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Ms. Christine Johnston <br />Mountain Coal Company <br />Page 18 <br />April 14, 1997 <br />analysis for inclusion within the Permit Revision No. 7 application. This stability analysis <br />report consists of a one and a half page summary report and numerous pages of XSTABL <br />computer generated printout for critical sections A-A' through I-I'. The computer <br />reiterated analyses of these critical sections appeaz to be based on two generalized material <br />property chazacterizations, purported to represent "Bedrock Parameters" and "Soil <br />Parameters". These generalized material parameters were reportedly "obtained from <br />laboratory data and prior experience", although no supporting data or text explanation of <br />these experiential deductions are included. The critical sections appear to have been <br />alternatively modeled as infinite isotropic soil masses and infinite isotropic bedrock <br />masses." <br />As the Division observed in it's original adequacy comment #71, based upon Map SGP- <br />13, "Sylvester Gulch Facilities Stability Cross Sections", the Division assumes that <br />analytical cross sections A-A', B-B' and D-D' were chosen to represent axial sections <br />through known areas of historic and modem landsliding adjacent to, and upslope from, <br />the Sylvester Gulch access/haul road. The Golder Associates' stability analysis, which <br />MCC continues to characterize as "valid", concludes that the static Factor of Safety (FOS) <br />for these three landslide cross sections vary between 1.7 and 2.0 for the assumption of an <br />infinite soil slope and between 2.7 and 3.0 for the assumption of an infinite bedrock slope. <br />As discussed above, each of these slopes has recently been unstable. In there current <br />configuration, they have zero to perhaps ten foot high cut slopes adjoining the 14 foot <br />wide Sylvester Gulch access road. Because of their recent instability, the Division <br />continues to suspect that each is meta-stable at best, with a static slope safety factor <br />probably very close to 1.0. Golder Associates' analytical critical cross sections, each <br />reflecting proposed increased cut slopes to accommodate the widening of the Sylvester <br />Gulch access/haul road, conclude that their modified static slope safety factors will exceed <br />1.7. Based upon this relationship, the Division continues to conclude that these stability <br />analyses are unrepresentative of the actual stability of the analyzed slope crass sections, <br />which Golder and Associates purports to adequately represent both the operational and <br />reclaimed configuration of the chosen cross sections. Analogous demonstrations and <br />analyses are proposed to support approval of the electrical substation and light use <br />electrical powerline access road. <br />As mentioned in original adequacy comment #71, the Division recognizes that the <br />methodology of slope and structural stability analysis is imprecise. The analytical results <br />are influenced by numerous parameters, including material shear strength parameters <br />(angle of internal friction, cohesion), degree of saturation (water table configuration, pore <br />pressure), configuration of slope structure (bedding, jointing, faulting), and externally <br />applied forces (surcharge loading, vibration, seismicity), among others. For this reason, <br />it is incumbent upon the analyst, to attempt to correlate their analytical postulations with <br />empirical observations. One common method to accomplish this task is to back calculate <br />