Numerical Modeling Methodology for Weak Rock Masses in Nevada gold mines
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Many Nevada gold deposits are found in highly fractured and faulted host rock with RMR ranging from 10 to 50. Various ground control design issues in weak rock masses are faced by most underground mines operating in Nevada. Numerical models complement field observations, instrumentation techniques, and testing, by providing a better insight into the fundamental behavior and response of weak rock mass and its interaction with support systems. It facilitates development of a better design methodology and guidelines for excavation span and support design. MPBX arrays were installed in underground excavations and the displacements at MPBX nodes were observed for previous numerical model calibration. Time dependent numerical models were designed to take into consideration the effect of jointing using discrete fracture network (DFN) and rock bolts in 3DEC. Sensitivity studies were performed on over 200 time dependent numerical models to understand the effect of various rock support design parameters on different excavation sizes and depths. Rock bolt length and spacing was varied for numerical models to understand the effect of each parameter on to stability of the underground excavations. A strain dependent criterion is considered for predicting squeezing instability in the numerical model at the end of 4-year period. Over 200 numerical models were developed as a part of this study for studying the effect of various rock parameters, joint parameters, support design parameters, excavation size and depth parameters. Rib squeeze of 90 mm in each rib for a 12x12 ft opening which is equivalent to 4.65% maximum principal strain rate in the semicircular region in ribs is chosen as cut-off point for defining instability in squeezing ribs at the end of four-year period. Among the rock and joint modeling parameters, Young’s modulus, Poisson’s ratio and joint normal stiffness are critical for time dependent rock squeezing. Modulus numbers for rock in particular are critical. The rock squeezing over time increases at lower value of Young’s modulus leading to squeezing instability in underground excavations.