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5 t <br />SMITH WILLIAMS CONSULTANTS, INC. <br />• Surface disturbances from caving of the underground workings are visually evident in some <br />areas of the Phase 5 VLF area. These surface disturbances are primarily evident near the <br />existing substation and on Squaw Mountain. In many areas, the underground excavations remain <br />open or are collapsed. Some of the mine workings near the substation (e.g. Thompson and Kitty <br />Lane workings) were partially backfilled as part of the Phase IV VLF construction <br />Appendix D presents a site reconnaissance of the underground mine workings within the Phase 5 <br />VLF area, which was conducted for the Phase IV VLF. Remediation of underground workings <br />will be required to ensure the stability of the foundation of the Phase 5 VLF, as was completed <br />for Phase IV and submitted as Amendment No. 8. As part of the Phase 5 VLF design effort, <br />Smith Williams undertook additional detailed evaluations of the area. The primary activities that <br />were completed included the following: <br />^ A detailed review of the available historical records, including the historical mining <br />plans filed at CC&V and in the public record. <br />^ A review of the Phase IV VLF site reconnaissance and remediation. <br />These data, combined with the experience gained during the construction of Phase IV, were used <br />to develop the mine working remediation guidelines. The general approach to remediate the <br />mine workings is as follows: <br />For open excavations where the depth to bedrock is less than 7 feet below the final <br />SLF subgrade surface: Backfill open shafts with coarse shaft backfill to <br />approximately 10 feet below the SLF subgrade surface and then place concrete, <br />cemented rock fill, and structural fill to achieve SLF subgrade surface. <br />For open excavations where the depth to bedrock is greater than 7 feet below the final <br />SLF subgrade surface: Backfill the open shafts with coarse shaft backfill to 3 feet <br />below the soil/bedrock interface: Then place concrete, cemented rock fill, and <br />structural fill. <br />^ For collapsed excavations where the depth to bedrock is less than 7 feet below the <br />final SLF subgrade surface: Excavate collapsed material from shafts to a depth of <br />25 feet below the SLF subgrade surface, then backfill with coarse shaft backfill to <br />approximately 10 feet below the finished grade, and then place concrete, cemented <br />rock fill, and structural fill. <br />S:\PROJECTS\1125 CCBV PHASE 5 VLF\H2 - DESIGNV\REQUA VLF\VLF PHASE 5 FINAL REPORT V2.DOC I S <br />