Design of pile groups - Piling Engineering - page 183

Figure 5.2 Block failure of pile groups.

Instead of failure of the individual piles in a group, block failure may occur under
axial or lateral load. Soil between the piles may move with the piles, resulting in
failure planes which follow the periphery of the group (or parts of the group) as
shown in Figure 5.2. In general, block failure will be associated with close spac-
ing of the piles. However, in certain cases, such as the axial failure of a pile group
with a ground-contacting pile cap, block failure may be enforced at quite large pile
spacings.
In the past, the difference between the capacity of a group of piles, and the sum of the
individual pile capacities, has been characterized by a group 'efficiency'. This concept
has led to a certain amount of confusion. As stated above, the capacity of a pile within
a group may be different from that of an equivalent isolated pile, and, in that sense,
the pile may be thought of as more or less efficient when acting as part of a group.
However, the block capacity of a group should not be confused with the sum of the
individual pile capacities by the use of an 'efficiency' factor. Independent calculations
should be made of both the block capacity and the individual pile capacities, to ensure
that there is an adequate factor of safety against both modes of failure.

5.1.1 Axial capacity

The axial capacity of a pile group failing as a block may be calculated in a similar
fashion to that for an individual pile, by means of equation (4.1), but now taking A b
as the base area of the block and A s as the block surface area. In general, the enclosing
block as shown in Figure 5.2(c) should be taken, but, where the pile spacing in one
direction is much greater than that in the perpendicular direction, the capacity of the
group failing as shown in Figure 5.2(b) should be assessed. It should be noted that the
settlement needed to mobilize the base capacity of the block will usually be very large
(for example, 5 to 10% of the width of the group).
Since the end-bearing pressure q b is much greater than the average skin friction

τ s ,
block failure only becomes more likely than the failure of individual piles where the
increase in base area, A b , is offset by a much larger decrease in surface area, A s .