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

Project Description2.8.3.1 LocationThe bridge over the Hackett River will be located:• at a narrow point on the river• at right angles, where possible• at a point where the banks are lowOption 3, which is a variation on option 1, crosses Hackett River at a point where the banks arelow and is the best crossing point for all options (Figure 2.8-1). West of the river, the road mustbe aligned at about 240 o to miss a large rocky outcrop, which would, however, provide a quarryfor road fill. However, option 3 is only appropriate if option 1 is the preferred routing.Option 1 crosses the river at a point where the banks are so high that a crossing would bedifficult. The banks of the river immediately north and south of this point are equally high anddo not allow for any improvement in the crossing.Option 2 crosses the river at a point where the river has moderately high banks. The west bank,which is close to the river, must be notched to allow the road to reach the bridge at a reasonablylow level. The east bank is sufficiently far from the river to allow the road to climb up to thebench on that side. The river banks are lower to the north of this crossing and higher to the south.The road alignment in this location is constrained by the need to miss the two small and one largelake on either side of the river. An improvement in the location of this crossing could be made ifthe smaller lakes, which are often shallow and seasonal, could be filled in.2.8.3.2 Design ConstraintsLow-volume roads demand a different approach to bridge engineering, and this is particularlytrue for construction in remote locations where environmental issues are paramount. The designof the Hackett River bridge will be dominated by transportation issues, equipment availability,and the selected erection method. The small traffic flow to and from the Hackett River mine willrequire the provision of only a single traffic lane along the access road and across the bridge. Thedesign of the bridge will be selected to suit:• the site location• the specified requirements of the crossing• the method of transportation of the materials and erection of the girders• environmental concerns.For remote sites, bridges commonly use either twin steel I-girders and precast concrete decks orshear-connected precast concrete beams.January 2008 Hackett River Project – Project Proposal Sabina Silver CorporationReport Version B.1 2–39 Rescan Environmental Services Ltd. (Proj. #833-2)