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Garrison Dam
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Gavins Point Dam
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Garrison Dam Structural Movement Survey
near Pick City, North Dakota
Garrison Dam, located in central North Dakota, forms the reservoir known as Lake Sakakawea – the largest Army Corps of Engineers’ lake on the Missouri River. The US Army Corps of Engineers monitors the structural movement, to the nearest 0.001-foot, taking place over time on dams under its control on a periodic basis. Through the efforts of a number of different delivery orders from the US Army Corps of Engineers, Omaha District, Eisenbraun and Associates has conducted extensive structural movement surveys on the Garrison Dam.
The E/A Approach
Vertical movement was determined on 60 movement points in and around the powerhouse and surge tanks. Eisenbraun and Associates determined elevations for the intake structure's movement points and piers on the service bridge. Challenging confined space surveys in the dam’s penstock tunnels were performed to monitor potential vertical movement. E/A also determined vertical movement on crest and slope markers on the mainstem dam and peizometers along the upstream face of the Snake Creek embankment. Again, all elevations were determined to the nearest 0.001-foot using electronic digital levels. Extensive monitoring of horizontal and vertical movement was also completed on the massive spillway structure.
Precise global and state plane coordinates were determined on all permanent control points in the project area with static GPS surveys. Two first-order NGS stations were incorporated in the initial survey network. The primary control survey included 55 baselines, all redundantly observed. State plane coordinates were determined on all crest markers and unusual movement points using fast-static GPS surveys. The fast-static survey included 287 baselines that were processed and adjusted using Trimble Geomatics Office.
The Outcome
The vertical and horizontal surveys of Garrison Dam were successfully completed and results documented thoroughly. In fact, E/A crews have repeated much of the monitoring survey work several times in the past decade.