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Memo to Tom Schreiner 2 November 21, 2003 <br />Slope Stability Analysis Permit No. M-1978-056 <br />Given the above list of issues, the Operator must be notified that the Division will consider mining at an <br />excavation slope of steeper than 1.25H:1 V within 80 feet (two-times the anticipated maximum pit <br />depth) of a structure, to be a serious violation of the permit that will initiate an immediate enforcement <br />action. The following items must be provided by the Operator: <br />1. A description, in terms of the mechanics of earth moving, of how the 1.25H:1 V pit slopes will <br />be excavated. <br />2. A description of survey control to be employed to assure that the maximum excavation slope <br />angle is not exceeded. <br />3. A description of management procedures to be used to ensure heavy equipment operators aze <br />continuously aware of the excavation slope angle requirement. <br />4. A description of the oil and gas pipelines referenced in the amendment application. In <br />particulaz, aze all of the pipelines within 80 feet of the excavation limits low capacity, low <br />pressure, small diameter flow lines? Or aze there any high pressure lines in the vicinity? If <br />there are high pressure lines, additional protections and monitoring programs maybe required. <br />To evaluate the Operator's proposed setback and excavation slope angle in terms of protection of <br />structures, the Division has completed quantitative slope stability analyses, with the following results. <br />Pit D th Failure Surface Phreatic Line Safe Factor File Name Anal sis Number <br />25 Feet De Pit Flooded 1.21 Durham 25 1.25 1 <br />25 Feet Shallow Pit Flooded 1.10 Durham 25 1.25 2 <br />25 Feet . Shallow Likel Case 1.14 Durham 25 1.25 3 <br />25 Feet De Likel Case 1.14 Durham 25 1.25 4 <br />25 Feet De Hi _ _ _ _ 1.02 Durham_25 1.25. 5 <br />25 Feet Shallow Hi 1.14 Durham 25 1.25 6 <br />40 Feet De Hi 1.03 Durham 40 1.25 1 <br />40 Feet Shallow Hi 1.07 Durham 40 1.25 2 <br />40 Feet Shallow Likel Case 1.08 Durham 40 1.25 3 <br />40 Feet Dee Likel Case 1.14 Durham 40 1.25 4 <br />40 Feet Dee Pit Flooded 1.21 Durham 40 1.25 5 <br />40 Feet Shallow Pit Flooded 1.04 Durham 40 1.25 6 <br />A full set of the analytical results are enclosed with this memo. Given the lack of geotechnical <br />information for this pit, the following assumptions were input to the analyses: <br />The geology of the pit is a simplified sequence of horizontal strata, with a thin layer of surface <br />soil overburden overlying 25 to 40 feet of sand and gravel overlying shale bedrock, with the <br />upper three feet of the bedrock being weathered. <br />When the weathered shah is unloaded by mining, the shear strength deteriorates to the residual <br />strength. <br />