would occur with the pile acting individually, or as a group? Neglect
bearing at tip of the pile. All piles are 10m long. Take m=0.7 for shear
mobilization around each pile.
A n-pile group has to be proportioned in a uniform pattern in soft clay
with equal spacing in all directions. Assuming any value of c,
determine the optimum value of spacing of piles in the group. Take
n=25 and m=0.7. Neglect the end bearing effect and assume that
each pile is circular in section.
Design a friction pile group to carry a load of 3000 kN including the
weight of the pile cap at a site where the soil is uniform clay to a
depth of 20m, underlain by rock. Average unconfined compressive
strength of the clay is 70 kN/m2. The clay may be assumed to be of
normal sensitivity and normally loaded, with liquid limit 60% and
eo=1. A factor of safety of 3 is required against shear failure.
For the design data in question 3, compute the settlement of the
group assuming the load to be transferred at 2/3 length of the pile.