Rock mass rating (RMR) plays important role in design and selection of support system (Ghosh, 2000). For stability assessment of rock mass it is very important to know the amount of rock load mobilized around the development gallery which is estimated using RMR (Singh et al., 2003, Barton et al., 1974, Bieniawski, 1984, and Ghosh et al., 1992). In Indian coal mines, Central Mining Research Institute-Indian School of Mines rock mass rating (herein after referred to as CMRI-ISM RMR) is mostly used for formulating design guidelines for supports. In this paper an attempt has been made to correlate CMRI-ISM RMR values and rock load of galleries and junctions for different gallery widths, ranging from 3.6 m to 4.8 m, at different densities of roof rocks. The proposed empirical expression can help in quick design of support system for underground coal mines working in the same regime.
Roof and side falls in underground coal mines constitute the major reason for underground accidents and fatalities even today. Statistical analysis reveals that the share of roof and side falls contributes to 28.5% of the fatalities [
The case studies incorporated in this paper are taken from different mines of Bharat Coking Coal Limited and Tata Steel Limited situated in Jharia coalfield (Figure
Location of Jharia coalfield in India.
Rock mass rating (CMRI-ISM RMR) [
Parameters of CMRI-ISM rock mass rating.
Parameter | Maximum rating |
---|---|
Layer thickness (cm) | 30 |
Structural features | 25 |
Weatherability (1st cycle slake durability index) | 20 |
Compressive strength (Kg/cm2) | 15 |
Groundwater condition (mL/min) | 10 |
Adjustments for depth, lateral stress, induced stresses, method of excavation, and gallery span are made for accounting their positive, neutral, and negative contribution to RMR values as given in Table
Adjustment factors for RMR.
Parameter | Adjustment of RMR | Adjusted RMR |
---|---|---|
(1) Depth | ||
Less than 250 m | Nil | RMR |
250–400 m | 10% reduction | RMR |
400–600 m | 20% reduction | RMR |
More than 600 m | 30% reduction | RMR |
(2) Lateral stresses | ||
Small | 10% reduction | RMR |
Moderate | 20% reduction | RMR |
High | 30% reduction | RMR |
(3) Induced stresses | ||
No adjacent working |
Nil | RMR |
Extraction areas within |
10% reduction | RMR |
Extraction areas within |
Up to 30% reduction | RMR |
Working with 10–20 parting | 10% reduction | RMR |
Working with |
Up to 30% reduction | RMR |
(4) Method of excavation | ||
Continuous miner | 10% increase | RMR |
Undercut and blasting | Nil | RMR |
Blasting off the solid | 10% reduction | RMR |
(5) Gallery span | ||
Less than 4.8 m | Nil | RMR |
4.8–6.0 m | 10–20% reduction | RMR |
After due adjustment, adjusted RMR is used for estimation of rock load in galleries and junctions from the following equations:
Field studies were conducted in different mines and RMR was determined. For determination of RMR three parameters, namely, layer thickness, structural features, and groundwater, were collected during geotechnical studies of roof rocks in the mine site. Compressive strength and 1st cycle slake durability index were determined in the laboratory. The details of investigations carried out are provided in Table
RMR and rock load estimation for galleries and junctions for different mines.
S. number | Name of the mine | Density |
RMR | Rock load in gallery |
Rock load in junction |
---|---|---|---|---|---|
For 3.6 m gallery width where roof rock density varied from 2.2 t/m3 to 2.4 t/m3 | |||||
1 | Huriladih | 2.19 | 57.6 | 1.83 | 2.89 |
2 | Block IV | 2.21 | 60.3 | 1.56 | 2.55 |
3 | Sudamdih | 2.4 | 61.2 | 1.59 | 2.65 |
4 | Huriladih | 2.23 | 55.8 | 2.07 | 3.19 |
|
|||||
For 4.2 m gallery width where roof rock density varied from 1.27 t/m3 to 2.55 t/m3 | |||||
1 | Bagdighi | 2.55 | 62.05 | 1.86 | 2.82 |
2 | Jogidih | 2.38 | 61.2 | 1.85 | 2.76 |
3 | Khas Kusunda | 2.18 | 58.7 | 1.99 | 2.86 |
4 | Khas Kusunda | 2.32 | 63 | 1.59 | 2.44 |
5 | Damoda | 2.4 | 61.2 | 1.86 | 2.77 |
6 | Sendra Bansjora | 2.05 | 55.8 | 2.22 | 3.07 |
7 | Dobari | 1.82 | 60.4 | 1.49 | 2.19 |
8 | Tetulmari | 2.37 | 60.8 | 1.88 | 2.8 |
9 | Sendra Bansjora (Bot) | 1.43 | 38.07 | 3.49 | 4.22 |
10 | Gopalichack | 1.48 | 43.3 | 2.94 | 3.66 |
11 | Kusunda | 1.36 | 36.7 | 3.49 | 4.19 |
12 | Bagdighi | 1.36 | 40.8 | 2.9 | 3.66 |
13 | East Bhuggatdih | 1.52 | 42.4 | 3.13 | 3.88 |
14 | Damoda (BJ Pit) | 1.27 | 36 | 3.34 | 4 |
15 | Amlabad | 1.44 | 41.6 | 3.06 | 3.78 |
16 | Alkusha | 1.49 | 43.2 | 2.97 | 3.69 |
17 | Nichitpur | 1.54 | 42.4 | 3.17 | 3.93 |
18 | Industry | 1.41 | 36.9 | 3.59 | 4.32 |
19 | Ramkanali | 1.44 | 41.6 | 3.07 | 3.78 |
20 | Mudidih (Pit number 3) | 1.51 | 43.74 | 2.94 | 3.67 |
|
|||||
For 4.8 m gallery width where roof rock density varied from 1.35 t/m3 to 2.4 t/m3 | |||||
1 | Godhur | 1.48 | 45 | 3.2 | 3.5 |
2 | South Balihari | 1.39 | 42.4 | 3.27 | 3.6 |
3 | Huriladih | 1.35 | 41.6 | 3.28 | 3.6 |
4 | Godhur incline | 1.47 | 43.2 | 3.35 | 3.8 |
5 | Mahamaya mine, SECL | 1.9 | 61.2 | 1.58 | 2.2 |
6 | Churcha mine, SECL | 2.4 | 56.7 | 2.82 | 3.6 |
7 | Shivani mine, SECL | 2.15 | 57.6 | 2.4 | 3.0 |
8 | Baherabad mine, SECL | 2.26 | 63 | 1.76 | 2.4 |
The correlation analysis done between CMRI-ISM RMR and estimated rock load of galleries (with the suggested rock load equation (
Correlation between RMR and rock load for 3.6 m gallery width.
Correlation between RMR and rock load for 4.2 m gallery width.
Correlation between RMR and rock load of 4.8 m gallery width.
The correlation analysis done between CMRI-ISM RMR and estimated (estimated using (
Correlation between RMR and rock load of junctions (3.6 m wide galleries).
Correlation between RMR and rock load of junctions (4.2 m wide galleries).
Correlation between RMR and rock loads of Junctions (4.8 m wide galleries).
The estimated rock load which is obtained from CMRI-ISM RMR is correlated with the estimated rock load determined for both galleries and junctions which is arrived at by best fit equations in the analysis for 3.6 m, 4.2 m, and for 4.8 m galleries as well as for junctions formed with 3.6 m, 4.2 m, and 4.8 m galleries (Table
Estimated rock load for 3.6 m, 4.2 m and 4.8 m wide galleries and junctions.
Estimated rock load |
Estimated rock load |
|||
---|---|---|---|---|
Gallery (t/m2) | Junction (t/m2) | Gallery (t/m2) | Junction (t/m2) | |
1.8 | 2.8 | 2.1 | 2.9 |
For 3.6 m |
1.5 | 2.5 | 1.8 | 2.7 | |
1.5 | 2.6 | 1.7 | 2.6 | |
2.0 | 3.1 | 2.2 | 3.1 | |
|
||||
1.8 | 2.8 | 1.7 | 2.6 |
For 4.2 m |
1.8 | 2.7 | 1.8 | 2.6 | |
1.9 | 2.8 | 1.9 | 2.8 | |
1.5 | 2.4 | 1.6 | 2.5 | |
1.8 | 2.7 | 1.8 | 2.6 | |
2.2 | 3.0 | 2.1 | 2.9 | |
1.4 | 2.1 | 1.8 | 2.7 | |
1.8 | 2.8 | 1.8 | 2.6 | |
3.4 | 4.2 | 3.3 | 4.0 | |
2.9 | 3.6 | 3.0 | 3.7 | |
3.4 | 4.1 | 3.4 | 4.1 | |
2.9 | 3.6 | 3.1 | 3.8 | |
3.1 | 3.8 | 3.0 | 3.8 | |
3.3 | 4.0 | 3.5 | 4.1 | |
3.0 | 3.7 | 3.1 | 3.8 | |
2.9 | 3.6 | 3.0 | 3.7 | |
3.1 | 3.9 | 3.0 | 3.8 | |
3.5 | 4.3 | 3.4 | 4.1 | |
3.0 | 3.7 | 3.1 | 3.8 | |
2.9 | 3.6 | 2.9 | 3.7 | |
|
||||
3.2 | 3.5 | 3.1 | 3.6 |
For 4.8 m |
3.2 | 3.6 | 3.3 | 3.7 | |
3.2 | 3.6 | 3.4 | 3.8 | |
3.3 | 3.8 | 3.3 | 3.7 | |
1.5 | 2.2 | 1.9 | 2.7 | |
2.8 | 3.6 | 2.3 | 2.9 | |
2.4 | 3.0 | 2.2 | 2.9 | |
1.7 | 2.4 | 1.8 | 2.6 |
Regression equations between CMRI-ISM RMR and rock load of galleries and Junctions.
Case number | Parameters | Regression equations |
|
---|---|---|---|
1 | RMR versus rock load |
|
0.94 |
2 | RMR versus rock load |
|
0.98 |
3 | RMR versus rock load |
|
0.86 |
4 | RMR versus rock load |
|
0.94 |
5 | RMR versus rock load |
|
0.96 |
6 | RMR versus rock load |
|
0.72 |
Correlation between estimated rock loads for 3.6 m galleries.
Correlation between estimated rock loads for junctions.
Correlation between estimated rock loads for 4.2 m galleries.
Correlation between estimated rock loads for junctions.
Correlation between estimated rock loads for 4.8 m wide galleries.
Correlation between estimated rock loads for 4.8 m wide galleries and junctions.
The significance of
S. number | Group 1 versus group 2 | Mean group 1 | Mean group 2 |
|
df |
|
Remarks |
---|---|---|---|---|---|---|---|
1. | RMR versus calculated rock load |
67.5 | 21.8 | 2.0 | 8 | 0.05 | Number of data are less |
2. | RMR versus calculated rock load |
67.5 | 22.6 | 2.0 | 8 | 0.05 | |
3. | RMR versus calculated rock load |
51.0 | 7.3 | 7.5 | 40 | 0.000000 | Number of data are good |
4. | RMR versus calculated rock load |
51.0 | 8.1 | 7.4 | 40 | 0.000000 | |
5 | RMR versus calculated rock load |
51.3 | 2.7 | 15.0 | 14 | 0.000000 | Number of data are moderate |
6 | RMR versus calculated rock load |
51.3 | 3.2 | 14.8 | 14 | 0.000000 |
An analysis is carried out to study the influence of varying RMR on rock load estimated from best fit equations to assess the behavior of the developed equations for the both galleries and junctions with the varying width from 3.6 m to 4.8 m (Figures
Plot between assumed RMR and rock load for 3.6, 4.2, and 4.8 m galleries.
Plot between assumed RMR and rock load for 3.6, 4.2, and 4.8 m junctions.
This study indicates that the rock load of galleries and junctions of various coal measures rocks of India can be estimated by using simple empirical relationships after substituting only the value of RMR. All the six cases showed linear relationship with each other. The empirical expressions for rock load estimation in coal measure roof rocks for 3.6 m, 4.2 m and 4.8 m, gallery width are as follows:
Strong coefficient of determination was found in all the six cases shown. Developed equations are applicable for 3.6 m, 4.2 m, and 4.8 m gallery width with density in the range of 2.2 t/m3–2.4 t/m3for 3.6 m gallery width, 1.27 t/m3–2.55 t/m3 for 4.2 gallery width, and 1.35 t/m3–2.4 t/m3 for 4.8 m gallery width. Equations are practical, simple, and reasonably accurate to apply. This study, coupled with judicious judgment, can be helpful for arriving at the initial estimates of rock loads in development galleries and junctions of underground coal mines and thus can help in support design with greater safety and stability for Indian geomining conditions. The variation in rock load behavior in different gallery widths can be attributed to variation in roof rocks density and RMR range. Lack of enough data sets also lead to this variation thus pointing to the need for including more data for realistic predictions. A relook into the parameters considered for rock load estimation is also necessary to make more wholesome predictions.
Central Mining Research Institute-Indian School of Mines
Rock mass rating
Rock load
Bharat Coking Coal Limited.
The authors declare that they do not have conflict of interests regarding of the publication of this paper.
The authors would like to thank the Director of CIMFR, Dhanbad, and Director of ISM, Dhanbad and also the Mine Management for their kind support during the research work. The authors are also thankful to Mr S K Singh Senior Principal Scientist and Head, Geo-mechanical Lab. CIMFR, Dhanbad for his sincere help during testing of rock samples. The work forms a part of the Ph.D. work of the first author being carried out at ISM, Dhanbad, Jharkhand, India.