Haul Truck Cycle Time and What It Costs per Tonne

One cycle is the round trip: spotting and loading at the face, the loaded haul out of the pit, dumping at the tip or crusher, the empty return, and any time spent queuing at the loader.

Loading and dumping are roughly fixed — a matter of passes and manoeuvring, typically a couple of minutes for loading and around a minute to dump. Queuing depends on how well the fleet is matched to the loader. The travel legs are where the geometry lives, and they are what changes as the pit deepens.

The asymmetry is the point. A loaded truck climbing a ramp is slow and power-limited; the same truck returning empty and downhill is fast and limited by braking and safety rules instead. Averaging the two directions into one speed hides the leg that actually costs you.

A loaded truck's climbing speed comes from the rimpull its engine can deliver against total resistance: the grade it is climbing plus the rolling resistance of the surface. Rolling resistance is not a constant. A well-maintained haul road might cost 2%, a soft or rutted one several times that, and every extra point acts exactly like an extra point of grade.

This is why road maintenance shows up in production numbers. Adding two points of rolling resistance to a 10% ramp is the same as making it a 12% ramp for every loaded truck, every cycle, all shift.

It is also why grade choices are not free. A steeper ramp is shorter and reaches depth quicker, but it slows every loaded climb; a flatter ramp is faster to drive but longer and consumes more of the wall. Both effects land in the same cycle time, which is why the comparison should be made in seconds rather than in degrees.

As the pit deepens, the haul gets longer and the cycle time grows, so the same fleet moves fewer tonnes per period. Plotting capacity against ramp length is the single most useful chart in a haulage study: it turns the schedule from a wish into a constraint.

This is also the link between road design and the mine plan. A ramp layout that saves 300 m of one-way haul does not just save distance; it raises the tonnes per hour the fleet can sustain for every period after it is built.

The match factor is worth checking at the same time. Too few trucks and the loader waits; too many and the trucks queue. Both show up as tonnes that were paid for and not moved, and the recommended fleet size is a suggestion to test, not a decision to accept blindly.

Cost per tonne follows once the cycle is known. Take the hourly cost of running the truck — fuel, tyres, maintenance and operator — divide by the cycles it completes in that hour, and divide again by the payload. The result is a number you can put next to the ore it moves.

Reporting it per tonne-kilometre as well is useful because it separates the two things that inflate haulage: an expensive truck-hour and a long haul. A fleet can look cheap per hour and expensive per tonne simply because the road got longer.

The life-of-pit figure needs the schedule, not just today's geometry. The cost per tonne in the first period and in the last period can differ substantially, and the weighted average across the periods is the number that belongs in an economic comparison.

Tyres are consumed by work, and the work is the product of load and speed. The tonne-kilometre-per-hour figure is how tyre manufacturers express what a tyre can absorb before heat becomes the limit, and a haul profile that exceeds it does not fail immediately — it just shortens tyre life until the cost appears somewhere else.

Long, hot, fast hauls with a full payload are the combination that pushes it. Where a route is close to the limit, the levers are the same ones that affect cycle time: shorten the haul, reduce the speed on the long legs, or accept a shorter tyre life as a costed decision rather than a surprise.

Most of these numbers can be computed from a design rather than measured after the fact. The ramp is a line on a surveyed surface; its length, grade at each station, and tightest curve are all readable from the geometry.

The check that matters is per station, not per average. One 14% pinch in an otherwise 8% ramp sets the speed of the whole loaded climb, and one curve tighter than the truck's minimum turning radius makes a route that cannot be driven as drawn. An average grade hides both.

Speed should be attributed too. When a leg is slow, it is worth knowing whether the limit was engine power, the site speed limit, the curve, or sight distance — because only one of those is fixed by physics.