Civil Engineering Reference
In-Depth Information
intrusive methods of inspection (such as coring a pile), and cheaper indirect methods
(e.g. seismic methods) when all the piles in a group can be inspected at a modest cost.
The intrusive methods can be more specific in their diagnosis of an integrity problem
than an indirect method, but many piles in a group would be left untested. It should
also be appreciated that despite the high cost of, for example, core drilling a pile
shaft, this method of inspection is unlikely to detect defects such as lack of cover to
reinforcement on the perimeter of a pile although it is appropriate for inspecting the
conditions at a pile toe.
The statistical probability of detecting a faulty pile in a group with a given number
of defective units for various testing levels is shown in Table 9.1 (Chapter 9).
Today, low-cost methods of integrity testing applied to the entire pile group have
become accepted as routine procedure. It is however important to be wholly satisfied
with the skill of the testing operative and the equipment employed as there is an
element of interpretative skill involved in understanding the output from such tests
that is generally outside the experience of engineers engaged in civil engineering
construction.
In recent years a number of experiments have been carried out with artificial 'faults'
within piles. The results of the application of various methods of integrity test have
been published. Whilst such experiments have a certain limited validity, decisions
about the outcome of integrity tests are frequently facilitated by the study of a fam-
ily of results. One or more anomalies lead to a search for the potential cause and its
implications. This may involve pile head excavation, coring, and very particularly,
a study of the ground conditions and construction equipment. Under these circum-
stances, well-instrumented construction equipment and integrity testing can prove to
be a powerful and informative combination.
To apply these tests to the best advantage, the merits and limitations of the methods
of integrity test available need to be known in order to select a test appropriate to
the pile type and ground conditions. Where a particular type of defect is considered
more likely than another, a test can be specific to this particular problem, be it trans-
verse cracking in driven concrete piles, or lack of end bearing in bored cast-in-place
piles.
8.1.1 Occurrence of faults in piles
The occurrence of faults in piles is infrequent, but when they are discovered, perhaps
in one pile, doubt is then likely to be cast on the other piles on a site. The result may
be delays to the contract, dispute as to causes and remedies, and probably an increase
of cost. As shown by Table 7.4, the largest single cause of damage is that inflicted on
pile heads after completion of the pile.
It is wrong to imagine that modern integrity testing can discover each and every fault
that may exist in a pile. For example, cover to reinforcement, small zones of segregated
concrete or small voids may be difficult to detect, but most of the successful systems
can give strong indications of the presence of serious problems. The consequences of
a major defect in an important pile are such that costs of remedial measures may far
exceed the cost of an integrity survey, whereas early detection may lead to relatively
inexpensive repairs or indeed modification of the method used so as to obviate further
difficulties.
