Question 19 of 27

Question 3.10 operational contingency (2 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.

Although Question 3.9 found this specific cut feasible with margin, the same geometry method shows the margin is not large (5.15 m of stacking-height headroom), so as overburden thickens in later cuts of the same pit (a common progression in a shallow-dipping seam) the SAME dragline can run out of capability. The chosen contingency here is extended bench (Question 3.1.5): split the overburden vertically — the dragline continues to side-cast the LOWER portion nearest the coal by its normal single-pass method (still well within its digging-depth rating), while the UPPER portion of overburden is removed separately by an auxiliary dozer/scraper fleet and cast (or trucked) further out onto a WIDENED spoil area beyond the dragline’s own reach. This keeps the SAME objective (overburden ultimately placed in the void of the previously-mined-out area, opposite the advancing coal face) while reducing the depth and height the dragline itself must handle in one pass, restoring the stacking-height and reach margins found tight in 3.8–3.9. The extra cost is the auxiliary equipment fleet and the double-handling of the upper overburden slice; the benefit is continued use of the existing, already-amortised dragline rather than a costly re-rig or replacement.