Average bolt installation time is a significant step on the critical path for underground tunnel development. In this case study, a ProdFinderTM time and motion study was conducted to estimate bolt installation time for an inflatable bolt trial for development at an underground gold mine. Because installation time data isn’t ‘normally distributed’, using the numerical mean to calculate the average installation time isn’t correct. In fact, it is rare for time trial data to be ‘normally distributed’ i.e. non-Gaussian.
The time taken to unblock holes dominated the time and motion study. Because all, bar one, of the 54 holes drilled during the trial required unblocking prior to bolt installation, the number of bolts installed was limited to 23. Because unblocking time distribution is not normally distributed (non-Gaussian), the mean (average) time taken to unblock holes was estimated by using maximum likelihood to fit an exponential decay function of the form;
where, the mean of an inverse exponential distribution is given by
1/λ, such that, 1/λ = 1/0.00132 = 756 seconds
PRODFINDERTM probabilistic techniques provided a reliable estimate for the overall effect of bolt installation upon development cycle time. Whilst in this particular case study, the capital development wasn’t affecting revenue, early in the mine life capital development cycle time directly affects the time taken to reach the orebody and start generating cash. Correct evaluation of cycle time components, enables correct decision making.
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Cavities found in wall at Location.
Hole blocked with quartz and altered andesite fragments.
Typical amount and type of fragments cleared from each hole.