Civil Engineering Reference
In-Depth Information
Figure 23.6
Pile driving formulae.
tests, loads will be applied in excess of the design working load and the deflections
measured. The loads may be applied in stages and maintained at each increment (like
in an oedometer test) or applied at a constant rate of penetration. The latter method
is found to give more consistent results and better definition of failure loads.
The capacity of a pile can be inferred from its resistance to driving. The basis of
these so-called pile driving formulae is that the work done by the hammer (less any
losses) is equal to the work done as the pile penetrates the ground. For the simple drop
hammer weight
W
falling through
h
shown in Fig. 23.6 the pile capacity
Q
is related
to the set
s
(i.e. the displacement) for a single blow by
Qs
=
Wh
(23.9)
Equation (23.9) is a very simple driving formula, too approximate to be used in prac-
tice, but it is the basis of other formulae which include terms to take account of energy
losses in the hammer and in the pile.
23.5 Capacity of pile groups
In a group of piles like that shown in Fig. 23.1(d), there will be interactions between
neighbouring piles so that the capacity of each pile in the groupwill be reduced. A group
efficiency
η
is given by
V
=
n
η
Q
(23.10)
where
V
is the total load on the group,
n
is the number of piles in the group and
Q
is
the capacity of an individual pile on its own. Values for the efficiency
η
decrease with
reduced spacing of the piles, roughly as shown in Fig. 23.7(b).
If the pile spacing is relatively close, as shown in Fig. 23.7(c), it is more appropriate
to consider the group as an equivalent foundation of base area A and depth
L
g
, where
L
g
2
3
L
. The bearing capacity
q
c
of the block is calculated using the methods for
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