HAUL ROADS
mine’ s haul roads can range from 35 t to 363 t or more. These huge loads can quickly degrade dirt and gravel haul roads that mine operators rely on.
“ This stress often results in rutting and surface erosion problems that can lead to safety issues and production delays – drivers have to reduce their speed and load size to ensure their own safety and to avoid vehicle damage. This slows down the overall mining process and decreases operational efficiency of the mine.”
In the western US, one mine’ s fleet of heavy trucks, along with other equipment, were rutting its haul roads so extensively that the‘ safe’ travel speed had to be cut in half, to less than 10 mph. The mine owners were assisted in the rebuilding of these roads to improve overall productivity by using two layers of Tensar geogrid combined with local aggregate material.
“ Installing a geogrid layer within the subgrade created a strong road structure that could handle heavy loads at 15-20 mph. This increase in speed allowed for higher productivity on the mine site due to the increased ability to safely haul more material. The smoother roadway surface from the reinforced MSL of the road section also reduced the wear and tear on the equipment and truck tyres. This translated into major savings for the mine owners.”
After completion of the project, management at another mine in the same region heard about the stabilised haul road and wanted to learn more about the solution and the installation process. In these cases, by reinforcing the MSL, Tensar states that these companies reduced the cost of ongoing road and truck maintenance, increased the safety of the haul roads, and increased the long-term stability and performance of their haul roads.
Mechanical ground stabilisation was also used to address an environmental issue at a third mine site in the western US. This mine had encountered variable soils while designing a holding pond for contaminated water and snowmelt. Part of the soil included hard travertine but there were also pockets of soft soils that needed to be addressed.
Reall comments:“ Consulting engineers on the project worked to design a platform for the pond to stiffen the softer areas before construction. Over these highly variable soils, project managers installed a bottom layer of largeaperture geogrid and topped by two layers of triangular-aperture geogrid. The project was a success and addressed a key issue – to ensure that contaminated water did not seep through the soft soil into a nearby stream. Mine operators are increasingly addressing road stabilisation and environmental issues by using geosynthetic solutions to increase mining productivity.”
Analysing and managing rolling resistance
In a paper titled Building the economic case for mine haul road improvements: sheeting selection and rolling resistance, Roger Thompson of the Western Australian School of Mines at Curtin University sets out to establish the economic case for haul road improvements through reduced rolling resistance as a result and therefore increased truck haulage productivity. The paper asks how can rolling resistance be managed and reduced on mine roads. Rolling resistance is the result of road deformation under the tyre and tyre penetration into the road surface, with resulting tyre deformation effects due to the road surface.
“ For existing roads, first, you need to know what is wrong before you can decide to fix it. For new and existing roads,‘ fixing’ requires an evaluation of what’ s needed and the associated component design or respecification. On‐board systems – and near real‐time monitoring can be used to record the truck and tyre response to the road, and when linked through FMS, gives the first indication of where and what the haul road problems are.” On ramps a 1 % increase in rolling resistance can translate to a 10 % speed reduction and on normal surface roads as much as 26 %. And of course the opposite is true in terms of the effect of reducing rolling resistance by 1 %. So the benefit of improved haul road conditions on operations include haul cycle time improvements, increased t / h and tkph, reduced fuel consumption and decreased unit costs.
Using haul cycle time estimates from FMS data( actual), OEM performance charts or software( targets), simulation software( targets) and then applying this data to generic equations for haul truck speed estimation, Thompson found that
Rolling resistance has a major effect on truck speed( source Roger Thompson, WASM, Curtin University)
each 1 % increase in rolling resistance above a 2 % base case increases cycle times by 20 % on flat roads and 8 % on ramp roads. A 2 km ramp cycle went from 13.06 minutes to 15.24 minutes( laden and return). And a 2 km flat haul cycle from 5.12 minutes to 7.54 minutes( laden and return). The haulage component of cycle time increases by approximately 21 %.
Then applying the data to fuel, each 1 % increase in rolling resistance above a 2 % base case increased fuel consumption by 25 % on flat roads and 5 % on ramp roads. Ramp cycle consumption went from 85 to 103 litres( laden and return) and the flat cycle consumption from 35 to 60 litres( laden and return). The total fuel cost increase was approximately 28 %.
From FMS or on‐board data the mine can determine the‘ steady‐state’ full throttle speed of the truck on various segments of road and from OEM performance data, determine total resistance( TR %). From survey data, the mine can determine grade resistance( GR %). The difference( TR‐GR)% is rolling resistance( against the grade). Mine roads can have high rolling resistance due to poor design and / or build specifications such as poor geometrics, structure( layer works and materials), and functional wearing course, ie the surfacing materials.
It is also possible to compare the‘ cost‐sensitivity’ of each segment of the haul cycle to determine which part of the haul cycle is most sensitive to increases in rolling resistance. Users can then focus maintenance resources on this segment to get best‘ value’ for road maintenance. Mine staff can visually assess haul road‘ defects’
74 International Mining | AUGUST 2017