Geotechnical News • March 2012
23
GEOTECHNICAL INSTRUMENTATION NEWS
Introduction by John Dunnicliff, Editor
This is the sixty-ninth episode of GIN. One full-length article this time,
and a series of brief articles about remote methods for monitoring
deformation.
Response values (a.k.a.
trigger levels and hazard
warning levels)
In the September 2011 episode of GIN
I wrote, “I’m working with a col-
league to put together answers to the
question, ‘How should we determine
response values?’ and hope to include
this in a later GIN”. The following
article by Mike Devriendt helps us to
face this challenging task.
Remote methods for
monitoring deformation
In the December 2011 episode of GIN
I wrote that I was planning to provide
an overview of various remote meth-
ods for monitoring deformation in
one or more later GINs—a one-page
overview of each and a concluding
article with a comparative analysis
of the various techniques. Here’s an
introduction by me and the first four
one-page articles on:
• Terrestrial laser scanning (light
detection and ranging): TLS Ter-
restrial LiDAR, by Matthew Lato.
• Terrestrial interferometric synthetic
aperture radar: TInSAR, GBIn-
SAR, by Paolo Mazzanti.
• Robotic total stations (automatic
total stations, automated motorized
total stations): RTS, ATS, AMTS,
by Rob Nyren, Ryan Drefus and
Sean Johnson.
• Reflectorless robotic total stations:
RRTS, by Damien Tamagnan and
Martin Beth.
In the next GIN we’ll have three more:
• Satellite interferometric synthetic
aperture radar: SInSAR, including
DInSAR and PSInSAR, by Franc-
esca Bozzano.
• Digital photogrammetry, by Raul
Fuentes and Stuart Robson.
• Differential global positioning
system: D-GPS, by Rob Nyren and
Jason Bond.
As one of my colleagues said to me,
“The basic
difference between these
remote sen
sing techniques and our
stuff is that
they
measure on the
outside, whereas
we
measure on the
inside. E.g. for a landslide, they mea-
sure the effect, we measure the cause”.
Not too shabby!
The next continuing
education course in Florida
This is now scheduled for April 7-9,
2013 at Cocoa Beach. Details of this
year’s course are on
-
ences.dce.ufl.edu/geotech. The 2013
course will follow the same general
format but with significant updating,
including remote methods for measur-
ing deformation. Information will be
posted on the same website in late
summer this year.
Closure
Please send contributions to this
column, or an abstract of an article for
GIN, to me as an e-mail attachment in
MSWord, to
co.uk, or by mail: Little Leat, Whis-
selwell, Bovey Tracey, Devon TQ13
9LA, England. Tel. +44-1626-832919.
Zivili! (Serbia)
Trigger levels for displacement monitoring
Mike Devriendt
Introduction
This article discusses the use of trigger
levels for monitoring geotechnical or
tunnelling projects. Trigger levels are
also known as response values and
hazard warning levels
.
The content of
the article focuses primarily on trig-
ger levels for instrumentation used to
monitor strain or displacement. How-
ever, some of the principles would
also extend to trigger values relating
to other parameters such as water
level, pressure or temperature. The
article refers to the measurement of
‘displacement’ throughout much of the
text, while later sections use the term
‘deformation’ to indicate the inter-
pretation of measured displacements
to calculate a strain or other form of
distortion of a structure.
Trigger level systems
This section provides a framework for
defining trigger levels.
A trigger level is a pre-defined value
of a measured parameter. If an instru-
ment reading is higher than this value,
then a pre-defined action is carried out.
It is common to use two or more trig-
ger values during monitoring of con-
struction to denote different levels of
response, given the magnitude of the
reading and urgency or significance of
the required response.
From the author’s experience the
adoption of a ‘traffic light’ system is
most effective, with the use of Green,
Amber and Red trigger levels. The use
of such a system is useful to provide a
simple and robust system that is clear
for monitoring and non-monitoring
specialists. Some practitioners propose
