It is usually incorrect to assume that the group failure load equals the sum of the pile failure loads with the piles acting individually. The difference is represented by an efficiency factor η:
勘查工程专业英语
For piles driven into loose or medium dense sands the effect of compaction will lead to an efficiency η greater than 1 with higher efficiencies for closer pile spacing and in looser sands. For piles driven into dense sands the efficiency is unlikely to exceed 1 and could be less if driving causes disturbance. For bored pile groups in sand the overlapping disturbance zones between piles is likely to reduce efficiency.
The model tests of Whitaker on piles in clay，see Figure 9. 4，showed the significant effect of the spacing of the piles on efficiency. For a square group of piles the efficiency at a spacing of 8 diameters or more，even with a rigid pile cap was close to unity giving equal pile loads. As the spacing decreases the efficiency decreases but not too significantly.
With wider spacing the pile group reaches a state of failure referred to as individual pile penetration，see Figure 9. 5，with the soil remaining static and the piles alone moving down. This，of course，assumes a‘free-standing' group. However，as the pile spacing decreases，at a critical pile spacing the soil between the piles moves down with the piles causing ‘block failure' when the efficiency diminishes dramatically，as shown in Figure 9. 5. The efficiency was also found to decrease for longer piles and for larger pile groups.
Figure 9. 4 Efficiency of freestanding pile groups
Figure 9. 5 Modes of failure for free-standing pile group