18
Geotechnical News • September 2016
GEOTECHNICAL INSTRUMENTATION NEWS
General role of instrumentation, and summaries of instruments
that can be considered for helping to provide answers to
possible geotechnical questions. Part 4.
John Dunnicliff
Introduction
This is the fourth in a series of articles
that attempt to identify:
• The general role of instrumentation
for various project types.
• The possible geotechnical questions
that may arise during design or
construction, and that lead to the
use of instrumentation
• Some instruments that can be
considered for helping to provide
answers to those questions.
Part 1, covering internally and
externally braced excavations, was in
December 2015 GIN. Part 2, in March
2016 GIN, covered embankments on
soft ground. Part 3, in June 2016 GIN,
covered cut slopes and landslides in
soil and in rock. This Part 4 is about
driven piles and bored piles (also
called drilled shafts).
Four introductory points were made in
December 2015 GIN (
-
calnews.com), for Part 1 of this series
of articles, and these also apply here.
Driven piles
General role of instrumentation
The subsurface length of a driven pile
cannot usually be inspected after driv-
ing; thus, its physical condition and
alignment are unknown. Subsurface
geotechnical conditions are rarely
known with certainty, and therefore
the design of driven piles involves
assumptions and uncertainties that
are often addressed by conducting
instrumented full-scale tests. Tests
may examine the behaviour of the pile
under load applied to the pile head or
under load caused by settlement of soil
with respect to the pile.
Defects in piles can be created during
driving, and inspection procedures are
available for examining the condition
and alignment after driving. Certain
types of driven pile cause large dis-
placements and changes of pore water
pressure in the surrounding soil, and
these may in turn have a detrimental
effect on neighboring piles or on the
stability of the site as a whole. Instru-
mentation can be used to quantify the
consequences of pile driving and thus
to assist in planning any necessary
action.
Summary of instruments that can
be considered for helping to provide
answers to possible geotechnical
questions
Table 8 lists the possible geotechni-
cal questions that may lead to the use
of instrumentation for driven piles,
together with possible instruments
that can be considered for helping to
provide answers to those questions.
Table 8. Some instruments that can be considered for
monitoring driven piles
Possible geotechnical questions
Measurement
Some instruments that can be considered
What is the load-movement relationship
of the pile
Displacement at head
Load at head
Displacement at toe
Stress along pile
Dial indicators with reference beams
Wire/mirror/scale
Surveying methods
Remote methods
Load cell
Telltales
Embedment or surface-mounted strain gauges
(Fibre-optic instruments)
Has the capacity of the pile been reduced
by defects caused during driving?
Curvature of pile
Condition of pile
Inclinometer
Integrity testing
