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24
Geotechnical News • June 2014
www.geotechnicalnews.com
GEOTECHNICAL INSTRUMENTATION NEWS
usefulness in targeted applications
with frequent monitoring requirements
or large anticipated deformations.
SAA instruments are linear arrange-
ments of linked elements with MEMS
accelerometers manufactured to
prescribed lengths for installation in
geotechnical environments. By relat-
ing the segment lengths and the tilt
(calculated from the sensor inclinations
with respect to gravity) of each seg-
ment with reference to a fixed end, the
spatial position of the array can be cal-
culated. As the array moves with time,
subject to geotechnical effects, defor-
mation along the array is measured,
providing information on both the rate
and magnitude of the movements. Fig-
ure 1 shows a typical up-hole system
cabinet and a team installing a SAA
sensor in a vertical borehole.
Performance
Based on an assessment of SAA
data from several project sites and
applications, the system performance
has been found to be sufficient for
transportation applications, particu-
larly where relatively large deforma-
tions (meters) are being measured.
After seven years, Minnesota Depart-
ment of Transportation
(MnDOT)
SAA sensors, except for those that
have been sheared off by exceptional
deformations, continue to function
well. SAA systems appear to provide
similar accuracy (at the ground or
structural surface) to that achieved by
robotic total station systems, with the
added advantage that movement inside
soil masses is characterized. In our
experience, we have not observed any
systemic data quality effects due to
sensor compression, extension, align-
ment, twist, temperature sensitivity, or
other inherent ‘device’ characteristics.
SAA sensors do display occasional
spurious readings due to electrical or
other effects—absolute data integrity,
as with most electronic sensors, is
not perfect. Filtering or engineering
judgment may be required to prop-
erly interpret SAA response. As SAA
sensors are manufactured to speci-
fied lengths, the array length cannot
be changed “on the fly” in the field if
changes are made to the monitoring
program. Advanced planning of instal-
lations is required; the fixed length
of the sensor arrays can also limit the
efficient reuse of the sensors at new
sites.
Automation,
particularly when
everything is
working properly,
can lead to
undesirable user
complacency and
poor practice.
Automation
A particular strength of the SAA,
which is also present in in-place incli-
nometers, is the ability of the sensor
to remain in-situ and for data to be
automatically collected and transmit-
ted to a web-based data storage and
presentation system in near-real-time.
However, a warning is appropri-
ate. Automation, particularly when
everything is working properly, can
lead to undesirable user complacency
and poor practice. Schedules should
be established to regularly check
that automated systems are properly
functional. Additionally, automation
should not be seen as a substitute for
site visits and application of the obser-
vational method. Fewer site visits may
have the unintended consequence of
removing opportunities for important
field observations and better under-
standing of mechanisms and triggers
causing deformation. Geo-engineering
requires an appreciation of site char-
acteristics that are not always well
captured at-a-distance.
Related beneficial aspects to sen-
sor automation include the ability to
establish movement thresholds, event
triggers, and automated warnings. A
bi-monthly interval was previously
considered ‘frequent’ for traditional
inclinometer installations. With system
automation, it is now possible to read
sensors several times daily—allowing
for the collection of data sets capable
of accurately depicting movement
trends, such as seasonal variation, and
discrete events such as those induced
by contractor operations, rainfall, or
earthquake. SAA systems can also
provide near-real-time information to
multiple users at multiple locations
(via the web) with comparative ease,
making the systems especially useful
for construction monitoring where
contractors and owners, in different
locations, have an interest in immedi-
ate information for decision making.
With increased data frequency there
is also the potential for improved
data interpretation. If a particular
Figure 1. Typical SAA up-hole cabinet (left) and crew installing a SAA sensor in a
borehole (right).