Question 4 of 13: 4 (6 marks, compulsory)

Question 1.4 (6 marks, compulsory)

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.

1.4.1 – Key cut. A key cut (or “keyway”) is a narrow starter trench dug at one end of a new panel, normally by an auxiliary machine (dozer, scraper, or a smaller excavator/dragline) rather than the main production dragline, to create the very first spoil-side room the dragline needs before it can begin its own normal casting cycle. It is necessary at the start of every new panel/strip and at box-cuts, because a dragline sitting on virgin, unmined overburden has nowhere to cast its first bucket – the key cut gives it an initial void to throw into.

1.4.2 – Extended bench. An extended-bench operation lengthens the dragline’s working bench (increases the along-strike length mined per machine position) beyond a normal single-pass width, typically by having the dragline work a wider swath through a combination of extra reach and lateral tub repositioning along the strike before advancing forward. It is used where higher production per dragline is needed from a fixed capital fleet, or where geometry (a locally wide, uniform overburden section) makes it more efficient to sweep a longer bench than to add another machine.

1.4.3 – Advanced bench. In an advanced-bench (or “pre-strip”) arrangement, a second bench sits ahead of and above the main dragline bench and is stripped in advance (by dozer, scraper, or a smaller machine) so the dragline only ever has to cast the remaining lower portion of the overburden column. It is necessary whenever total overburden thickness exceeds the dragline’s effective digging depth plus its safe stacking height – splitting the column between an advanced upper bench and the dragline’s own lower bench lets a single dragline handle overburden that would otherwise be beyond its geometric capability (directly analogous to the stacking-height shortfall found for this dragline’s own geometry in Question 4.5–4.6).