Laserfiche WebLink
Grand Island Resources,LLC Page 11 <br /> Idaho Tunnel Portal—Slooe Stability Analysis <br /> 3.0 STABILITY ANALYSIS <br /> 3.1 Approach <br /> The stability analyses were conducted as two-dimensional limit-equilibrium analysis using commercially <br /> available software. Three cases were considered at which the slope was observed to be stable and <br /> therefore must exhibit a Factor of Safety(FoS)greater than unity;at the end of excavation,during spring <br /> thaw, and with an open void present. For the end of excavation scenario,the slope reinforcement was <br /> neglected in the analysis and the slope was assumed to be fully drained. For the spring thaw scenario, <br /> the presence of groundwater in the slope was considered in the analysis. The open collapse void and <br /> backfilled void were considered separately. <br /> The actual FoS should be higher than the results presented for 2D analyses section due to the concaved <br /> slope orientation and 3D edge effects. Studies have shown that these 3D effects can become significant, <br /> often increasing the FoS by 10-20 percent, or even morel. This effect tends to become more significant <br /> as the amount of slope curvature increases, particularly as the ratio of the slope width to slope height <br /> drops below 3. In the case of the Idaho Tunnel Portal,the excavation has a relatively narrow open width <br /> of approximately 30 ft at the base of the mouth of the excavation relative to a height ranging from 15 to <br /> 28 ft. These effects have been considered qualitatively in the results discussion. <br /> 3.2 Software <br /> The stability analyses were conducted using the RocScience SLIDE2 software,a 2D slope stability program <br /> for evaluating the safety factor or probability of failure,of circular and non-circular failure surfaces in soil <br /> or rock slopes. Slide2 analyzes the stability of slip surfaces using vertical slice or non-vertical slice limit <br /> equilibrium methods like Bishop, Janbu, Spencer, and Sarma, among others. Search methods can be <br /> applied to locate the critical slip surface for a given slope. The Bishop method of slices for circular failures <br /> surfaces while the Janbu method of slices for satisfying both moment and force equilibrium was adopted <br /> for non-circular surfaces. <br /> The Slide2 software also allows the effects of slope reinforcement to be included in the analyses. <br /> 3.3 Model Input <br /> 3.3.1. Slope Geometry <br /> An idealized representative two-dimensional cross-section was considered for analysis. This section <br /> consisted of the profile along the axis of the tunnel included on Figure 2, at the maximum cut slope on <br /> the left(south) side of the portal excavation. The idealized slope consisted of a 28-ft high excavation at <br /> an angle of 75-degrees then natural ground sloping at approximately 40 ft to the edge of the 20-ft wide <br /> County Road. Figure 3 presents the idealized slope stability cross-section superimposed on the tunnel <br /> profile section. <br /> Included on this figure is the assumed material distributions as described in the following section. <br /> 2 Zhang,Y.,Chen,G.,Zheng,L,Li,Y.,and Zhuang,X,.2013;"Effects of geometries on three-dimensional slope stability."Canadian <br /> Geotechnical Journal.Vol. 50,No.3,pp.233—249. <br /> Applied Geot.ogic LLC 5/7/2020 <br />