Pile Design and Construction Practice, Fifth edition

Ground investigations, contracts and testing 535
Typical load–deflection curves for cyclic tests are shown in Figure 11.20.
EC7, Clause 7.7, does not prescribe a method of deriving the ultimate limit state design
transverse load, F trd, as is the case for compression and uplift loading. As for all ultimate
limit state pile design EC7 does require the transverse ground resistance to be equal to or
greater than the transverse load.
11.5 Tests for the structural integrity of piles
From time to time doubts are thrown on the soundness of pile shafts. Excavations for pile
caps may show defective conditions of the type illustrated in Figures 3.40 and 3.41, and
questions are immediately asked about the likelihood of similar defects at greater depths and
in other piles on the site. Where preformed piles such as precast concrete or steel tubular
sections are used, defects can readily be explored by lowering inclinometers down guides
fixed to the interior (see Section 2.2.4) or by inserting a light or TV camera down the interior
of a hollow pile. In the Netherlands it is the practice to check the soundness of precast
prestressed concrete piles by embedding a thin electric cable down the shaft. After driving,
a test for electrical continuity is made quite simply by incorporating a light bulb in the
circuit. If the pile shaft is broken it shows as a break in the circuit. Such explorations cannot
readily be made with cast-in-place concrete piles.
Turner (11.27) has classified and described integrity testing techniques under two main heads:
Direct examination
(1) Visual, during and after installation, including excavation and extraction of the pile
(2) Load testing, static, dynamic and internal compression
(3) Drilling, coring or probing, alongside the pile or into the pile.
Indirect examination
(4) Internal, using the drill holes or preformed ducts for sonic logging and nuclear
backscatter and gamma-ray techniques, CCTV inspection, water or air pressure testing
and calliper dimensional logging
(5) External from the top or side of an exposed pile using low strain acoustic integrity tests,
dynamic load measurements for high strain integrity tests, ultrasonic pulses and
electrical resistivity tests
(6) Remote, in a borehole alongside the pile and installation of sonic probe for ‘parallel
seismic’ techniques.
Satisfactory evidence should be provided by the specialist that a particular method of
non-destructive testing or integrity testing will be appropriate to the site and type of pile.
A complete pile can rarely be examined economically by excavation or extraction. Piles
are frequently installed in soft or loose ground, making excavation difficult and costly
particularly below the water table. It would be unthinkable to examine all piles on a site by
excavation down the shaft.
Test loading is a positive method and its value in detecting defects in a shaft is illustrated
in Figure 11.13e and f. However, test loading all the piles on a site is a costly operation,
particularly if heavy kentledge loads are required (Figure 11.10). Figure 11.21 shows the
much simpler arrangements required for the SIMBAT dynamic load test on small-diameter
piles. The method of shaft compression has been described by Moon (11.28) . A rod or cable is