Hauling ore and waste rock typically accounts for about 60 per cent of the cost of obtaining ore from an open-pit mine—so just shaving a few percentage points off that cost can make a difference to the mine’s bottom line. In many cases, relatively low-cost changes in layout, structure, and operational procedures for haul roads can provide significant savings, which continue through the life of the mine. This is the old adage, “spend a penny, save a pound,” brought to life.
Another adage, “there is no free lunch,” also applies. Wise haul road management involves balancing several factors against each other. For many mines, productivity can be improved by understanding these balances and taking action based on that understanding. Many mine roads “just happen” without adequate planning and design, and the results are found in prematurely aged trucks, higher tire and fuel costs, and lower productivity.
The question of balance is a big one in the layout of the road network. This includes the gradient and the width of the ramp used to haul ore and waste to the surface. A steep climb results in shorter haul distance and takes less space, so 10 to 12 per cent gradients are common. However, a truck’s efficiency declines significantly at steeper grades. Good layout considers engine power and the load weight in deciding on the grade of the road out of the mine.
The width of the road is dictated by the truck size and the traffic density. For example in Quebec, this width is 1.5 times the width of the biggest truck in the fleet for one traffic lane, and three times for two traffic lanes. Where traffic density allows, i.e. temporary roads, a one-lane road could be considered.
At the surface, one factor many mining companies miss has to do with small rises along the route. While a truck driver will downshift before a large hill, she or he may try to take a small rise without shifting, which can put unacceptable strain on the motor. Downshifting has its own penalties in increased turnaround time, so good road layout minimizes small rises as well as the larger ones.
Another common design problem in haul roads has to do with water management. Failing to study drainage patterns ahead of time, and install culverts and other water-management measures, can mean washed-out road surfaces. This reduces vehicle efficiency and increases maintenance costs, so “pennies” invested in water management measures pay off in “pounds” over the life of the mine.
The structure of the roadbed on the in-pit ramp is generally simple - just a layer of gravel to boost traction, laid on the bare rock. Good engineering can help determine the optimal thickness and gravel type. At the surface, if the route lies on soil rather than rock, there is more need to pay attention to the road bed so it does not deform under the weight of the trucks.
An area where some mine road designers miss the mark - with possibly dangerous results - has to do with curves in the road. Many curves are designed, with posted speed limits to match, to reflect the need to prevent vehicles from skidding off the road due to inadequate side friction. However, these measures are sometimes not enough to deal with the danger of rollovers. As a result, a curve that is safe at a given speed regarding friction may still cause a truck, loaded high with ore, to tip.
Tight curves also result in outward centrifugal forces, compensated by side friction between tires and road surface. Along with good surfacing materials, a good design should include banking (super-elevation) of the road through the curve.
One of the biggest sources of improvement for haul roads lies in their operation.
A too rough or soft road surface increases rolling resistance, slowing vehicles, and causing unnecessary stress. This means that procedures must be established so grading is done at appropriate intervals. In many mines, grading is done on a schedule - “If it’s Tuesday, we must be grading road four” - rather than on an as-needed basis. Mines in South Africa are among the world pioneers in regular checks of each road’s surface, with grading carried out only when needed to keep an acceptable surface on each part of the road.
Better training for shovel operators can help make sure that each truck is loaded as full as possible, but not beyond a limit imposed by the engine capacity, the roadbed surface, safety, and other factors. Training can also help make sure that grader operators deliver surfaces that are rough enough for good tire grip and smooth enough for vehicle efficiency.
Mohamed Bouna Aly is a member of the mining practice of Golder Associates Ltd., based in Val-d’Or, Quebec. Robert Douglas specializes in haul road engineering and is a member of the transportation practice of Golder Associates Ltd., in Mississauga, Ontario.