Building a road bridge is not your typical weekend project, but considering the circumstances and available alternatives, officials of the Montana Department of Transportation (MDT) decided a recent group of bridge replacements were just that.  Three old bridges were removed, and new permanent bridges were constructed on U.S. Highway 12 in Deep Creek Canyon were installed, over the course of one weekend each.  Precision planning and full use of modern materials and techniques helped the department and contractors get the job done. 

Planning for Replacements

Deep Creek Canyon owes its tight, winding topology to erosion over millennia by Deep Creek, which originates east of the Big Belt Mountains and flows west to join the Missouri River.  The section of U.S. Highway 12 between Townsend and White Sulphur Springs runs along the narrow floor of the canyon and was built in the 1930s, when timber bridges were constructed at eight locations where the highway crosses the creek.   

Spring flooding in 2011 damaged bridges in Deep Creek Canyon.  High runoff flows resulted in significant scour of the bridges and serious erosion of the highway embankment. There was an emergency response to prevent structural loss after the damage, and ultimately it was decided to replace three bridges at mileposts 17.3, 19.6 and 19.8 in this project.

It wasn't immediately clear how to proceed, however.  Construction of temporary detours in the narrow canyon would have posed safety hazards for both workers and motorists.   There would also be significant environmental impacts, and very high project costs. Alternatively, closing the road entirely would result in a 150-mile detour for up to nine months.

In light of those considerations, said one MDT official, "it was a big deal" when bridge design engineer Morrison-Maierle, Inc., of Helena, Montana, proposed the weekend bridge replacements, and convinced MDT that the idea would work.  

The Innovation of ABC

There is a trend to look for alternatives to traditional methods of bridge construction.  The Federal Highway Administration calls it Accelerated Bridge Construction (ABC), and defines it as "bridge construction that uses innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the onsite construction time that occurs when building new bridges or replacing and rehabilitating existing bridges." 

The FHWA notes that about one fourth of all the 600,000 bridges in the United States are in need of rehabilitation, repair, or total replacement.   It notes that the direct and indirect costs of traffic detours that result from the loss of a bridge during construction can exceed the actual cost of the structure itself.  Add in the safety impacts or partial lane closures and other bridge activities that occur alongside adjacent traffic and there are powerful arguments to make minimizing traffic disruption a high-priority goal in planning bridge projects.  

MDT's construction contract with prime contractor Dick Anderson Construction called for three full closures to complete the bridge replacements, with each closure to begin at 6 p.m. on a Friday and end at 7 a.m. the following Monday.

The contract made time into money very explicitly, with a bonus of $1,600 per hour for early completion, and a reduction of $1,600 per hour for late completion. The schedule gave the contractor just 60 hours to remove the existing bridge, install a new bridge, align and grade the roadway approaches, and rebuild the creek.  At Deep Creek Canyon it took innovative design and meticulous planning to meet that goal.  

A Well Executed Plan

According to MDT, each of the single-span structures range from 54 to 70 feet long.  They decided to use tri-deck beams with off-alignment early foundation construction and precast bridge substructure elements. The drilled shaft foundations were located in the shoulders of the roadway, which was outside of the existing bridge footprints. That allowed construction of the shaft foundations to be done in advance of the bridge removal.  That required single-lane closures during working hours, with two-lane traffic resuming at night and non-work hours.

The drilled shaft foundations were combined with precast conventionally reinforced concrete grade beams, wingwalls, and prestressed concrete tri-deck beams. Concurrent grouting of the connection and superstructure keyway saved time by avoiding separate cure times for both.

The short timeline and geography of the canyon made detailed planning and scheduling paramount to success. The remoteness meant all materials had to be onsite and nothing could be overlooked. Lack of cell coverage posed a communication challenge to be accommodated.

Prior to work beginning, multiple meetings with MDT, the contractor, subcontractor, and the designer were held to review critical path activities. Staging of materials and equipment had to be efficient. Each person had to know what he or she would be doing when and where. This included the contractor and subcontractor, MDT inspectors and project manager, the designer, traffic control, and a stream restoration specialist.  Careful planning and staging were necessary to protect worker safety. Along with everything else, the activity included two cranes operating in a very tight space to place the nearly 80,000-ton precast beams.

Environmental impacts were also an integral consideration in the project.  Natural resources staff worked with the design engineers to identify critical environmental resources and found ways to minimize and avoid impacts. Stream restoration included riprap, re-vegetation - planting 40,000 willows and seeding five other plant species, and regrading. The efforts went to mitigate impacts from the current project as well as the road construction done in the 1930s.

According to the plan, the first four to six hours after the road was closed would be spent removing the existing bridge. Then came the stream work and abutment excavation which would proceed for the next six to ten hours. The new bridge was then constructed within 24 hours, followed by the bridge end backfill ranging from six to 18 hours. Guardrail and additional riprap placement would occur on Monday under single lane traffic and asphalt approach surfacing would be done during the following week.

The careful planning was rewarded with success.  The first bridge replacement was completed six hours prior to the Monday morning deadline. The second two bridges each beat their deadlines by nearly 16 hours.  The $2.75 million project has received several statewide and regional awards, most recently a regional award in the 2015 America's Transportation Awards competition in the "Best Use of Innovation, Small" category from the American Association of State Highway and Transportation Officials (AASHTO).