Geotechnical News September 2011
19
GEOTECHNICAL INSTRUMENTATION NEWS
Geotechnical Instrumentation News
John Dunnicliff
Introduction
This is the sixty-seventh episode of
GIN. Two articles this time.
Interchangeability of
Inclinometer Probes
The first article, by Brian Tigani and
Rolando Rongo of Monir Precision
Monitoring Inc., Ontario, Canada,
provides useful practical guidance
on monitoring with MEMS digital
inclinometer probes. I welcome such
nuts-and-boltsy help from experienced
users. Any more out there?
Monitoring of Surface
Deformation with Robotic Total
Stations Using Reflectorless
Measurements
The second article, by Damien
Tamagnan and Martin Beth of SolData
Group in Spain and France tells us
about a recent development whereby
measurements of vertical deformation
can be made by robotic total stations
without the need for prisms. This allows
us to monitor ground surfaces such as
road pavements without obstacles on
the surface and consequent interruption
to traffic.
Yes, I know that I don’t normally
publish articles that are written by au-
thors with a commercial interest in the
subject, in an effort to keep GIN as a
totally professional source of infor-
mation. But I decided that this article
added enough to our toolbox so that I’d
make an exception.
Confusion about Initial
Readings and Baseline
Readings
Some years ago I participated in
writing a guide instrumentation
specification for a major construction
project. Funding regulations mandated
that various tasks, including reading
the instruments, had to be included
in the general contractor’s scope of
work. In the specification for reading
instruments I adopted the term “formal
initial readings” (FIRs). These were
intended as readings to which all
subsequent readings would be referred,
hence indicating changes. The FIRs
needed to be taken after all installation
effects had disappeared, such as
‘settling down’ after drilling, grouting,
welding etc., (remember that swelling
of bentonite can cause either reduction
of pore water pressure by drawing
water out of the pores or increase of
pore water pressure by pressing on
the soil, and that equilibrium may not
be reached for a while), and therefore
wording was included to specify timing
with respect to installation. FIRs also
needed to take into account any non-
repeatability from reading to reading,
and therefore wording was included to
specify how many individual readings
were required and how to use these to
create an FIR.
This wording has been copied for
other construction projects. I’ve re-
cently learned that others may not fully
appreciate the logic behind FIRs, and
are confusing them with baseline read-
ings. So I’ll try to define what baseline
readings are and why they are entirely
different from FIRs.
Baseline readings are readings taken
over a period of time, before any con-
struction starts, to help in the defini-
tion of changes that occur from causes
other than construction. For example,
seasonal changes in groundwater levels
often cause deformation of structures.
Tidal and moisture content changes can
do the same thing. Climatic changes
such as temperature and incidence of
sunlight can cause substantial defor-
mation of structures. If these naturally
occurring changes are not documented,
the task of evaluating measured chang-
es is severely hampered, and it requires
significant engineering judgment to
adjust day-to-day measured changes to
discount those that have nothing to do
with construction.
In summary, formal initial readings
and baseline readings are entirely dif-
ferent things, and formal initial read-
ings come first.
On a Related Subject
I’m working with a colleague to put
together answers to the question, “How
should we determine response values
(RVs, a.k.a. trigger levels and hazard
warning levels)?”, and hope to include
this in a later GIN. A few thoughts now:
• Don’t ignore changes during the
green RV period by simply waiting
for the green flag to change to am-
ber. Trends during the green period
can give useful forewarning.
• Early RVs can be based on calcu-
lated changes, whereas later RVs
can be based on (unrelated) toler-
able changes.
