Management of outburst in underground coal mines

International Journal of Coal Geology 35 Ž1998. 83–115

Management of outburst in underground coal mines R.D. Lama

a,1

, J. Bodziony

b,)

a

Mining Technology Kembla Coal and Coke Pty. Limited, PriÕate Mail Bag 14, Campbelltown, NSW 2560, Australia b Strata Mechanics Research Institute, Polish Academy of Sciences, ul. Reymonta 27, 30-059 Krakow, Poland Received 12 November 1996; revised 16 May 1997; accepted 16 May 1997

Abstract The paper outlines the phenomenon of outbursts of gas and coal in underground coal mines and provides general statistics of their occurrence. Various factors that influence this phenomenon such as geological conditions, physical properties of coal, gas content and gas pressure are discussed. The indices used in different countries for the prediction of conditions that can produce and cause outbursts are presented. Control measures and prediction techniques are discussed with an emphasis on the management systems, decision making and the use of risk analysis to predict the consequences of outbursts. Where control methods cannot be adopted and mining has to be done, procedures for outburst mining are given. Data collection sheets on the occurrence of outburst are provided. An example of a technological development in the remote control of continuous miners is given. q 1998 Elsevier Science B.V. Keywords: outbursts; prediction indices; prediction techniques; risk assessment

1. Introduction An outburst is sudden ejection of gas and coal from a coal face. There are known cases of outbursts of gas and rock resulting from the disintegration of the immediate roof or floor rock of the worked coal seam. One of the causes of difficulties connected with the definition of an outburst is the lack of distinction between the quasi-static and the dynamic phenomena induced by mining. Ryncarz and Majcherczyk Ž1986. define the outburst as a gas-geodynamic phenomenon. It is a phenomenon which may be instantaneous or may last over several minutes. The amount of material ejected may vary from a ) 1

Corresponding author. Deceased.

0166-5162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 6 - 5 1 6 2 Ž 9 7 . 0 0 0 3 7 - 2

Australia ŽBowen . Australia ŽIllawarra . Belgium ŽSouthern . Bulgaria ŽBalkan . Canada ŽNanaimo. Canada ŽCrows Nest. Canada ŽCanmore. Canada ŽSydney . China ŽShanxi Prov. Chongquing . China ŽShanxi Prov. Yangquam . China ŽHenan Prov. Jiaozuo . China ŽSichuan Prov. Nantong . China ŽSichuan Prov. Kaiping . China ŽLiaoning Prov. Beipao . China ŽLiaoning Prov. Beipao . China ŽHuainan . China ŽLiu Zhi. Czech Republic ŽOstrava-Karvina. Czech Republic ŽSlany .

1 2 3 4 5 6 7 8 9

19

16 17 18

15

14

13

12

11

10

2

1

Country Žbasinrfield .

Table 1 Worldwide occurrence of outbursts

815

80

coal Ž CH 4 , CO 2 . rock Ž CO 2 .

- 100 - 100

coal ŽCH 4 . coal ŽCH 4 . coal ŽCH 4 .

coal ŽCH 4 .

coal ŽCH 4 .

coal ŽCH 4 .

coal ŽCH 4 .

100 – 200

100

coal ŽCH 4 . coal Ž CH 4 .

200 703

130 180

95 400 250

4

Min. depth of out-bursts Žm .

coal Ž CH 4 . coal ŽCH 4 . coal ŽCH 4 . coal ŽCO 2 , CH 4 . rock ŽCH 4 , N 2 , C 2 H 6 .

coal ŽCO 2 , CH 4 . coal ŽCO 2 , CH 4 . coal ŽCH 4 .

3

Mineral mined Žgas type .

Slany

J.Sverna

a26

Collinsville Metropolitan

5

colliery

First outburst

7 220 450

1986

1894

1955

815

226

8

1989 1927 1928 1954 1984

1986, 1990

1894, 1988

1969, 1976 1959, 1975 1950, 1981

1951, 1986

600

Till, 1983

Till, 1992

1982, 1986

1951, 1971

1933, 1921, 1903, 1944, 1977,

1954, 1980 1895, 1995 1847 – 1908, 1954–1965

8

469

84 211 9845

1688

y1141

268

466

596

Slany

J. Sverna

a26

1990

1983

1973

1983

1967 1922 1904

Tverdnica Cassidy Carbonado

) 250 268 ) 200 43 37

11

10

Collinsville ŽBowen . 1954 Ž220 . West Cliff ŽBulli . 1976 1959

Year Ž depth . Žm .

9

colliery Žseam .

Largest outburst

) 220 449 357, 130

number

Outburst experienced depth period Žm .

1933 before 1921 1903 1944 200 1977 707

1954 1895 1847

6

year

4310 Ž 96000.

580

12780

100 Ž 3000 .

1500

525 Ž 18750.

316 Ž750 .

180 1200 3500 Ž60000 – 140000 .

500 q 500 Ž14000 . 400 Ž18000 . 1600

12

Size: coal q rock Žt. Žgas Žm 3 ..

84 R.D. Lama, J. Bodzionyr International Journal of Coal Geology 35 (1998) 83–115

200 183

41 Ukraine ŽDonetsk . coal Ž CH 4 . 42 United Kingdom ŽWest Wales. coal ŽCH 4 .

Novaya Smolanka Jarrow

Kozlu

110

Kapitalnaya Bursunka a2 – 5

280

Twistdraai

Cezar Zofia Manifest Lipcowy

Miike Yotsuyama

200 160

coal coal coal coal

coal ŽCH 4 . coal ŽCH 4 . coal ŽCH 4 .

80

400

1150 140

270

de Brassac Fontanes

Issac

Ž CH 4 . ŽCH 4 . ŽCH 4 . ŽCH 4 .

34 Republic of South Africa ŽMain Karoo . 35 Russia ŽKuznetsk . 36 Russia ŽPechora . 37 Russia ŽYegorshynskoKamenski. 38 Russia ŽFar East. 39 Taiwan ŽTaiwan . 40 Turkey ŽZonguldak .

coal ŽCH 4 .

ŽCH 4 . ŽCH 4 . ŽCH 4 . ŽCH 4 .

33 Poland ŽUpper Silesian .

coal coal coal coal

ŽCH 4 . ŽCH 4 . ŽCH 4 . ŽCH 4 . ŽCO 2 . ŽCO 2 . ŽCO 2 , CH 4 . ŽCH 4 .

coal ŽCO 2 , CH 4 .

Germany ŽIbbenburen ¨ . Hungary ŽMecsek . Japan ŽHokkaido and Kyushu . Kazakhstan ŽKaraganda .

coal coal coal coal coal coal coal coal

32 Poland ŽLower Silesian .

28 29 30 31

20 Czech Republic ŽRosice-Oslavany . 21 France ŽNord-Pas-de-Calais. 22 France ŽLorraine . 23 France ŽLoire . 24 France ŽDauphine . 25 France ŽCentre . 26 France ŽGard . 27 Germany ŽRuhr .

300

200

340

80

573

323

1906 728 ; 1830

1962

1944 1927

1993 1943 1950

1979

1894

1970 1894 1926

1843 1940 1856 1879 1903

1963 1977 1986 1993

1946, 1988 1901, 1980

1944, 1927, 1972, 1962,

1993, 1994 1946, 1988 1950, 1989

1979, 1986

1894, 1994

1970, 1993 1894, 1989 1926, 1986 until 1988

Till, 1987 1912, 1959 1919, 1964 Till, 1980 Till, 1985 Till, 1986 1879, 1989 1956 – 1970, 1971–1993

4689 219

190 72 60 58

5 132 127

7

1731

Karadon Gagarin ŽMazurka . Cynheidre

Twistdraai Ža4 .

Manifest Lipc. Ž403r1 .

Nowa Ruda ŽFranciszek .

Ibbenburen ¨ Zobak Yubari

Haupt-schacht

21, 98 240 ; 600 920 45

Fontanes

Ricard 40 26 275 172 ) 6300

5

220 Ž66000 .

5600 Ž100000 .

1330 Ž130000 .

1270 Ž400000 .

1969 Ž710 . 1971

1975 Ž360 .

1994

1985

1958 Ž550 .

14500 Ž600000 . 400 Ž60000 .

700 Ž11000 .

200

95 Ž5000 .

5000 Ž750000 .

1975 2500 Ž12000 . 1981 1800 Ž27000 . 1981 Ž1138 . 5200 Ž600000 .

1958

1921

1983

1938 Ž240.

R.D. Lama, J. Bodzionyr International Journal of Coal Geology 35 (1998) 83–115 85

86

R.D. Lama, J. Bodzionyr International Journal of Coal Geology 35 (1998) 83–115

fraction of a ton to tens of thousands of tonnes resulting in damage to the excavation, equipment and loss of life. It may occur during the drilling, cutting process, with blasting and with a delay from a few seconds to several minutes after the operation has ceased. The amount of gas released may be from a few to several times the amount of gas present in the ejected coal. Outbursts of gas, coal and rock is a worldwide phenomenon. Perhaps over 30,000 outbursts have occurred in the world coal mining industry. The most outbursts, almost one third of the total, have occurred in China followed by CIS countries leading the world in the frequency of occurrence of outbursts. Table 1 summarises the occurrences worldwide. Outbursts have been the cause of major disasters in the world mining industry. The disastrous mine outburst resulted in 187 deaths in the Piast area of Nowa Ruda Colliery, Lower Silesian coal basin in 1941 ŽPolak and Szewczyk, 1983.. The largest outburst in a coal mine ejected 14,500 tonnes of coal with 600,000 m3 of gas and occurred in Gagarin Colliery, Donetsk basin in Ukraine. The largest outburst in a salt mine occurred in Werra salt ŽMenzengraben mine, Germany. in 1953 when 100,000 tonnes of salt was ejected with 700,000 m3 of carbon dioxide ŽDuchrow, 1958.. Outbursts have occurred at depths as little as 41 m ŽKowing, 1977.. The outburst ¨ intensity and frequency have increased with depth of mining. However, there is no clear correlation with depth mostly because of advances in understanding the phenomena, application of new technologies and control measures. Due to the variability in the size of the outbursts and the material involved, the shape and size of the associated cavities, vary greatly. The cavity may have a small core or covers a very large area particularly when a geological structure is associated with it.

2. Factors influencing outbursts The most important factors that influence the occurrence of outbursts are: Ž1. gassines of the coal seam, Ž2. tectonics, Ž3. properties of the coal and rock and Ž4. vertical and lateral stresses occurring in the seam andror rock or part of the coal seam. The role of each factor may vary from coalfield; hence, the emphasis that needs to be placed in any investigation will also vary. In a virgin area it is not easy to define the role of these factors. Notwithstanding these, the basic factors defined above are the starting points in any investigation. The interpretation and the relative role of these factors will become clear at a later stage when preliminary excavations are driven Že.g. shafts, crosscuts or roadways. in the coal. It is almost impossible to predict whether an outburst will always occur and if so what size. However, it is possible to predict the probability of occurrence with some degree of confidence if sufficient data have been collected. 2.1. Geological factors Geology plays a very important role in the outburst process. Coal seams of complex geological structure are liable to outbursts if high gas conditions are prevalent. Taylor Ž1852–1853., while investigating methane explosions in a north England coalfield ŽNorthumberland and Durham., was the first to discover the phenomenon of sudden gas

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and coal outbursts. He observed that outbursts occurred in the immediate vicinity of tectonic disturbances and that the ejected coal comes from that part of the seam which is soft and disturbed by the loss of its texture. Taylor’s observations have been confirmed by statistical analyses of data from different coalfields. In the statistical analysis of the outburst risk the whole complex of the geological factors should be taken into consideration. Such an approach has been proposed by Cyrul Ž1992., Dybciak Ž1994. and Dubinski ´ et al. Ž1994.. However, because of the lack of statistically representative data, the statistical analysis is, in general, limited to the correlation of individual geological factor with the parameters characterising the outbursts, Že.g. the size, the intensity or the frequency of their occurrence.. From among all the geological factors two groups can be distinguished: Ž1. parameters characterising directly the occurrence and geometry of the coal seams and Ž2. parameters characterising the tectonic disturbances of the coal seams and neighbouring rocks. This division, however, is not adequate. The first group comprises factors characterising directly the occurrence and geometry of coal seams. These factors include: the depth of occurrence, angle of dip, thickness of the seam, etc. These represent the geometrical features of the geology of a coal deposit and may be easily and accurately determined. Examples of parameters associated with the depth of occurrence of coal seams include: Ž1. vertical stress vector acting upon the skeleton of the coal substance and along with it the two horizontal stress vectors, Ž2. gassiness of the coal seam and together with it the state of free and sorbed gas in coal, Ž3. amount of energy in different forms, accumulated in coal and rock substance and in the gas contained in the pore space, Ž4. nature of the coal, its metamorphic state, degree of coalification, volatile matter and other features which change with the state of metamorphism and Ž5. change in temperature and its effect on the desorption process of gases from coal. The role of depth in the occurrence of sudden outbursts has already been indicated by Gerrard Ž1899–1900. in his observations concerning exploitation of the Lancashire coalfield, UK. According to Hargraves Ž1983a,b. the dependence of outburst intensity at depth is more pronounced in the case of steep dip-angles since under these circumstances a number of seams can be encountered. The effect of depth is also more obvious in the case where vertical and horizontal stress is dominant. The effect of dip of the seam or the so-called gravity effect ŽNekrasovski, 1951. acts in various ways on the initiation and propagation of outbursts. In steep seams, outbursts occur more often at lower gas content when compared with flat seams. Caverns created as a result of an outburst in steep seams, in general, run parallel to the dip direction and rarely along the strike ŽPlatonov, 1989.. Outbursts in roadways driven parallel to the dip occur rarely. In general, thicker seams are more liable to outbursts than the thinner beds. In thicker seams the probability of softer coal increases and the total amount of gas available for the transportation of outbursted coal is higher. The amount of accumulated energy that can initiate an outburst is higher when compared with a thinner seam. Nevertheless, Swift Ž1964. in his study of outbursts in South Wales, where thickness of seams varied from 0.6 to 4.6 m, found no correlation between seam thickness and outbursts. He confirmed, however, that the size of the outbursts was much larger in thicker coal seams.

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Ujihira and Higuchi Ž1986., citing the work of Ujihira and Hashimoto Ž1976., give correlation between the number of outbursts and the following parameters in Japanese mines: Ž1. number of coal seams per 100 m depth of occurrence, Ž2. total number of coal seams and Ž3. total thickness of the coal seams. The coefficients of correlation quoted by them for the above three parameters are 0.739, 0.693 and 0.620, respectively. The second group comprises factors characterising the disturbances of the primary geometry of the coal seams as a result of the action of tectonics and volcanism. These factors include: folds, fracturing, faults, shear zones, changes in seam thickness and magmatic intrusions. The zones of coal seams, subjected to the effects of tectonics and volcanism, form areas which are often very difficult to detect. The occurrence of the areas of the interaction of tectonics and volcanism may have a great influence on the process of initiating and on the progress of outbursts. In these areas the geometry of the coal seams and the mechanical, physical and physico-chemical properties of the coal, which directly favour the occurrence of outbursts, undergo sudden changes. Fracturing is the basic element of tectonic disturbances. Studies on fractures in coal show that these can be divided into two main classes: Ž1. micro- and macro cleat systems and joints and Ž2. large joints and low angle shears. These fractures together with the bedding planes result in anisotropic behaviour of coal both from the geotechnical as well as from the gas filtration point of view. Ammosov et al. Ž1957. distinguished two mechanisms for the origin of cleat in coal. Endogenetic cleat is formed during the process of physical changes in the properties of coal during the metamorphic process. Exogenetic cleat is formed as a result of the external stresses on the coal seam, particularly tectonic stresses. Close Ž1993. has introduced a third class of the cleat system which is both endogenetic and exogenetic and has called it duogenetic. Coal seams liable to outbursts have high percentage of clarain. These have highly fractured coal plys with high density of micro-cracking Ž) 7 mmrmm2 .. Coal seams not liable to outbursts have crack density - 3 mmrmm2 . Normal faults are often not outburst prone when these are clean and not associated with gouge. However, in many mines, even normal faults of ) 0.1 m displacement have given rise to outbursts. Normal faults, which have a throw greater than the coal seam thickness, result in complete stoppage of gas flow and are hence more dangerous than those with throw less than the seam thickness. Faults without any gouge are less dangerous. The greater the thickness of the gouges, the greater is the danger of outbursts. Faults which produce torsion in the coal seam that can result in changes in the seam thickness are very important. These faults can also result in locked stresses. The thrust faults of 0.1 m thick are not outburst prone in some mines. However, this is not a definitive conclusion, as the thickness of a gouge may change suddenly at the same place through the coal seam thickness from roof to floor. Strike-slip faults generate more gouge than thrust faults. Gouge thickness is lower on normal faults. When a mine area is split into two subareas by a major fault, it is essential that both these subareas are examined separately. It is quite possible that one of the subareas, particularly on the downthrown side may be characterised by different conditions related to gas regime. Shear zones occurring in one coal seam may be completely absent in the neighbouring seam. In-seam shears may be concentrated in one coal seam. For example, in Ibbenburen, a53 seam is a massive bed, 0.2–0.3 m thick. This bed consists of ¨

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interbedded coal and mudstone, while the rest of the coal seam is completely sheared. The neighbouring a54 seam is much more massive and much less sheared ŽPaul, 1980.. Cleat in coal, particularly the shearing of the cleat and presence of the sheared material between the cleat surfaces ŽWood and Hanes, 1982., is an indication of high stress and deformation. Studies in Crows Nest coalfield, Canmore district, Alberta, Canada have shown that in most coal seams liable to outbursts, the cleat system was destroyed; in one of the coal seams Ža4 seam., the cleat system was remarkably well preserved. The movement required along a fault to obliterate the cleat in a coal seam may not exceed a few millimetres ŽNorris, 1958.. The Royley seam, Broad Oak Colliery, Lancashire, UK has a sheared layer of coal 0.45 m thick, and all outbursts occurred in this seam ŽGerrard, 1899–1900.. In core samples, zones of sheared coal within the bedding planes may be visible in hand specimens. Where coal seams are interbedded with sandstone beds, there is high probability that the shearing will occur in the seam. Shaley softer, less stiff rocks will tend to protect the coal seam from shearing and, hence, create conditions less liable to outbursts ŽNorris, 1958.. In West Wales coalfield, it is seen that coal seams with a sandstone and siltstone roof rocks are liable to outbursts. The Green Vein coal seam with shale roof rock has experienced no outbursts ŽPrice, 1959.. On the South Coast of Australia, mining of the Bulli coal seam with massive sandstone and siltstone roof rocks, has shown that the frequency of outbursts is much higher when compared with mines with mudstone roof rock. The frequency of occurrence of shear zones in the part of the coal seam with massive sandstone is also much higher. In Donetsk coalfield, Ukraine, virtually all coal seams liable to outbursts have strong roof and floor rocks Žsandstones and sandy shales. ŽNekrasovski, 1951.. Thus, it is important to examine the characteristics of roof and floor rocks. Sandstone beds interbedded with the coal seams of low tensile strength and high porosity can lead to outbursts from the roof and floor. Sandstones with ) 7% porosity are highly liable to outbursts if their tensile strength is lower than the pressure of gas contained in the pores. Seam thickness and sudden change in the thickness of the coal seam by pinching out, splitting of coal seams by sandstones and the presence of stone rolls favour mine outbursts. Outbursts have occurred in coal seams as thin as 0.5 m in China. When considering changes of seam thickness, it is important to develop the rate of change over short distances, for example 20–50 m. Outbursts are also more frequent where coal seams start thinning out or at points where the maximum thickness is reached ŽShepherd et al., 1981.. Changes in the structure of the coal seam can be studied both at microscopic and submicroscopic level. Microporosity of coal is important as it determines both the capacity of the coal to sorb gas, as well as its permeability at matrix level. Coals containing mean pore size diameter - 75 mm Žporosity based upon mercury porosimetry. are more liable to outbursts. Structural changes in coals can be classified using one or more of the following criteria: Ž1. disturbance in stratification, Ž2. average distance between microfractures, Ž3. changes in thickness of coal beds and Ž4. changes in strength of the coal. The higher the degree of change, the greater the proneness to outbursts. Coal seams where the structure of coal has been damaged or completely obliterated are most liable to outbursts. For example the Roman seam in Lower Silesian

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coalfield, Poland ŽSuchodolski, 1977. and Upper Marsh seam in Canmore, Alberta, Canada ŽNorris, 1958.. Strength of coal Žply by ply basis. is important in assessing outburst potential of coal seam. Variations in the strength of plys and in thickness over distances of 20–50 m are important. Large variations Ž20–40%. are an indication of susceptibility to outbursts ŽPetrosyan et al., 1983.. Coal seams with a Protodyakonov strength index of f - 0.5 are highly liable to outbursts ŽZhang, 1995.. Outbursts generally do not occur in low rank coals though gas related phenomena have been reported in Valenje Lignite Mine, Slovenia ŽZavsek et al., 1995.. Coal rank ) 1.2% Žvitrinite reflectance. is considered the baseline ŽWilliams and Rogis, 1980.. The presence of dikes and sills and sudden changes in the rank of coal as a result of contact with the volcanic rocks result in changes in gas sorption capacity, disrupt normal flow of gas during mining, and change gas pressure regime. Dikes may also be associated with some minor faulting, hence pulverisation of coal. Geological structures play a very dominant role in outbursts. Thus, delineation of structures is very important. Determination of the existence of faults and igneous intrusions such as dikes and sills is a common practice in general geological investigations prior to the opening of a coal deposit. Seismic 2D or 3D methods presently cannot be used to predict structures of less than 5 m Žperhaps 3 m. of thickness, particularly if the displacement of the fault is less than the whole coal seam thickness. Thrust faults or strike-slip faults cannot be predicted prior to opening a coal deposit. Herein lies the greatest difficulty as even small faults with less than 100 mm vertical displacement can be the loci of an outburst. 2.2. Gas content An essential element in an outburst of gas, coal and rock is the gas content of the coal seam. There is a certain minimum gas content that must be present if an outburst is to occur in a coal seam. This critical value is dependent upon the overall strength of the coal seam or part of the coal seam, permeability of the coal seam, and other geological conditions associated with it. In general, a gas content greater than 8 m3rt Žresults are quoted on dry and ash free DAF-basis. is considered enough to initiate an outburst if other conditions are favourable ŽBrandt, 1987; Lama, 1995.. While the methods for the determination of gas content vary ŽEllicot, 1983. an important point to note is the variation in the gas content in different plys of the seam. Some plys may have a gas content 3–4 m3rt higher than other plys. These plys, if they contain high percentage of durain and high density of cracking Žsofter., will become the loci of outbursts even when the rest of the seam is not liable to outbursts. Consideration should also be given to the effects of weathering and gas content. Weathering changes the sorption capacity of the gases considerably. Also the effect of weathering is different for various gas types and on different coal macerals ŽBeamish and Crosdale, 1995.. 2.3. Coal permeability Seams or sections of the coal seam with permeability ) 5 mD are not liable to outbursts. Permeability studies need to be conducted in-situ. Slug tests in vertical

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boreholes drilled from the surface is a useful technique in estimating permeability of the seam. Variation in the seam permeability through its section is more difficult. Use of packers installed in a section of the seam is possible, but very difficult in moderately thick seams. The analysis of the data obtained for such tests is also questionable. When the permeability is determined from core samples, the coals with permeability - 10y3 mD are highly outburst prone and the coals with permeability ) 10y1 mD are least ´ prone to outbursts ŽGil and Swidzinski, 1988.. ´ Laboratory permeability studies should be conducted on large samples ŽHarpalani and McPherson, 1988. under simulated stress conditions and field moisture levels ŽDabbous et al., 1976.. Moisture maintenance in samples is a problem. Data will need to be corrected for the effect of moisture which causes swelling of coal, hence a decrease in permeability. In a wet coal seam relative permeability of coal to water and gas is needed for modelling purposes ŽHyman et al., 1992.. These tests are costly and cumbersome, but some information on this aspect is helpful. Laboratory permeability data are often lower by 1–3 orders of magnitude as compared to in situ test results if cleats are not filled with gas and water. This must be taken into account in the analysis. While laboratory studies are conducted, these should be taken as an adjunct to the field studies. The directional permeability of coal due to the presence of a cleat and joint system influences gas pressure gradients ŽPomeroy and Robinson, 1967; Summers, 1993.. These data, along with the lateral and vertical stress direction, can also show the likelihood of location of outburst sites when outbursts are stress related. Permeability can also be estimated from the desorption data obtained from core samples. This result however is under no stress conditions. Desorption under stress is another alternative to field studies. Diffusion of gas through the matrix of coal controls flow as well as the desorption rate at cleat and matrix interface. Tests to determine diffusion coefficients can be conducted in the laboratory or data from field core results can be used for the purpose. These tests should be conducted for various plys of the coal seam separately. Coals that are susceptible to outbursts have higher diffusivity coefficients Ž10y7 –10y8 cm2rs.. 2.4. Gas pressure It is important to estimate the gas pressure that is likely to occur during mining. Gas pressure ahead of a coal face is dependent upon the rate of mining, but at high rates of advance Ž; 10 mrshift. the effect of rate of mining on gas pressure at 2–3 m ahead is minimal. Gas pressures ) 0.3 MPa have resulted in outbursts in coal seams which are very soft ŽNanovska et al., 1988.. In strong seams, the acceptable gas pressure ahead of the coal face may be as high as 0.6–1 MPa ŽZhang, 1995.. Gas pressure measurement from surface exploration boreholes is feasible and can be accomplished easily. Gas pressure transducers can be installed in exploration boreholes. Gas pressure build up data can also be used for gas permeability calculations. Gas pressure may exceed the hydraulic head in some exceptional cases particularly at medium depths ŽOsipov, 1978.. At higher depths, gas pressure is in general lower than the hydraulic head. Where gas composition changes rapidly, gas pressure may be quite different even at close distances.

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2.5. Stress conditions Vertical as well as lateral stress play an important role in outbursts particularly rock outbursts. While various methods of stress measurements are available from underground, the assessment of stress conditions from surface boreholes is best done by one of the following methods: Ž1. hydraulic fracturing, Ž2. observations of fracturing in boreholes using borehole cameras or calliper logging and Ž3. study of the core conditions. Study of the core conditions, particularly when associated with discing, has always proved to be a reliable method in predicting outbursts of sandstone beds. Discing reflects anomalies in the in-situ stress tensor, existence of high gas pressure andror induced reduced strength ŽBelin, 1981; Lama, 1995.. Measurement of the volume of cuttings is another method that can be used to assess both the state of stress and the strength of rock in place ŽJahns, 1965; Barsznica et al., 1980; Ryncarz and Majcherczyk, 1982.. It should be kept in mind that a point measurement of stress is only an indication and cannot be taken as a value that will be applicable everywhere in the area. Stresses can jump considerably across major fractures. It is therefore essential to build a geological and geotectonic model of the area under investigation. From an outburst point of view we are concerned both with the general areas of increased stress as well as the local points of elevated stress field. Studies indicate that coal seams liable to outbursts show an increase in stress from a few MPa to 1.5 times the normal stress field. 2.6. Strength of coal The strength of coal is an important contributing factor in the outburst potential of a coal seam. Coal strength is dependent upon maceral composition, hence the strength of various plys is quite variable. Because of the requirement to classify the various plys of the coal and their potential to outbursts, tests for strength are done using small samples from various plys. The preparation of regular specimens in such a case is not feasible. Irregular samples can be tested to determine the strength of various plys. Tests such as the Protodyakonov index and ISI are very useful and most commonly used. Where cores are available, tensile strength tests using the Brazilian method can be adopted. It may be pointed out here that it is the tensile strength of coal that is important to an outburst and not the uniaxial compressive strength that is commonly determined from geomechanical point of view. The presence of gas under pressure influences the strength of some coals. Tests under the influence of gas pressure can be conducted using coal-sample discs and indenting these when saturated with gas under pressure. A drop in strength in the presence of gas at pressure is an indication of high outburst potential of a coal ply. The strength of coal seams liable to outbursts is low. Low strength is also associated with the maturity of coal and with the effect of tectonic movement that coal seams have undergone. Shearing of the coal is seen in many seams as potential for outbursting. Strength tests should be related to the occurrence of geological disturbance. Sampling must include zones of disturbance and the structural studies should be related to the strength tests.

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2.7. Sorptionr desorption properties of coal The relationship between the amount of gas sorbed in coal and the gas pressure varies depending upon the coal rank and maceral ŽBeamish et al., 1993.. This information is needed in gas drainage modelling as an important input parameter and the calculation of likely pressure gradients that will occur during mining. High pressure sorption studies can be conducted in the laboratory using volumetric or gravimetric methods. It is important that ply samples for these studies are sealed to avoid oxidation and transferred to the laboratory as soon as possible. Studies should be commenced in - 72 h to avoid deterioration of samples. The gases used for sorption should be those occurring in-situ. Gas composition can be obtained from desorption tests on cores used for gas content measurements. The temperature at which sorption studies are conducted should be the temperature at which the coal seam occurs. This data can be obtained from borehole temperature logs. High pressure sorption studies should form a part of investigation in other gas parameters such as various desorption indices that are used to predict the liability of a coal seam to outbursts. A number of sorptionrdesorption indices have been used to predict the proneness of coal outbursts. These indices quantify the nature of coals and the values of these indices must be related to other factors such as structure of coal, gas content, stress, etc. Ettinger Ž1952. was the first to point out the role of high rates of sorptionrdesorption of gas in outbursts based on laboratory experiments. The method of Ettinger’s sorption index consists of crushing a sample, drying it at 608C and evacuating any residual gas. The particle size, 0.25–0.5 mm, was chosen as the best to differentiate the outbursting from non-outbursting coals. The sample is sorbed with gas to equilibrium at 0.1 MPa pressure at 308C. Ettinger suggested that the sorptionrdesorption rate in the first 30 s should be taken as the reference value. Ettinger et al. Ž1953. presented the following additional data. Instead of measuring the rate of sorptionrdesorption as a percentage of the gas sorbed at 0.1 MPa pressure, they suggested the measurement of gas pressure build up in an enclosed chamber of definite dimensions in the first 30 s. They defined the D P0 – 30 index on the same basis as Lidin et al. Ž1954.. They presented data in categorising sections of seams and zones of a single coal seam. This basic research of Ettinger and his co-workers has formed the basis of virtually all sorptionrdesorption indices. A modified D P0 – 60 index for field studies has been used in a number of countries for the estimation of the liability to outburst of an advancing coal face ŽJanas and Winter, 1977; Paul, 1977.. Polish desorbometer measures D P values in terms of water gauge, using a 3 g sample of size fraction 0.5–1.0 mm. Samples are taken from a 42 mm diameter borehole drilled to a depth of 3 m. Cuttings are collected from the last 10 cm of the drillhole. Fractions are sealed and the test starts within 35 s. Observations are made over the next minutes. The D P equal to 120 mm of H 2 O gauge over a 2 min period has been found to be the limiting value for defining the conditions of an imminent outburst in mines with CO 2 in the Lower Silesian coalfield, Poland ŽTarnowski, 1968; Kozłowski and Polak, 1978a,b.. The K T index is a measure of the change in the desorption rate of a coal sample. The method of sampling consists of drilling holes and collecting fractions of particles in the

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range of 0.4–0.63 mm. The critical K T value relates to a gas content of 9 m3rt ŽJanas and Winter, 1977; Janas, 1979; Noack et al., 1995.. For determining the D P express index a coal sample of about 70 g in the range of 0.25–0.5 mm for bituminous Ž2–3 mm for anthracite. is enclosed in a chamber. The sample is evacuated for 2 min and then methane is allowed to enter into it to raise the gas pressure to 0.2 MPa as quickly as possible. The gas flow into the chamber is closed. The chamber is immediately connected to a manometer and the change in pressure after 1 min is read, which gives the D P express index ŽPaul, 1977.. The gas emission V-index is a measure of the volume of gas in the early stages of desorption of a coal sample under atmospheric pressure. The method is based on the early work by Ettinger Ž1952.. Somnier Ž1960. used coal samples of about 5.0 g in the range of 0.5–0.8 mm grain size. If gas content, gas pressure, coal seam permeability and high pressure sorption data are available, together with other parameters that influence gas pressure gradients, then the above tests can be conducted in the laboratory by simulating field conditions. The tests are conducted by taking appropriate sample fractions, subjecting these to high pressure sorption and then measuring the various indices following the methods. The important point to keep in mind is that samples are subjected to the gas pressure and moisture levels that are likely to be met underground during the mining process. If the test conditions reproduced in the laboratory are different from those in the field, the results obtained will contain errors and corrections will need to be applied.

3. Design of control measures If the results of investigations show that a coal seam is liable to outbursts, then a consideration needs to be given to control measures to be adopted when a coal mine is opened. In coal seams liable to outbursts, the best control method is pre-drainage of the seam either by using surface boreholes and hydraulic fracturing ŽLidin, 1987. or directional drilling and if possible by pre-working a protective seam. All other measures that are used to predict outbursts and require local action to control outbursts are much less effective and impact on productivity. Longhole drilling from underground or directional drilling from the surface requires that the boreholes be stable. This, in turn, requires a strong coal seam. Stability of the boreholes must be considered if outburst control measures are considered. Factors which influence stability are: strength of coal, variability in strength of different plys, gas pressure, gas content and stresses in the coal seam. The economic viability of the project will very much depend upon the control measures that need to be adopted. In areas where pre-drainage as a control method is not possible due to low permeability of the coal seam, problems of drilling longholes, maintenance of longholes and lack of availability of technology for drilling of longholes, other control methods need to be adopted. These may include the use of complex indices such as sorptionrdesorption, cutting volume, gas flow rate from boreholes and camouflet blasting. There is a need to very clearly define control measures and develop methodologies to achieve them. The management system should address this clearly.

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4. Prediction of outbursts As mentioned above, the four most important and widely accepted factors are gas content, geological disturbances, stress regime and material properties. All four factors combine to produce an outburst. Gas content can be measured or estimated fairly reliably. Being a function of gas pressure and porosity it determines the amount of energy that is available for an outburst to manifest itself and transport the material involved for a shorter or longer distance away from the place of occurrence. Gas content influences the properties of coal and rock. As the name of the phenomenon implies, gas content is always an important factor. Without the presence of its certain critical value, outbursts of gas, coal and rock will not occur at all. Tables 2 and 3 summarise various sorptionrdesorption indices based upon local conditions. The role of stress must be judged in association with the strength of coalrrock. The strength of the material determines the stress level required at which an outburst will be initiated. Stress levels that are sufficient to fracture rock to a state of almost pulverisation cause intense outbursts. The measurement of stress is not easy and almost impossible on a regular basis under operating conditions in mines. Discing is a phenomenon, which describes the status of stress, but it is influenced by gas pressure and the rate of drilling ŽSpackeler et al., 1960; Kvapil, 1961; Gimm and Pforr, 1964; Josien and Luneau, 1989.. Radio imaging is based upon the dielectric resistance of coals ŽThomson et al., 1990, 1995., but its value is highly dependent on the moisture levels. Its use in the prediction of structure is dependent on moisture levels anomalies. Methods that have been developed to predict outburst conditions can be divided into various groups depending on the interaction of the factors influencing the method. Fig. 1 shows a general characteristics of the predictive methods used. It is very difficult to categorise each method precisely. The type of the method used depends upon local conditions. Some mines may use more than one method for continuous prediction. For regional prediction invariably more than one method is used.

5. Management systems for control of outbursts The purpose of the management systems is to ensure that the procedures in place are precisely followed and to ensure that under no circumstances mining proceeds without endangering operations. A management system thus relies on checks and balances to achieve the desired result. It also ensures that the system operates independent of the people who developed it or have been driving it. It helps to arrive at the desired results efficiently by defining the methodologies to achieve the results and when the desired results are not achieved, it can help vary the procedures without sacrificing safety. Most countries around the world have developed guidelines for mining under outburst conditions. The mining of coal seams liable to outbursts requires the development of special procedures to ensure that the risk to miners and equipment is eliminated or reduced. Most mines have laid out basic parameters which, when followed, can allow safe mining

3–4 3 2.9

2 bituminous ŽPoland . anthracite ŽPoland . bituminous ŽPoland . bituminous ŽPoland . 2 fireclay seams, Lower Silesia ŽPoland . 2 Cevennes ŽFrance . Cevennes ŽFrance. France bituminous ŽWest Germany . bituminous

ŽWest Germany . not critical bituminuos ŽBelgium .

1 1 2 3 4 5

6 7 8 9

10

11

14 Australia .

1.5 – 1.8

bituminous ŽMetropolitan Australia . 1.5 – 1.8 bituminous ŽLeichhardt,

13

12

anthracite ŽCynheidra, UK .

12

y0.63 – q 0.50

2 Žon face . 8 – 10 Žsides . 35 – 70

70

70

35 – 70

120 – 60 0 – 70 t1 s 2 t 0

earliest possible but not critical ; 600 35 – 70

30

35 35 – 70 35

60 to 90

120 35 t 0 - 35

y1.00 – q 0.60

y1.8 – q 0.60

50

50

cuttings Ž V u .

60 – 360

60 – 360

large

V ) 1.5 CH 4

V ) 0.5 – 1.0 CO 2

V s rV u ) 4 CH 4

K ) ) 0.75 CH 4 V 0 ) 1.0 cm 3 CH 4 Žoutburst danger. V0 ) 1.0 cm 3 CH 4 Žserious outburst.

V 0 ) 4 cm 3 rminrg

V 0 ) 1.0 cm CH 4 V ) 2.0 cm 3 CH 4 V 0 ) 0.6 cm 3 CO 2

3

V ) 3 cm 3 CO 2

V ) 0.75 cm 3 rg CH 4 V ) 1.44 cm 3 rg CO 2 V ) 1.44 cm 3 rg CO 2 where V s V 1Ž t 1 y t 0 .r Ž t 1 y t 0 . V ) 3 cm 3 CO 2

Time elapsed Period of Outburst index between sample observation Ž s. removal and start of test Žs . 5 6 7

cores and cuttings sealed in place Ž V s . and unsealed sample-

y0.63 – q 0.40

q0.80 – q 0.50 q0.80 – q 0.50 y0.80 – q 5.0

2.7 – 3.0 3 2.7–3.0

y1.00 – q 0.50

y1.01 – q 0.50

y1.00 – q 0.50 y1.00 – q 0.50 0.80 – q 0.50

Distance from Fraction size face to point Žmm . of sample collection Žm . 3 4

S. Type of coal Žrock . No.

Table 2 Characteristics of sorptionrdesorption Žvolumetric . indices used to predict outbursting conditions

Tarnowski, 1970 Kozłowski and Polak, 1978a,b

9

Refs.

Loison and Belin, 1965 de Vergeron and Belin, 1966a,b Somnier, 1960

Suchodolski, 1990

Vandeloise, 1964

Janas, 1976; Janas and Winter, 1977

weight of sample s 4 g

weight of sample s 4 g

Leichhardt Colliery Žformal communication, 1978 .

Hargraves, 1962

samples taken at 3 m intervals in each hole of 12 m length on longwall face V s s average gas content for minerg, predetermined V u s gas content from cuttingsrg Davies and Jenkins, 1978

weight of sample s 10 g

K ) s slope of curve represented by gas desorbed plotted against time k ) s Žln V t 1 y ln V t 2 .r Žln t 2 y ln t 1 .

weight of sample s 10 g, V s cm 3 rminrg Janas and Winter, 1977

simultaneous measurement of gas pressure V0 s CDPr870 C-volume, DP s pressure, measured for 3 g sample weight of sample s 10 g 3 g sample

3.5 g sample Barsznica et al., 1980 simultaneous measurement of gas pressure Hałamaj and Szwajgier, 1965

3.5 g sample 5 g sample

8

Remarks

96 R.D. Lama, J. Bodzionyr International Journal of Coal Geology 35 (1998) 83–115

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97

of outburst-prone coal seams. These conditions may be based on the achievement of a critical threshold level value ŽTLV. of gas content levels or D P0 – 60 , gas flow and D P, or K T values etc., and the methods to achieve these depend upon local conditions. In Australia the Department of Mineral Resources, in the State New South Wales, Australia has provided Outburst Mining Guidelines ŽDMR, 1995. which are designed to help draw up an outburst management plan for operations in underground mines. The basic outline of a management plan is shown in Fig. 2. 5.1. Key elements of an outburst management plan 5.1.1. Definition of the problem Definition of the problem and the conditions necessary to overcome it form the key element of the management plan. Outbursts in a mine may be associated with high gas, high stress, and structures. The solution to the problem may lie in reducing gas, destressing, and defining structures. The solution may need simultaneous reductionrelimination or recognition of one or more factors that determine the cause of an outburst. The management system should address these aspects and clearly define the conditions that need to be achieved to mine safely. Procedures required for sampling and testing should be defined unambiguously. It is essential to ensure that the persons testing are independent of the users of the information. 5.1.2. Methodology to achieÕe a good management system A management system must define the procedures that will be necessary to achieve the conditions for safe mining. For example, when gas content levels have to be reduced to a certain critical level ŽTLV. by gas drainage, the design of gas drainage measures for varied conditions should be clearly documented. The results of sampling for gas content or any other threshold values established in accordance with the safety requirements of mining of the outburst seam must be certified by the engineerŽs. responsible ensuring that the standard operating practice has been followed to collect the data. This must be supported by appropriate records. All this information, together with other prior information, must be presented on the mine plan. It should be certified by the surveyor responsible for the preparation of the plans ensuring that the information relating to placement of holes, sampling and location of structures is correct. Only when all the above information has been assembled together, the person responsible can authorise it in writing. This authorisation should be supported by data on prediction of structure, flow date, volume of cuttings, the layout of the holes, spacing between the holes, drainage time, methods to drilling and drilling technology. These provide a successful placement of holes in order to achieve the results and data on other indices as well as results obtained as certified by the testing authority. 5.1.3. Management plan A management plan should incorporate standard operating procedures pertaining to all technical aspects of the management plan such as: Ž1. equipment operation, checking and maintenance; Ž2. details on gas content and gas composition; Ž3. threshold index

Type of coal Žrock .

2 bituminous Ž USSR .

bituminous ŽPoland .

bituminous and anthracite ŽWest Germany .

anthracite ŽPoland .

S. No.

1 1

2

3

4

Field test ;3 m

laboratory tests

laboratory tests

laboratory test Žlocation not critical.

3

Distance from face to point of sample collection Žm .

100

100

5

Saturation pressure for 90 min ŽkPa .

y6.0 – q 0.5

y0.5 – q 0.25 100 Ž for bituminous. y3.0 – q 2.0 Žfor anthracite .

y0.5 – q 0.25

y0.5 – q 0.25

4

Fraction size Žmm .

Table 3 Characteristics of desorption Žpressure. indices used to predict outbursting conditions

120

10 – 60

0 – 60

10 – 60

6

Period of observation Ž s.

8 weight of coal s 3.5 g, free volume s 4 cc Žpartial value of D P , considered as filtration property of coal. weight of sample s 3.5 g, free volume s 4 cc Žtotal value of D P , considered as an overall property of coal. weight of sample s 10 g, total volume s 26 cc

weight of sample s 3 g, maximum time elapsed between collection of sample and start of test s 35 s

D P - 15 mm Hg Žno outburst., 15 - D P - 25 mm Hg Žsporadic outburst., D P ) 25 mm Hg Žcertain outburst., gas: CH 4 or CO 2

D P - 40 mm Hg Žno outburst., 40 - D P - 75 mm Hg Žsporadic outburst., D P ) 75 mm Hg Žcertain outburst., gas: CH 4 or CO 2

D P - 15 mm Hg Žno outburst., D P ) 15 mm Hg Žlikely outburst., gas: CH 4

D P ) 120 mm H 2 O, gas: CO 2

Remarks

7

Outburst index and type of gas

Kozłowski and Polak, 1978a,b

Paul, 1974

Tarnowski, 1966

Lidin et al., 1954

9

Refs.

98 R.D. Lama, J. Bodzionyr International Journal of Coal Geology 35 (1998) 83–115

bituminous ŽFrance .

anthracite ŽFrance .

bituminous ŽBulgaria . bituminous ŽCzechoslovakia . bituminous ŽBelgium .

5

6

7

8

9

laboratory test

laboratory

laboratory tests

laboratory tests

field test ;3 m

q0.5 – y 0.25

q0.5 – y 0.25

y0.5 – q 0.25

100

100

100

y3.0 – q 2.0 100 ŽCevennes basin . y0.50 –q 0.25 Žother basins.

y0.8 – q 0.5

0.60

10 – 60

0 – 60

0 – 60

35 – 70

Use of both D P , V and micro-seismic measurements sample weight s 3 g, ) 95% values lying above this limit

sample weight s 3 g

sample weight s 3 g

sample weight s 3 g

D P ) 14, gas: CH 4 D P ) 16 ) , gas: CH 4

D P - 25 Ž25 – 32 ., gas: CH 4 D P ) 25 Ž25 – 35 ., gas: CH 4

D P - 15 Žno outburst. 15 - D P - 30 Žslight suspect. 30 - D P - 45 Žsuspect. 45 - D P - 60 Ždangerous. D P ) 60 Žhighly dangerous ., gas: CH 4

Vandeloise, 1964

Anon, 1964b

Anon, 1964a

Loison and Belin, 1965

de Vergeron and Belin, 1966a,b

R.D. Lama, J. Bodzionyr International Journal of Coal Geology 35 (1998) 83–115 99

100

R.D. Lama, J. Bodzionyr International Journal of Coal Geology 35 (1998) 83–115

Fig. 1. Flow chart and classification of outburst prediction methods.

values such as threshold gas content, K T , flow rates, D P0 – 60 , seismic signal values as applicable at the mine; Ž4. structure prediction techniques and reliability of the methods used; Ž5. drilling strategies; Ž6. gas drainage standards such as hole integrity, surveying of holes, flow monitoring; Ž7. testing strategies and methodologies and Ž8. mining standards such as ventilation, barrier widths, protection equipment for miners. These standards, sometimes called ‘current best practices’ must be documented and made available to the persons involved in various operations of the plan in part or as a whole.

Fig. 2. Flow chart of the basic outline of a management plan.

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Fig. 3. Flow chart on decision making on the type of mining.

5.1.4. Decision making process The process of decision making has to be logical. The use of flow charts in decision making is very helpful at virtually all levels. Figs. 3 and 4 show examples of decision making processes. In the decision making process, whether normal mining can be done or mining will have to be conducted under special conditions such as mining under outburst conditions where an outburst may occur, need to answer a number of questions. Some of these are noted below ŽAllonby, 1995; Kelly, 1995.. Do physical observations show any of the following signs which may indicate the existence of an outburst prone zone? Increase in frequency of cutters Žjoints. or fracturing, presence of fault planes, calcite bands, change in coal colour, presence of mylonite, coal hardening or softening, change in gas composition, gas blowers, presence of dike stringers, faulting in coalrroof, cindered coal, greasy backs, and water issuing from face. Is the data reliable and representative of the areas to be mined? Have all structures been detected? Is it known accurately enough where holes intersected structures? Is it known accurately enough where cores were taken? Are core analyses low due to the proximity of gas drainage holes? Is the density of coring adequate to detect localised gas variations, i.e. are we confident we know what is in our next cut? Are K T and D P values reliable and have not been affected by moisture levels? Is there sufficient data available to statistically satisfy the variability of the coal seam? Is there a need to drill for additional data? If data are unreliable or not representative, will additional drilling provide the necessary confidence? If structures are present, are

102

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Fig. 4. Flow chart of decision making for mining in outburst prone area.

they outburst-prone as determined by following a standard decision-making flow chart? If the gas content or other indices as practised in an area are close to outburst mining threshold, can the mining operations be re-scheduled to allow further gas drainage or reduction in the value of the controlling indices? Can development be re-sequenced andror priorities changed to allow delay in mining in marginal areas? Are additional drainage holes or other measures advantageous? Is there an option other than to work under outburst mining procedures? Can the drivage be significantly delayed to allow further mitigation? Is there an alternative drivage which could eliminate the ‘outburst’ drivage? Is there a need to collect more data? If so, what and how shall it be collected? 5.1.5. Organisation, responsibility and authority The management systems are always driven from the top. Any management system, when introduced, must be an integral part of the policy statement of the mine or company. For example, ‘‘it is a company policy that mining shall proceed only when the gas content threshold values are below the critically defined values’’. Authorisation to mine shall be given by the senior personnel at the mine and the elements that lead to authorisation must be precisely defined. For example: Ža. assessment of the geological structure which forms the basis of the geological conditions influencing an outburst must be obtained from a variety of sources and confirmed in writing by a geologist. Žb. Pre-mining placement of drill holes, their location plan, their status based upon surveying of holes, results of monitoring of any abnormal behaviour must be certified and must then be provided to the district or panel in charge, placed in the district as well as filed for records. The management system thus defines both the responsibility and authority, and these must always be separated. The person collecting the information is not the person who authorises mining.

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5.1.6. Flexibility A management plan cannot be too rigid. Invariably, conditions do change and sometimes very rapidly. A system of data collection and decision making may become invalid which will require changes in the mining plan. It is thus quite possible that the management system may contain procedures for different types of mining. For example, normal mining, or mining under conditions where an outburst may be expected, remote mining and use of camouflet blasting instead of normal machine mining. Under such conditions, the mining methodology and procedure should be specified. This may include special equipment. For example, a specially prepared canopy of a continuous miner. The management plan can stipulate that authorisation for mining can be given by a person senior to the one normally responsible. 5.1.7. Participation No matter how clearly the principles and methodologies are defined and practised to achieve desired results, abnormal conditions will arise when the safety indices have not been achieved. Under such conditions, there will be a need to renew the procedures and upgrade the methodologies. The management system must be supported by representation from all levels of employees, miners, management and technical staff so that everyone is well equipped to advise and make ongoing changes, implement and maintain them. 5.1.8. Failure of the plan It is always possible that conditions may occur which were not expected or an outburst may occur when all procedures and analysis had indicated that normal conditions exist and that no outburst is expected. This is taken as failure of the plan. In such circumstances, the management plan must incorporate detail procedures to be adopted by the crew working at the face and by the senior management for decision making. Simultaneously, there should be enough equipment available on site and in ready form together with appropriate precautionary procedures. For example, when no structure is expected in the area authorised for mining and suddenly a structure appears at the face then normal mining should be stopped. The mine geologist and the official in charge of the panelrsection should be notified. Further mining can only be authorised by the senior person responsible for authorisation of mining under abnormal conditions Žoutburst mining., which will require a different set of equipment. 5.1.9. Training The training of persons working in areas prone to outbursts is an essential part of any management system. Persons working at the coal faces need to be trained in the recognition of conditions liable to outbursts, the use of special equipment provided for safety if abnormal and if unexpected conditions develop Žfailure of the plan. or any special equipment that is used for different types of mining. Training will need to be updated if a certain type of mining is not practised as a part of normal mining. For example, mining under especially defined conditions such as outburst mining, requires a limited number of people who are allowed at the face. The location of the rest of the crew is suitably located at a base station. Persons, who will use the special equipment such as compressed air masks and operate the special

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communication systems between the machine driver and the section in charge, must be given special training first. Training must also cover first response of the crew in the event of an outburst. 5.1.10. Auditing system The purpose of the auditing system is to ensure compliance as well as adequacy of the management system. The auditor must assess all parts of the management system, procedures, equipment, availability at the site, methodology used, inspection of records and frequency of training. Auditors must investigate causes of plan failure, non-conformity to plan and develop procedures for corrective actions. Auditors must be qualified persons well versed with the system. Auditors may be external or internal. Internal auditors must be independent and without any direct responsibility to any aspects of the management plan. External auditors are those that are independent of the mine operation. Auditing methods would include on-site inspection and oral questioning of the crew as well as participation in the decision making processes. Results of the audit should be communicated orally and in writing and records of all audits must be maintained. 5.1.11. CorrectiÕe action procedures A good management plan is a living document. It will need to be corrected for shortcomings, improvements, developed procedures for circumstances not foreseen in the plan and an updated system with new developments in technology. The plan must have procedures to meet these requirements. The methodology for initiation of amendment should be specified and authorisation procedures outlined. The plan may incorporate assessment by an expert committee represented by the members from within the colliery and outside experts. 5.1.12. Information control documentation All information relating to various aspects of the management plan, decision making, information collection and operation of the procedures should be well documented and communicated to all involved in the management. The information about the management plan must be available at locations, in part or whole, where the operations are conducted. For example, mining procedures and drilling procedures must be displayed on site where mining or drilling is carried out. Changes to the management plan must be carried out efficiently and all obsolete information removed promptly. 5.2. Procedures during outburst mining When there is any risk associated with mining through an area where there exists a slight possibility of an outburst, mining is needed to be conducted under special procedures. This is termed outburst mining. Procedures differ from mine to mine, but general requirements of outburst mining when developing roadways with continuous miners and shuttle cars are as follows: Ž1. A base station in the fresh air at the inbye end of the last cut-through shall be established and no person shall proceed beyond this point without the supervisor’s instruction.

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Ž2. Unless remote mining is adopted, only the machine driver operating the machine will be at the coal face. In the case of a continuous miner, he shall remain in the cabin with full mask and in radio communication with the base station. The continuous miner is equipped with a specially designed canopy. Ž3. Cutting commences with the instruction of the district supervisor and continues till the shuttle car is filled. The shuttle car driver proceeds to the face only after cutting has stopped. Cutting shall be done from roof to floor and the continuous miner boom shall be raised when cutting stops. Ž4. The number of persons setting the support shall be limited and they may be asked to operate with full mask or with sufficient Žequal. number of persons at the fresh air base equipped with self contained breathing equipment to support any emergency. Ž5. The access to the panel shall be limited and there shall be no work conducted on the return side of the panel. Ž6. The panel is provided with special equipment such as CH 4 and CO 2 monitors, self rescuer ŽFenzy oxygen escape units., first response equipment such as escape units, rescue units, communication equipment and face masks. Ž7. An appropriate amount of radio communication equipment to communicate between the fresh air base, continuous miner operator and shuttle car operator. One of the developments in outburst mining has been the protection of the continuous miner driver. This is now virtually a requirement when mining under outburst conditions in spite of the fact that safe levels have been achieved. In the presence of a structure many outburst management plans require that mining proceed under outburst conditions. This requires provisions of special equipment. The continuous miner driver protection is the most important aspect of this provision. The continuous miner cabin is specially designed with the following requirements ŽNicholls, 1987.: Ž1. Ensure physical protection from outbursted material, Ž2. ensure a constant supply of breathable air, Ž3. ensure both secondary and tertiary supplies of breathable air should the basic supply be cut off during a major violent outburst. It should automatically switch on and expel the air in the cabin within 5 s. Ž4. Maintain a positive air pressure in the capsule in the event of a major gas outburst preventing ingress of gases into the capsule. Ž5. Enable the mining operation to be conducted by the miner driver by developing a system of remote control for the shuttle car flughts. Ž6. Ensure continuous radio voice communication between the miner driver, deputy and other crew members at all times. Ž7. The whole system to expose a minimum number of men to risk at any time. A specially designed continuous miner cabin has a rear entry, laminated glass, a streamlined cabin structure to deflect ejected material with no flat face and radio control communication within the cabin. The machine also has additional lighting for better visibility and truly one man operation. A schematic of the cabin is given in Fig. 5. 5.3. Remote operation of cutting equipment The operation of the ploughs on longwall faces from the gate roadway has been a common practice in thin coal seams. More recently, the shearer operation has been fully mechanised where it cuts the face in close tolerance between the roof and floor. The

106

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Fig. 5. Photograph of Joy 12 CM 20 machinery equipped with an outburst protection cabin, Tahmoor Colliery.

shearer can be pre-programmed by completing a cut manually, which is then stored in the memory of the computer control system. In subsequent cuts, the shearer maintains its horizon along the coal face as planned. The memory run can be updated every day or every shift as the case may be. In other systems, roof and floor proximity sensors based upon natural radioactivity of the roof and floor rocks guide the shearer to cut within "40 mm from the roof and floor. The supports are activated by the shearer as soon as the leading or trailing drum goes past them. The shearer, support withdrawal, setting and conveyor push sequence is computer controlled by a central computer placed in the main gate. Use of this technology has the following advantages: Ž1. allows better horizon control, Ž2. better roof control and Ž3. eliminates workmen from the face. The last aspect is very important if outbursts may occur on the longwall faces. In many countries, only plough machines are allowed for use when mining seams liable to outburst and drum shears are forbidden. Limitations are also introduced in the rate of advance of face per day and width of the cut to be taken at a time by the machine. In outburst mining when using continuous miners, cutting is done from top to bottom taking only 0.30 m of slice and full heading machines are preferred. More important, however, is the remote operation of the equipment in the development headings. The problem here becomes complex when multiple headings are to be driven. Here it is required not only to cut coal remotely, but also transport it from the back of the remotely operated continuous miner to the boot end of the conveyor. This is much easier when the conveyor is in line with the continuous miner but when it has to transfer to other excavations requiring negotiating corners, the system requires guidance of the shuttle cars. Recently remote operation of the continuous miner has been successfully demon-

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Fig. 6. Present system for remote operation of a continuous miner ŽWynne and Case, 1995..

strated ŽWynne and Case, 1995.. The system is based upon the location of all controls in a portable chamber which is ventilated by a filtered fresh air stream and allows use of non-intrinsically safe and non-flame proof equipment in the hazardous zone. Two video cameras, one focused on the cutting head of the miner ŽABM-20. and the second on the shuttle car transfer the image to the video screen. The lowering and raising of the miner canopy, operation of the shuttle car flights and cutting operation are done through a radio transmitter. An audio system is also incorporated via the video link. The transmission of all information is through microwave video transmission. A general layout of the system is shown in Figs. 6 and 7. 5.4. Outburst data collection A management plan must have a reporting system on the occurrence of outbursts in the colliery. The system should be designed so that all relevant information that can help in reviewing the risk is reported, assessment of the situation that management procedures were followed and parameters of the outbursts can be recorded. The reporting document may be in more than one part. Part A may contain information that must be filled in on-site underground and part B is filled in at some later date by the person responsible for reporting Že.g. section in charge, geologist, gas drainage engineer, geotechnical engineer.. An example of an outburst data report is given in Appendix A ŽDMR, 1955.. 5.5. Risk analysis Quantitative risk analysis techniques have been used in the chemical and nuclear industries for about 50 years. These techniques are now being used in outburst management and risk analysis in many companies. These techniques are based on the

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Fig. 7. Schematic plan showing the proposed system Žtop. and a separately ventilated purged control room Žbottom. ŽWynne and Case, 1995..

interaction of the various elements that impact on each other. They assess the effect of failure of one or more than one element simultaneously on, for example, the ultimate risk of a fatality. The analytical system takes into account the following: Ž1. procedures for mining, manning, location of the various crew, communication systems between the crew, etc., Ž2. outburst assessment procedures, such as drilling, drainage, sampling, geological assessment and the probability of assessment of outburst conditions using various technologies, Ž3. failure of the system, non-compliance of the outburst management plan procedures, failure of the technological standards, and Ž4. probability of the

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size of an outburst, location of an outburst and effect of outbursts on people Žasphyxiation.. Based upon the assessment, the system can define parameters that have the greatest impact on reducing the risk of fatality. The system can analyse any changes to the outburst management plan and their impact on decreasing Žor increasing. the risk under particular conditions. The historical risk to employees engaged in development work operations at West Cliff Colliery Žwhich experienced the largest number of outbursts in Australia. was of the order of 1,000 chances of fatality per million man years and at Tahmoor operations the historical figure was almost 1,500 initially. Application of this technique in Kembla Coal and Coke ŽAustralia. operations showed that the introduction of a series of improvements in the management plan reduced the risk to less than 50 chances of fatality per million man years at both the collieries. The Outburst Hazard Analysis Sub-Committee appointed after the 1992 outburst in the South Bulli Colliery ŽNSW, Australia. concluded that the short term target risk of 350 chances per million fatality per year was acceptable, while the long term target was set at 100. Use of risk analysis had helped to achieve and even exceed this target. Appendix A

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