Deep Creek Canyon Bridges
Three Bridges in Three Weekends
High flows during the 2011 spring runoff resulted in significant erosion of the US Highway 12 embankment and damaged bridges crossing Deep Creek. The subsequent flooding required emergency maintenance to prevent structural loss and triggered a project to replace three bridges in the initial phase of a three-phase construction program.
After careful analysis, the Montana Department of Transportation (MDT) contracted with Morrison-Maierle to replace the Deep Creek bridges, located in Deep Creek Canyon between Townsend and White Sulphur Springs, Montana, with new single-span, precast concrete structures between 54 and 70 feet long. The existing timber bridges, which were built in the 1930s, were constructed in a narrow canyon through the Big Belt Mountains where the 24-foot-wide, two-lane highway competes with the creek for space on the canyon floor between steep rocky walls.
With advance planning and sound engineering decisions, Morrison-Maierle and its partners designed a bridge foundation and superstructure system that could be built using accelerated bridge construction (ABC) techniques during weekend closures that came in under budget and did not interrupt the public’s traffic patterns over an extended construction season.
Services & Highlights
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Structural engineering
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Permitting
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Wetlands and riparian habitat rehabilitation and engineering design
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Accelerated bridge construction (ABC)
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Roadway and bridge design
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Environmental review and documentation
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Hydraulic surveys and modeling
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Right-of-way design
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Utility coordination
Weekend closures of the road required the bridges to be constructed with a modular system to minimize construction time. Each bridge consists of early drilled shaft foundations combined with precast conventionally reinforced concrete grade beams, wingwalls, and prestressed concrete tri-deck beams. The new 54’-0” Deep Creek bridges are single-span structures and provide two 12’-0” lanes and 2’-0” shoulders.
Two of the bridges are located on tangent roadway alignments. One has a normal crown cross-section while the other holds a 2% transverse slope for the length of the bridge because of its proximity to a superelevation transition. The third is located in a horizontal curve resulting in a 6% transverse cross slope.
Additional shoulder width was needed to account for one of the bridges being located on a horizontal curve. The bridge is tangent, so an extra 4” of width was added to provide the minimum 2’-0” shoulder width along the full length of the bridge. The extra width was added to the other structures to simplify design and fabrication.
Speed of Construction and Closures
Drilled shafts were chosen for the foundations over piles and spread footings because of the sloping bedrock at each of the bridge sites. The 3-foot diameter drilled shafts were socketed into the bedrock on each side of the road just beyond the existing shoulder. Placing the drilled shafts just off the road allowed the construction to occur under single-lane traffic during the day; the two-lane traffic resumed at night and during non-work hours.
All of the drilled shaft construction was completed several weeks before the weekend road closure. When the road was closed, the design allowed the contractor to quickly excavate the buried drilled shafts, clean the surface, connect the steel reinforcing to embedded mechanical couplers, and place the shim and expansion joint material before setting the grade beams.
One of Morrison-Maierle’s major design challenges was to create a drilled shaft and grade beam connection that added no construction time. The window of time for bridge construction during the closure was not long enough to accommodate separate grout cure times for the grade beam connection and the superstructure keyway connection. Additionally, the connection had to handle the deflection and rotation of the grade beam cap under dead and live loads because the span between the drilled shafts is long.
The challenges of this connection were addressed by using a pin connection with a keyway at the top of the drilled shaft. This design allows the tri-deck beams to be placed on the grade beam immediately after the grade beam is placed on the drilled shafts without grout. The keyway allows live load rotation about the longitudinal axis of the structure and provides resistance to rotation about the transverse axis of the structure. Following beam placement, the detail allowed concurrent grouting of the connection and superstructure keyway.
Communication, Traffic, and Schedule
MDT held meetings with the public, Montana Fish Wildlife & Parks, U.S. Forest Service, and local emergency services. Weekend closures of the roadway and the ABC technique were preferred by all agencies and the public due to lower costs, reduction in construction duration, and less environmental impact. Emergency service providers accepted the closure and staged equipment and personnel on each side of the canyon to serve the local communities. The weekend closures were advertised on various media outlets. Signs were also placed on the surrounding highway system to inform the traveling public of the closures.
The schedule for the weekend road closure started on Friday at 6 p.m. for each bridge, and the road was scheduled to reopen at 7 a.m. the following Monday. Each road closure began on schedule, with actual closure times shortening as the contractor became more familiar with the process. The first structure was completed and the road was opened to traffic at 3 a.m. on Monday, the second at 3 p.m. on Sunday, and the third at 1 p.m. on Sunday. The first 4-6 hours after the road was closed was spent removing the existing bridge. Stream work and abutment excavation followed the bridge removal for the next 6-10 hours. The new bridge was constructed within 24 hours, followed by the bridge end backfill ranging from 6-18 hours. Guardrail and additional riprap placement occurred on Monday under single-lane traffic and asphalt approach surfacing occurred during the following week.
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