1. Hkkjr dksfdax dksy fyfeVsM
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. Hkkjr dksfdax dksy fyfeVsM
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. 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 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
V Very Poor
IV Poor
III Fair
II Good
I Very Good
Hkkjr dksfdax dksy fyfeVsM

4. Hkkjr dksfdax dksy fyfeVsM
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 ( RMR x RMR2)
= x 3.6 x ( x x 55 x 55) = 2.15 te/sq.m
supposing gallery width = 3.6m
rock density = 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. Hkkjr dksfdax dksy fyfeVsM
Total Resistance required = te/m2 X 7.2 m2
= 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. Hkkjr dksfdax dksy fyfeVsM
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
R 1.02
For Goaf Edge
r. H 0.54 X K 0.49 X W 0.89
R 0.79
Hkkjr dksfdax dksy fyfeVsM

7. Hkkjr dksfdax dksy fyfeVsM
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 = X B X D1.7 ( 1-RMR/100)5.6 (mm/day)
B = roadway width
D = density of roof rock
Hkkjr dksfdax dksy fyfeVsM