Geotechnical News • December 2016
27
GEOTECHNICAL INSTRUMENTATION NEWS
Vibrating wire MPBX, despite being
tedious to install properly when com-
pared to other solutions, were chosen
because we would have a single type
for all instruments. This gave us the op-
portunity to greatly reduce cabling and
to facilitate integration into the data-
loggers.
Given the long cabling distances in this
project, using vibrating wire instru-
ments sidesteps the issues of voltage
drops that occur with potentiometer-
based MPBX.
Installation
In this project, the MPBX were in-
stalled directly into the bedrock and
there was very little risk of using an
improper (too soft) grout mix.
Results
This project showed that confidence in
the instruments and their reliability can
prove critical. Indeed, every MPBX
installed in this project gave nearly-
constant and consistent measurements
over months. In October 2014, the
measured values of one MPBX jumped
to more than 5 mm, above the alarm
threshold (figure 3 (a)). A rapid inves-
tigation found that a worker had exca-
vated just beyond the planned limit and
hit the head of the instrument.
Relying on redundancy and histori-
cal data, the engineers were confident
enough in the instrument and in the re-
dundancy we had implemented to not
immediately stop work despite going
against their internal processes.
Damage to the head occurred on a few
occasions, an example of which is de-
picted in the photograph of figure 3 (b).
It can be seen that, in this case, half the
head of the instrument was torn off.
The simple fact that the instrument was
nearly destroyed shows their vulner-
ability in a demolition and restoration
project.
Recommendations for future use
Performing long term-monitoring to
build confidence in the instrument and
the measurements is strongly recom-
mended whenever possible. This con-
fidence helps the engineers to make the
right decision when unexpected jumps
or breaks in the data occur.
Tiltbeams
Purpose and description of
instruments
Vulnerable walls were monitored with
vibrating wire tiltbeams (Geokon mod-
el 6350). Though tiltmeters are com-
monly used in structural health moni-
toring, tiltbeams were selected because
they were to be installed on masonry
walls which can flex due to their mortar
joints. Using long (2 m) tiltbeams av-
erages out localized tilts and provides
a better image of the behaviour of the
walls.
Vibrating wire tiltbeams were selected
over electrolytic or MEMS sensors,
two other common types of tiltmeters.
First, they are less sensitive to tempera-
ture effects than electrolytic tiltmeters.
Second, integration is facilitated by
using a single signal types and by re-
quiring very little power over long dis-
tances.
Sources of inaccuracy
Temperature-induced errors are the
main sources of inaccuracy in this type
of instrument. First, temperature affect
Figure 3. (a) Anchor displacement
of a damaged MPBX
(b) Photograph of the head of a
damaged MPBX).
Figure 4. Tilt angle over time (blue) and temperature (orange).
