Pile Design and Construction Practice, Fifth edition

528 Ground investigations, contracts and pile testing
by the reaction of an explosive force designed to raise weights mounted on the pile head to
a height of about 2.5 m at accelerations of up to 20 g. It is claimed that over 10 MN of force
is generated by the explosion of fuel in a combustion chamber beneath the stack of
cylindrical weights. It is also claimed that the duration of the explosive reaction of about
100 milliseconds can reproduce the effect of static loading up to the working load, with little
divergence at twice the working load; thus simulating a static rather than a dynamic load test
of the type described in Section 7.3. The force on the pile is measured by a load cell and the
deflections of the pile by a laser beam and light-sensitive sensor.
11.4.2 Interpretation of compression test records
A typical load/settlement curve for the CRP test and a load/time/settlement curve for the ML
test are shown in Figure 11.12. The ultimate or failure load condition can be interpreted in
several different ways. While there is no doubt that failure in the soil mechanics sense occurs
when the pile plunges down into the ground without any further increase in load, from the
point of view of the structural designer the pile has failed when its settlement has reached
the stage when unacceptable distortion and cracking is caused to the structure which it supports.
The latter movement can be much less than that resulting from ultimate failure in
shear of the supporting soil.
With reference to Figure 11.12, some of the recognized criteria for defining failure loads
are listed as follows:
(1) The load at which settlement continues to increase without any further increase of load
(Point A)
(2) The load causing a gross settlement of 10% of the least pile width (Point B)
(3) The load beyond which there is an increase in gross settlement disproportionate to the
increase in load (Point C)
(4) The load beyond which there is an increase in net settlement disproportionate to the
increase of load (Point D)
(5) The load that produces a plastic yielding or net settlement of 6 mm (Point E)
(6) The load indicated by the intersection of tangent lines drawn through the initial, flatter
portion of the gross settlement curve and the steeper portion of the same curve
(Point F) and
(7) The load at which the slope of the net settlement is equal to 0.25 mm per MN of
test load.
EC7, Clause 7.6.2.2, prescribes a method for assessing design pile loads from the
load/settlement curves obtained from a series of static load tests as described in Section 4.1.4.
With experience the load/settlement curve from a compression test can be used to
interpret the mode of failure of a pile. A defective pile shaft is also indicated by the shape
of the curve. Some typical load/settlement curves and their interpretation are shown in
Figure 11.13.
A method of analysing the results of either CRP or ML tests to obtain an indication of the
ultimate load is described by Chin (11.26) . The settlement � at each loading stage P is divided
by the load P at that stage and plotted against �/P as shown in Figure 11.14. For an undamaged
pile a straight line plot is produced. For an end-bearing pile the plot is a single line
(Figure 11.14a). A combined friction and end-bearing pile produces two straight lines which