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Rock Mass Rating (RMR) for Support Plan Design in Underground Mining

Understand RMR concept & its role in designing mine support plans. Learn how to create an RMR-based support plan for B&P development workings at a depth of 300m. Discover RMR adjustment factors & roof classification based on RMR values. Explore RMR applications in support load estimation & monitoring strata support in underground mining.

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Rock Mass Rating (RMR) for Support Plan Design in Underground Mining

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  1. Question: What do you understand by the term RMR? How it is used for designing a support plan for u/g mine workings. Draw a RMR based support plan for B&P development workings at a depth of 300m. Hkkjr dksfdax dksy fyfeVsM

  2. Answer: RMR means Rock Mass Rating. Rock Mass Rating (RMR) is the sum of the five parameter ratings. • Layer thickness 30 • Structural features 25 • Weatherability 20 • Rock strength 15 • Groundwater 10 • If there are more than one rock type, then the combined RMR is obtained as : Σ(RMR of each bed x bed thickness) Combined RMR = ----------------------- Σ (thickness of each bed) Hkkjr dksfdax dksy fyfeVsM

  3. RMR so determined / obtained is further adjusted for various geo-mining conditions such as: • Depth • Lateral Stress • Influence of adjacent and overlying workings • Mode of drivage Finally roof is classified on the basis of adjusted / final RMR as given below: Sl. Range of RMR Class of Roof Description of roof • 0-20 V Very Poor • 20-40 IV Poor • 40-60 III Fair • 60-80 II Good • 80-100 I Very Good Hkkjr dksfdax dksy fyfeVsM

  4. Use of RMR in designing a support plan for underground mine workings: 1. Rock Load is derived with following empirical formula :- Expected rock load = rock density x gallery width x (1.7-0.037 RMR + 0.0002 x RMR2) = 2.215 x 3.6 x (1.7 - 0.037 x 55 + 0.0002 x 55 x 55) = 2.15 te/sq.m supposing gallery width = 3.6m rock density = 2.215 te/m3 ( derived in lab test) Taking factor of safety = 2 Resistance applied = Rock Load X FOS = 2.15 X 2 = 4.30 te/m2 Let Area of Exposure = 3.6m X 2m = 7.2m2 = Hkkjr dksfdax dksy fyfeVsM

  5. Total Resistance required = 4.30 te/m2 X 7.2 m2 • = 30.96 te • suppose one roof bolt takes load of 8 te • Number of roof bolts to be jammed = 30.96/8 • = 4 nos • This way RMR is used for determining the support design. 2. Estimation of Support Load Density(SLD) in depillaring district – RMR is also used to estimate the SLD by following empirical formula: For slice junction, r.H0.50 X K0.64 X W1.17 • SLD = ----------------------- R0.90 Hkkjr dksfdax dksy fyfeVsM

  6. within slice, r. H 0.67 X K 0.84 X W 1.74 • SLD = ------------------------ R 1.42 In the Split Gallery r. H 0.52 X K 0.59 X W 1.12 • SLD = ------------------------ R 1.02 For Goaf Edge r. H 0.54 X K 0.49 X W 0.89 • SLD = ------------------------ • R 0.79 Hkkjr dksfdax dksy fyfeVsM

  7. Where, r = weighted average rock density of the immediate roof (te/m3) • H = depth of cover (m) • K = ratio horizontal to vertical • W = width of split or slice (m) • R = weighted average RMR of the immediate roof rock. 3. Monitoring of Strata Support RMR is also used in calculation of theoretical maximum convergence and convergence velocity of strata • Cm = 40 X B X D 1/3 ( 1-RMR/100)3 (mm)  • Cv = 0.55 X B X D1.7 ( 1-RMR/100)5.6 (mm/day) B = roadway width D = density of roof rock Hkkjr dksfdax dksy fyfeVsM

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