PILE GROUPS
In most practical situations, piles are used in groups. They are arranged in geometric patterns (squares, rectangles, circles, and octagons) at a spacing, s (center to center distance), not less than 2D (where D is the diameter or width of the pile). The piles are connected at their heads by a concrete pile cap, which
Pile groups.
may or may not be in contact with the ground (Figure ). If the pile cap is in contact with the ground, part of the load will be transferred directly to the soil.
The load capacity for a pile group is not necessarily the load capacity of a single pile multiplied by the number of piles. In fi ne-grained soils, the outer piles tend to carry more loads than the piles in the center of the group. In coarse-grained soils, the piles in the center take more loads than the outer piles. The ratio of the load capacity of a pile group, (Qult)g, to the total load capacity of the piles acting as individual piles, (nQult), is called the effi ciency factor, he; that is,
where n is the number of piles in the group and Qult is the ultimate load capacity of a single pile. The effi ciency factor is usually less than 1. However, piles driven into a loose, coarse-grained soil tend to densify the soil around the piles, and he could exceed 1.
Two modes of soil failure are normally investigated to determine the load capacity of a pile group. One mode, called block failure (Figure ), may occur when the spacing of the piles is small enough to cause the pile group to fail as a unit. The group load capacity for block failure mode is
Block failure mode.
ESA:
TSA:
where the subscript gb denotes block mode of failure for the group. The other failure mode is single pile failure mode or punching failure mode. The key assumption
in single pile failure mode is that each pile mobilizes its full load capacity. Thus, the group load capacity is
where the subscript gs denotes single pile mode failure.
The values of cp to use in determining Nq in Janbu’s equation depend on the s/D ratio and the friction angle. Janbu (1976) showed that
The value of cp is not signifi cantly affected by s/D < 2.5. As a reminder, Janbu’s equation may signifi cantly underestimate the end bearing capacity because the failure mechanism assumed in developing Equation may not develop.