Executive Summary
The mine haul road network is a critical and vital component of mine operations. Truck haulage costs can account for up to 50% of the total operating costs incurred by a surface mine. Under-performance of a haul road will impact immediately on mine productivity and costs. Operations safety, productivity and equipment longevity are all dependent on well-designed, constructed and maintained haul roads.
Adopting Renolith nanotechnology in mine haul road construction or upgrade enables a thin, resilient and stiff pavement with multiple performance advantages. This can significantly reduce both capital costs and operational costs.
Mine Haul – Aims, Requirements & Challenges
Safe and efficient mine haul operations are essential to mine viability. There are many costs to consider:
Figure 1: Components of Road User Costs (After Chatti & Zaabar 2012)
With the right technology choices, it is possible to reduce all costs. For simplicity, we will focus on the most critical.
The running costs of heavy-duty mining equipment are strongly influenced by fuel consumption. Even the smallest improvement in fuel economy has a large impact on overall running cost. This also has a follow-on effect that the total pollution omitted can be reduced (Roche & Mammetti 2015).
In Design, Construction, and Maintenance of Haul Roads Thompson outlines the key drivers for mine haul road design optimisation:
The mine haul road network is a critical and vital component of the production process. As such, under-performance of a haul road will impact immediately on mine productivity and costs. Operations safety, productivity and equipment longevity are all dependent on well-designed, constructed and maintained haul roads.
Truck haulage costs can account for up to 50% of the total operating costs incurred by a surface mine and any savings generated from improved road design and management benefit the mining company directly as a reduced cost per tonne of material hauled. Central to the cost of truck hauling is the concept of rolling resistance (expressed here as a percentage of Gross Vehicle Mass (GVM)). It is a measure of the extra resistance to motion that a haul truck experiences and is influenced by tire flexing, internal friction and most importantly, wheel load and road conditions. Empirical estimations of rolling resistance based on tire penetration specify typically a 0.6% increase in rolling resistance per centimeter tire penetration into the road, over and above the 1.5% (radial and dual wheel assemblies) to 2% (cross-ply or single wheel assemblies) minimum resistance. In addition to tire penetration, road surface deflection or flexing will also generate similar results, with the truck tire running “up-grade” as the deflection wave pushes ahead of the vehicle.
In simple terms, to achieve the most efficient mine, the aim is to minimise the total costs across construction, operations and maintenance by optimising rolling resistance.
Figure 2: Aim to minimise total costs by optimising rolling resistance
In A review on Australian mine haul road design procedures Strack explains the relationships between rolling resistance, fuel use and vehicle operating costs. Fuel costs increase and production declines with increasing rolling resistance. Tyre penetration is mostly a function of the wearing course, whereas pavement deflection is governed by the stiffness of the base.
Figure 3: Mine Haul – Rolling Resistance
A pavement is ‘A road surface that can adequately support the weight of traversing traffic without excessive deterioration of the surface caused by the traffic.’ (Kaufman & Ault 1977) A typical pavement consists of a wearing course, a base, subbase and subgrade layer. All layers work together to provide a suitable road.
The strains induced within flexible pavements are mostly elastic (i.e. recoverable) however, every vertical strain is not fully recoverable. Therefore, after many load repetitions permanent deformations accumulate at the subgrade level and throughout all pavement layers. These deformations may be seen in the form of rutting along the wheel path and surface roughness (Jameson 2012).
Unbound materials are susceptible to permanent deformation. In mine haul applications, multiple thick layers are required to disperse loads to the subgrade.
Figure 4: Mine Haul – Stress Distribution
There are many design methods available for mine-haul roads. This chart below shows the Kaufmen Aults method, which is a conservative mechanistic method. Note how thick the pavement subbase needs to be with unbound materials, especially if the subgrade is poor and the vehicles are heavy.
Figure 5: BMA CBR Design Cover Curve (BMA Projects Group 2012)
Various design methods
- Structural analysis (critical strains or stresses)
- Kaufman and Ault’s Method – Empirical CBR method
- Ahlvin’s Method – Thickness over Subgrade CBR
- Tannant & Regensburg’s Method – Critical Strain / Resilient Modulus
- Thompson’s Method – Mechanistic
- Austroads Sublayering
- CIRCLY – layered elastic
According to Austroads Guide to Pavement Technology Part 4A: Granular Base and Subbase Materials (AGPT04A-24):
The functions of a granular subbase layer in a pavement are as follows:
- provide sufficient stiffness to distribute traffic loads transmitted through the pavement base, reducing their intensity to a level which will not cause excessive permanent deformation of the subgrade
- provide a working platform on which base materials can be transported, placed and compacted to the required standards
- depending on the pavement design requirements, drain the base and/or protect the subgrade from moisture infiltration, e.g. the lower subbase may be relatively impermeable whilst the upper subbase may be more permeable provided it is constructed in conjunction with appropriate sub-surface drainage.
The behaviour of granular materials in service is governed by many factors which are related to the following:
- the intrinsic properties of coarse particles, including hardness, surface friction and contamination, and the geological origin and history of the source rock from which the material is derived
- manufactured aggregate properties such as particle shape, size and surface texture, particle size distribution, fractured faces, nature and quantity of fine particles, and fillers – these factors are related to processes used during manufacture to produce the final product
- compacted layer properties such as density, moisture content and particle orientation, which are in turn related to the construction and compaction processes
- boundary conditions such as in situ moisture and temperature regimes, and the stresses applied at the boundaries of the constructed pavement – these are external influences that will influence both short and long term behaviour.
Figure 6: Pavement Material Requirements
To build an unbound granular haul road base asks a lot of the material properties. Sometimes suitable material is available near the mine, sometimes not.
Figure 7: Wearing course – requirements and conventional options
The road surface is slightly different to the other pavement layers. Not only should it provide a comfortable (smooth) wearing course it should also take into consideration dust control, traction and rolling resistance. A good running surface will prevent increased vehicle and maintenance costs and assist the vehicle to safely traverse the designed route (Strack, 2015). All conventional options have significant limitations.
Nanoengineered Mine-Haul Roads
Cementitious stabilisation improves the strength of pavement materials and reduces pavement deflection. However, the tendency of cemented materials to crack from drying shrinkage imposes limitations on the design. Cemented materials enhanced by Renolith nanotechnology achieve better performance and tend not to crack, even with high cement content. See https://renolith.com.au/crack-free-roads/.
This means that high strength heavily-bound base layers can be formed from a wide range of materials. The tensile strength, compressive strength and elastic modulus of Renolith-enhanced heavily bound materials is much higher than unbound or lightly bound materials. In a haul road application, this means that the pavement can simultaneously be thin, hard and stiff.
Thin pavements cost less to construct. Hard, stiff pavements have low tyre penetration, low deflection and low rolling resistance. Low rolling resistance means reduced operating costs.
Both capital and operational costs can be reduced.
Further, nanoengineering enables the wearing course, base course and subbase to be combined into a single monolithic layer. This further reduces costs, improves performance and addresses the key disadvantages of unbound compacted materials.
Table 1: Mine haul road surface – Unbound granular vs Nanoengineered
Conclusion
The mine haul road network is a critical and vital component of mine operations. As such, under-performance of a haul road will impact immediately on mine productivity and costs. Operations safety, productivity and equipment longevity are all dependent on well-designed, constructed and maintained haul roads.
Adopting Renolith nanotechnology in mine haul road construction or upgrade enables a thin, resilient and stiff pavement with multiple performance advantages. This can significantly reduce both capital costs and operational costs.