In a truck shovel operation, all we ever talk about is the excavator. We schedule the excavator, measure it’s productivity, compare various models and make judgements about which ones perform best. But In fact, the excavator doesn’t put any material in the dump or stockpile, its the trucks that do all the work and incur most of the cost of moving the material. So we should really be looking at which is the best truck to use. Assuming that the truck and excavator size are appropriately matched (between 3 and 5 bucket passes to fill a truck), the performance of the excavator only comes into the equation during loading which is perhaps 2 minutes out of a total truck cycle time of 20 minutes. So if the excavator hits some hard dig and takes 3 minutes to load a truck, the excavator productivity will fall by 50% but the truck productivity will only fall by 5% which hardly affects the unit cost of the operation.
What does affect the unit cost of the operation is minimising the truck cycle time and carrying the maximum payload in the truck all the time. This is where truck selection and allocation matters. Electric drive trucks are generally faster on grade than mechanical drive trucks because where a mechanical drive truck would have to sit on 7kph in first gear up a ramp to avoid hunting gears, an electric drive can run at 12kph. So put the electric drive trucks on the deeper hauls. Mechanical drive trucks are often faster on flat hauls as they don’t incur the power losses through generation.
Mixing different truck types on a circuit will incur significant inefficiencies as the faster trucks wait for the slower trucks on every cycle. Separate truck types and consider not only the make and model, but also the installed power. Many trucks come with different power options and a fleet of the same model truck will often have a range of engine power specs and hence truck speeds. Group the fast and slow trucks together when allocating them to the excavators. The final point is to work hard on reducing haul length through dump and haul road planning. Often it is possible to achieve a lower overall cost by installing temporary roads to shorten hauls, even when this incurs some rehandle. There are several haulage optimiser software programs on the market that greatly assist in the task of minimising haul times.