20
Geotechnical News September 2011
GEOTECHNICAL INSTRUMENTATION NEWS
• RVs must recognize the changes
that occur from causes other than
construction.
• RVs
s
hould be several times larger
than the accuracy of measured
changes (those last four words are
very carefully chosen).
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 john@dunnicliff.
eclipse.co.uk, or by mail: Little Leat,
Whisselwell, Bovey Tracey, Devon
TQ13 9LA, England. Tel. +44-1626-
832919.
Alla salute! (Italy)
P.S. For those of you who are not
long term readers of GIN, here’s the
background to the line just above.
Soon after GIN was born in 1994 a
colleague gave me a beer mat inscribed
with about a dozen drinking toasts, in
different languages. We agreed that
they would make appropriate endings
to GIN ‘columns’. “Alla salute!” is the
sixty-seventh different toast to end a
column.
Alla salute!
Interchangeability of MEMS Digital
Inclinometer Probes
Brian Tigani and Rolando Rongo
This article examines the data collected
with
Micro-Electro-Mechanical
Systems
(MEMS)
inclinometer
probes, using inclinometer probes
manufactured by RST Instruments Ltd.
History
Inclinometer systems consist of casings
with alignment grooves, inclination
sensing probes, communication cables
and readout devices. The casing is
placed into the ground or attached to a
structure which is anticipated to move
and the equipment is used to monitor
any deformation perpendicular to the
alignment of the casing.
Stanley D. Wilson, creator of the
“slope inclinometer” in 1954 and co-
founder of Slope Indicator Company
produced the first production model
inclinometer in 1957. Wilson original-
ly attached his inclinometer casing to
sheet piling. There has been a tendency
to use inclinometers more for dam and
soil shear measurements. The major-
ity of inclinometers at Monir Precision
Monitoring Inc. are used for monitor-
ing support of excavation walls.
The Survey Process
Analogue vs. Digital (MEMS)
Analogue inclinometer probes have
been in use since 1957; however they
are not interchangeable. Each probe
has its own characteristics and is
sensitive to shock and temperature
(range: -20 to +50 deg. C) which
amplify these characteristics. As a
result, the probe used to make an initial
reading was thereafter the only probe
which could be used reliably to survey
that installation. Unlike analogue
systems, the MEMS are less sensitive
to shock and temperature (range: -40
to 70 deg. C), minimizing such probe
characteristics. Also the MEMS system
which was tested aids in technician
repeatability. For example, the cable
grip ensures all technicians read at the
same top reference mark, unlike the
pulley/cleat assembly typically used
with analogue systems.
Data Gathering and Analysis
If different probes survey installations
differently, data gathering with only one
probe may be a liability in the event of
later unavailability for reasons such as;
damage, loss, calibration or scheduling
conflicts. To address this concern,
Monir chooses to take initial readings
of every installation with two probes;
in the past with analogue and presently
with digital. This ensures accurate
surveys could always be collected. If a
probe is away for its yearly calibration
or simply not available, a survey can
then be taken without delay.
When first introduced at Monir, we
employed the same protocols with the
MEMS system, as it was understood
that these probes were also not inter-
changeable. The manufacturer states
data gathered from
one
probe are re-
peatable over 25m of depth to within
2mm, (RSTmanual, October 12, 2010).
When we make initial surveys of an
installation, multiple sets of surveys
are taken using two probes to confirm
the casing initial position within 1mm
over 25m of depth (as compared with
2mm for the manufacturer’s specifica-
tions). This practice was adopted when
attached (not borehole) installation
depths in the Toronto area were short,
typically 15m. Installation depths for
this study ranged from 6.7m to 32.3m.
As we gathered data using different
MEMS equipment we began to see a
clear trend of interchangeability based
on our above criterion. With this trend
we questioned the duplicated survey
approach and decided in September of
2008 to further analyze our data. It was
one thing to get repeatable initial sur-
veys but another to ensure such repeat-
ability for moving installations.
The only way to show that probes
were interchangeable was to take con-
secutive surveys with
multiple
systems
and use our above criterion for repeat-
ability. So in addition to two sets of ini-
tial readings with different probes, we
