Geotechnical News • December 2016
29
GEOTECHNICAL INSTRUMENTATION NEWS
the reading itself, but this can be cor-
rected to an extent with proper calibra-
tion curves. Second, temperature can
have an effect the monitored structure
and induce real local tilt, often hours
after air temperature (i.e. the tempera-
ture measured by the on-board ther-
mometer) has changed.
Installation
For phase (2) and (3), effects of the ex-
cavations on the walls were monitored
with tiltbeams. Like MPBX, vibrating
wire tiltbeams are robust instruments
that can be relied upon over long pe-
riods of time provided that they are
correctly used. Avoiding exposure to
sunlight is often recommended as lo-
cal heating of the structure can induce
a small amount of tilt from local sensor
and structure deformation. Putting tilt-
beams in the shade is not always pos-
sible since the outer walls of a building
are often more accessible.
Results
Measuring variations over weeks or
months before work starts can prevent
a lot of head scratching because the ef-
fect sunlight on the system can be quan-
tified before work begins. The graph in
figure 4 shows the effect of tempera-
ture and sunlight on tilt measurements
for a single tiltbeam in the inner court-
yard. From April to December 2014,
the measured tilt variation (blue curve),
with respect to an initial measurement
(blue curve) steadily decreased as tem-
peratures went down (orange curve).
Starting in the spring of 2015 values
remained low while the temperature
increased again. It was impossible to
accurately measure the value of tilt un-
til temperatures had climbed back to as
high as the initial value. Hour-to-hour
comparisons, when temperatures are
similar, should give smooth increases
and decreases that are repeated day af-
ter day. Any sharp or fast change might
indicate a blow to the instrument or an
actual shift in the wall. Slow and long
term tilting can be difficult to detect
without a proper base line.
Recommendations for future use
Installing the tiltmeters indoors or in
the shade, though often not possible,
can improve the quality of long-term
measurements.
In addition to this, when monitoring
an already existing structure, a long
enough baseline will allow engineers
to work out the relationship between
temperature and tilt and thus enable
the analysis of all subsequent data
acquired during the project. In short,
baselines are a simple but often over-
looked method of improving the reli-
ability of instruments such as tiltbeams
and MPBX.
There are several points to take into
consideration when choosing between
competing technology when choos-
ing an instrument, such as signal type,
accuracy, reliability and temperature-
dependence.
Conclusion
In conclusion, restoration of Parlia-
ment Hill’s West Block is an unusual
project for geotechnical instrumenta-
tion. In a demolition and restoration
project, instruments are constantly put
at risk. Communications cables, liq-
uid lines and instruments heads can
all be damaged. It is therefore critical
to protect the cable and lines, use reli-
able and trustworthy instruments, plan
for redundancy and perform long-term
baselines. Applying these measures to
any project, and to restoration projects
in particular, will greatly improve any
monitoring in restoration-related proj-
ects.
Vincent Le Borgne
GKM Consultants,
2141 Nobel Street
Suite 101
Sainte-Julie,
Québec Canada J3E 1Z9
Tel. (450) 441-5444 ext. 207
Email: vleborgne@gkmconsultants.
com
General role of instrumentation, and summaries of instruments
that can be considered for helping to provide answers to
possible geotechnical questions. Part 5.
John Dunnicliff
Introduction
This is the fifth and last 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 exter-
nally braced excavations, was in De-
cember 2015 GIN. Part 2, in March
2016 GIN, covered embankments on
soft ground. Part 3, in June 2016 GIN,
covered cut slopes and landslides in
soil and in rock. Part 4, in September
2016 GIN, covered driven piles and
bored piles (also called drilled shafts).
This Part 5 is about tunnels.
