Question

The modified Lauffer diagram relates roof span, RMR and stand-up time. For a roof span of 4 m, if the RMR of the rock mass changes from 40 to 60, the stand-up time increases by a factor of ____________ (rounded off to two decimal places).

Hint:

When using the Lauffer diagram, always read stand-up times on the logarithmic axis and compare the curves for the two RMR values at the same span.

Answer 1

Correct Answer: 50

From the modified Lauffer diagram:
For roof span = 4 m:
- At RMR = 40, the stand-up time (read from the lower curve) is approximately: \[ T_{40} \approx 1\ \text{hr}. \]
- At RMR = 60, the stand-up time (read from the middle curve) is approximately: \[ T_{60} \approx 60\ \text{hr}. \]
Thus the factor increase in stand-up time is:
\[ \text{Factor} = \frac{T_{60}}{T_{40}} = \frac{60}{1} = 60. \]
\[ \boxed{60.00} \quad (\text{acceptable range: } 50.00\text{–}70.00) \]