Mystic Lake Flowline and Trolley Bridges
Bridge Design for a Hydroelectric Plant
Morrison-Maierle provided engineering services to design and develop construction documents for the new steel trestle bridges along the Mystic Lake flowline hydroelectric project.
An initial site visit was necessary to determine the span of the new bridges and finalize abutment details. The new bridges were required to span an existing rock slide on a location with competent bedrock. Morrison-Maierle provided a survey scan of the project site and developed topographical drawings to lay out the bridges.
Morrison-Maierle’s team analyzed and detailed two steel truss bridges and developed a feasible construction methodology for the remote site. One bridge accommodated the existing flowline, while the other accommodated a tram track.
Furthermore, Morrison-Maierle developed a construction cost estimate for the anticipated work. The team also coordinated design review meetings to facilitate efficient critical milestone reviews.
Final deliverables included stamped construction drawings and specifications for the two steel truss bridges.
Services and Highlights
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Survey
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Structural engineering design
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Project coordination
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Construction sequencing
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Quality control
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Two steel bridges were designed to span the rockslide area
Building Bridges in the Wilderness
The Mystic Hydroelectric Project is a 12-megawatt plant located in the Beartooth Mountains, roughly 75 miles southwest of Billings, Montana. The project was constructed in 1925 and consists of two pelton-wheel-type units with over 1,000 feet of head and a reservoir with active annual storage. The plant’s powerhouse is connected to the dam by a 57-inch diameter flowline, which runs for over two miles and traverses a steep hillside. Adjacent to the flowline, NorthWestern Energy maintains a small rail car track and cable-hoist tram for maintenance and access to the dam and other facilities.
The rail car tracks and tram traverse steep mountainside areas with rockslides and varying subgrade conditions. In recent years, NorthWestern Energy observed that one section of the tracks had settled. To address this, they decided to construct new steel truss bridges to span the rockslide area and mitigate future settlement issues.
Previously, the support structure for the flowline consisted of concrete footings with steel saddles spaced about 12 feet apart. The concrete footings bear directly on the grade, while the trolley tracks are supported by steel beams, columns, and concrete footings.
Construction in a Remote Location
The main powerhouse is located about 75 miles southwest of Billings in Stillwater County, Montana. To reach it, vehicles must drive approximately 14 miles on rough dirt roads. From this location, access to the work area is restricted to cable trolley car, foot, or helicopter. As a result, the team had to carry all personnel, lightweight equipment, and materials to the site. All other equipment and materials were flown to the site via helicopter.
The team created a staging area near the main powerhouse to facilitate the transportation of both lightweight and heavy materials to the project site. Performing the work also required a full plant outage due to disassembling the flowline pipe. This full plant outage required NorthWestern to replace the power that would have been generated from the plant with other sources, including market purchases, so resuming plant operation according to schedule was critical.
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