2020-05-12_REVISION - M1977410 - Laserfiche WebLink

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Grand Island Resources,LLC Page 1L Idaho Tunnel Portal—Slope Stun iy,,rzr; 2.0 PROBLEM DESCRIPTION Figure 2 presents a plan and profile of the Idaho Tunnel based on 10-ft topographic contours of the original ground surface. Superimposed on this figure are the approximate position of the rehabilitated tunnel portal following excavation. The material encountered during portal excavation and currently exposed in the wing walls consisted of regolith and colluvium, with some decomposed rock encountered at depth. The excavated slopes stood unsupported following excavation and were dry at the time (Photograph 7). The maximum height at the taller left (south) wing wall is 28 ft, sloping at an angle of 70-80 degrees from horizontal. The top of the excavation is approximately 40 ft from County Road 128 (Caribou Road) at the closest point and 20 ft lower in elevation. This creates potential long-term concerns for stability of the road. ' • • f ash ,�. ,.444' «,. Photograph 7—Idaho Tunnel Portal following excavation The excavated slope above the portal opening and wing walls were reinforced with 10-and 20-ft soil nails, 6-gauge wire mesh,and nominal 6-inches of fiber-reinforced shotcrete. Grouted threadbar spillings were also installed at 12-inch spacing above the portal. These were reportedly 35 ft long,extending into harder ground and grouted. Unfortunately, little as-built documentation is available. As of May 2, 2020, the sink hole above the collapse has crown slightly in size. Otherwise, there are no obvious signs of slope stability issues with the excavation or adjacent slopes,such as cracking or slumping, despite the occurrence of spring thaw and presence of some ground water. Locations which had been seeping water have stopped (Photograph 2) and the ground exposed in the non-shotcrete covered margins of the excavation has dried considerably.