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MEMORANDUM <br />FROM: Kent Pease, P.E. <br />TO: File <br />DATE: January 22, 2008 <br />SUBJECT: Parkdale Quarry Slope Stability Summary <br />This memorandum summarizes results of slope stability evaluations and calculations for the proposed <br />granite quarry of Front Range Aggregate's Parkdale Quarry. Calculations by Lyman Henn evaluated the <br />stability of rock slopes and the stability of fill placed on individual benches as part of mine reclamation. <br />Calculation sets for both these evaluations are attached. A summary of Lyman Henn recommended <br />approach to slope stability, is included in our report presenting the maximum build out configurations <br />for the proposed mine and intermediate phases. <br />Rock Slopes <br />The rock slopes were evaluated based the kinematic stability of rock blocks, slabs, and wedges defined <br />by the rock joint patterns. Evaluation of rock mapping data by Lyman Henn and J A Cesare shows that <br />the most prevalent joints are vertical to sub-vertical in two to three sets. Additionally, several sets of <br />random joints at different strike orientations and dip angles were noted to be present. Most of the joints <br />were determined to be fresh to slightly weathered, and moderately rough to stepped. With these <br />characteristics, the joints were determined to have a friction angle of between 39 and 50 degrees. <br />For the vertical and sub-vertical jointing, there is the potential for toppling and sliding of slivers, but <br />these are expected to be limited to individual benches and highwalls. Large scale instabilities involving <br />multiple benches and the overall quarry slopes are not expected as a result of these joints. <br />For the non-vertical random jointing, there is the potential for sliding of blocks, slabs, and wedges in <br />situations where the potential slide angle is steeper than the joint friction angle. This condition is not <br />likely in the upper portion of the quarry, above elevation 5800 where the quarry walls are inclined at a <br />dip angle of 39 degrees; which is the lower bound of the likely joint friction angle. For the lower <br />portion of the quarry, below elevation 5800 the quarry walls are steeper, with a dip angle of 51 degrees <br />and rock instabilities are kinematically possible. However, for a slide to occur, the joint would have to <br />be relatively continuous and connected to other release joints to isolate a free block or wedge. Because <br />these joints are random, it is not expected that these conditions would be persistent throughout the <br />quarry. As such, there could be isolated areas of slope instability, but it is not expected that the random <br />joints would result in slope instabilities on a large scale that would cause persistent problems. <br />For any rock mass there is the possibility of large scale random joints with a low strength such as from <br />historic sliding, weathering, or clay infilling. If such a joint or several joints exist and if these joints