The selection of either ultra-class (payload 360–400 tons) or large-class (payload 180–240 tons) of haul trucks is an important financial decision in terms of open-pit surface mining operations. An investment in a single 400-ton haul truck is multi-million dollar in nature, and operation cost during the lifetime of 15 to 20 years can be more than $40 million for a single truck. Though a higher payload will reduce cost per ton in transportation, but these benefits are subject to geological structure, pit road size, and loading shovel suitability.
The fleet of ultra-class haulage machinery can make huge economies of scale due to lower trip frequency needs and low unit cost of operation. The use of high-capacity loading shovels and large loads makes such fleets 20%-30% more efficient than smaller fleets with average labor productivity of 54 tons per man-hour. High tonnage deposits of copper or iron ore where the production targets are greater than 60,000 tons per day will profit from tall benches and ultra-class fleets that can maximize the NPV of the project. Selective or narrow vein deposits suffer from ore dilution and high stripping ratios at tall benches. Shorter benches and large class trucks can protect ore quality and economics of deposits in such geological conditions.
Deployment of ultra-class trucks entails wider pit haul roads to accommodate an expanded ramp width of between 26 and 43 meters, allowing for increased manoeuvrability. Considering that the bench heights lie within the 15–20-meter mark, there is a need to provide drill blastholes of larger diameters, that is, from 295 to 365 millimeters. The reduction in cost of drilling per unit volume is compensated by the need for heavy duty shovels to achieve the match 3-to-5 pass criteria. Also, drivetrain design plays an important role in determination of the productivity of the truck at different pit configurations. While mechanically driven trucks, for instance, Caterpillar 797F, have direct power transmission and faster speed in straight roads, electric AC drive trucks like Komatsu 980E produce dynamic retardation of up to 6000 horsepower.
The productivity of the system relies on maintaining an equilibrium between the loading rate of the shovels and the scheduling of the trucks, which can be determined formally by the Match Factor. A Match Factor of 1.0 indicates that there is no equipment idling. An over-allocation of trucks causes long lines of waiting trucks at the loading faces, whereas under-allocation denies loading shovels work and significantly decreases production from the pit. Large class fleets offer flexibility in the allocation of trucks and balancing of the length of the lines. Tyres are estimated to cost about 5% of the budget of the open-pit mines, with ultra class 63-inch tyres costing more than $42,500 each.
Ultra-class haul trucks provide significant scale economies, lower fuel consumption per ton of material, and higher net present value of projects in large ore bodies with deep pit configurations. Large-class haul trucks provide greater flexibility, lower capital risk, smaller road requirements, and better grade control in difficult and selective ore bodies. The choice of haul truck type must consider geological continuity, pit configuration, and shovel compatibility. Electric drives are suitable for deep pit configurations with steep downhill haulage, while mechanical drives perform well in flatter pit configurations. Payload management, tyre monitoring, and fleet balance continue to be critical.
