Question 8 of 13: Truck-Haulage Efficiency Technologies

Question 2: Truck-Haulage Efficiency Technologies (20 marks, optional)

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.

2.1 – The four technologies. Overhead trolley assist draws power from an overhead wire via a pantograph on a truck's sustained haul-road ramp grade, boosting speed and cutting diesel burn on the steepest, most fuel-intensive part of the haul; typical application is a long, fixed ramp out of a deep pit. Benefit: large fuel savings, faster ramp speed, lower emissions/engine wear. Disadvantage: high capital cost for the wire and substation infrastructure, and only trolley-compatible trucks benefit; it affects mine planning because the wire's fixed location constrains ramp/pushback sequencing – relocating it as the pit deepens is itself a large cost. Truck dispatch (Question 2.4–2.7) assigns trucks to shovels in real time by computer rather than by fixed loop; benefit is fleet reduction and production gains (quantified in Question 3 below) for comparatively low capital; it affects planning by allowing shovels to be positioned more flexibly since routing adapts dynamically rather than needing simple fixed loops. In-pit crushing and conveying (IPCC) stations a mobile or semi-mobile crusher within or near the pit and conveys crushed material out by belt instead of truck; benefit is a large reduction in diesel fleet size and haul cost per tonne once distances are long; disadvantage is high upfront capital and a conveyor system that is far less flexible to relocate than a truck route, so shovel and pushback locations must be planned around keeping the crusher within reach. Direct loading of a short portable conveyor feeding a slurry plant crushes/grinds ore near the face and pipes it as a slurry rather than trucking or dry-conveying it; benefit is avoiding a diesel haul fleet entirely for that material stream; disadvantage is suitability only for slurry-amenable material, high fixed capital for grinding/pumping/pipeline infrastructure and the associated water balance, and the loading point must stay within the short portable conveyor's reach of the plant, again constraining sequencing.

2.2 – Cost/productivity comparison and long-payback implementation. Trolley assist and IPCC both carry large FIXED upfront capital (wire/substation; crusher/conveyor) but deliver materially lower unit operating cost per tonne than straight truck haulage over the life of the investment, since diesel, tyres and truck maintenance dominate haul opex and both technologies remove a large share of that cost. Truck dispatch has comparatively low capital (server, onboard hardware) relative to the fleet, typically delivering a 15–25% fleet-size reduction for the same output (Question 3 shows a 25% reduction numerically) – the best capital-efficiency ratio of the four. Slurry/portable-conveyor systems sit in between, with capital scaling with pipeline length and plant throughput. Because a moving open pit makes trolley wire and IPCC infrastructure position-dependent (both become stranded assets as the pit advances past them), implementing either requires committing the pushback sequence to keep that infrastructure in reach for the FULL length of its payback period – i.e. the decision must be made years ahead of construction, using a long-life reserve and price forecast, not opportunistically as each bench is mined.

2.3 – Future of truck haulage. Likely modifications: continued growth in ultra-class truck size (400+ tonne payload, spreading fixed driver/dispatch overhead over more tonnes); autonomous haulage systems (AHS) removing the operator and enabling continuous, tightly dispatched operation; and a shift from pure diesel toward trolley-assisted, battery-electric, or (for very large fleets) hydrogen drivetrains to cut fuel cost and emissions. Technologies most likely to displace conventional truck haulage outright are IPCC and slurry/pipeline conveying (Question 2.1) – as pits deepen and average haul distance grows, a fixed conveyor or pipeline's cost per tonne-km falls well below a truck's once distance exceeds a break-even threshold, and very large, long-life operations may add fixed rail haulage on surface for the longest hauls.

2.4 – BP, LP and DP interaction (10 loaders, 60 trucks). At this fleet scale, full real-time optimization of every dispatch decision is computationally impractical, so the three methods are used HIERARCHICALLY rather than competitively. Linear Programming (LP) sets the SHIFT-LEVEL target allocation of trucks to loaders that respects grade-blend and stripping-ratio constraints (Question 2.7) – a global, slower-horizon optimization solved periodically. Dynamic Programming (DP) re-solves the truck–loader assignment sub-problem at each dispatch decision point using the CURRENT fleet state (which of the 60 trucks and 10 loaders are where, right now), constrained to track the LP's shift targets – a medium-horizon, state-based layer. Best Path (BP) then makes the actual moment-to-moment call – a cheap, greedy rule (send the next empty truck to whichever assigned loader has the shortest queue) – because a fleet this large generates far too many dispatch events per minute for LP or DP to be re-solved on each one. The three therefore compose: LP sets the target, DP tracks it against real-time state, and BP executes the individual truck-by-truck decision within that envelope.

2.5 – Match factor. Match factor (MF) is the ratio of the truck fleet's DEMAND for loading time to the shovel fleet's SUPPLY of it: $$MF=\dfrac{N_{trucks}\times T_{load}}{N_{shovels}\times T_{cycle}}$$ $MF=1$ is the theoretically matched fleet (neither waits on the other, on average); $MF<1$ means the shovel(s) wait on trucks; $MF>1$ means trucks queue (Question 1.2.2's over-trucked case).

2.6 – Closed out, dispatched, spotting. Closed out permanently dedicates each truck to one shovel-to-dump loop; dispatched pools the whole fleet and assigns each truck dynamically to whichever shovel needs one next, exploiting shorter cross-routes a closed-out loop never uses (quantified in Question 3.4–3.5); spotting (Question 1.2.3) is the manoeuvre of positioning an empty truck at the shovel's loading point.

2.7 – Dispatch hardware; grade control and stripping ratio.

• GPS/GNSS receivers on every truck and shovel for real-time position.
• Onboard computer/display in each truck cab, receiving routing assignments from the dispatch server.
• Payload-weighing system (suspension or strut-pressure based) confirming actual load size.
• Radio/wireless data network carrying telemetry and dispatch commands.
• Central dispatch server running the LP/DP/BP hierarchy of Question 2.4.
• Condition-monitoring sensors (engine, hydraulics, tyres) feeding the maintenance database.

Grade control and stripping-ratio management are encoded as CONSTRAINTS inside the LP/DP layer: each active face is tagged with its current block-model grade, and the dispatcher biases ore-truck assignments to blend faces so crusher feed grade stays inside the mill's target band, while capping the share of trucks sent to waste-only faces versus ore faces in a given period to keep the realised ore:waste ratio on the planned stripping-ratio schedule.