24-MMP-A5 Surface Mining Methods and Design · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Mining and Mineral Processing Engineering, 09-Mmp-A5 Surface Mining Methods and Design, 2013-May. 3 hours duration; one handwritten 8.5×11 in reference sheet permitted (not an open-book exam); only approved Sharp or Casio calculators allowed. Question 1 is compulsory (40 marks, parts 1.1–1.7); candidates then select FOUR of the six optional Questions 2–7 (15 marks each) to complete the paper.
Reference texts: Hartman & Mutmansky, SME Mining Engineering Handbook, 3rd ed. (dewatering, slope stability classification, dragline stripping geometry, truck dispatch, mine closure); Hustrulid, Kuchta & Martin, Open Pit Mine Planning and Design (moving-cone and Lerchs–Grossmann pit optimization, capital-cost estimating, truck-shovel match factor); Lerchs, H. & Grossmann, I.F. (1965) “Optimum Design of Open-Pit Mines,” CIM Bulletin (the graph-theoretic 2-D worked example this question is drawn from); O’Hara, T.A. (1980) “Quick Guides to the Evaluation of Orebodies,” CIM Bulletin, Feb. 1980, and Mular, A.L. & Poulin, R. (1998) CANCOST, CIM Special Volume 47 (capital-cost formulae); Bieniawski, Z.T. (1989) Engineering Rock Mass Classifications (RMR system).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
1. Bieniawski RMR (2 marks). The Rock Mass Rating sums five intrinsic parameters – uniaxial compressive strength of intact rock, RQD, discontinuity spacing, discontinuity condition (roughness, infill, weathering, aperture) and groundwater condition – each scored on its own table, then adjusted by a discontinuity-orientation correction relative to the excavation. The result is a 0–100 index grouped into five classes from “very good” (81–100) to “very poor” (<21) rock. RMR was developed for tunnel support design, and its stated slope-angle guidance (roughly $\text{slope angle} \approx \text{RMR} - \text{constant}$, e.g. an empirical band of 35–85° across the classes) is a coarse, first-pass planning tool only – it is applicable to slope stability as a screening index but is not a substitute for a limit-equilibrium or numerical stability analysis using the actual discontinuity orientations, because RMR’s orientation correction is calibrated for underground openings, not for the specific kinematics (planar, wedge, toppling) that govern a pit wall.
2. Slope Mass Rating (SMR), a system familiar for pit-wall use. SMR (Romana, 1985) is the slope-specific derivative of RMR built exactly for this purpose: $SMR = RMR_{basic} + (F_1 \cdot F_2 \cdot F_3) + F_4$.
(a) Three-dimensional discontinuity/slope relation. Factors $F_1$ (parallelism between discontinuity strike and slope face strike), $F_2$ (discontinuity dip, tied to shear-strength mobilization) and $F_3$ (relation between discontinuity dip and slope dip – whether the joint daylights favourably or unfavourably) together capture the full 3-D kinematics of planar and wedge failure, distinguishing a joint that merely exists in the rock mass from one that is actually oriented to slide out of the face.
(b) Groundwater. SMR’s $RMR_{basic}$ term already carries Bieniawski’s groundwater-condition parameter (dry through flowing, scored 0–15); a wet or flowing discontinuity lowers $RMR_{basic}$ directly, because pore pressure on the failure surface reduces the effective normal stress and hence the mobilized shear resistance holding the wedge or plane in place.
(c) Blast damage. $F_4$, the method-of-excavation adjustment, adds a bonus for pre-split/smooth-wall blasting and a penalty for poor (production-style) blasting, recognizing that fly-rock overbreak, excessive powder factor, short delay timing between adjacent holes and inadequate stand-off distance from the final wall all create new fractures and loosen the existing joint set immediately behind the face, degrading stability independent of the natural rock mass quality.
(d) Spatial variability with elevation and sector. Because SMR (like RMR) is scored bench-by-bench and sector-by-sector around the pit rather than as one pit-wide number, a mine divides its walls into structural domains by lithology, alteration and structural orientation, and re-scores each domain independently at each elevation – so a competent, dry upper bench and a sheared, wet lower bench in the same wall carry different design slope angles rather than one averaged value.
