(3) Weight per unit length of pile
(4) Weight and fall of hammer (or rated energy per hammer blow)
(5) Efficiency of hammer
(6) Weight of helmet, packing, and any dolly or follower used
(7) Elastic modulus of packing and of any dolly used
(8) Elastic modulus of pile
(9) Coefficient of restitution of packing (and dolly if used)
(10) Elastic compression (quake) of soil
(11) Damping properties of soil
(12) Required ultimate driving resistance.
Of the above input data, items 1, 2, 3, 4, 6, and 11 are factual. The efficiency of the
hammer is obtained from the manufacturer’s rating, but this can decrease as the working
parts become worn. The elastic modulus and coefficient of restitution of the packing may
also change from the commencement to the end of driving. The elastic compression of the
ground is usually taken as the elastic modulus under static loading, and this again will
change as the soil is compacted or is displaced by the pile. Thus the wave equation can never
give exact values throughout all stages of driving, and its continuing usefulness depends on
amassing data on correlations between calculated stress values and observations of driving
stresses in instrumented piles. Further refinements of the calculation procedure may also be
made to allow for the changing dynamic characteristics of the hammer–pile–soil system
during driving. Smith(7.2)states that the commonly accepted values for the soil compression
(quake), and the damping constants for the toe and sides of the pile are not particularly
‘sensitive’in the calculations, i.e. small changes in these values produce a smaller change in
the final calculated results.
The basic Smith idealization represents a pile being driven by a drop hammer or a single-
acting hammer. Diesel hammers have to be considered in a different manner because the
energy transmitted to the pile varies with the resistance of the pile as it is being driven down.
At low resistances, there are low energies per blow at a high rate of striking. As the pile
resistance increases the energy per blow increases and the striking rate decreases.
Manufacturers of diesel hammers provide charts of energy versus rate of striking. When
predictions are being made of the ability of a particular diesel hammer to drive a pile to a
given resistance consideration should be given to the range of energy over which the
hammer may operate. Goble et al.(7.3)have published details of the GRLWEAP computer
program which models diesel and other hammer behaviour realistically. The program
proceeds by iterations until compatibility is obtained between the pile–soil system and the
energy/blows per minute performance of the hammer. Because of the problems of interpret-
ing data from the pile driving analyser when operated with diesel hammers, the present
tendency is to use hydraulic hammers which give a reasonably constant energy of blow.
However, the resilience of the cushioning material can change with use causing variations
in the energy transmitted to the pile head.
Pile driving resistance can be computed from field measurements of acceleration and
strain at the time of driving by using the dynamic pile driving analyser in conjunction with
the CAPWAP program(7.4). Pairs of accelerometers and strain transducers are mounted near
the pile head and the output of these instruments is processed to give plots of force and
velocity versus time for selected hammer blows as shown in Figure 7.3. The second stage of
the method is to run a wave equation analysis with the pile only modelled from the
Structural design of piles and pile groups 381