Haul roads are engineered in a way such that their surface remains stiff, properly drained, and geometrical smooth so as to minimize the energy spent in fighting the rolling resistance and also the time taken for braking and making repairs. It has been proven that the road gradient remains constant due to the geometries of the mine, while the rolling resistance is variable.
Geometry first
Geometric elements include a safe grade, adequate width to accommodate the largest truck, and a curve radius that does not require an extreme slowdown. With the Canadian haul road standards, the maximum grade was constrained at 10%, usually less than 8%. Road widths were roughly four times the width of the trucks, and crossfall ranged between 2%. Increased road widths, smoother road alignments, and reduced intersections improve traffic flow and help reduce cycle times.
Surface structure
Rolling resistance largely depends on the stiffness of the road surface and roadbed materials when trucks are loaded. As stated in the Womp article, even well-constructed mine roads will have a default rolling resistance of 2% to 2.5%. However, soft, wet, or poorly constructed roads will have significantly higher rolling resistances since the tires sink into soft ground surfaces. According to the UBC guidelines, as payload capacities for trucks grew, there was a need for increasing thicknesses and stiffness in layer construction to minimize deflections. This requires competent material selections, proper compaction, and adequate layer thicknesses according to axle loads and tire pressure.
Drainage and moisture
One of the key methods of minimizing rolling resistance is through good drainage. According to the UBC manual, deterioration results mainly from factors such as precipitation, runoff, high groundwater table, and spring thaw, while potholing, ruts, settling, and skidding are some of the common problems. The use of a 2 percent crossfall and ditches, as well as correct culverts, will ensure that any water on the surface is drained before the layers become soft and deformations occur. Any accumulation of water will increase rolling resistance dramatically.
Maintenance actions
Maintenance is more than repairing failed components; it is maintaining the road close to its minimum-cost operating condition. According to the Womp article, regular maintenance for a good road design ensures low average rolling resistance, but inadequate maintenance will lead to an increase in rolling resistance until haulage expenses exceed the cost saving associated with inadequate maintenance. The UBC manual indicates that mines employ road maintenance activities such as grading, resurfacing, plowing, scarifying, sanding, and spot fill-cutting operations, with dust reduction measures achieved through watering or chemical application.
Cycle-time impact
There is also a direct link between rolling resistance and truck cycle time since this parameter leads to reduced speeds on ramp hauls and level hauls. According to the Womp, an increase in rolling resistance by 1% leads to a 7% increase in cycle time on a 10% ramp haul, as well as an increase of 20% in cycle time on a level haul. Hence, road maintenance is a production function, not a civil works function since the condition of the roads impacts performance in terms of increased cycle times, increased consumption of fuel, tire wear, and driveline work.
Practical operating rule
The ideal haul road involves a comprehensive process that includes design, construction, monitoring, and maintenance of the road. The road must be constructed in the right manner with the appropriate width, slope, superelevation, crown, and drainage features; then the road needs to be monitored concerning surface condition and rolling resistance so that maintenance can be carried out before the performance becomes unacceptable. The increase in the weight of trucks calls for either reconstruction of the road or reinforcing the surface with stronger pavement structures since a surface with similar conditions will experience greater deformation from heavy axles.
