Unbound granular materials in haul mining roads
The controversy around use of the terminology ‘unbound’ in granular materials in road applications has involved opposition between proponents against stating hydraulic binding of the fine fraction of the aggregate by moisture. On the contrary protagonists for, base their argument on lack of bitumen, cement and other binding agents. Evaluation of unbound granular materials in haul mining roads will focus on their properties, design, performance criteria and maintenance. The lack of binder in unbound granular materials will be highlighted extensively whilst understanding the contribution of engineering principles and sustainable practices to enhance the service life of haul mining roads.
Mining Haul Road Applications
Unbound granular materials in haul mining roads consist of different aggregate size distributions according to gradation curves based on sieve analysis. The contribution of the variation in size distribution is reflected in their ability to sustain load of the granular mix. Interlock of the interactive coarse particles creates a primary load sustaining structure whereas the secondary structure of fine-grained particles confers stability through occupying the gaps between the coarse particles. A packing theory approach based on particle-to-particle contact, particle size distribution and particle arrangement provide adequate support for unbound granular material. Void ratio of skeleton, disruption potential, porosity, coordination number, grain size and content of fines are the parameters considered assuming constant dry density and specific gravity for all sieve sizes.
Air and water lodge within pore spaces of unbound granular materials. Voids within the primary and secondary structure are the two types of free volumes. The disruption potential is related to secondary structure particles present within the gaps of the primary structure particles and how it affects the stability of the load bearing structure. Low disruption potential relates to low volume of secondary structure particles in the voids in primary structure particles leading to unstable granular material conditions. Excessive amounts of fines reduce grain to grain contact between coarser particles leading to high disruption potential. Grain size distributions form the basis of gradation properties governed by grading envelopes amongst the proportions of coarse and fine particles. Mixtures of unbound granular materials are classified as dense, poorly, uniform, gap and open graded. To consolidate the properties, behavior of unbound granular materials is significantly affected by the average number of contact points per particle as the applied load is distributed over the particle surface.
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Key Design Elements
Geometric, road and drainage design are key components of haul mine roads because construction is limited to the available space and for specific load vehicles as the rule of thumb dictates that widest vehicles proposed determine the pavement width. The elements of geometric design include road width, stopping and sight distances, maximum sustained grade, vertical curve configuration, super-elevation, cross-slope for drainage, horizontal curve alignment, extra width on curves, harmonization between horizontal and vertical alignments, and spacing between safety road-edge berms. Critical to geometric design is implementation of the mining plan with alignment and maximum grade designed prior construction. The other parameters depend on road service within acceptable ranges without strict design obligations. The trade-off is between an ideal layout relative to permissible mine geometry and hauling economics.
Road design focuses on load carrying capacity of the pavement and response to applied loads. Key is reduced deflection on the surface and better wearing course performance whilst making sure deformation is eliminated in the sub-grade or in-situ. The transfer of load is based on the layers in the pavement as each layer has either a structural or functional role. Critical to most mining haul roads are the wearing course layers as there might not be the luxury of proper pavement design owing to material unavailability. Drainage design varies from one application to the other depending on how to direct flow of water to the critical points in the drainage system as well as how to deal with floodwater. Water should be kept off the roads or at the very least led off the road as soon as possible. This is considered a function of geometry and local topographic drainage patterns in the vicinity of the mining haul road. Moisture primarily concentrates in the secondary structure particles as a water bridge regime in the pores of granular particles held at the contact points by capillary forces.
Failure Mechanisms and Measures
Resilient and permanent deformation are the two main deformation modes for unbound granular materials. Resilient deformation can either be elastic or inelastic and is responsible for fatigue cracking whilst permanent deformation is associated with rutting in the wheel tracks as a result of small contributions caused by each cycle through a wheel motion. Recoverable resilient deformation tends to be more pronounced than non-recoverable permanent deformation. The failure modes of unbound gravel materials include inadequate stiffness which leads to depressions of the surface under trafficking. Weaker granular materials can succumb to shear deformation owing to inadequate granular material shear strength which appears as dilative heave adjacent to the wheel track. Shear deformation within the subgrade can lead to granular layer deflection and the effect of studded tyres can contribute to particle damage through attrition. All these modes of failure are collaborative leading to the need for well thought performance criteria and maintenance practices.
Performance criteria can be implemented using mechanistic design and California bearing ratio cover-curve approaches. The former utilizes limiting vertical strain suited for traffic volumes, type and service life of mine road whereas the latter uses California bearing ratio values to determine thickness of successive underlying layers and the truck wheel load. Maintenance measures implemented seek to improve and restore ideal operational conditions of mine haul roads in order to allow for safe and efficient operations at reasonable costs. Material replacement should cater for reduced rolling resistance, deterioration rates and restoration of pavement thickness of the wearing course. The absence of compaction contributes to loose materials which leads to dust as a serviceability failure resulting in loss of fines. Mitigation through use of dust suppressants has to meet safety and health requirements whilst improving cost-efficiency in the haul mine road cost-benefit analysis.
Safety and Cost
Maximizing safety, utilizing materials and equipment available whilst minimizing rolling resistance and total-road user costs is critical for haul mine road performance. Safety considerations should involve reducing excessive shear forces, truck instability, damage to tyre and chassis, dust and slippery running surfaces. The management of mining haul roads also must balance the need for safe operations versus the need for cost effective pavements and surfaces. Managing surfaces only by watering and grading lead to the loss of fines, lowering the road profile, and consequently introducing drainage problems in rain events. Safe and efficient mines need to find the balance point between quality, safe haul speeds, water availability, and the best way to manage road structural and surface issues like roughness, failures, and visibility (dust).
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