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Geotechnical News • June 2014
www.geotechnicalnews.com
GEOTECHNICAL INSTRUMENTATION NEWS
slowly creeping landslide during flood
conditions played an important role in
quantifying the stabilizing influence
of high water levels on the project.
SAA sensors can be installed within
embankments, walls, or slopes imme-
diately below travelled roadways.
Monitoring then occurs without traffic
interruption—an important consider-
ation from cost, roadway user incon-
venience, and safety standpoints.
SAA systems have also been useful
in monitoring construction deforma-
tion of spread footing foundations
in response to loading. Construction
projects are often active, dynamic,
and unsafe. As SAA systems can be
installed and buried with only a small
up-hole cabinet for support, high qual-
ity, frequent data, can be obtained to
monitor the influence of construction
sequencing safely and with minimal
contractor impact. In general, auto-
mated sensors and systems (of all
types) have been shown to be highly
beneficial on projects where the
influences of construction staging are
of interest and frequent monitoring
intervals are desired.
System costs
In common uses, the cost to install
a borehole for a vertical monitoring
system in native ground is roughly the
same for a traditional inclinometer as
it is for a SAA. A traversing probe may
be used at multiple installations—it is
possible for a $6K probe to be used to
effectively monitor either one borehole
or ten nearby boreholes. Conversely, a
SAA system for one similar hole may
cost $15K and perhaps $10K for each
additional nearby hole for the sens-
ing element (sharing some up-hole
resources). It may therefore appear that
SAA systems may only be appropriate
for very specialized projects. However,
the difference in total monitoring cost
for a project depends on more than the
initial cost. If a site with ten instru-
mented boreholes was four hours from
the closest project office, it could take
a technician an entire day to collect
data, with associated time and travel
expenses. For a five-year project with
monthly reporting, sixty site trips
would be needed; weekly reporting
would require 260 trips, and daily
reporting would require 1825 trips. On
inspection of the cost of daily trips, this
option might not even be considered
as a plausible alternative—it would
be clearly “cost prohibitive.” Here,
automation can bring previously dis-
counted options back to the table. SAA
systems become more cost competi-
tive if system components, or entire
systems, can be redeployed elsewhere
at the end of a monitoring program;
costs can be amortized across projects.
Several MnDOT systems have been
removed from service on initial proj-
ects and repurposed at new sites.
Ten observations on SAA system
performance
SAA systems have both positive and
negative attributes. “Lessons learned”
from a number of installations and
projects include:
1. Predetermined sensor lengths
require advance planning and can
limit the potential efficient reuse of
the sensors at new sites.
2. Reliance on computers, coding,
and system support is greater than
with traditional systems; electronic
systems have a distinct “learning
curve.”
3. Cellular modems and power sup-
plies are often “weak links” in
system.
4. Sensor readings can be subject to
some spurious (electrical or other)
effects—the data quality, as with
most electronic sensors, is not per-
fect. Filtering or engineering judg-
ment is required in some cases.
5. Installation procedures are similar
to traditional inclinometers.
6. SAA sensors are very robust in
large-displacement environments.
7. With seven years of operational
experience, data can be acquired
in severe environmental condi-
tions (during floods, below ice and
snow, under roadways).
8. Automation allows high frequency
readings for better data analysis
of rate information and capture of
seasonal variation and unexpected
events.
9. SAA systems provide generally
well-behaved data-sets in hori-
zontal applications, especially as
compared to many common settle-
ment sensors;
10. Relatively high initial costs are
offset by improved data quality,
near-real-time event reporting,
and life cycle savings in manual
labor and travel costs, particularly
if components can be re-used on
future projects. Systems can also
improve safety with fewer field
visits and reduced field construc-
tion conflicts.
As with other automated systems,
users should take care that SAA moni-
toring systems compliment site visits,
observations, and thoughtful evalu-
ations of the geologic character of
project sites to better evaluate causes
and impacts of the ground movements
being monitored.
Derrick Dasenbrock
Minnesota Department of
Transportation
1400 Gervais Avenue
Maplewood, MN 55109
E-mail:
Tel: (651) 366-5597
Editor’s Note
I’ve had some concerns about including this article in GIN because it may appear to favor one of the items in our tool box
too strongly. If you have experience with the SAA instrument and have anything to add about performance, pro or con, will
you please send me a discussion of this article, and I’ll consider it for a future GIN?
JD