TRANSPORTATION
Railway crossing safety Incorporating accelerated bridge construction techniques
into the grade separation process. By Art Ivantchouk, Ph.D., P.Eng., BDS
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 acci- dents. The only strategy to completely eliminate the risk of accidents, and to guarantee safety, is grade separation through construction of an over- pass 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 accel- erated 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. GSS procedure 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 bear- ing 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 be- tween 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). Modifications 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 typi- cally cast monolithically, resulting in a rigid and continuous connec- tion 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.
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csengineermag.com
july 2018
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