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

Identification of Potential EnvironmentalEffects and Proposed Mitigation7.3.1.3 Ground Stability and PermafrostPotential EffectsPotential project-related impacts to ground stability and permafrost include:• Alteration of active layer and permafrost thickness• Slope stability in quarries and borrow areasThe Project involves ground disturbances at watercourse crossings, cut and fill locations on theaccess road to the proposed BIPR road and borrow areas where excavation of sand and gravelwill occur. Any ground disturbance may result in a change in the thermal regime of the groundactive layer.Modification to the thermal regime may induce a shift in the active layer and melting of groundice, resulting in thaw settlement. Depressions caused by these settlements could form, leading toerosion and ponding of water.Where thicker layers of engineered fill are placed on the road, the permafrost will rise over time,possibly creating a damming effect in the seepage through the active layer that results in pondingbehind the road berm. This would not be expected to occur at well drained locations, but mayoccur at low-lying sections that form natural drainage courses.Permafrost-related phenomena, such as mass wasting and erosion, are occurring under naturalconditions; however, Project activity may accelerate these processes.Proposed MitigationEffects to permafrost will be mitigated by reducing the extent of cut and fill areas. At low lyingareas where roadbed fill is greater than 1 m, the permafrost can be expected to rise to ameaningful degree. Swales or culverts can be installed as part of road construction andmaintenance to prevent the ponding of water that may arise from the damming effect of theraised permafrost level in the road bed. To address this issue upon closure, swales can be left inplace, or alternatively, the road bed can be breached to allow drainage.Increased soil erosion will be mitigated by controlling soil disturbance. In areas where soildisturbance is unavoidable, mitigation will focus on stabilizing cut and fill areas with gentleslopes less prone to erosion. Implementation of sediment and erosion control measures includingcompaction, silt fences and erosion control blankets will help mitigate effects to ground stability.Cut and fill areas are expected to be relatively small in extent. If they are located nearwatercourse crossings, there is increased need for sediment and erosion control measures toprevent siltation of the watercourses.The area impacted from borrowing of materials will be more extensive by comparison. Effortswill be made to concentrate borrow activities to limit the area of disturbance. This will beJanuary 2008 Hackett River Project – Project Proposal Sabina Silver CorporationReport Version B.1 7–6 Rescan Environmental Services Ltd. (Proj. #833-2)