Civil Engineering Reference
In-Depth Information
4.1.4.1 Pile driving formulae
The most widespread method of estimating the dynamic capacity of piles is the use
of some form of pile driving 'formula', relating the measured permanent displacement
(or 'set') of the pile at each blow of the hammer, to the pile capacity. Driving formulae
are based on an energy balance between the (dynamic) input energy of the hammer, and
the (static) work required to move the pile permanently a small distance,
s
. As such,
they offer a compromise (often rather a poor one) between the static and dynamic
capacities of a pile.
There is a variety of pile driving formulae in common use, many of which have been
reviewed by Whitaker (1975), who comments that, in some situations, pile capacities
predicted by the different formulae may differ by a factor of 3. Rather than describe
all the different approaches here, the basic approach will be outlined and some of the
main variables discussed.
A simplified picture of the driving process, and variation of pile resistance with
displacement is shown in Figure 4.19. This picture gives rise to the fundamental pile
driving formula
η
Wh
R
=
(4.26)
(
s
+
c
/
2)
where
R
is the pile resistance.
η
is the efficiency of the hammer (allowing for energy loss on impact).
W
is the weight of the hammer.
Figure 4.19
Schematic diagram of pile driving.
