Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
δ res
= residual (or permanent) pile head settlement upon unloading from maximum
load
Q max
= maximum test load
W L
= design working load of pile
L = pile length
Ap = cross-sectional area of pile
Ep = Young's modulus of pile
D = least lateral dimension of pile section (mm)
9.2.5.4 Instrumentation
a) General
Information on the load transfer mechanism can be derived from a loading test if the pile is
instrumented. To ensure that appropriate and reliable results can be obtained, the pile
instrumentation system should be compatible with the objectives of the test. Important
aspects including selection, disposition and methods of installation should be carefully considered.
It is essential that sufficient redundancy is built in to allow for possible damage and
malfunctioning of instruments. Where possible, isolated measurements i.e., survey leveling method
should be made using more than one type of equipment to permit cross-checking of results. An
understanding of the ground profile, proposed construction technique and a preliminary
assessment of the probable behaviour of the pile will be helpful in designing the disposition of the
instruments. Limitations and resolutions of the instruments should be understood.
b) Axial loading tests
Information that can be established from an instrumented axial loading test includes the
distribution of load and movement, development of shaft resistance and end-bearing
resistance with displacement. A typical instrumentation layout is given in Figure 9.6.
Strain gauges (electrical resistance and vibrating wire types) can be used to measure local
strains, which can be converted to stresses or loads. Vibrating wire strain gauges are generally
preferred, particularly for long-term monitoring, as the readings will not be affected by changes in
voltage over the length of cable used, earth leakage, corrosion to connection and temperature
variation. In case measurements need to be taken rapidly, e.g. in simulation dynamic response of
piles, electrical resistance type strain gauges are more suitable.
A variant form of vibrating wire strain gauges is the 'sister bar' or 'rebar strain meter'. This is
commonly used in cast-in-place concrete piles. It consists of a vibrating strain gauge assembled
inside a high strength steel housing that joins two reinforcement bars at both ends by welding or
couplers. The sister bar can replace a section of the steel in the reinforcement cage or be placed
alongside it. Such an arrangement minimises the chance that a strain gauge is damaged during
placing of concrete. The electrical wirings should be properly tied to the reinforcement cage at
regular intervals.
To measure axial loads, the strain gauge stems are orientated in line with the direction of the load
(i.e. vertical gauges). One set of gauges should be placed near the top of the pile, and preferably
in a position where the pile shaft is not subject to external shaft resistance, to facilitate calculation
of the modulus of the composite section. Gauges should also be placed close to the base of the
9-24 March 2009