Geomechanics and Ground Control · Ground Control · 1 mark · Numerical answer
A metal-mine drive has roof span 4 m. Using the modified Lauffer chart, find the factor increase in stand-up time when RMR increases from 40 to 60, to two decimals.
Draw a horizontal line at roof span 4 m. Read stand-up times where it intersects the RMR 40 and RMR 60 curves: approximately 20 hours and 1200 hours. The factor is the ratio 1200/20≈60.00. The horizontal time scale is logarithmic, so the ratio must be formed from actual times rather than measured horizontal distances; the graphical reading is approximate.
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