Home to the world famous Polar Bear Express, the Ontario Northland Railway (ONR) passes through several hundred kilometres of wild and pristine northern landscape, extending all the way north to Moosonee, on the shore of James Bay. Near Smooth Rock Falls, at Mile 36.0 of the ONR's Island Falls Subdivision, is Trappers' Creek Bridge, a 1920's vintage, 7-span steel structure. Prior to a major repair designed by Hatch and undertaken in 2010, the bridge's southernmost bent was supported by 2 shallow-founded concrete pedestals that had settled and rotated out of plumb. These pedestals were replaced with 3 new pile caps supported on micropiles constructed by Geo-Foundations under subcontract to Sema Inc.
The physical and geological settings of the site, as well as railway operational constraints all made this project ideally suited for micropiles. All of the micropiles had to be installed immediately adjacent to the existing pedestals, and the close proximity of the creek precluded construction of large access pads. In the interim before the new pile caps were ready to be put into service, the pedestals had to be preserved undisturbed in order to maintain rail service throughout construction. To add to these multiple constraints, the site was only accessible via rail, with the micropile drill rig having to be hoisted into place from a rail-based crane positioned on the bridge deck.
Each micropile consisted of a 245 mm diameter, thick-walled upper casing embedded 5 m deep, complete with grout-flushed R51 central reinforcement over its full 11 m embedment depth. In total, 30 permanent micropiles were constructed. Each micropile was designed for 315 kN (service load) of both tension and compression. Static compressive testing to 630 kN was carried out at each of 2 pre-production, sacrificial micropiles.
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