Civil Engineering Reference
In-Depth Information
Figure 5.1
Piles as reinforcement for the soil.
foundation, in order to reduce settlements to an acceptable level. This approach is
discussed further in section 5.3.
At the end of the chapter, there is a discussion of some of the main principles in the
design of single piles and pile groups. The discussion includes comments on appropriate
factors of safety to use for piles in different circumstances, and reviews the potential
benefits and hazards of using raking piles.
5.1 Capacity of pile groups
A group of piles may be viewed as providing reinforcement to a particular body of
soil. Failure of the group may occur either by failure of the individual reinforcing
members (the piles) or as failure of the overall block of soil. When considering failure
of the individual piles, it must be remembered that the capacity of each pile may be
affected by the driving of subsequent piles in close proximity. This has been demon-
strated experimentally by Vesic (1969) and Lo (1967), who showed that the shaft
capacity of piles driven into sand could increase by a factor of 2 or more. Physically,
this may be attributed to higher normal effective stresses acting on piles driven in a
group, than on single piles. The beneficial effects of driving neighbouring piles may
be particularly noticeable in loose sands, where compaction of the sand can lead to
very low contact stresses around the pile shaft. Vibration and soil displacement due
to driving of subsequent piles is likely to reduce the tendency for the soil to arch
around each pile, leading to higher contact stresses, and thus shaft capacities, than for
single piles.
In other situations, the capacity of a pile within a group may be reduced by compar-
ison with a single, isolated pile. In particular, for piles driven into sensitive clays, the
effective stress increase in the surrounding soil may be less for piles in a group, than
for individual piles. This will result in lower shaft capacities.
