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MEMORANDUM <br />FROM Kent Pease PE <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 Aggregates 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 subvertical 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 subvertical 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 nonvertical 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