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Grand Island Resources, LLC Page 13 <br />Idaho Tunnel Portal – Slope Stability Analysis <br />Applied GeoLogic LLC 5/7/2020 <br />3.3.2. Material Distribution <br />An “Alluvial Rock” soil unit was assumed to comprise the first 40 ft of the original tunnel as depicted on <br />Figure 2. Based on the rock materials exposed at the base of the portal excavation (Figure 7) and currently <br />exposed in the tunnel ribs (Photographs 3 and 4), this is a conservative assumption as at least some <br />portions of this interval will include decomposed or weathered rock. <br />For the stability analysis, the “Decomposed Granite” unit was assumed to comprise the next 33 ft at the <br />tunnel horizon. The transition from “Decomposed Granite” to “Weak Hard Rock” was modeled to coincide <br />with the change in the type of ground support used in the tunnel. This is slightly further into the slope <br />than the geology depicted on Figure 1 and therefore, more conservative. <br />This layered profile was then carried up the height of the slope for the stability analysis section as depicted <br />on Figure 3. In reality, these layers are likely thickest at the toe of the slope down at the portal level and <br />taper in thickness moving higher up the slope and this assumption will also be conservative. <br />3.3.3. Material Properties <br />The analyses incorporated shear strength parameters for the soil material, decomposed rock and weak <br />weathered rock mass separately. Since the slope height is not great, the shear stresses will be low. For <br />the low range of stresses present, equivalent linear Mohr-Coulomb shear strength parameters were <br />assumed. <br />During excavation the regolith and colluvium “Alluvial Rock” unit was observed to stand near-vertical for <br />up to 28 ft without ground support. From an engineering perspective this material consists of poorly- <br />graded sandy gravel with cobbles, silt and clay (GP). For the purposes of the stability analysis this material <br />was assigned a friction angle of 38 degrees and 500 psf cohesion with a moist unit weight of 125 pcf. <br />Areas which contain a higher proportion of coarse rock fragments will exhibit higher shear strength, and <br />the overall average strength is likely higher, however, if failure were to occur it will tend to pass through <br />the weaker materials which offer less resistance. <br />From an engineering perspective the “Decomposed Granite” unit consists of rock which has been <br />weathered and decomposed in situ, but has not been disturbed and retains the original rock fabric. This <br />material represents a weak rock mass for which the Hoek-Brown criterion3 was used to estimate the <br />average rock mass strength across this material based on a large body of empirical data. Assumed rock <br />mass parameters for Decomposed Rock: <br />Intact Rock UCS = 1000 -2000 ksf (7,000 – 14,000 psi) <br />GSI = 15 (Disintegrated with highly weathered surfaces with soft clay coatings or infilling) <br />mi = 25 <br />D = 0 <br />3 E.Hoek and E.T.Brown, 2018; “The Hoek–Brown Failure Criterion and GSI – 2018 Edition.” Journal of Rock Mechanics and <br />Geotechnical Engineering, Volume 11, Issue 3, June 2019, Pages 445-463