Pile group design

Piles are almost always used in groups joined by a pile cap. A group does not simply carry the sum of the individual pile capacities: the zones of soil stressed by neighbouring piles overlap, the group settles more than a single pile, and in clay the whole block of soil enclosing the piles can fail together.

In clays, two checks are made. The first sums the capacities of the individual piles, multiplied by an efficiency factor if one is used. The second treats the group as a single block, with skin friction on its perimeter and end bearing on its base. The lower value governs. Close spacing tends to make block failure govern; wider spacing makes the individual-pile mode govern but enlarges the cap. In sands, driving piles densifies the soil and group efficiency can exceed one; for bored piles it is usually taken as one or less.

Spacing is usually not less than about 2.5 to 3 pile diameters for friction piles, and less for end-bearing piles on rock. The cap distributes column load to the piles; for a rigid cap under vertical load and moment, each pile load equals the average plus a term proportional to its distance from the group centroid. Lateral loads are shared similarly, with group effects reducing the lateral stiffness of trailing piles.

Settlement of a group is estimated by treating the group as an equivalent raft at a depth, commonly two-thirds of the pile length in friction piles. IS 2911 gives design and construction rules for piles in India, and load tests confirm the assumptions.