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<br /> <br />Memo to Gary Curtiss -2- June 20, 1997 <br />recommend that the applicant commit to a more conventional maximum <br />slope for mining, such as 3H:1V. If the applicant would agree to <br />such a commitment, the issues listed below would no longer apply. <br />2. It is stated in the 11/8/96 letter report that the safety factor is <br />1.36 for a failure surface that intersects the pipeline easement boundary <br />at the proposed 30 ft. set-back distance, the gravel bank saturated, and <br />O.5v:1H pit slopes. It is further stated that the 1.36 safety factor is <br />considered adequate. Theoretically, any safety factor greater than unity <br />is indicative of a stable slope. Safety factors greater than unity are <br />universally specified due to uncertainties that are inherent in stability <br />analyses of inhomogeneous materials. The minimum acceptable safety factor <br />specified for a particular analysis depends on the degree of certainty <br />that may be applied to the input data and on the nature of the <br />consequences associated with a particular failure. <br />a) In the case of the Del Camino Pit stability analysis, the <br />applicant has used assumed values for shear strength of the earthen <br />materials that would correspond with typical, or expected values <br />that can be found in the literature. For the purpose of analyzing <br />slope stability, the applicant has selected an angle of internal <br />friction for the pit gravels of 38 degrees. The Division has been <br />provided no testing information to justify the selection of this <br />friction angle, although I would agree that 38 degrees angle of <br />internal friction would be typical for dense sand and gravel. <br />However, friction angles can range as low as 17.7 degrees in gravels <br />with clay and as low as 31.4 degrees in silty gravels. Based on the <br />potential for friction angle variability and the lack of site <br />specific testing, I would place only a moderate degree of confidence <br />in the shear strength inputs to the Del Camino Pit stability <br />analyses. <br />b) In terms of the consequences of failure, the Sinclair pipeline <br />is a structure that can be considered extremely hazardous if it is <br />disturbed, with potential consequences of a slope failure affecting <br />the pipeline ranging from property damage and disruption of service, <br />to injury or death of any nearby persons or animals. Structures <br />with high failure consequences merit higher safety factors. It is <br />stated in the AGRA Earth and Environmental report dated August 28, <br />1996, and submitted to the Del Camino Pit application, that "slope <br />factors of safety in excess of 1.5 are considered acceptable for <br />high hazard slopes." I would agree with this statement, and would <br />specify that the minimum acceptable safety factor for pit slopes <br />that are within 200 feet of the Sinclair pipeline, should be 1.5. <br />3. It is stated in the 11/8/96 letter report that the pipeline easement <br />has been considered essentially inviolable for the purposes of the <br />stability analyses. This is an appropriate consideration given that the <br />applicant has not secured a compensation agreement with the pipeline <br />company. It is further stated that the easement line leaves a minimum of <br />