Haul road design and maintenance have a huge impact on truck haulage cycle efficiency, costs and production. Site planning and operations management influences the critical variables, but every aspect of truck haulage offers continuous improvement opportunities for site personnel.
Proper road design is the first step in ensuring good truck productivity and low operating costs. Critical elements are grade, cross-slope and super-elevation of curves. The goals are to maintain proper weight distribution of the load and to minimise lateral forces on tyres. The same design that enhances truck productivity also reduces component wear and optimises fuel burn, as the truck remains stable at optimum speeds.
The steeper the grade, the more weight bears on the rear tyres. The goal is to keep the weight distribution at about one third on the front tyres and about two thirds on the rear duals. Ideally, the grade would not exceed eight per cent. The grade should be constant and the road smooth to minimise rapid weight distribution changes, minimise transmission shifts and maintain higher average speed. Such a road also promotes smooth braking when trucks are returning to the loading area.
Optimum road design also helps to minimise spillage of rocks on the road. Approximately 75 per cent of tyre failures are caused by cuts from rocks and impacts with rocks.
Roads must also drain properly to reduce slippery conditions and to help minimise rolling resistance. On flat terrain, the minimum cross-slope maintains drainage for the expected rainfall at the mine. If conditions permit, consider a two per cent constant cross-slope. Four per cent cross-slope can be used in rainy areas. Alternatively, the road should be crowned slightly to drain water to both sides of the road. On grades, minimal cross-slope or crowning is needed, because the grade itself helps direct water off the road.
Corners should have the maximum practical radius to help maintain speed and to minimise side force on tyres. Side force generates heat in the tyres and reduces casing life. High side forces scuff the tyres and accelerate tread wear too. Corners should be constant and smooth to reduce steering corrections, operator fatigue and component wear.
If truck speed exceeds 15 kilometres per hour, the curve should be super-elevated to negate centrifugal force that puts side loading on the truck. The amount of super-elevation needed depends on curve radius and the speed at which it is negotiated. Tables published in the Caterpillar Performance Handbook show the amount of super-elevation required to produce zero lateral forces for given curve radii and truck speeds. Super-elevation greater than 10 per cent should be used with caution due to the potential for slipping toward the lower edge of the roadway in wet conditions when truck speeds may be reduced.
In addition to countering centrifugal forces, corners should be designed so that truck operators can see and avoid obstacles when travelling at normal speeds. The same consideration should be given to crests of hills. These calculations should use the worst case scenarios: smallest obstacle, highest expected speeds, longest stopping distances, etc.
Road width is another factor that affects safe operating speeds and can help minimise tyre contact with safety berms and spilled rocks. On one way roads, Caterpillar recommends road widths of two to 2.5 times the width of the widest truck used at the site. On two way roads, road width should be a minimum of three to 3.5 times the truck width on straight sections and 3.5 to four times in corners.
It is important too to construct the road from material that will compact properly and will not become soft when wet. Quarry trucks create compaction forces of more than one metre deep. Therefore, it is important for the road base to compact well to maintain a solid road. Minimising rolling resistance can promote high productivity. In practice, a five per cent increase in rolling resistance can result in a 10 per cent decrease in production and a 35 per cent increase in production costs.
For reference, trucks using radial-ply tyres experience approximately 1.5 per cent rolling resistance on hard, well maintained roads, and about three per cent rolling resistance on firm, smooth, rolling roads. When tyres penetrate 25mm, rolling resistance is approximately four per cent, and 50mm penetration means about five per cent rolling resistance. Tyre penetration of 100mm means about eight per cent rolling resistance, and 200mm penetration means 14 per cent rolling resistance.
Frequently, high rolling resistance is associated with wet conditions. The road material becomes soft when wet. To avoid a saturated road base, adequate drainage structures should be built, such as ditches, to carry away the maximum expected rainfall. Properly sloping the roadway and carrying water away will help minimise water puddling, potholing and entry of water into the road base.
It should also be remembered, too, that the haulage road extends to the loading face and to the dump point. Bench widths should be adequate for a loaded truck to clear the loader under full acceleration and for an empty truck to avoid tight, high speed turns. The minimum bench width should equal the machine turning radius plus space needed for the safety berm. Dump points should have enough space for the truck to align properly with a minimum of manoeuvring.
OPERATING FOR LONG TYRE LIFE
Most truck tyre failures are caused by contact with rocks spilled on the road, in the loading area or at the dump point. Machines to remove spillage and a communication system that promotes fast action are needed at every mine. But, in addition to good mine design, proper loading and operating techniques should reduce the need for cleaning up.
The loading machine operator plays a key role in getting long life from truck tyres. Centring the load properly in the truck body, and not overloading the truck, allows the tyres to work within their intended load limits. A correctly sized and properly placed load also is less likely to lose rocks onto the roadway.
Training motor grader operators in proper road construction and maintenance is important. Water truck operators should receive thorough training, too. Watering to suppress dust also helps maintain compaction in most climates. But too much water can create slippery conditions, and water can increase the likelihood of tyre cuts from sharp rocks. Caterpillar suggests using intermittent watering patterns on slopes to reduce the risk of slippage.
To optimise truck productivity and truck tyre life, do not overlook the dump areas. Dump areas should have a smooth floor that allows trucks to maintain speed until they reach the dump zone where they enter parallel to the edge or crusher opening and brake in a straight line before turning, stopping and reversing to the dump point.
Technology aids available from Caterpillar and the local Cat dealer can help assess haulage road design and condition. Fleet production and cost software can compare actual cycle times with theoretical values and point to areas for improvement. The Vital Information Management System (VIMS) and the Truck Production Management System (TPMS) onboard trucks can help with payload management and can help identify areas where braking or shifting are severe.
From common sense to the technologically advanced, there are many options for optimising haulage roads and truck productivity and costs.
Source: Caterpillar Asia Pacific