Variation in performance can be a major contributor to reduction in efficiency and it is this variation which must be understood and controlled if the operation is to achieve their strategic goals. In statistics, a result within three standard deviations of the average is considered to be under control. Given a normal distribution of results, 0.14% of cycles should be expected to be more than 3 standard deviations above the average. Under normal circumstances the key “interaction parameters” – Wait on Trucks and/or Wait on Loaders, will not be normally distributed and should be skewed strongly to the right or “positively skewed”. (Skew or skewness is the lack of symmetry in a frequency distribution. Positive skew has a long tail to the right of the peak – high percentage of results with a low result.) Most mines have both wait on truck and wait on loader very strongly and significantly skewed to the right. In truck and loader operations most wait on truck and wait on loader results, up to 5% of cycles can be more than three standard deviations above the average. However, to meet one of the two key strategies our mines follow most of the time you actually only want one of these strongly skewed.
A high skew (maximum frequency of low values with a long tail to the right) on both parameters is required for optimising efficiency but this will deliver neither maximum output nor minimum cost. The higher the measure of skew the more efficient the operation. It is not unusual for the value of skewness statistic divided by standard error of skew to be over 100. A significant difference in skewness statistic / standard error of skew for wait on trucks cf wait on loader is an indicator of overtrucking (skew of wait on truck is stronger than wait on loader) or undertrucking (skew of wait on loader is stronger than wait on truck) and one of these is what is required for most mines.
Another way of measuring the efficiency of truck and loader usage is the proportion of time where the truck and loader wait for less than 30 seconds (excluding spotting). To optimise the mine’s execution of strategy it is often necessary to have these two measures significantly different. For example you might find that wait on trucks is less than 30 seconds 90% of the time and wait on loader is less than 30 seconds 35% of the time. This demonstrates a strongly over-trucked scenario. This will deliver high system output but will not be the most cost effective way to operate the fleet. However, if it is your mine’s strategy to optimise output at any cost then being overtrucked is a good thing. These results can be reported on a month by month basis to demonstrate the strength of the overtrucking (maximum wait on truck events less than 30 secs and minimum wait on loader events less than 30 secs).
As already discussed most operations usually follow one of two strategies in relation to matching number of trucks to the loading unit. These operations either follow an over trucked or an under trucked approach. Under trucking is a lower cost option, delivering a higher utilisation on the trucks while sacrificing the loading units utilisation. Over trucking will cause a higher cost per tonne, will have a higher utilisation on the loading units and a lower utilisation on the truck fleet although moving more material. Sometimes an approach will be taken to attempt to optimise output and cost but this usually ends in underperformance in profit and/or output.
The challenge for all mines is how best to represent this match of numbers of trucks reporting to each loading unit in a way which makes the outcome meaningful for ongoing optimisation. The optimal matching of trucks is the critical element for a loading unit to achieve its required production rate and it is essential that the supply of trucks to the loading unit is sufficient to meet the required output. In most cases the average haul distance varies from the beginning of a new bench to the end of the bench and truck numbers need to taken into account as well as managing the dump areas (long and short dumps).
Wait on truck delays (loading unit entered delays) that are less then 2 minutes in duration are generally considered to be part of normal operational delays. Wait on truck is typically where there are no trucks available to be loaded by the loading unit and highlights one or more of the following problems in the circuit:
· The loading unit is under trucked.
· The trucks are being delayed in the circuit by one or more of the following:
· Delays on the dump, waiting for dozer work or queuing.
· Haul road grades too steep, poor road conditions, grading of roads etc. slowing trucks down.
· Sub optimal Operator performance / speed / technique
· Dust / weather / blasting etc.
If the trucks are queued, waiting to load, this is called wait on loader. Most mines that have relatively high wait on loader time (queue time) are over trucked.
Some mines operate day-to-day using a Match Factor, i.e. where a MF of 1 means that the number of trucks are perfectly matched to the digger such that the cycle times are integrated and should no delays occur, trucks will arrive and depart in a perfect scenario exactly matching the digger’s truck requirement. The method of calculating MF varies but the formula used by GBI is
MF = (1-%wait on loader time)*(1-%wait on truck time)
This is only of value where a mine is passing through this phase of balancing output and cost. Fleets operating a strategy of optimising the balance between loaders and trucks should achieve an MF >=0.70. This allows for the typical mining delays and means that as a result of them, some time the digger waits for trucks, and other times trucks are queued at the same digger (normal every day mining). For most mines the important factor is a comparison of either wait on truck or wait on loader. This will give them a direct indication of how well they are meeting their strategy.
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