Dam Monitoring with 3D Tracker GPS - Jewell Instruments

Dam Monitoring with 3D Tracker GPS
CASE STUDY
30-SECOND SUMMARY: Libby Dam is a straight axis
concrete gravity dam on the Kootenai River approximately 17
miles upstream of Libby, Montana. The dam, constructed in
1961, is part of the Columbia Basin network of dams operated
by the US Army Corps of Engineers Seattle District and is
located upstream of Chief Joseph Dam.
SYSTEM CONFIGURATION
In February 2002, the Army Corps completed the installation of a
3D Tracker real-time GPS deformation monitoring system. The
system is comprised of six GPS installations spaced across the
crest of the dam with two reference stations; one located on a
ridge above the left abutment and a second on a promontory
upstream from the dam at approximately equal elevation with the
crest of the dam.
This system configuration allows operators to run multiple
independent processing tasks to each of the remote receivers on
the dam crest while also maintaining reference station integrity
monitoring by running processing tasks between reference
stations.
Figure 1. There are 6 GPS stations on the crest of Libby
Dam. A reference station is located on stable ground on
each side of the dam.
Figure 2. 3D Tracker Screen Shot.
All data is processed in real-time at a PC located in an
instrumentation office at the dam with data directly accessible
through WAN connectivity to Corps offices in Seattle. Good sky
visibility at the site, in combination with two reference stations,
produces measurement data of approximately 2-4 mm in realtime
and 1-3 mm over a 24-hour period. This data is being used
to compare monitoring results from other conventional
monitoring technologies installed at the dam. These include uplift
pressure cells, plumb lines, joint meters, tiltmeters,
extensometers and inclinometers monitoring rock slide areas on
the left abutment. The 3D Tracker GPS system deployed at
Libby was specifically designed to replace a laser alignment
survey which is conducted inside of the dam.
During real-time processing, the 3D Tracker software computes
and displays estimated error envelopes for the current position
solution through the analysis of the current GPS satellite
geometry. The error envelopes are representations of a calculated
covariance
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RELATIVE DISPLACEMENT (m)
RELATIVE DISPLACEMENT (m)
0.015
0.010
GPS Upstream/Downstream − Left Base to Mono 35
Suspended Plumb Line − Mono 35
GPS 7-Day Running Average
0.010
0.005
GPS Upstream/Downstream − Left Base to Mono 23
Suspended Plumb Line − Mono 23
GPS 7-Day Running Average
0.005 0.000
0.000 -0.005
-0.005 -0.010
-0.010 -0.015
-0.015
APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR
2002-2003
-0.020
APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR
2002-2003
Figure 3. Comparison of Plumb Line and GPS data over a 1-year period.
value for each displacement vector roughly equivalent to
one standard deviation. The Army Corps designed the GPS
monitoring system with two GPS reference stations to
ensure that at least one is running at all times and to
provide integrity monitoring. The integrity monitoring comes
in the form of two independent solutions for each point on
the dam computed from the separate reference stations. To
further monitor system integrity, plumb line measurements
collocated with GPS installation locations were collected
and analyzed.
MULTIPLE REFERENCE SYSTEMS
A byproduct of using two reference stations is the ability to
gauge system accuracy and precision. The two reference
stations allow for two independent solutions to be derived
for each remote station at any given time. Agreement in the
real-time computed solutions for the two independent
processes serve as an immediate indicator of the overall
system precision and accuracy. Disagreement between
independent solutions, however, will alert an operator
to pursue an immediate investigation of the cause for the
deviating solutions. Installing reference stations on
opposing slopes of the Kootenai River canyon has
introduced confirmation processes from each respective
receiver station.
An extremely close agreement between independent processes
for each remote station on the dam reinforces
confidence in accuracy and precision of the computed
remote positions. The difference between the computed
solutions roughly equates to
one standard deviation for 24-hour data sets. These
independent processes accurately represent the definitive
position of each remote station within the operating error
envelopes for the monitoring system.
PLUMB LINE MEASUREMENTS
Four of the six GPS instruments were collocated with
existing plumb line instruments contained within the dam.
The suspended plumb lines consist of wires suspended in
vertical shafts. Manual readings are taken at the bottom of
the shaft. The collocation of GPS and the plumb lines
provides not only a way to compare the two measurement
systems, but also allows the GPS system to provide
integrity monitoring for the plumb line system and viceversa.
While GPS can measure vertical motion as well as
horizon- tal motion, the plump line system can only
measure horizontal motion. The graphs above show good
agreement between the lateral GPS measurements and
the plumb line measurements at the dam crest.
THE RESULTS
An analysis of error envelopes, accuracy/precision of the
computed solutions using multiple baselines with multiple
reference stations, and comparisons of the collocated
plumb line instrumentation and GPS provides three unique
indicators of system performance. These three indicators
have been defined at Libby Dam and are serving the US
Army Corps of Engineers system operators to monitor
motion of the dam at the millimeter level.
Figure 4. Dam monolith cross-section.