1. SOIL MECHANICS AND FOUNDATION ENGINEERING-II (CE 311)
Lecture – 9
SOIL MECHANICS AND FOUNDATION ENGINEERING-II (CE 311)

2. GROUP ACTION IN PILES

5. GROUP ACTION IN PILES Efficiency of Pile Group (hg) 𝜂 𝑔 = 𝑄 𝑔 𝑛× 𝑄 1
𝜂 𝑔 = 𝑄 𝑔 𝑛× 𝑄 1
where Qg = load carrying capacity of pile group
Q1 = load carrying capacity of single pile
Design of Pile Group
𝑄 𝑓𝑔 =𝑐𝑃𝐿+A 𝑟 𝑏 Group Action
𝑄 𝑓𝑖 =𝑛(𝑚𝑐𝑃𝐿+A 𝑟 𝑏 ) Individual Action
Settlement of Pile Group
𝜌= 𝐻 𝐶 𝑐 1+ 𝑒 0 𝑙𝑜𝑔 𝜎 𝑓 𝜎 0

6. PROBLEMS
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.