Productivity of the underground truck - how can it be measured?

By Dmitry Przhedetsky*

It seems even more challenging given the trend of the last 12 years, to upsize trucks, has reached a limit whereby any further increases would require uncommon mine designs and practices.
Truck Size – Determining factors for truck size are the overall width, length, highest point and height over the tray. The current mine regulations may vary from state to state. According to the Western Australia Mine Regulations the gap to the wall from each side of the truck must be at least 900mm and 600mm from the highest point of the truck to the roof of the tunnel. Truck size is obviously very important because the existing mines already have definite decline sizes, while the new ones must minimise the cost of development. The Tray Height is a factor determining efficiency of truck loading with an Underground Loader.
However, two trucks with similar dimensions may have different pay loads due to different design.
Turning Radius, Loading Area, Dumping Area – Articulated Dump Trucks will fit into more turning points than Rigid Frame Trucks. Lower dumping area will be required for the trucks with lower Box Raised Height, whilst the Ejector (Teledump) Trucks can unload practically anywhere underground, which is a handy feature with certain kinds of mine designs.
Pay Load – This seems to be simple at the first glance. The load capacity of the underground truck is what the manufacturer claims in specification sheets. However the average payload of most trucks varies and quite often proves to be less, rather than more than that of the original specifications. The variations may be due to the method used by manufacturers. The current standard SAE J1363 provides a method for rating or estimating the effective volumetric carrying capacity of a truck body. It gives definitions of Struck and Heaped truck. Most manufacturers tend to publish the payload for SAE Heaped truck. However, in reality none of the trucks can be practically loaded heaped. Moreover, during the haulage some ore will inevitably spill. What is the actual payload? A true payload formula could be:
For instance, for Ore Density 2.0 tonne/m3 the Dump Box (Tray) Volume for the Rear Dump 50 tonne Truck is the following: The SAE Heaped Rate is 27.4 m3. The SAE Struck Rate is 22.6 m3. The Semi Heaped Rate is 25.0 m3.
Speed on grade – the higher speed is – the shorter cycle time will be. Manufacturers publish the speed on grades either as a table, or as a performance curve. However, the actual performance of a truck may vary if a different payload is used, as well as the road surface; tyre model and wear; operator’s technique and other factors.
Cycle Time – This is a measure for the one complete cycle of a mine truck operation including the following phases:
Manoeuvring to a loading spot; loading with a bogger; chute or conveyor; travelling (in most operations where trucking is the main haulage method – up hill); dumping; returning to the loading spot.
The Cycle Time includes only pure continuous operating time. Breaks, service, downtime and other interruptions are covered by availability and utilisation.
Utilisation – The definition of the utilisation is a debatable topic, because it also used to be calculated as the Operating Time/Scheduled Time, also known as the Agreement Utilisation. In this case, the total indicator of utilisation will be always lower than the Operational Utilisation; that is the truck is not used even when it is available. This indicator depends on the particular mine scheduling and methods and is more appropriate for calculation of overall mine productivity, rather than productivity of the truck.
TKPH (tonne-kilometre per hour) – again, because there are no Australian Standards for definitions associated with this term we suggest a formula designed to estimate the overall capacity of a truck to move a certain amount of load over a certain number of kilometres per hour. Over the last few years, the TKPH factor has become very popular in mining for determination of the amount of works for contractors performing trucking and for indication of the overall productivity of the mine.
Operating & Maintenance Cost – it is well known that the running cost of mining equipment depends on the mining conditions it works in. One needs to consider both the operating cost (including fuel, oil and grease, and tyres) and maintenance cost (including regular maintenance & repairs, breakdown repairs and tray rebuilds). These costs will be interdependent with Ownership Cost.
A cheaper surface truck re-designed for underground applications may be initially cheaper to buy, but finally will last shorter and will require more investments in parts and service. Serviceability of trucks and parts support of a particular supplier are other things which are hard to quantify, but impossible to ignore.
Ownership Cost – is a combined value including many factors, such as Capital Cost, Operating & Maintenance Cost, Depreciation, Resale Value and a few others.
The real cost of any industrial equipment can be estimated only in conjunction with its efficiency. Productivity of a mine truck depends on its capability to transport a certain amount of ore (or mullock) within a definite time, thus reliant on the parameters explained above, that is payload and speed on grade.
General design of the machine and number of features offered may be crucial for economical and safe operation of a mine truck fleet. The absence of some features may lead the owner to the higher operating and maintenance cost due to premature failures. Alternatively, investing into additional innovation may bring remarkable savings and safety improvements.
Operator’s Wage and Mine Overheads – every extra underground truck requires three or sometimes four operators. It goes without saying that minimising the number of trucks underground is essential from the environmental and occupational health and safety points of view. Fewer emissions and less working machines underground is an unbeatable argument in favour of the modern high capacity underground trucks.
However, as previously stated, the underground trucks seem to have reached their size limit. The largest available trucks are ranging from 50 to 60 tonnes. A series of trials of an 80-tonner proved its inefficiency (one mine which accommodated such a trial had to change the common mining practices – the smaller laden trucks going uphill had to stop to give way to the bigger one cruising downhill empty).
The skyrocketing cost of truck tyres and their shortage have also questioned the efficiency of the trucking haulage. Few will argue that the cost of diesel will continue to rise. This means that alternatives, such as shafts; conveyors; slurry systems or the good old rail car should be reconsidered.
Australian underground mines are going deeper and while Australia has managed to keep one of the lowest ratio of underground mines in proportion to open cut mines in the world, undoubtedly this will soon change and the viability of the new underground operations will be…in the truck’s tray.

* Dmitry Przhedetsky (M Eng (Mining), FAusIMM) is a director of Rock Cognition Pty Ltd. Contact him at: dmitry@rockcognition.com.au