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Geotechnical News • June 2014
25
GEOTECHNICAL INSTRUMENTATION NEWS
data point from a sensor monitored
quarterly appears ‘out of line’ with
earlier readings, the data quality may
be immediately suspect. With auto-
mated systems, the trend leading up
to an apparently anomalous point can
be recorded and it may be far easier to
assess incongruous data as to whether
it is erroneous or representative of
actual physical conditions. It has been
MnDOT’s experience that there is
less user-intervention (assessment,
validation, and correction) required
with SAA installations, as the “human
element” has been largely removed
from the data acquisition and process-
ing portions of the process—especially
when considering the relatively large
number of readings associated with
frequently acquired SAA data.
SAA systems are useful for monitoring
remote sites. There is no significant
cost increase to poll the sensors several
times daily as compared to monthly or
longer intervals, (as was more common
with traditional manual systems). In
seven years of operations, and today
with about 30 operational systems,
there has been no data loss related to
IT or server problems, although there
have been data gaps due to system
faults related to power or telecommu-
nications. Electronic components do
fail, and while SAA sensors and related
architecture of data collectors, uplinks,
servers and web-interfaces, have been
shown in our experience to be robust,
some up-hole electronics are suscep-
tible to influences including lightning,
flooding, rodent infestation, tempera-
ture, humidity, and vandalism. It is
easy to become accustomed to a level
of reliability, only to find after several
months have passed, that a modem
developed a fault or wiring has become
part of an industrious bird’s nest.
System automation of data acquisi-
tion, monitoring, and reporting tasks
also requires some degree of specialty
computer support. SAA system set-up
has a distinct learning curve. Reliance
on computers, coding, and infrastruc-
ture design and support is greater than
with manual systems. In general, the
most challenging aspects of automated
systems have been related to initial
system deployment, cellular modem
telecommunication set-up and service,
and maintaining system power at
remote locations.
Large deformation applications
MnDOT first installed three SAA
systems in the summer of 2007 in
Crookston, MN at a site where large
known movements were occurring.
Several traversing-probe inclinometer
installations at the site had crushed,
sheared, and in one case trapped a
probe in the ground. An early conclu-
sion from that project was that the
SAA systems appeared to be ductile
in nature and the sensors could report
exceptionally large deformations
while maintaining operational integ-
rity. Deformations of several feet were
observed in two of the three sensors
along relatively narrow shear bands
(slip planes); the other SAA was
installed outside the active slide area.
Based on the success of that project,
two additional sensors were installed
to monitor a nearby project where
a roadway embankment failed only
a few months after the monitoring
program began in the summer of 2008.
Lateral movements of over 100 inches
(2.5 m) were accurately recorded by
the two SAA sensors—well outside
the operational boundaries of typical
traversing probe systems. The frequent
monitoring of the SAA sensors at this
second site enabled MnDOT to close
the roadway and begin building emer-
gency bypasses prior to the collapse of
the westbound portion of the high-
way. Figure 2 shows the embankment
failure area where the SAA systems
identified multiple slip surfaces and
recorded significant lateral deflections.
MnDOT has also successfully installed
SAA systems below roadway embank-
ment surcharge operations to monitor
large settlements over soft compress-
ible soils. Based on project data, it
appears likely that the curvature of
traditional plastic conduit would have
been challenging for a horizontal tra-
versing-probe to negotiate (especially
with single-end entry). On this project,
contractor operations were controlled
based on embankment settlement
response. Monitoring the SAA systems
and other sensors to review construc-
tion activities on a daily basis had
significant project benefits, particularly
as it could be done via the internet.
Broader application of
automated systems
While not immediately appreciated,
it became clear that SAA systems had
additional advantages over manual
systems. The sensors can be polled
even in poor weather conditions such
as when the installation area may
be covered with snow and ice. SAA
systems also continue to function
when the sensing elements are below
floodwaters. The ability to monitor a
Figure 2. SAA systems reported and recorded a significant landslide event in near-
real-time to geotechnical offices five hours away. The SAA systems remained fully
operational as over 100 inches (2.5 m) of lateral deformation was measured.