Abstract
Rolling resistance of mine haul roads influences fuel efficiency, vehicle speed and cycle time (Joseph and Szymanski (2013)). It is affected by many factors including the non-elastic compression of tires, the compression of the ground, slippage between the tire and the road surface and friction in moving parts. Rolling resistance of mine haul roads depends upon the depth of sinkage into the road and the potential for rut formation. Rolling resistance varies from 2% for firm, flat, well-constructed roads to as much as 20% for soft and rutted roads. A 2% reduction in rolling resistance can produce up to 40% reduction in fuel costs and up to 26% increase in maximum haul speed (Thompson 2019, Tannant and Regensberg 2001). Lowering rolling resistance by improving the competence of the driving surface also results in longer intervals between required road maintenance and lower maintenance costs as well as a decrease in CO2 emissions with improved fuel efficiency.
The addition of natural, enzyme soil additives to materials improving compacted road quality is an emerging construction strategy. Haul roads constructed using local materials often contain a significant component of silts and clays which are typically considered unsuitable building materials. Enzymes are natural organic compounds that act as catalysts and include biodegradable surfactants that reduce the water surface tension in soil pore spaces and promote enzymatic reactions. An enzyme soil product reduces the amount of water that bonds to clay particles. A drier compacted soil, in turn, results in greater soil stiffness and a higher bearing capacity. The authors have experience with enzyme-based additives being applied to haul roads with positive results at Shenhua Group Mines among many others.
To support anecdotal evidence from construction projects, the authors performed laboratory tests to measure the effect of the addition of enzyme-based stabilizers on the rolling resistance of simulated compacted roadbeds. Tests were performed on three different soils, with and without enzyme treatment, and included two different methods of soil compaction, a vibratory plate compactor and a jumping jack compactor or rammer to represent the range of poorly constructed roadbeds to well-constructed roadbeds. All three soils were predominantly fine-grained silt-clay mixtures with between 28 and 56% clay-sized particles, selected to represent marginal quality road construction materials. The rolling resistance tests involved measuring the force required to pull a 2.6 or 4.9 kN trolley at a constant speed over a 14 cm thick compacted soil bed. All tests produced a reduction in rolling resistance for the enzyme treated soil of between 1.7% and 2.0% compared with untreated soil.
All three series of tests performed by the authors indicated an approximately 2% reduction in rolling resistance value for the treated soil. This research supports the empirical evidence from construction projects that showed improvement in haul road quality with enzyme treatment. The potential benefits are in the reduction of fuel consumption, increase in maximum vehicle speed and the reduction in maintenance costs. In addition, enzyme products are biodegradable and pose no environmental risks with their use.