What is Rock Mass Rating?

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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.

2. Answer:
RMR means Rock Mass Rating.
Rock Mass Rating (RMR) is the sum of the five
parameter ratings.
1.Layer thickness 30
2.Structural features 25
3.Weatherability 20
4.Rock strength 15
5.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)
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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 RMRClass of RoofDescription
of roof
1.0-20 V Very Poor
2.20-40 IV Poor
3.40-60 III Fair
4.60-80 II Good
5.80-100 I Very Good
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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 RMR
2
)
= 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
=
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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.H
0.50
X K
0.64
X W
1.17
•SLD = -----------------------
R
0.90
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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
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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 D
1.7
( 1-RMR/100)
5.6
(mm/day)
B = roadway width
D = density of roof rock
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