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Geotechnical News • December 2013
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GEOTECHNICAL INSTRUMENTATION NEWS
• Were the budget and field plan
fixed with no allowance for sub-
surface uncertainties?
• Was no sampling carried out during
the drilling for inclinometer instal-
lation?
• Were the inclinometers only at-
tached to piles that did not extend
fully through the soft soils?
• Was the instrumentation part of a
“low-bid-wins contract”? Were
the least-costly personnel chosen
for the work to meet a low-bid
budget?
Authors’ Reply
In response to Mr. Boone’s ques-
tions we first have to emphasize that
this was from the perspective of the
instrumentation contractor. Typically,
the field technicians are provided the
depth to which the instrument will be
installed such as a borehole incli-
nometer, or it is set in the case of an
attached inclinometer to a pile. The
field technicians were present for the
installation of the instrument after
drilling was completed and achieved
that depth. Upon further review, the
inclinometer was confirmed to be
founded in stable ground at the correct
depth; however, due to large horizon-
tal deflections in the casing caused
by the installation of adjacent drilled
shafts, the inclinometer probe was not
able to reach the bottom of the casing
and the “zero” anchoring point was
lost.
No sampling was done during the
installation; however, independent
sampling was done by the geotechni-
cal engineer.
The work was of an emergency nature
and there was a negotiated rate for
the work and not a bidding process,
so a low bid contract was not a fac-
tor. We concur that low bid is not the
best route to a successful monitoring
program.
The engineers designed the monitor-
ing program with redundancy in mind,
knowing that the combination of tight
site access and difficult geotechnical
conditions could result in damage
to monitoring instruments. The team
used the full complement of instrumen-
tation to analyze the unusual incli-
nometer movements, therefore there
was no reason to stop the job and add
additional inclinometers.
Lesson Learned 3
Lesson Learned 3 states that innova-
tive thinking was able to provide a
solution whereby surveying was used
to locate the horizontal position of the
top of the inclinometer. While survey-
ing of inclinometer tops can be useful
to adapt to the situation the authors
describe (and many others), accurately
surveying horizontal positions is
often far more difficult than com-
monly understood. Even with modern
and highly precise surveying instru-
ments, such measurements can vary
by +/- 20 mm or more, reflecting the
combination of instrument, skill level
of some operators, set up, and sight-
ing angles to reference points among
other factors. With the right instru-
ments, highly skilled operators and all
other details carefully controlled the
systematic variability in horizontal
survey measurements can be reduced
to +/- 3 mm or so. However, “the devil
is in the details” and, unfortunately,
details are often missed. The published
sample inclinometer plot illustrates
five virtually parallel lines of subsur-
face displacement data and they do
not appear to indicate a discernible
pattern, at least in comparison to the
illustrated dates. For example, the
first and last dates show the minimum
and maximum displacements, respec-
tively. The penultimate reading (#4),
however, illustrates less displacement
than the other two intervening reading
dates (#2 and #3 in date order). Are
the displacements real or might they
be a figment of survey error? If the
differences between individual and
parallel inclinometer survey event
plots are not figments of survey error,
certainly there must be some other
rational explanation for the changes.
Answers to these questions may
provide additional valuable lessons
learned. It would be very useful in a
future episode for the authors to also
illustrate how the other instrument
data was used in combination with an
understanding of the construction pro-
cesses and soil mechanics to rational-
ize the measured displacements.
Authors’ Reply
We agree that the type of instrument
and skill of the operators is vital to
achievement of accurate data. In
our experience the survey measure-
ments achieved are within ±1 mm with
proper procedures in place. Of course
there can always be bad readings and
lessons learned from those experi-
ences. In the inclinometer plot, the
product was a sample created for the
column. A re-zeroing of the survey
data for the top of the inclinometer
occurred that was not factored into
the sample plot. In the actual report-
ing, minimal movement had occurred.
Fortunately, these top-anchored incli-
nometer plots were supplemental data
to the pile-attached inclinometers.
Further research can be done in more
controlled conditions rather than in
emergency situations.
Author of discussion:
Storer J. Boone
Golder Associates Ltd., 309 Exeter
Road, London, Ontario, Canada
N6L 1C1,
E: sboone@golder.com
Authors of June 2013 article
and the above reply:
Marcelo Chuaqui and Wing Lam
Monir Precision Monitoring Inc.,
2359 Royal Windsor Drive, Unit 25,
Mississauga, Ontario, Canada
L5J 4S9, 905-822-0090,
E: marcelo@monir.ca,
E: wing@monir.ca