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Cross Mine(M1977-410) Technical Revision No.5 <br /> 5. Description of the Proposed Technical Revision <br /> 5.1. Portal Collapse Repair <br /> In late February 2020,two additional new steel sets were installed in the area of the tunnel portal beneath <br /> the collapse and lined with lagging on the ribs and back. As with the initial two steel sets,these consist of <br /> W6 x28 wide flange steel beams and posts installed on 4-ft center-to-center spacing. Lagging consists of <br /> 3-in.x 8-in. treated Douglas Fir planks. This design and installation were inspected and approved by Mr. <br /> David Hallman,a geological engineer with 37 years of experience and registered as Colorado Professional <br /> Engineer No.26076. <br /> The narrow gap between the new ground supports and the existing ground has been closed using pieces <br /> of lagging, plywood, polyurethane foam, and caulk to create a tight seal. The remainder of the void will <br /> be completely backfilled with pervious cellular concrete to provide permanent support to the opening <br /> and overlying slope.The remaining void is estimated to be approximately 65 cubic yards. <br /> 5.2. Tunnel Rehabilitation <br /> During rehabilitation,the tunnel will be enlarged to a nominal 10-ft. x 10-ft.opening in order to replace <br /> the existing ground support which is failing and remove the loosened rock surrounding the present <br /> opening. The enlarged tunnel opening will be supported by steel sets installed at 4-ft center-to-center <br /> spacing with full lagging on the back and ribs. The steel sets will consist of W6x28 wide-flange I-beams <br /> and support posts. The lagging will consist of 3-in.x 8-in. Douglas Fir planks. Figure 2 depicts the typical <br /> portal ground support to be employed. <br /> Figure 3 depicts a 1954 geologic map' of the initial portions of the Idaho Tunnel. GIR has explored the <br /> first 200 ft of the Idaho Tunnel in order to investigate the corresponding ground conditions that can be <br /> anticipated during the rehabilitation efforts. Starting from the back of the last steel set, the existing <br /> ground support consists of timber sets with full lagging on the back and ribs for the next 41 ft, followed <br /> by rock bolts and chain link mesh. The timber sets retain loose soil and rock, obscuring the undisturbed <br /> intact ground. Loose material has also fallen onto much of the chain link and pulled it from the roof in <br /> places. The ground mass and rubble observed consists of granular fragments of decomposed monzonite <br /> transitioning more to angular pieces of highly fractured weathered rock with increasing distance into the <br /> tunnel. On average,this portion of the tunnel has collapsed to a section of roughly 9 ft x 9 ft or 10 ft x 10 <br /> ft. In several areas almost complete collapse has occurred, including one section in decomposed rock <br /> which has created an open 12 ft x 12 ft void above the tunnel horizon approximately 110-125 ft into the <br /> tunnel. <br /> At approximately 200 ft from the new portal there is a collapse after which,the tunnel exhibits a more <br /> regular 6 ft x 6 ft opening, there is no visible ground support and the rock exposed in the sides of the <br /> tunnel(ribs)appears fresh and less fractured. This portion of the tunnel will also be enlarged to a 10-ft.x <br /> 10-ft. opening such that where necessary,the same ground support elements can be used. There is 2.5 <br /> to 3 ft of standing water in the tunnel beyond this point(Photograph 4)due to the muck pile which, as a <br /> result,GIR has not gone further into the tunnel. <br /> 1 Moore,F.B.,Cavender,W.S.,and Kaiser,E.P., 1954;"Geology and Uranium Deposits of the Caribou Area,Boulder <br /> County,Colorado."US Geological Survey Trace Elements Investigations Report 228,March 1954 <br /> 7 <br />