Synergy between dust suppression and stabilisation in unsealed mining roads presents opportunities to formulate and develop products that meet a multi-faceted criterion. Dust suppression and stabilisation can be achieved using multi-purpose dust suppressants and the contribution of haulage compaction to stabilisation as a result of unsealed mining road usage further enhances the intactness of the layers. Opposed to intactness of the layer is the loss of fines and eventual debonding of the material leading to instability of lower layers. The extent to which the rolling effect of the tyre dislodges wearing course particles resulting in dust and destabilisation will be assessed critically. Mechanical and chemical methods available to ameliorate dust whilst achieving stabilisation in the unsealed mining roads will be discussed. Chemical methods available through the application of non-traditional chemical dust suppressants and mine haul road stabilizers offer more variety in the field whereas mechanical methods offer limited but site-specific options to achieve the best out of their use.
Rolling resistance as a measure
Mine haul road performance is principally based on rolling resistance, an opposing force to the rolling motion, which prevents vehicle tyres from moving. Rolling resistance is expressed as a percentage weight of the vehicle and can be added to grade resistance to obtain total resistance. Contributing factors to rolling resistance are related to aspects of structural design in deformation of haul road under the effect of the tyre, functional design with regards to tyre penetration into the surface and a function of maintenance design in tyre deformation effects on the road surface. The inevitable outcome which leads to dust generation and destabilisation of mining haul roads revolves around the balance of design aspects – as no amount of maintenance can fix a poorly designed mine haul road. However, it is easier said than done because although an integrated design approach is a critical contribution, much of what is experienced is post-exposure of the surface to haul traffic which makes the process of servicing and remediation of mine haul roads dynamic and continuous.
Different modeling approaches
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Vehicle-soil interaction models shape the approach used to understand the behaviour of soil surfaces on contact with mining haul vehicles. The first model is the soil material model, which uses the Drucker-Prager yield criterion to model soil utilising stress, friction angle and cohesion. Secondly, the soil pressure-sinkage model categorizes the indentation displacement relationship into three zones. Zone one is a linear elastic region, zone two is strain-hardening region in which the pressure-bulb developed under the indenter has not reached the bottom of the snow cover and zone three begins when the pressure bulb has reached the bottom of the snow cover. Specifically, for soil, the first two zones are applicable utilizing the same parameters in the Drucker-Prager yield criterion. Thirdly the tyre-soil interaction model uses the traditional rigid wheel model popularly applied for sand as it is much softer than the tyre, which renders deformation of the tyre negligible. Practically, this corresponds to the situation where maximum ground pressure generated at the tyre sand interface is less than the contact pressure, which is close to the inflation pressure of the tyre and between a tyre and rigid ground.
Improving physical characteristics of haul roads
Methods to ameliorate the dust and stability of haul mining roads can be either mechanical or chemical. Mechanical methods involve appropriate selection of blending material based on grading, plasticity and strength in the specific operating environment. For example, the use of clay additives such as bentonite which agglomerates with fine dust particles and generally increases the dry strength of materials under dry conditions with one treatment every five years. The conditions and limitations to its application include the tendency of slippery surfaces when wet and on over application which leads to more contents of fines. Geogrid mechanical stabilisation of a granular layer involves stone particles of the granular layer effectively penetrating and interlocking within the rigid apertures of the monolithic geogrid under vertical loads and in the process restricting the vertical and horizontal movement of the granular particles. Structurally, the geogrid evenly distributes any applied vertical load over the underlying subgrade whilst functionally reducing the development of irregular surface deformations and rutting. The dislodging of granular material is minimized through protection from migrating and being lost into the underlying weak subgrades whilst improving the overall performance of the mining haul road.
Chemical stabilisation and dust control
Chemical methods are divided into traditional and non-traditional with the focus of our discussion on non-traditional chemical stabilizers and dust control agents. Traditional stabilisers include fly ash, lime, bitumen, and cement. Limitations to their use include negative environmental, health and safety issues, with some even toxic. Non-traditional stabilizers provide good dust control, environmental friendliness, improve mechanical properties of soil and strengthen the structural performance and durability of unsealed haul mine roads. Examples of these stabilisers promoted in the industry include acids, enzymes, lignosulfonates, liquid polymers, resins, silicates, salts, synthetic fluids and ions. Preference for liquid polymers, lignosulfonates, synthetic fluids and resin stabilizers is enhanced owing to their better performance in preventing dust and improving structural strength. Adhesive and cohesive binding mechanisms with increases in shear strength whilst achieving dust control and material stabilisation is typical of synthetic fluids. Lignosulfonates bind surface particles together whilst retaining effectiveness during long dry periods with low humidity. Resins bind and agglomerate surface particles whilst reducing moisture sensitivity. The polymeric component of the liquid polymers improves saturation, penetration and bonding of the surface layer post evaporation of water.
Balancing investment and production
The interdependence of economic impacts and mining haul road performance is unequivocal. Fuel costs tend to increase due to slower travel in lower gears, whereas the per ton haulage costs soar owing to lower productivity and tyre, equipment and maintenance costs spiral upwards as a result of greater wear and tear leading to potential failure. The analysis into different dust and stabilisation methods highlighted a preference for non-traditional stabilisers through interrogation of the most feasible and complementary approach. Balancing the performance through enhancing functionality from a design perspective reduces the need for extreme case mitigation but rather a more integrated and solution-driven utilisation of multi-purpose dust suppressants in road stabilisation of haul mine roads.
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REFERENCES
Anand, A. 2012. Scaled test estimation of Rolling Resistance. Master of Science in Mining Engineering thesis. University of Alberta. Edmonton, Alberta.
Jones, D., Kociolek, A., Surdahl, R., Bolander, P., Drewes, B., Duran, M., Fay, L., Huntington, G., James, D., Milne, C., Nahra, M., Scott, A., Vitale, B., and Williams, B. 2013. Unpaved Road Dust Management, A Successful Practitioner’s Handbook. FHWA-CFL/TD-13-001.
Lee, J.H., and Gard, K. 2014. Vehicle-soil interaction: Testing, modeling, calibration and validation. Journal of Terramechanics, 52, 9-21.
Solovyev, G.V., and Vatchnadze, K.I. 2017. Improving of performance characteristics during mechanical stabilization of quarry haul roads with stiff polymeric Tensar Triax hexagonal geogrid. Procedia Engineering, 189, 666-672.
Thompson, R.J. 2011. GSFM-An Integrated Approach to Mine Haul Road Design. Western Australian School of Mines at Curtin University.
Xu, G. 2017. Research and application of non-traditional chemical stabilizers on bauxite residue (red sand) dust control, a review. Sci Total Environ.
