Laserfiche WebLink
Stockpile Stability Evaluation -4- Pernrit N . M-1984-001 <br />Surface Rock Pit O sober 6, 2000 <br />shear strength is used, it is necessary to err on the conservative end of the spectrum. The ombination of the <br />steep stockpile slopes, an estimated and conservative shear strength, and an evaluation of orst case seepage <br />potential may result in an analytical result demonstrating that the stockpile is not stable. ince this result <br />would contradict the performance of the stockpile over the past 14 years, the Operator m chose to conduct <br />laboratory analysis of the shear strength of the stockpiled material, which is likely to yiel a higher or much <br />higher result than the conservative assumed values. <br />Another form of relief that may benefit the Operator's attempt to demonstrate that the slo a is stable comes <br />from the consideration of the consequences of a slope failure. Stability of slopes is usual) analyzed by <br />methods of limit equilibrium, which yield a safety factor that is the ratio of the forces resi ting failure, the <br />shear strength of the soil, and the forces driving failure, or the applied shear stress. Thes results provide <br />information on the likelihood of a slope failure, but no information on the magnitude of p tential slope <br />movements. Any safety factor greater than one indicates that the slope will be stable and r the conditions <br />that were input to the analysis. However, safety factors much greater than one are usual) specified for <br />slope stability demonstrations depending on: <br />• the degree of uncertainty in the shear strength measurements, slope geometry, and oth~r conditions; <br />• the costs of flattening or lowering the slope to make it more stable, and; <br />• the costs and consequences of a slope failure. <br />For example, if the Operator can demonstrate that the type of slope failure that may be an cipated for the <br />stockpile under worst case conditions would not adversely affect the operation of Highwa 141, then the <br />safety factor required in the stability demonstration would be smaller. A table is attached to this <br />memorandum listing recommended safety factors under vazious conditions. <br />In summary, the Operator may apply to change the reclamation plan and allow the stockp le to remain in <br />place pending a demonstration of its long-term stability. The demonstration must include field observations <br />and measurements combined with test pitting to evaluate the nature and consistency of th stockpiled <br />materials. Using the field information, a stability analysis based on conservative estimate of shear strength <br />may be sufficient to complete the required demonstration. If it is not, the Operator may c ose to conduct <br />sheaz strength testing and repeat the stability analysis. The analyses must consider worst ose conditions for <br />potential seepage and flooding. The value of the required safety factor will depend on th consequences of a <br />slope failure. If it appears that a failure in the stockpiled material would not impact High ay 141 a <br />relatively lower safety factor would suffice. If the analyses fail to demonstrate that the st ckpile will be <br />stable in the long term, the Operator may consider modifying the approved reclamation pl by regrading, <br />bumessing, or otherwise stabilizing the slope. <br />attachment(s) <br />Source of the attachment is "An Engineering Manual for Slope Stability Studies, " Yirgini~t Tech <br />Department of Civil Engineering, March 1987. <br />c:\windows\personal~surracc rock pit.doc <br />