Pile Design and Construction Practice, Fifth edition

382 Structural design of piles and pile groups
Force/velocity
Time
Force
Velocity
Figure 7.3 Force and velocity versus time for hammer blow on pile.
instrument location downwards. Values of soil resistance, quake, and damping are assigned
and the measured time varying velocity is applied as the boundary condition at the top of
the pile model. The analysis generates a force versus time plot for the instrument location
and this is compared with the measured force versus time plot. Adjustments are made to the
values of resistance, quake and damping until an acceptable agreement is reached between
computed and measured values. At this stage the total soil resistance assigned in the analysis
is taken as the resistance at the time of driving. The latter is a reliable assessment of the static
resistance in soils and rocks where time effects are negligible.
The instrumentation described above including the field processing equipment which
produces the force/time plot is a useful method of field control of pile driving in difficult
ground conditions. The shape of the plot (Figure 7.3) can give an indication of a broken pile
and a check can be made of the stresses in the pile induced by driving.
When assessing the results of wave equation analyses made at the project planning stage
for the purpose of predicting the capability of a particular hammer to achieve the required
penetration depth, due account should be taken of the effects of time on pile resistance as
discussed in Section 4.3.8. Sufficient reserve of hammer energy should be provided to overcome
the effects of set-up (increase of driving resistance) when redriving a partly driven pile
after a delay period of a few hours or days. If pile driving tests are made at the planning stage
it is helpful to make re-strike tests in conjunction with wave equation analyses at various
time intervals after the initial drive.
Too much reliance should not be placed on immediate readings of the output from the
field processing unit. Rigorous analysis of the data by experienced engineers is required in
conjunction with the appropriate computer programs. Wheeler (7.5) described experiences of
a field trial competition in the Netherlands when a number of firms specializing in dynamic
pile testing were invited to predict the ultimate bearing capacity of four instrumented precast
concrete piles driven through sands and silts to penetrations between 11.5 and 19 m. A
wide range of predicted capacities was obtained. In the case of one pile the range was 90 to
510 kN compared with a failure load of 340 kN obtained by static testing. Reliable estimates
of ultimate bearing capacity may not be possible if the available hammer has insufficient
energy to overcome the resistance mobilized by the soil against penetration of the pile.
A downward movement of the toe of piles up to about 1 m in diameter of 2.5 mm or
preferably more is required to mobilize sufficient soil resistance to obtain reliable results.