Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 241

average load per pile when failure of the complete group occurs, to the load at failure of a
single comparable pile. The various efficiency ratios are based simply on experience without
any relationship to soil mechanics principles. For this reason the authors do not consider this
to be a desirable or logical approach to the problem and prefer to base design methods on the
assumption that the pile group behaves as a block foundation with a degree of flexibility
which depends on the rigidity of the capping system and the superimposed structure. By
treating the foundation in this manner, normal soil mechanics practice can be followed in
the calculations to determine the ultimate bearing capacity and settlement. Load transfer in
shaft friction from the pile shaft to the surrounding soil is allowed for by assuming that the
load is spread from the shafts of friction piles at an angle of 1 in 4 from the vertical. Three
cases of load transfer are shown in Figure 5.3a to c.

Test pile

(a) (b)

Fill or

weak soil

Soft compressible clay

Compact

stratum

Figure 5.2Shear failure of pile group (a) Test load on single isolated pile when soft clay is not stressed
significantly (b) Load applied to group of piles when soft clay is stressed heavily.

Spread of load at 1 in 4

(a) (b) (c)

Soft

clay

Base of equivalent raft

foundation

D

D

(^2) / 3 D
(^2) / 3 D
Figure 5.3Load transfer to soil from pile group (a) Group of piles supported predominantly by shaft
friction(b) Group of piles driven through soft clay to combined shaft friction and end
bearing in stratum of dense granular soil (c) Group of piles supported in end bearing on
hard rock stratum.