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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.

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.

Answer:RMR means Rock Mass Rating. Rock Mass Rating (RMR) is the sum of the five parameter ratings.

1. Layer thickness 302. Structural features 253. Weatherability 204. Rock strength 155. 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

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 drivageFinally roof is classified on the basis of adjusted /

final RMR as given below: Sl. Range of RMR Class of Roof

Description of roof1. 0-20 V Very Poor2. 20-40 IV Poor3. 40-60 III Fair4. 60-80 II Good5. 80-100 I Very Good

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• 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 = 3.6mrock density = 2.215 te/m3 ( derived in lab test)Taking factor of safety = 2Resistance applied = Rock Load X FOS

= 2.15 X 2 = 4.30 te/m2Let Area of Exposure = 3.6m X 2m = 7.2m2

=

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• 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

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within slice,r. H 0.67 X K 0.84 X W 1.74

• SLD = ------------------------R 1.42

In the Split Galleryr. H 0.52 X K 0.59 X W 1.12

• SLD = ------------------------R 1.02

For Goaf Edger. H 0.54 X K 0.49 X W 0.89

• SLD = ------------------------• R 0.79

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• 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

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