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Memo to Bill Carter <br />Sanborn Creek - New Pile Preliminary Review <br />page 3 <br />If OXBOW can complete the excavation and determination prior to final <br />application I recommend that they do so. If they cannot, I recommend that <br />the Division address this concern by placing a stipulation upon the <br />approval. That stipulation should require OXBOW complete the excavation <br />and determination of depth to competent bedrock upon mobilizing to the <br />site. Once the determination is completed the plans should be amended to <br />ensure foundation of the concrete sediment wall on bedrock. In the event <br />that depth to bedrock is great enough to favor an alternative construction <br />method, OXBOW shall provide an amended plan design to the Division for <br />review. In no case shall OXBOW proceed with construction of the concrete <br />pond wall until approval is obtained from the Division. <br />Potential Impact of Previous Underground Workings <br />You asked me to consider whether the possible subsidence of previous <br />underground workings within the historic underground of the Somerset <br />Mine might potentially impact the proposed Sanborn Creek Waste Pile. <br />Examination of the latest extent of mining maps for the Somerset Mine <br />and the Sanborn Creek Mine determined that no underground workings <br />existed close enough to the proposed waste pile footprint to potentially <br />impact the pile. <br />In order to prevent potential impacts due to future workings of the <br />Sanborn Creek Mine the Division should stipulate that no underground <br />extraction will be approved in the future which could potnetially subject <br />the pile to subsidence settlement. <br />Recommendation <br />With the imposition of the two above stipulations, I would recommend <br />approval of the geotechnical aspects of the proposed new Refuse Pile at <br />the Sanborn Creek Mine. <br />