Incorporating accelerated bridge construction techniques into the grade separation process
A grade or “level” crossing is an intersection where railroad tracks cross a roadway at the same level. Currently, there are approximately 37,000 level crossings across Canada. Despite constant investment in upgrades and new installations of crossing gates and warning signals, level crossings continue to be the primary cause of all railway accidents.
The only strategy to completely eliminate the risk of accidents, and to guarantee safety, is grade separation through construction of an overpass or underpass. However, current industry standard construction methods are unreasonably costly and time consuming.
With the goal of improving cost efficiency and constructability of grade separation structures, ART Engineering, Inc. launched a research and development initiative. This endeavor led to the incorporation of accelerated bridge construction (ABC) techniques into the grade separation process and the development of Grade Separation Systems (GSS), a construction procedure that can shorten construction schedules by as much as 50 percent and reduce overall project costs by as much as 45 percent.
In the commencing step of the GSS construction procedure, pairs of caisson liners are installed along the length of the track, outside of the railway clearance envelope (see Figure 1). The clearance envelope is determined by the governing railway, but must be a minimum of 9 feet from the centerline of the tracks according to AREMA standards. The caisson liner installation does not need to interfere with railway operating schedules.
Segments of the future substructure (either pier cap or abutment) are prepared on or off site. They weigh approximately 35 metric tons so they can be easily transported if necessary. Once onsite, the precast segments are joined to steel trench boxes. Following installation, the trench boxes provide a safe working area adjacent to the tracks for construction of the cast-in-place elements of the substructure. These trench boxes also act as formwork (see Figure 2).
Railway traffic is closed for a four- to six-hour period (typically overnight, but not necessarily) to allow for placement of the precast assembly. A trench is dug across the track, around each pair of liners. A precast assembly is placed in the trench so that the caisson liners are sitting within the trench boxes. The trench is then backfilled and the tracks are reinstated. Railway operations can return to normal as work moves to the trench boxes.
To complete construction of the substructure, reinforcing steel is placed throughout the caisson liners and trench boxes, post-tensioned steel or mechanical couplers are used to integrate the previously placed precast elements with the cast-in-place elements, concrete is poured, and bearing pads are placed with millimeter precision. While work continues within the trench boxes, the bridge span(s) can be constructed onsite at a safe distance from the railway. The GSS technology supports any bridge span design, whether concrete or steel (see Figure 3).
Once the substructure and superstructure are constructed, the railway is closed for another series of four- to six-hour periods. A trench is dug between abutments and/or pier caps, the trench boxes are removed, and the permanent bridge span is placed to sit on the bearing pads. Depending on the size and weight of the span, a tandem crane lift or a lateral slide can be used for placement. Once the span is in place, the trench is ballasted, tracks are reconnected, and railway traffic is reinstated. The final step in the construction procedure is excavation beneath the railway bridge and construction of the under-passing roadway (see Figure 4).
The procedure outlined above is a general summary of steps rather than an all-encompassing methodology. The technology was designed with options for modifications so that a wide range of projects could be accommodated.
Rigid frame variation — A rigid frame structure is one that is typically cast monolithically, resulting in a rigid and continuous connection between the substructure and superstructure. For this adaptation, a slab segment is precast rather than a pier cap or abutment segment. This slab segment is placed during a short closure and then expanded upon once railway traffic has been reinstated. Following placement, caisson liners are drilled, cast-in-place concrete is poured, and then excavation and construction of the under-passing roadway occurs to complete the project.
Multiple lines of track — All examples discussed so far have assumed a single line of track. This technology can be modified to accommodate any number of tracks; however, the exact process for doing so will vary from project to project.
Bridge rehabilitation — Most bridge construction is focused on rehabilitations and replacements rather than new structure builds. GSS can be modified to apply to bridge replacement projects that require construction of new abutments. To accomplish this, abutments are constructed in the manner described previously and anchored to the existing bridge abutments. GSS can typically keep more traffic lanes open throughout construction when compared with alternative approaches.
Staging — In dense population areas, it may sometimes be beneficial to lengthen a construction schedule to allow an existing roadway to remain in use throughout the project schedule. This is possible with GSS in project scenarios with multiple road lanes in each direction, requiring two or more bridge spans to be installed. To maintain roadway function, traffic is staged to occupy half of the roadway at a time while the other half goes through the construction process as outlined previously. Once the bridge spans have been installed and the underpass constructed, traffic is then rerouted to use the underpass while the second half of the roadway undergoes construction.
GSS is a modern technology that utilizes a combination of ABC techniques and conventional bridge construction practices to limit onsite construction and reduce overall construction costs. The technology increases the feasibility for grade separation construction while also improving construction conditions for road users. This technology has been designed to be highly adaptable so that it can be used in a variety of construction projects. The procedure has the potential to significantly reduce project costs and shorten construction schedules.
INDOT makes $125 million available for rail overpass projects
The Indiana Department of Transportation (INDOT) is making available at least $125 million for high-priority railroad safety projects on local roads statewide through the agency’s new Local Trax matching grant program. Local Trax provides state matching funds for Indiana cities, towns, and counties interested in pursuing high-priority railroad grade separations, crossing closures, and other safety enhancements at railroad intersections with local roads.
“Much like Community Crossings, Local Trax is an innovative approach to infrastructure funding that creates a partnership between the state and communities willing to put skin in the game toward improving their local roads,” Governor Eric. J Holcomb said. “Eliminating at-grade rail crossings on local roads makes our transportation network safer, reduces congestion, and better connects our communities.”
INDOT began accepting project proposals from local agencies in May. The application window will stay open until Aug. 31, with the awarding funds expected to be announced in late summer. Local Trax requires local governments to provide only 20 percent of funding for land acquisition and construction with the state providing the other 80 percent.
More information about Local Trax is available at www.in.gov/indot/2390.htm.
Art Ivantchouk, Ph.D., P.Eng., BDS, is president of ART Engineering, Inc. (www.artengineering.ca), structural consulting engineers based in Carp, Ontario.