Question 25 of 27

Question 5.2 mine planning and scheduling — waste schedule (13 marks)

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.

Given. Waste reserve by elevation and phase (Figure 5.2.1, Mt); pre-production waste 6 Mt in Year −1, 12 Mt in Year 0, up to 12 Mt/yr thereafter; the completed ore schedule from Question 5.1.1.

Approach. Apply the same NW-corner sweep to the waste reserve, but at the STATED pre-production/annual rates (6, 12, then up to 12 Mt/yr), continuing Phase 1→Phase 2 as each is exhausted, and cross-check every year’s waste against that year’s ore for the “waste ≤ ore” guidance.

1. 5.2.1 — completed waste schedule:

$$\boxed{\text{Waste mining spans Years } -1 \text{ to } 6 \text{ (84 Mt total), finishing 2 years AHEAD of ore (which runs to Year 8)}}$$

5.2.2 — elevations mined when Phase 1 deep ore is mined. Phase 1’s deepest ore (elevations 1925/1910) is mined in Year 4 (Question 5.1.1). At that same time the waste schedule (Year 4) is working elevations 1955→1940 in Phase 2 — SHALLOWER than the ore being mined, confirming waste stays properly ahead of/above the ore it exposes. Safety: waste blasting at these higher, Phase 2 elevations directly above active Phase 1 ore shovels (the scenario the paper’s introduction specifically warns about) risks fly-rock and fragment roll-down onto working equipment below; mitigation is blast-timing exclusion zones (no ore-shovel operation during and immediately after an overlying waste blast), catch berms on intermediate benches, and geotechnical confirmation that inter-bench separation is adequate. Ore tonnage impact: a properly-managed exclusion window causes at most a short, planned stoppage of the Phase 1 shovel, not a lost tonnage event, provided blasts are scheduled outside the shovel’s active loading window.

5.2.3 — truck-hours over mine life. Truck-hours track total material moved (ore+waste) per year, so the sketch rises through the heavy Years −1 to 5 (12 Mt/yr waste plus 7 Mt/yr ore once ore starts) then drops sharply in Years 6–8 once waste is exhausted and only ore continues.

To hold truck-hours roughly CONSTANT, the schedule could smooth the transition by deliberately spreading a small residual waste tail into Years 6–7 (mining slightly under 12 Mt/yr in the peak years and carrying a little waste later) rather than running waste flat-out to exhaustion in Year 6, avoiding the abrupt fleet-utilisation drop the current NW-corner schedule produces.

5.2.4 — used trucks and productivity effects. Purchase/rental of used trucks COULD help smooth the fleet through the Years −1–5 peak without committing capital to new units for a temporary demand bulge, provided the used units’ remaining component life and support/parts availability are adequate for the (roughly 6–7 year) peak-demand window. Time reduces truck productivity through progressive component wear (tyres, engine, transmission) raising unscheduled downtime as hours accumulate (Question 1.1); depth reduces productivity because deeper pit levels mean longer, steeper (adverse-grade) haul routes to surface destinations, lengthening cycle time and lowering payload-per-hour even with unchanged truck condition — both effects argue for periodically re-assessing fleet size against the CURRENT (not original) cycle-time and availability, not just against the original schedule’s truck-hour plan.

5.2.5 — annual stripping ratio. Waste/ore by year (from the two schedules): Years 1–5 each carry 12 Mt waste against 7 Mt ore: $$SR_{max} = \dfrac{12}{7} \approx \boxed{1.71}$$ Year 6 carries only the residual 6 Mt of waste against 7 Mt ore (SR=0.86), and Years 7–8 carry ZERO waste (already exhausted) against 7 Mt and 1 Mt ore respectively: $$SR_{min} = \boxed{0 \text{ (Years 7\text{-}8, once waste is exhausted)}}$$ Lifetime average SR = 84/50 = 1.68. Knowing the SR profile lets the planner see the fleet-hours imbalance coming (5.2.3) well in advance and plan either a used-truck bridge (5.2.4) or a deliberately-smoothed waste tail, rather than discovering the Year-6 cliff only when it arrives.

5.2.6 — sequence check. Years −1 and 0 mine ONLY waste (zero ore) — by design, this is the pre-production stripping period, not a rule violation. Checking every ore-production year against its co-scheduled waste confirms waste NEVER lags behind the ore it exposes anywhere in this NW-corner schedule (Question 5.2.2) — so no scheduling rule is broken. The one notable feature, not a violation, is the REVERSE situation: waste finishes in Year 6, two full years before ore (Year 8), meaning Years 7–8 mine ore with no waste-stripping activity that year at all — a safe surplus of exposure, not a rule breach, but worth flagging as the schedule’s one soft spot (fleet under-utilisation, Question 5.2.3).

5.2.7 — Phase 2 above Phase 1. Yes: in Year 4 the schedule has Phase 1 ore shovels working the deep 1925/1910 benches WHILE Phase 2 begins at the much higher 1970/1955 elevations directly above — exactly the geometry the paper’s introduction warns can blast rock down onto the earlier phase.

Safety and productivity are protected the same way as 5.2.2: strict blast-timing exclusion of the lower shovel during/after upper blasts, adequate geotechnical bench separation and catch-berm capacity, and (where practical) sequencing so Phase 2’s FIRST blasts in an overlapping column are scheduled during a planned Phase 1 shovel move/maintenance window rather than during active lower-bench loading.

5.2.8 — opinion on the simple NW-corner scheduler. As a FIRST-PASS planning guide, NW-corner scheduling has real value: it produces a physically feasible, easy-to-communicate schedule that respects the “waste ahead of ore” rule automatically (as confirmed in 5.2.2/5.2.6) and gives the planning engineer a solid, quickly-built starting point for short-term (this year’s bench sequence) through long-term (multi-year phase transition) planning. Its shortcoming, evident in this very schedule (the Year-6 waste cliff, the Year-4 double-transition burden), is that it optimises NEITHER truck-fleet utilisation NOR discounted cash flow — it is a feasibility heuristic, not an economic one (the same limitation already identified for the moving cone vs. Lerchs–Grossmann/NPV-scheduling gap in Question 1.8.2). Recommendation: use the NW-corner result as the STARTING sequence, then apply a smoothing/optimisation pass (either manual re-balancing of a few benches between adjacent years, or a proper mixed-integer/LP mine-scheduling optimiser) to flatten the truck-hour profile and, ideally, maximise NPV rather than simply feasibility.