Safety Factor Design Analysis: Integration of Bolts, Mesh, and Shotcrete Support in Weak Rock Masses, Turquoise Ridge Mine, Nevada

"CURRENT BASIS OF GROUND SUPPORT DESIGN ANALYSIS The current strategy used in determining what kinds of support are needed is based on empirical design charts (Grimstad and Barton, 1993). Figure 1 is an example of the topcut analysis used for a 14 foot wide (4.3 meter) drift, which is the current design width used for topcuts at TRJV mine. The majority of our topcuts are situated in ore of the Type IV and Type III weak rock class categories that necessitate the use of bolts, mesh and 2”-4” of shotcrete. This type of empirical chart gives excellent guidelines for the kinds of support, and it is based on case histories of civil tunneling and mining properties. It is a good starting point for determining the kinds of support needed, but not to the detail needed to optimize the number of bolts, type of mesh, or combination of types or strengths of shotcrete. This report is an attempt to help quantify the safety factor and optimum design parameters to include mesh and shotcrete as part of the design criteria and subsequent safety factor.SAFETY FACTOR ANALYSIS: BOLTS ONLY One type of safety factor analysis used at TRJV for the past 7 years is described as the “dead weight” method, developed by Pakalnis (2008). It involves the support of a conservative “dead weight” of rock held in place by bolts in which the depth of failure is approximated as a wedge whose height is one-half of the span. The failure or dead weight wedge resembles a triangular wedge that must be supported by the remainder of the bolts going through the wedge (see Figure 2).The support capacity of the system is determined by the bond strength of the bolts holding up the wedge, which is in turn based on the RMR. The method assumes the total support capacity is the sum of the individual bolt values defined by either the breaking strength or the bond strength of the bolts (whichever is lower) passing through the failure surface."