2021-09-07_REVISION - M1973021 (7)

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5. Rock Mass Characterization The Precambrian granitic gneiss rock mass at Morrison Quarry is characterized using the Hoek-Brown Criteria(Hoek, Brown, 1980), Rock Mass Rating (Bieniawski, 1976), and Q- System (Barton, 1974). The evaluations are based on subsurface data collected during drilling, visual inspections made during annual inspections, and geologic reconnaissance mapping. The section describes inputs and methods used for the rock mass characterization methods. Additional information is included in Appendix C on these methods. 5.1 Hoek-Brown The Hoek-Brown failure criteria method is an empirical rock mass model that uses results of laboratory strength testing and qualitative field observations to develop non-linear strength criterion for a rock mass. The method uses the Geological Strength Index (GSI) as one parameter to characterize the rock mass. The criterion is given by the following formula: 6 a 61 = 63 + 6ci (Mb 3 + S) 6ci where mb, s, and a are the rock mass material constants given by GSI — 100 m b = mi l ex P 28 - 14D s = exp[(GSI-l00)/(9-3D)] a= 1/2 + 1/6 (e-Gsv15_ e 2oi3) Key input parameters for the generalized Hoek-Brown criteria are the intact uniaxial compressive strength (6,,,), the GSI, material constant(m,), the disturbance factor(D), and either the intact modulus (EI) or the rock mass modulus ratio (MR). The parameters for localized bench failures and global conditions are summarized in Table 5-1. The input parameters for sillimanitic gneiss are summarized in Table 5-2. Additional information on the selected rock mass parameters is contained in Appendix C.1. These recommended input parameters result in the shear stress versus normal stress failure envelopes (known as the Hoek-Brown Failure Criterion)presented in Figure 5-1. 18 Aggregate Industries—Morrison Quarry GEI Consultants, Inc. AM-07 Geotechnical Stability Exhibit November 2020