2011-08-09_REVISION - M1981185 (52)

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Geotechnical Stability Exhibit Exhibit 6.5 Erosion and sedimentation BMPs will be installed prior to excavating or disposing the material in the designated disposal area. An augmentation pond diversion structure is be designed to contain approximately 5,000 gallons and designed to release excess flows through a 12 -inch HDPE gate valve dewatering pipe. Water will be discharged to a rip rapped channel constructed below the diversion structure which will discharge to the existing Idaho Spring channel. Adjacent to the diversion structure, a small pump house will be constructed to gravity drain water from the diversion structure to the augmentation pond through a 12 inch HDPE pipe (Attachment D -5). Water will be pumped from the augmentation pond to the diversion structure using a 4 -inch HDPE pipe to comply with SEO water right requirements. Geotechnical and hydrogeologic characteristics will be defined prior to finalizing the augmentation pond design and constructability work plan. Material characteristics will be determined to a depth of 10 feet below pond embankments, including strength, hydraulic conductivity, classification (Atterberg limits and grain size distribution), density and moisture content. The dead storage pond area will be constructed below ground surface where the live storage area will not extend more than five feet above ground surface. The dam height will be less than 10 feet thereby eliminating DWR's jurisdictional dam construction requirements. Since the dam embankment height will be less than 10 feet, the concern for dam failure through an engineered constructed berm is not probable. La Plata River Bridge • La Plata River Bridge is currently designed to support an AASHTO truck loading of 15 tons. (Attachment 6.5 -3). As the New Access Road will have a rating of 25 tons, the bridge requires to be upgraded. In order to consider a safety factor, the bridge will be upgraded to support an AASHTO truck loading of 25 tons (Sakura, 2009). Construction activities will be limited to reinforcing the bridge deck with steel I -beams and installing pedestal supports. As -built bridge drawings will be prepared and submitted 30 days after the bridge has been upgraded; and • The bridge is currently 36 feet long, 11 feet wide with four 12 "x12" beams. The bridge supports are approximately 33 feet on center. The bridge is capable of supporting an AASHTO truck loading type H15 -44 which is 6,000 pounds on the front axle and 24,000 pounds on the rear axle spaced 14 feet on center. Off Site Stability Impacts On the basis of our observations and engineering analyses, we conclude that there are no areas adjacent to the permit area where potential off -site stability impacts could occur. There are no geologic structures or earthen facilities that have the potential to impact offsite locations. The closest adjacent properties considered in the assessment include the area parallel to the access Wildcat Mining Corporation May Day Idaho Mine Complex — 112d Permit Application Revised August 2011 Geotechnical Stability Exhibit 6.5 Page 9of