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Byron map). The Wynona Shaft is located next to the CIVIC mill. The shaft collar has been backfilled, leaving <br /> only a pipe casing and manhole at the surface. Parts of the Wynona Mine are shown on the 90% design plans. <br /> The mine is not accessible. <br /> In 1986, the mine owners installed a 3-foot-thick concrete bulkhead into the Times mine as shown on the 90% <br /> design plans. The intent of the bulkhead was to enable the use of the Times and Wynona Mines to store water <br /> pumped up from Left Hand Creek. In 2019, Deere &Ault Consultants (now Schnabel Engineering) evaluated <br /> the capacity and status of the bulkhead for CIVIC in response to questions by DRMS. In the evaluation, D&A <br /> concluded that the bulkhead was adequate for the CIVIC proposed maximum operating elevation head 8364.5 <br /> ft. D&A also concluded that the bulkhead was not adequate for the maximum possible water level where the <br /> water flows out of the Wynona Shaft collar(8445 ft). In the Adequacy Review No. 4, DRMS approved storage <br /> only to the bulkhead design elevation head of 8360 ft. <br /> In 2022, Schnabel Engineering was hired to design an extension to the existing bulkhead that would function <br /> adequately under the maximum possible pressure. This extension would use the existing bulkhead as the back <br /> form to avoid disturbing the mine pool. <br /> SUMMARY OF DESIGN CALCULATIONS <br /> When designing or evaluating mine bulkheads, one must consider the possible failure modes. These include <br /> shear failure around the plug, hydraulic jacking of the surrounding rock mass, excessive seepage or piping <br /> past the plug, and structural failure of the plug (Lang, 1999; Abel, 1998). Seven scenarios were considered in <br /> the design of the new bulkhead. The discussions refer to the 6-foot-long bulkhead shown on the 90% design <br /> plans. The summary table also includes an 8-foot-long bulkhead and the existing bulkhead. Note that the <br /> bulkhead extension designs do not consider the contribution of the existing bulkhead. <br /> Perimeter shear or punching shear failure occurs when the bulkhead moves along the concrete/rock interface <br /> or adjacent rock due to water pressure from the mine pool. This failure can be avoided by locating the <br /> bulkhead in good ground, roughening the surface at the rock/concrete interface, keying into the rock, and <br /> grouting the interface. The shear capacity of the Times Mine Bulkhead can be calculated by multiplying the <br /> shear strength of the concrete by the total area that the concrete is in contact with the wall rock. Unless the <br /> ground is heavily altered, the weakest shear strength is in the concrete. At the Times Mine, the vein and <br /> hanging wall are heavily altered. Our proposed bulkhead design requires the removal of altered material. This <br /> will be possible in the hanging wall but not on the vein. This will still leave most of the perimeter as sound rock. <br /> Based on our calculations, the factor of safety (FS) against punching shear under hydrostatic load is 7.7. The <br /> FS for punching shear under hydrostatic load with water hammer(shockwave due to earthquake) is 6.2. <br /> Hydraulic jacking or hydraulic fracturing/fracking occurs when the water pressure behind the bulkhead is higher <br /> than the confining pressure of the ground in the area. The hydraulic jacking causes joints in the rock mass to <br /> open up, allowing more flow through them. This can be avoided by locating the bulkhead deep underground <br /> where the confining pressure (weight of rock above and related horizontal stresses) is high enough to resist the <br /> water pressure. There are two typical approaches to calculate the hydraulic fracturing potential. Abel (1998) <br /> suggests that under typical circumstances, the pressure required to hydraulically jack the rock is twice the <br /> Project 22690002.000 7/18/2022 Page 2 Schnabel Engineering,LLC <br />