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Fig. 8.23 A typical pile group.

Elevation

One characteristic of pile groups in cohesive soils is the phenomenon of 'block failure'. If the piles are placed very close together (a common temptation when dealing with a limited site area), the strength of the groups may be governed by its strength at block failure. This is when the soil fails along the perimeter of the group.

For block failure:

D = depth of pile penetration L = length of pile group B = breadth of pile group Nc = bearing capacity coefficient (taken generally as 9.0).

Whitaker (1957), in a series of model tests, showed that block failure will not occur if the piles are spaced at not less than 1.5d apart. General practice is to use 2d to 3d spacings.

In such cases:

Qu = En Qup where

E = efficiency of pile group (0.7 for spacings 2d-3d) Qup = ultimate bearing capacity of single pile n = number of piles in group.

8.10.11 Settlement effects in pile groups

Quite often it is the allowable settlement, rather than the safe bearing capacity, that decides the working load that a pile group may carry.

For bearing piles the total foundation load is assumed to act at the base of the piles on a foundation of the same size as the plan of the pile group. With this assumption it becomes a simple matter to examine settlement effects.

With friction piles it is virtually impossible to determine the level at which the foundation load is effectively transferred to the soil. An approximate method, often used in design, is to assume that the effective transfer level is at a depth of 2D/3 below the top of the piles. It is also assumed that there

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