Question 3 of 13: 3 (5 marks, compulsory)

Question 1.3 (5 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.

1.3.1 – Tracked equipment (cable/hydraulic shovels, most large excavators). Steel crawler tracks (a continuous chain of shoes driven by a sprocket, riding on rollers) spread the machine’s weight over a large ground contact area, giving high tractive effort and the ability to climb grades and cross soft or broken muck. This gives good short-term selectivity – the shovel can reposition itself metre-by-metre along the face to follow an ore contact – but propel speed is very slow (typically <2 km/h), so relocating from one bench to another (via a ramp) is a slow, deliberate operation, often requiring the ramp to be specifically prepared and, for very large shovels, the machine may need to be partially disassembled for a long move. Tracked shovels/excavators load trucks routinely and are the standard loading unit in most truck-and-shovel operations.

1.3.2 – “Walking” equipment (walking draglines). A walking dragline does not use tracks or wheels at all: it sits on a large circular base (tub) and, to relocate, lifts the whole machine on two eccentric-cam “shoes” (or a hydraulic-shoe mechanism) that pick the machine up and shuffle it forward in short (1–2 m) steps, then set it back down – hence “walking”. This gives essentially zero mobility for short-term selective mining (the machine works one long, fixed cast-radius pass at a time and cannot chase a contact sideways without a walk), and moving it from one bench to another is extremely slow and normally avoided altogether – walking draglines are designed to work a single bench for the life of the pit and are only walked to a new bench in exceptional circumstances (e.g. at the very end of a panel). A dragline does not load trucks in normal operation; it casts (throws) spoil directly to a dump area beside the cut, or occasionally loads a hopper/conveyor.

1.3.3 – Rubber-tire wheeled equipment (front-end/wheel loaders, some hydraulic shovels). Pneumatic tires give high travel speed (up to 30–40 km/h) and excellent short-term selectivity/mobility – a wheel loader can dart between multiple small ore pockets in a shift and is the equipment of choice for grade-control/selective mining. It also moves itself from bench to bench under its own power up a ramp in minutes, with no special preparation. The trade-off is lower tractive effort and higher ground-bearing pressure than tracks (poorer flotation in soft or wet muck) and generally smaller bucket capacity per machine at a given cost. Wheel loaders load trucks routinely, though cycle times are typically longer than an equivalent-capacity tracked shovel because the loader itself must reposition (drive in, load, back out) for every pass.

1.3.4 – Close mobile hopper/conveyor in place of trucks. Replacing the truck fleet with a shovel-fed mobile crusher/hopper and a conveyor (an in-pit crushing and conveying, IPCC, system) removes the haul-truck cycle entirely: material is crushed at or near the face and moves continuously by belt, which cuts diesel fuel burn and haul-road maintenance dramatically, removes truck-related traffic/safety exposure, and gives a smoother, more continuous feed to the plant. The cost is much lower flexibility – the conveyor route and the mobile crusher’s working area are fixed corridors that must advance with the face on a planned schedule, so short-term selective mining and rapid re-direction of material (e.g. sending an unexpected low-grade pocket to a different destination) are far harder than with a truck fleet, and the capital cost of the crushing/conveying infrastructure is front-loaded rather than spread machine-by-machine as trucks are added.