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JOHN F. ABEL, JR. 310 LOOKOUT VIEW COURT <br />MINING ENGINEER GOLDEN, CO 80401 <br />303-27&4901 <br />FAX 278-8163 <br />JFMN7®Cmrrat~rt <br />July 31, 2003 <br />Mr. Steve Fearn <br />Silver Wing Co, R <br />P.O. Box 299 ~~jV@d <br />Silverton, CO 81433 <br />~~ <br />Reference: Bulkhead design, Koehler Tunnel ~ ?~~~ <br />Dear Steve: 0lv~S~~~a Mne~~'rd0y~~£ <br />.,fir,. <br />This letter report presents the bulkhead design for the ~s• <br />Koehler Tunnel, A 12-ft thick reinforced concrete bulkhead is <br />recommended for the 8.5-ft wide, 9-ft high tunnel section between <br />Stations 3+02 and 3+14 from the outermost portal set, indicated on <br />the Figure 1 plan view and Figure 2 cross section. The bulkhead <br />sill elevation is approximately 11,193-ft and the maximum head to <br />the Carbon Lake Shaft collar is 497-ft. The bulkhead requires a <br />two-way cage of 60 ksi #10 rebar (1.270-in diameter) on 19-in <br />centers for tensile, shrinkage and temperature reinforcement at <br />the downstream side and a two-way rebar cage of #6 rebar on 12-in <br />centers for shrinkage and temperature reinforcement at the <br />upstream side. The bulkhead will include a 4-in schedule 40 <br />stainless steel bypass pipe and gate valve to allow the mine pool <br />to be drawn down if and when required. <br />DESIGN CRITERIA <br />The purpose of the bulkhead is to impound the acidic (range <br />from a pH of 1 to a pH of 3) and sulfate bearing water (range from <br />800 to 5500 ppm) that has flowed out the portal during annual <br />cycles. During the July 18, 2003 inspection the flow rate was 9 <br />gpm. Flow rate records for 1996 - 1999 indicate peak annual flow <br />occurred within one month, either May, June or July, with a <br />maximum of 185 gpm. The maximum measured flow during the other <br />nine months of those years was 66 gpm, generally considerably <br />less. Bulkhead construction should avoid the high flow period. <br />Figures 1 and 2 indicate a direct connection between the <br />approximately 2,530-ft long Koehler Tunnel and the ground surface <br />through a 255-ft long lateral toward the south to the Carbon Lake <br />Shaft at about 2,300-ft from the Koehler Tunnel portal. The <br />maximum static hydraulic head will develop on the bulkhead if <br />water flows out at the approximate 11,640-ft elevation of the <br /> <br />