PERMFILE72133 - Laserfiche WebLink

Home Browse Search

Morrison Knudsen Corporation • January 7, 1997 Page 3 During the December 5, 1996 meeting it was indicated to us that the portal road would remain in use and not be reclaimed after mining operations were complete. It was agreed that if the cut slopes remained stable over the life of the project (estimated at 20 to 30 years) then they would most likely remain stable after the mining operation were completed. For this reason, we concluded that the post-construction stability of both the road cuts and the topsoil pile area was most critical and therefor atotal-stress analysis was conducted. The following soil parameters were used in the program. Soil Type Unit Weight Angle of Internal Cohesion, c (pcf) Friction, ~ (degrees) (psf) Native Material 125 0 2,000 to (CL-GC) 3,000 Topsoil Fill 100 0 0 • Every attempt was made to conservatively estimate the subsurface conditions modeled during these analyses. Values of cohesion of 2,000 and 3,000 were used when analyzing the road cuts to evaluate the sensitivity of the factor of safety to variations in shear strength. This was done in anticipation of the significant changes in topography and surface drainage that will occur. With the exception of the construction of the portal road along the northern edge of the ridge, grading in the topsoil pile area will be minimal. For this reason a cohesion of 3,000 psf was used in the analysis of the topsoil sections. Bedrock in the topsoil area was conservatively modeled at an elevation of about 10 to 20 feet lower than that indicated by Boring 7. The topsoil itself was also conservatively modeled assuming zero shear strength. Laboratory testing indicated the natural soils to have strength values as much as twice what was used in these analyses. Ground water was measured at depths of 20 to 30 feet in only two of about fifteen borings located along the proposed portal road. Although the lack of water in the borings would indicate that cuts will most likely not encounter ground water seeps, it is probable that some seeps may be encountered or may develop due to the changed topography or seasonal ground water fluctuations. Recommendations regarding the mitigation of exposed ground water seeps are discussed in a later paragraph. Results of Stability Analysis: Since the portal road is to remain after mining operations are completed, it was agreed in the December 5th meeting that a minimum factor of safety of 1.3 would be required for cut stages along ttte portal road. VYlten modeled using a value for • cohesion of 2,000 psf the factor of safety varied from 1.4 to 1.5 for the three portal road cross- sections analyzed. For a cohesion of 3,000 psf, the factor of safety increased to between 2.1