Pile Groups.

It is sometimes necessary to drive a group of piles to support heavy loadings and it is important to notice two effects:

(1) The pressure bulb of the group affects deeper layers of soils than a single pile of the same depth (see Fig. 14.13) in a similar manner to a wide foundation.
(2) The load-bearing capacity of a group is not necessarily the product of the capacity of the single pile times the number of piles. There can be a pressure ‘overlap’ (see Fig. 14.14) and the capacity of the group could decrease as the difference between a pad and strip foundation.

A single pile, in driving, displaces soil which can result in heave at ground level and a group can cause greater heave and displacement; this fact should be checked and considered. Driving a single pile, too, in loose sand and fills will compact the soil around the pile to a diameter of approximately 5.5 times the pile diameter and make it denser. If a group of piles is driven it could create such a compact block of soil as to prevent driving of all the piles in the group. The central piles should be driven first and then, working out to the perimeter of the group, the remaining piles should be driven.

Fig. 14.13 Section through pile pressure bulbs.

Fig. 14.14 Plan on pile pressure bulbs.

Spacing of piles within a group

Approximate values for centre-to-centre spacing are as follows:
 
(1) Friction piles – not less than the perimeter of the pile.
(2) End-bearing piles – not less than twice the diameter of the pile.
(3) Screw piles – not less than 1.5 times the diameter of the blades.
(4) Piles with enlarged bases – at least one pile diameter between enlarged bases.
 
These values are affected by the soil conditions, the group behaviour of the piles, the possible heave and compaction, and the need to provide sufficient space to install the piles to the designed penetration without damage to the pile or group.