GENERAL55767 - Laserfiche WebLink

Home Browse Search

Koehler Tunnel bulkhead design Page 2 July 31, 2003 shaft collar elevation. The maximum static hydraulic head is 497-ft above the bulkhead, a hydraulic pressure of 215 psi. The design maximum 0.136 g horizontal earthquake acceleration was calculated from the maximum credible 6.5 magnitude earthquake along the Ridgeway Fault, 20 miles to the north. Previous earthquake studies for the Sunnyside Gold Corp. tailings at Gladstone (Sullivan, et al, 1980; Weisser, 1982; Knight Piesold Co., 1988) predicted a maximum credible horizontal acceleration of 0.087 g at Gladstone, 25 miles south-southeast from the Ridgeway Fault. It was conservatively assumed that the earthquake acceleration would act along the axis of the Koehler Tunnel. The tunnel axis is actually at a large angle from the line-of-sight toward the Ridgeway Fault which should reduce the maximum earthquake acceleration component along the tunnel axis. The Koehler Tunnel was inspected on July 18, 2003 to a distance of approximately 600-ft from the portal, Figure 1. The Koehler Tunnel was driven S74°E. The initial 600-ft of the tunnel was dry except for the water flowing along the invert, despite two steeply dipping joint sets. The water in the tunnel is flowing from further back in the tunnel. One of the steeply dipping joint sets is nearly vertical and is sub-parallel to the tunnel axis. Individual joints of this joint set locally form the entire right tunnel rib for lengths of up to 20-ft. The other joint set dips roughly 65° to the west and strikes nearly perpendicular to the tunnel axis. This joint set forms the roughly 100-ft high dip slope cliff behind the portal. The tunnel walls were coated with a generally thin layer of red mud to the level of water that had been impounded behind a debris dam in the portal area before the debris dam was removed for access. An up to 1-ft thick layer of red mud was resting on the tunnel floor, except where disturbed by walking in the tunnel. The location selected for the Koehler Tunnel bulkhead, between Station 3+02 and Station 3+14, is in a dry and apparently impermeable rock section of the tunnel. The ribsides and back at the bulkhead section selected are irregular and contain only short segments of the sub-parallel joint set. The joint spacing in the back and ribs at the bulkhead site appears to average approximately 1-f t, or more, in all directions and to be minimally altered. The irregularities in the back and ribs are more than sufficient to resist the thrust from the maximum static water head and maximum credible earthquake acceleration. The slight narrowing of the tunnel toward the portal within the selected bulkhead location will increase the bulkhead's thrust resistance. The porphyritic rock appears to be extremely competent and hard. Block samples were barred from the back and ribs at the bulkhead site for strength testing and have been sent for compressive strength testing.