The bottom section of a stoping block has dimensions 200 m $\times$ 40 m. If the modified RMR of the rock mass is 50, the appropriate method of mining on the basis of Laubscher’s chart in the figure is:
Show Hint
Supportable rock masses (moderate RMR and moderate hydraulic radius) are ideal for cut-and-fill mining because backfill provides continuous ground support during excavation.
To determine the suitable mining method, we use Laubscher’s chart, which classifies the stability of an underground excavation based on the hydraulic radius (HR) and the modified Rock Mass Rating (RMR). Step 1: Compute Hydraulic Radius (HR)
Given dimensions: 200 m $\times$ 40 m
Area:
\[
A = 200 \times 40 = 8000\ \text{m}^2
\]
Perimeter:
\[
P = 2(200 + 40) = 480\ \text{m}
\]
Hydraulic Radius:
\[
HR = \frac{A}{P} = \frac{8000}{480} \approx 16.67\ \text{m}
\] Step 2: Use Laubscher’s chart with RMR = 50 and HR ≈ 17
Locate the coordinate (HR ≈ 17, RMR = 50) on the chart.
This point falls inside the Supportable region, not the purely “Stable” region.
A rock mass classified as Supportable indicates:
• The rock mass is not weak enough for caving methods like block caving.
• It is not strong enough for unsupported stoping like shrinkage stoping.
• Moderate support must be installed during excavation. Step 3: Select the correct mining method
Cut-and-fill mining is specifically meant for supportable ground where:
• The rock requires controlled excavation.
• Backfilling provides local support.
• The rock mass rating is moderate (RMR 40–60).
Check each option:
(A) Shrinkage stoping → requires highly competent, stable rock → incorrect.
(B) Cut and fill → used in supportable rock masses (RMR 40–60) → correct.
(C) Block caving → requires weak rock (RMR<30) → incorrect.
(D) Sublevel stoping → needs very good rock (RMR>60) → incorrect.
Therefore, the correct method is Cut and fill. Final Answer: Cut and fill
