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overburden pressure. At a maximum mine pool water pressure of 44.6 psi and 26 feet of rock cover, the <br /> calculated FS against hydraulic fracture is 1.3. Another approach used to estimate hydraulic fracturing is the <br /> Norwegian Tunneling Method. This method yields a minimum cover of 47.7 feet, resulting in a 0.5 FS, <br /> insufficient for hydro jacking. If this were a new bulkhead, the hydro jacking problem would be solved by <br /> installing the bulkhead further inside the mine where the rock cover was greater. Since this is an extension of <br /> an existing bulkhead, the location is fixed. To mitigate the effects of hydro jacking in the vicinity of the <br /> bulkhead, the design includes rock mass grouting and a longer bulkhead that will provide a lower pressure <br /> gradient, as discussed below. <br /> Excessive seepage past the plug occurs when the higher upstream head finds fractures in the downstream <br /> rock mass or concrete-rock interface and bypasses the bulkhead. The worst case would be where the gradient <br /> and seepage are high enough to wash out material in joints or shears, leading to a piping failure. Seepage and <br /> piping can be avoided by placing the bulkhead in good ground, contact grouting the concrete-rock interface, <br /> and ring grouting the rock mass prior to installing the bulkhead. Based on guidance from Garrett and <br /> Campbell-Pit, 1961, the maximum allowable pressure gradient (pressure at leakage/length of bulkhead) should <br /> be between 23 and 57 psi/ft for a contact grouted bulkhead and 40 to 100 psi/ft for a grout mass grouted <br /> bulkhead. For a 6-foot-long bulkhead with a maximum mine pool pressure of 44.6 psi, the pressure gradient is <br /> 7.4 psi/ft, well within the maximum allowable ranges. Using a mid-range value of 40 psi/ft, the FS is 5.4 The <br /> seepage calculation does not consider water hammer since water hammer is a transient load. <br /> Structural failure occurs when the concrete plug itself fails due to deep beam bending or shear failure through <br /> the reinforced concrete due to water pressure or earthquake induced water hammer. It can be avoided by <br /> making the bulkhead long enough and adding rebar reinforcement at both faces. The Times Mine Bulkhead <br /> structural strength can be calculated per ACI 318 deep beam analysis, as described in Abel 1998. Based on <br /> our calculations the FS for the bulkhead under hydrostatic load is 4.0. The FS for the bulkhead under <br /> hydrostatic load with water hammer is 3.5. <br /> Failure Mode at Calculated Factor of Safety(FS) <br /> Maximum Pressure Based on design water head = 103 ft <br /> 8 ft 6 ft Existing <br /> Punching Shear 10.0 7.7 5.3 <br /> Punching Shear with <br /> 8.0 6.2 4.1 <br /> Water Hammer <br /> Hydraulic Jacking (Abel) 1.3 1.3 1.3 <br /> Hydraulic Jacking <br /> 0.5 0.5 0.5 <br /> (Norwegian) <br /> Seepage around <br /> 7.1 5.4 0.7 <br /> Bulkhead <br /> Flexural Bending Failure 5.3 4 1.9 <br /> Flexural Bending with <br /> 4.7 3.5 1.7 <br /> Water Hammer <br /> Project 22690002.000 7/18/2022 Page 3 Schnabel Engineering,LLC <br />