Geotechnical News • June 2016
27
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 3.
John Dunnicliff
Introduction
This is the third 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. This
Part 3 is about cut slopes and land-
slides in soil and in rock.
The following points were made in the
introduction to Part 1, and also apply
here:
• Of course it is recognized that
there may be additional geotechni-
cal questions and also additional
instruments that are not described
in this article.
• The sequence of geotechnical
questions is intended to match
the time sequence in which the
question may be addressed dur-
ing the design, construction, and
performance process, and does not
indicate any rating of importance.
• The suggestions for types of
instruments is 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 possible types
in parentheses.
• The tables include the term “remote
methods” for monitoring displace-
ment. An overview of these remote
methods is given in a December
2012 GIN article by Paolo Maz-
zanti
.
com/instrumentation_news.php).
Readers who want to learn more
about these methods may want to
consider participating in the annual
International Course on Geotech-
nical and Structural Monitoring
held in Italy (
-
calmonitoring.com), where they
are discussed in detail.
Cut slopes in soil
General role of instrumentation
It is imperative that, prior to plan-
ning an instrumentation programme
for a cut slope in soil, an engineer
first develop one or more working
hypotheses for a potential behaviour
mechanism. The hypotheses must be
based on a comprehensive knowledge
of the locations and properties of
stratigraphic discontinuities.
Instrumentation can be used to define
the groundwater regime prior to
excavating a slope. Results of mea-
surements during excavation can be
used as a basis for modification of the
designed slope angle. Measurements
of ground movement and positive or
negative groundwater pressure can
assist in documenting whether or not
performance during and after excava-
tion is in accordance with predicted
behaviour. Measurements can also be
used to document whether short- and
long-term surface and/or subsurface
drainage measures are performing
effectively. If evidence of instability
appears during or after construction,
instrumentation plays a role in defin-
ing the characteristics of the instabil-
ity, thus permitting selection of an
appropriate remedy.
A very important subset is the case
of a cut slope in clay. Here negative
pore water pressures generated during
excavation can give rise to temporary
stability, the lifetime of which will be
related to the height of the slope and
the slope angle. Therefore monitor-
ing the negative pore water pressures
is an effective way of assessing the
stability of a cut slope in clay. In some
instances the stability may be main-
tained for long enough to undertake
temporary works within the excava-
tion and thereby save on expensive
stabilisation measures.
Summary of instruments that can
be considered for helping to provide
answers to possible geotechnical
questions
Table 4 lists the possible geotechnical
questions that may lead to the use of
instrumentation for cut slopes in soil,
together with possible instruments
that can be considered for helping to
provide answers to those questions.
Landslides in soil
General role of instrumentation
If there is evidence of slope instability,
its characteristics must be defined so
that any necessary remedial measures
may be taken. The question
how much
ground is moving?
can be answered by
use of instrumentation. The question
why is the ground moving?
will not be
answered by instrumentation alone:
the answer of course also requires a
