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Mogul Mine bulkhead design Page 2 July 14, 2003 <br />south-southeast from the Ridgeway Fault. It was conservatively <br />assumed that the earthquake acceleration would act along the axis <br />of the Mogul Tunnel. The tunnel axis is actually nearly <br />perpendicular to the line-of-sight toward the Ridgeway Fault which <br />should significantly reduce the maximum acceleration component <br />along the tunnel axis. <br />The No. 1 Tunnel was inspected on July 2, 2003 to the dogleg, <br />850-ft from the portal, Figure 1. No. 1 Tunnel follows a vein <br />structure exposed in the back (roof). The initial 850-ft of the <br />tunnel had very few drips of water, despite the vein, accompanying <br />fracturing and the recent melt season. The water in the tunnel is <br />flowing from mine workings further back along the tunnel. The <br />vein structure ranges from one to about five feet in width, is <br />relatively closely jointed and locally altered and clayey. The <br />tunnel walls were coated with mud to the level of water that had <br />been impounded behind a debris dam in the portal area. The debris <br />dam had been removed for access. The location selected for Mogul <br />Mine bulkhead, between Station 2+q1 and Station 2+50, is in a dry <br />and apparently impermeable rock section of the No. 1 Tunnel. The <br />geometry of the bulkhead section selected is favorable for <br />resisting the thrust from the static head and earthquake <br />acceleration because narrows toward the portal increasing the <br />bulkhead's thrust resistance. The tunnel ribsides contain joint <br />irregularities which will prevent bulkhead movement when loaded. <br />The back of the tunnel is even more irregular which will resist <br />the predicted maximum thrust. <br />The joint spacing in the back at the bulkhead site appears tc <br />average approximately 6-in and to be minimally altered. The <br />average joint spacing in the ribs is on the order of 2-ft where <br />clearly visible above the mud coating. Mud coats the lower 5-ft <br />of the tunnel walls at the bulkhead site. Where the mud was <br />__,_~qn%9. !-h,? ioint spacing am„-~.~rec+ t~ be sim i.lar_ ThF rock in <br />the back and ribs was hard and competent and the joints were tight <br />and apparently cemented by some alteration product. It was <br />difficult to bar sample blocks for coring and compression testing <br />from the roof and ribs. However, the large blocks bared from the <br />ribs were easily broken on cemented joints with a 2-lb hammer into <br />sizes that could be hand carried out the tunnel. The cementing <br />agent along the joint surfaces is weaker than the mean measured <br />11,320 psi compressive strength of the rock substance. <br />BULKHEAD DESIGN <br />The specified bulkhead concrete strength is 4000 psi in <br />compression. The concrete controls the bulkhead design because it <br />is weaker than the rock. The bulkhead was designed on the basis <br />of 3000 psi compressive strength because of the difficulties <br />inherent in pumping and placing concrete underground. Type II <br />cement is recommended because of the sulfate in the Mogul mine <br />.., <br />.'' * :: <br />