Oil sand mining and coal mining involves trucks with payload capacity of more than 300 tonnes and costing upwards of six million dollars. A good haul road ensures low vehicle operating and maintenance costs. A well-designed road itself has a lower maintenance cost than a road with inadequate cover thickness.

A thesis from the University of Alberta examines new and better haul road designs for these large and expensive trucks. Here is the abstract from this thesis (slightly edited to improve clarity based on my reading of the remainder of the thesis):

Large haul trucks are used at surface mines in Canada thus requiring better haul roads. The mines use empirical design methods, which may not result in optimum road design. A road design method based on resilient modulus results in better haul road designs than the CBR-based method. Numerical modeling done to analyze the effect of material modulus, layer thickness and tire interaction on strain bulbs in a haul road, showed that putting the stiffest layer at the top results in least vertical strain—hence improved road performance and reduced truck wear and tear. Coal mines located adjacent to coal-fired electric power plants produce fly ash as a waste by-product. Tests of fly ash, kiln dust and aggregate mixes proved that fly ash significantly improves the strength and bearing capacity of aggregates thus enabling use of thinner layers for road construction.