080121-08MN006-Sabina Hackett River Project Proposal ... - NIRB

Project DescriptionTypical bridge design criteria must be modified for low-volume road bridges to account for thefollowing constraints:• Heavy industrial off-highway vehicular loading• Remote sites resulting in limited access to conduct geotechnical investigations• Limited access for construction equipment to transport materials to the site• Limited sources for cast-in-place concrete supply• Avoidance of environmental impact on the Arctic environmentTo overcome these and other constraints, a bridge structure will be developed that is:• easy to transport and install• requires only limited quantities of cast-in-place concrete• allows for construction flexibility if the ground conditions encountered are not asassumed.2.8.3.3 SubstructureDue to the remote location of the majority of the low-volume road bridges, it will probably not beeconomical to conduct a detailed geotechnical investigation with borehole drilling. Geotechnicalinformation will be deduced from site observations, geotechnical maps and past experience in thearea. In the initial stage of construction, the excavation of footings or driving of piles willindicate what lies below the surface. To account for the limited amount of geotechnicalinformation available, the preferred substructure will be precast concrete footings, founded on agravel-leveling surface and lowered in place using an excavator or crane. Steel pipe columns willbe bolted onto the footings and bracing will be welded in place, as required. The bridge bearingscan then be directly placed onto the columns, eliminating the need for a concrete cap.If rock is encountered and is considered suitable for direct bearing, small cast-in-place concreterock pads (bagged grout containing coarse aggregate) can be readily installed on site. Ifgeotechnical conditions indicate that precast concrete footings or cast-in-place concrete rock padsare not suitable, steel pipe piles will be considered.For small structures over smaller watercourses, where a crane would not normally be used forinstallation, precast concrete footings will be the most economical solution. However, culvertswill always be the first choice for small watercourses if of sufficient capacity.Innovative substructure designs are required for large river piers that are subject to significant iceforces. In such a situation, the use of large steel pipe piles and bolted steel diaphragms for bridgepiers would eliminate the use of cast-in-place concrete.2.8.3.4 SuperstructureIn the Hackett River area, good quality road fill will be hard to find. To avoid the construction oflarge abutments containing large amounts of road fill, the bridge deck length can be increased.January 2008 Hackett River Project – Project Proposal Sabina Silver CorporationReport Version B.1 2–41 Rescan Environmental Services Ltd. (Proj. #833-2)