Question 6 of 11

Question 1.6 (6 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.

The ultimate (“final”) pit produced by a floating-cone or Lerchs–Grossmann run at a single, fixed set of prices, costs and slope angles is a NECESSARY starting reference but is deliberately NOT itself a mine plan: it ignores the time value of money, contains no operational sequencing, and (in its bare graph-theoretic form) contains no haul ramps at all — ramps must be added afterward, cut into the theoretical outline, which both reduces the mineable volume locally and forces small local wall-angle relaxations near the ramp.

From that ultimate-pit envelope, the planner generates a NESTED series of smaller, intermediate “pushback” or “phase” pits (commonly by re-running the same optimisation at a sequence of progressively lower revenue factors, e.g. 100%, 80%, 60%… of the base price, or by directly parameterising smaller pit envelopes), each with its own ramp system, each mineable as a stand-alone stage, and each nested wholly inside the next-larger stage. Sequencing these phases from smallest/highest-grade outward (rather than mining the whole ultimate pit as one pass) exposes ore earlier, defers waste stripping, and gives the planner a genuine choice of MULTIPLE alternative sequences — e.g. a low-capital, slow start that mines a small high-grade phase first to build cash flow before committing to the large waste stripping needed to reach deeper phases, versus a high-capital, fast-push sequence that pre-strips several phases ahead to guarantee ore continuity and equipment utilisation later in the schedule.

Because the ore body here is approximately horizontal and extends laterally over several blocks, pushback selection has an additional degree of freedom: successive phases can step OUTWARD across the deposit at roughly constant depth as much as they step DOWNWARD, so alternative plans differ not only in stripping-vs-grade trade-off but in which lateral portion of the flat-lying ore is exposed in which period. The “final” (ultimate, single-price) pit remains a valid and necessary STARTING POINT for life-of-mine strategy — it fixes the outer envelope that every nested phase must respect and gives a first-order total reserve and stripping-ratio estimate — but the actual life-of-mine plan is built by optimising the SEQUENCE and TIMING of pushbacks inside it (Question 6.7 develops this NPV/scheduling refinement explicitly), not by mining the ultimate pit as a single, undifferentiated stage.