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-- - - - - 1ifATl QI r: Olrlll Ali<J Sit ~I 1. ul~ll i.YAM I•°v^~ III III III III IIII III DEPARTMENT OF NATURAL RESOURCES 999 D. Morale Pascoe, Executive D~renor NIINEU L:1NU IZH:(:LAA9A'I'l0N 423 Centennial Build ing, 13135herman Street Denver, Colorado 80203 Tel (30;3) 839-3567 David C. Shelton Director August I7, 1981 TO: Brian Munson \ -- FROM: Jim Pendleton J ', i' SUBJECT: Review of the St~~bility alysis for the Hayden Culch Mine's Overburden Exc~.ss~Spo~ Pile Pursuant to your request I have reviewed the report entitled; "Geotechnical Investigation of the Overburden Dump Area, Hayden Gulch mine, Routt County, Colorado", prepared for H-G Coal Company by Morrison-Knudsen Company, Inc. I forward the following comments for your consideration. The geotechnical and stability anlaysis.performed by M-K is relatively simplistic in nature. No materials testing was performed to determine shear strength of the spoil materials. In addition, no observation of actual hydrologic conditions within the pile was made. The consultant assumed that the pile is "well drained and no phreatic surface is to be found within". The computer technique utilized is acceptable. Dased upon the assumed material and hydrologic properties, the static slope factor of safety was calculated to be in excess of 2. In the absence of discrete spoil materials strength and hydrologic data concerning the phreatic surface within the pile, I believe it prudent to repeat - the computer stability analysis in order to determine the inter-related sensitivity of the two assumed input parameters. The consultant did complete two separate computer analyses assuming a variation of spoil material shear strength. The first analysis assumed an angle of the internal friction of 22 degrees and a cohesion of 100 pounds per square foot. The second analysis assumed an angle of internal friction of 28 degrees and a cohesion of 250 pounds per square foot. In my opinion this variation of shear strength is adequately conservative to be acceptable. However, the consultant should repeat the analysis projecting possible phreatic surface configurations other than the assumed dry pile. The consultant should project a maximum probable elevation of the phreatic surface within the pile. The computer analysis should be repeated utilizing that maximum probable phreatic surface assumption, as well as several other intermediate configurations. If the resulting factors of safety exceed the required 1.5, I would deem it acceptable to satisfy the requirements of Rule 4.09. If the resulting factors of safety are sub-standard, the operator should then be required to install the appropriate monitoring well within the pile to determine the maximum elevation of the existing phreatic surface. If this is proven to be necessary, it may be possible to utilize an existing monitoring well within the pile, which was originally designed to monitor an aquifer I . ~ Wry ~ i. ~.. I .