Geotechnical News • December 2015
35
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 1.
John Dunnicliff
Introduction
This is the first of 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.
Of course it is recognized that there
may be additional geotechnical ques-
tions and also additional instruments
that are not described in this article.
The sequence of geotechnical ques-
tions is intended to match the time
sequence in which the question may
be addressed during the design, con-
struction, and performance process,
and does not indicate any rating of
importance.
The suggestions for types of instru-
ments are not intended to be dogmatic,
because the selection always depends
on issues specific to each project,
and is influenced by the personal
experience of the person making the
selection. In the tables some of the
most likely instruments that can be
considered are listed, with other pos-
sible types in parentheses. The tables
include the term “remote methods”
for monitoring displacement. An
overview of these remote methods is
given in a December 2012 GIN article
by Paolo Mazzanti (
calnews.com/instrumentation_news.
php). Readers who want to learn more
about these methods may want to
consider participating in the annual
International Course on Geotechnical
and Structural Monitoring held in Italy
(
),
where they are discussed in detail.
Part 1 of this series focusses on inter-
nally and externally braced excava-
tions. Later parts will include:
• Embankments on soft ground
• Embankment dams
• Cut slopes and landslides in soil
• Cut slopes and landslides in rock
• Tunnels
• Driven piles
• Bored piles (drilled shafts)
Internally braced excavations
General role of instrumentation
The design of internally braced (strut-
ted) excavations is based for the most
part on empirical procedures and past
experience. The consequences of poor
performance can be severe and may on
occasion be catastrophic. A monitoring
programme may not be required if the
design is very conservative, if there is
previous experience with design and
construction of similar facilities under
similar conditions, or if the conse-
quences of poor performance will not
be severe. However, under other cir-
cumstances a monitoring programme
will normally be required to demon-
strate that the excavation is stable and
that nearby structures are not affected
adversely. Depending on the specific
needs of each case, the monitoring
programme may apply to the wall
and struts, to the ground beneath or
surrounding the excavation and/or to
adjacent structures or utilities.
Summary of instruments that can
be considered for helping to provide
answers to possible geotechnical
questions
Table 1 lists the possible geotechnical
questions that may lead to the use of
instrumentation for internally braced
excavations, together with possible
instruments that can be considered for
helping to provide answers to those
questions.
Externally braced excavations
General role of instrumentation
The general role of instrumentation for
externally braced excavations (using
ground anchors or tiebacks) is the
same as for internally braced excava-
tions. However, it is possible to make
regular visual inspections of internal
bracing, but external bracing cannot
be seen. Although confidence in the
performance of an externally braced
excavation is increased by conduct-
ing a proof test on every anchor, if an
anchor subsequently fails, the failure
may be progressive and catastrophic.
In general, therefore, instrumentation
plays a role in three phases of exter-
nal bracing that are not applicable to
internal bracing:
• Testing of
test anchors
during the
design phase or at the start of con-
struction, as input to design of the
project anchors.
• Performance
and
proof testing
of
anchors during construction.
• Subsequent
monitoring
of selected
representative anchors. This` phase
may be omitted if a conservative
design has been used.
