Pile testing - Piling Engineering - page 301

Table 9.1 Probability of selecting at least one defective pile from a group containing 100 piles

No. of piles not meeting No. of piles tested Probability of selecting

specification at least one low grade pile

2 2 0.04 (1 in 25)

2 5 0.1 (1 in 10)

2 10 0.18 (1 in 5.5)

2 20 0.33 (1 in 3.0)

10 2 0.18 (1 in 5.5)

10 5 0.41 (1 in 2.5)

10 10 0.65 (1 in 1.5)

It is noted that the errors in the position and alignment of the piles within a pile group
may significantly influence the behaviour of the completed foundation.
The selection of, and the number of, piles to be tested obviously greatly influences the
reliability of any conclusions drawn from the test results. Table 9.1 illustrates the effect
of sample numbers in the likelihood of detecting a defective pile. From this table it is
evident that to achieve a reliable assurance of the satisfactory quality of the group of
piles, a large number of piles have to be tested. In general, load testing is unsatisfactory
for quality control on the grounds of cost. It is usual to select at least one 'typical' pile
for testing, together with any pile that, from its construction records, is thought to be
unsatisfactory. Quality control of pile construction may be checked by adopting one
of the non-destructive testing techniques that are now available. A great deal of effort
has been put into the development of dynamic loading and integrity testing of piles; the
former to check, as reliably and simply as possible, the bearing capacity and the latter
to confirm soundness and structural continuity. These methods are now sufficiently
well-developed to be considered useful tools in any quality control procedure.
The behaviour of a single test pile may well be very different from that of the pile
working in a group. The results of observations of load distribution down the shaft
of a pile at a bridge in Newhaven, England, are shown in Figure 9.1 (Reddaway and
Elson, 1982). These results clearly demonstrate that under short-term test loading the
soil resisted a large proportion of the load as friction in the alluvium, in spite of a
bitumen slip coat which was applied to the upper 10m of the pile. On completion of
the bridge, the same pile (now working in a group of 32 piles), transmitted the entire
loading to the underlying chalk. In such circumstances one would seriously question
the usefulness of the pile test. Subsequent analysis of the situation, using an elastic
continuum model, demonstrated that the observed behaviour was a function of the
very large difference in stiffness between the alluvium and the chalk. It should be noted
that the designers had, not unreasonably, assumed that friction would be developed
in the gravel, which was moderately dense.
It is emphasized that any pile test programme should be carefully designed, and
that the purpose of each test should be clearly stated. A large amount of information
can be obtained from carefully conducted loading tests on test piles. The data can
be further augmented by instrumentation of the pile. The information may well lead
to refinements of the foundation design with a consequent possible cost saving, or
certainly greater assurance of the satisfactory performance of the foundation. On the