Induced seismicity of Indian coal mines

Physics of the Earth and Planetary Interiors, 44 (1986) 82-86 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

82

Induced seismicity of Indian coal mines R.K.S. Chouhan Department of Applied Geophysics, lndian School of Mines, Dhanbad-826004 (lndia) (Received June 4, 1985; revision accepted April 25, 1986)

Chouhan, R.K.S., 1986. Induced seismicity of Indian coal mines. Phys. Earth Planet. Inter., 44: 82-86. A number of coal mines in the Eastern Coalfields Ltd. have reported induced seismicity on account of mining activity. All these coal mines belong to the same Raniganj series of Gondwana system. Field investigations carded out using the coal samples show an average energy index value of about 9. The drilling yield tests conducted in the area generally show an erratic pattern which is quite consistent with the current state of seismic activity.

1. Introduction

Man made earthquakes have been reported from different parts of the world as a result of environmental changes caused by human activity. On the one hand, these localised activities of loading or unloading any part of the Earth are essential for the growth and development of mankind; on the other hand, they result in localised seismic activity; although in general, the magnitudes of the resultant earthquakes are quite small. However, at times the Richter magnitude may be as high as 5; nevertheless, because of the shallow nature of the events, even small earthquakes are harmful as they cause appreciable damage and hardship to local inhabitants. Surface blasting is one of the routine activities of the civil engineers in the construction of dams, tunnels, roads, etc. Blasting becomes inevitable where other methods like ripping fail to trim the rock especially in hard rock areas. These rocks occasionally contain fossil energy or deformational energy. When a rock undergoes deformation, the process of deformation continues until the accumulated deformational energy is dissipated in some form; however, the energy acquired during the course of geological history 0031-9201/86/$03.50

© 1986 Elsevier Science Publishers B.V.

might be retained in the rock developing into a critical state of stress; and subsequently released only under the action of some triggering agencies. The latter type of deformational energy available in the rocks is termed fossil energy; because the rock has been carrying this energy from the beginning of the rock's deformation. Seismic activity associated with such deformational energy is normally short lived, and almost always requires some triggering agent to get the release of energy in the form of seismic activity. In addition to civil engineering projects, other shallow activities associated with opencast mining, where tons of rocks are required to be stripped off, may also generate seismic activity; but with slightly higher intensities because of the involvement of huge quantities of rocks and dynamite. Underground excavation is another interesting and useful activity resulting in environmental change; this change could be in the form of subsidence, which is of widespread occurrence where mining activity has continued for some decades. However, subsidence may also be due to mine fire, especially, in coal mine areas which are quite prone to fire hazards. In the latter case the intensity of subsidence might be more intense. In addition to these surface manifestations, the excava-

83 tion of underground openings disrupts the local stress pattern which in turn may lead to the genesis of microearthquake activity, provided the stress on the rock mass are on the brink of unstable equilibrium as reported by Salamon (1974). The microearthquake activity in a metal mine is known as rockburst whereas in a coal mine it is known as bumps. These activities have been observed to increase with depth and in general becomes hazardous. Their genesis is attributed to faulty mining methods and a favourable geological environment, in addition to prevailing physical properties of the rocks. Thus the condition for the genesis of bumps as summarised by Rice (1934) and Holland and Thomas (1954) are as follows: (1) an overburden of 300 m or at least 150 m where the topography is irregular, (2) a strong overlying rock stratum within a distance of 10-15 times the thickness of the seam, (3) a structurally strong coal, which as a pillar does not crush easily, and (4) a strong floor material that does not heave readily. However, recent developments in the field of actual mechanism of rockburst and bumps have been commendable. According to Salamon (1983) a seismic event will occur only if a set of conditions pre-exist; the necessary conditions for this to happen are: (1) A region in the rock mass must be on the brink of unstable equilibrium either because, (a) the presence of an appropriately loaded pre-existing geological weakness such as a joint, fault, dyke or bedding plane; or because (b) the changing stresses are driving a volume of rock towards sudden failure; or because (c) some support system, for example, a system of pillars, approaches a state in which its unstable collapse is imminent. (2) Some induced stresses must affect the region in question and the magnitude of these stress changes, however small, must be sufficiently large to trigger off the instability. (3) Sudden stress change of sizeable amplitude must take place at the locus of instability to initiate the propagation of seismic waves. (4) Substantial amounts of energy must be stored in the rock around the instability to provide the source of kinetic or seismic energy. The origin

of this stored energy is work done by (a) gravitational forces, a n d / o r (b) tectonic forces, a n d / o r (c) stress induced by mining. 2. Geology of the area

The ancient crystalline rocks of later post Dharwar form the basement and it was over these rocks that the lower Gondwana group of sedimentary strata including the coal bearing beds were laid down. These sediments are river deposits that were deposited in slow sinking faulted troughs in more or less flat countries composed of Dharwar rocks. It was thus possible for several thousand metres of river stream deposits to accumulate in definite linear tracks like the Jharia and Raniganj coal basins. At the commencement of the lower Gondwana period, there was a glacial age in India as is evident from the glacial boulders deposited at the bottom of the Gondwana system of rocks and the presence of undecomposed feldspar grains in the associated sandstone above them. The climate rapidly warmed which gave rise to super abundant growth of vegetation which supplied material for the formation of coal seams in the succeeding series of rocks in the Jharia Coalfield and in the Barakar portion of the Raniganj Coalfield (after Sharma 1953-1954). Chinakuri colliery is the deepest coal mine in India that is mined into the Raniganj measures underlying the Panchet formation of the lower Gondwana series. This colliery is intercepted by some intrusive bodies like dykes and a major fault having a throw of about 75 m on the westside. The colliery produces Goal from the Dishergarh seam which dips 1 in 5 towards the west of the Damodar river. It occurs at a depth of 600-900 m and this mine is one of the most gaseous in the country. Because of the high emission of gas a serious explosion (gas outburst) took place in 1958 killing 158 people. This mine also produces coal from the Bharatchak seam; other coal seams present in the area include Gopalpur, Bharatchak, Lower Dhadka, Burradhemo, Raghunathbati, Borachak, Hatmal and Sanctoria. The Dishergarh seam has been beset with bumps since 1958 and gradually the activity has intensified with an increase in the depth of working that has eventually led to an

84 adverse working condition. The present study aims at presenting some of the investigations of seismicity associated with the Indian coal mines. 3. Seismicity

Most of the collieries mining the Raniganj coal seam show induced seismicity on account of mining activity with varying levels of seismic activity. The seismicity is usually sporadic in nature and the events (bumps) are well separated in time. Out of all the collieries, Chinakuri attracts immediate attention, because seismic activity in the form of bumps have been reported from this colliery since 1958. This activity continues with somewhat decreasing intensity and also a fall in the number of incidence of bumps. This change in the nature of seismic activity may be attributed to a number of factors; some of them are change in mining methods used for extracting coal, variation in the amount of coal production and proximity to geological structures. Occasionally, recently generated fossil energy is observed as a result of pillars left in earlier mine workings. If mining activity is resumed below 20 m of such a left over pillar, then this culminates in a very severe type of bumping. Such bumps have been attributed to the local concentration of stress which manifests itself into a critical state of stress in the underlying seam. Seismicity of this type is short lived; it has been reported from the U.K. by Kusznir and A1 Saigh (1982) and U.S.A. by Crouch et al. (1977). In India this type of seismicity was reported by Chouhan and Zorychta (1985) from the Girmint Colliery of Eastern Coalfields Ltd. The activity was most pronounced during 1980-1981 and gradually disappeared as soon as the load above the seam being working was reduced. Induced seismicity in such cases appears as a sympathetic shock that follows the production around. Production is responsible for triggering the shock as the coal block is under a critical state of stress. 4. Field investigations

In recent years various methods have been developed to assess t h e susceptibility of bumps in

coal mines. A basic characteristic displayed by a coal mine prior to a bump is an excessively stressed region in a localised part. The concentration of this localised stress may be detected either b y direct physical measurements in the working area of an underground mine or by using a remote sensing technique to measure micro-seismic noise with the aid of pressure sensitive detectors. Hence, by using microlevel measurements it is feasible to detect the stressed zone in a mine. Before the aforesaid microlevel measurements are resorted to in the field; it is pertinent ~to investigate the proneness of bump. Using physico-mechanical properties of coal, a method has been proposed b y Neyman et al. (1972) and Szecowka et al. (1973); to investigate the liability of bumps in coal mines. An energy index has been defined in numerical form expressing complex mechanical properties of coal and its liability to bumps. The energy balance in the process of axial compression of a coal sample gives a synthetic representation of the mutual relationship existing between basic mechanical properties of coal and determines the basic energy types present in the process as is the case in elastic hysteresis. The energy index Wet is defined as

where, f (~) is the loading curve, f/ (~) is the unloading curve, % is total axial strain, co is permanent non elastic axial strain, and c c - Eo is elastic strain. Thus, the proneness of bump in a seam might be assessed by taking coal samples from the active sections of the seam and then taking the average index as the index of bump liability. Coal samples were collected from the Dishergarh seam at Chinakuri and test samples (cores) prepared. The diameter of the samples varied from 50'mm to ~/5 mm in such a way that the length to diameter ratio was constant. Stress-strain relationship ~ der uniaxial loading and unloading was determined in a 200 kN universal testing machine. All 10 tests including the result of Singh (1980) have given an energy index ranging from 8 to 10.8.

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Some typical loading and unloading stress-strain curves are shown in Fig. 1. According to the Polish standard, an index greater than 5 is indicative of severe liability to bumps. Therefore, it may be construed that the Dishergarh seam is highly liable to bumps. Once it is established that the seam is liable to bumps, it becomes imperative to carry put microlevel measurements. For the Dishergarh seam, drilling yield t e s t - - a Polish method, proposed by Neyman et al. (1972), has been applied by drilling 42 mm diameter holes. By this method, the zone of rock loosening or destressed zone and the zone of increased pressure can be established with their respective locations. In this way the extent of stress concentration of any given time can be found from the progressive results of drilling and the associated variations observed. Drilling in a zone of increased pressure, produces compression in the borehole; as a consequence, the quantity of drilling ejected is increased; hence the increase in drilling eject per metre is indicative of the state of stress. The nearer to the side walls that the drill encounters such a zone, where the ejected drilling material has increased in volume, the greater is the danger of bumps in the working. A number of measurements for drilling yield tests were undertaken from 1977 by a group of

workers from this institute and continued until 1982. To arrive at an optimum depth of drilling, we drilled holes for various depths, the maximum being 15 m. The diameter of the hole drilled was 42 ram. It has been observed in these measurements that the information beyond 10 m depth does not show significant variation in the volume of the ejected material. Moreover, the stressed and destressed zones near the side walls are of prime importance as mentioned earlier; therefore, the depth of drill hole was confined to 10 m, however, a few were shallower depending upon the relative position of the drill holes. Distribution of drill holes was arranged in such a fashion that the spacing of drill holes varied from 4 to 10 m. It is difficult to follow a~systematic pattern of drilling because of multifarious activities and certain inherent problems of an underground mine. Some typical results obtained during these tests are shown in Fig. 2. Dashed lines in the Figure show safe bound for the particular colliery under discussion and in general all the observations barring a few lies outside the bound. From the Figure it is

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Fig. 2. Some results of drilling yield tests as obtained from the Chinakuri Colliery when the diameter of the drilled hole was 42 ram. Horizontal axis shows the depth of drill hole in metres and the vertical axis shows the volume of drilling eject in litres per metre, respectively. Dashed line parallel to the horizontal axis shows bounds of the drilling eject; upper line represents the safe limit of the drilling eject; one curve which crosses the upper bound shows a critical state of stress and it actually culminates into a bump.

86 apparent that, in general, the observations show erratic behaviour in the volume of drilling yield. In the present set of data there is only one observation which m a y be interpreted in the light of Wilson and Ashwin's (1972) view according to which a pillar is divided into two zones; a central inner core subjected to triaxial stress conditions surrounded b y a yielding zone which constrains the inner core.

Colliery for providing the facility to carry out the present work. The author acknowledges with thanks suggestions and help received f r o m Prof. R.D. Singh and Prof. A.K. Ghose. The author also thanks A.K. Sharma for his help in various ways. Financial support from the Ministry of Energy is gratefully acknowledged.

References 5. Conclusions A study of Indian Coal mine bump, as reported above, is preliminary in m a n y respects because of the limitations on the measurements being carried out in an u n d e r g r o u n d coal mine which happens to be the deepest in the c o u n t r y with a hostile environment. A n energy index test has been f o u n d quite useful to assess the liability of b u m p s in a coal mine. At present because of closure in the underg r o u n d opening, the incidence of b u m p s has been reduced appreciably. Some of the limitations of drilling yield test stems f r o m the very nature of drilling yield measurements side b y side with the p r o d u c t i o n of coal. A l t h o u g h the measurement involved is quite cumbersome; in G e r m a n y a legislation has been passed, as reported b y Will (1982), to carry out drilling yield test prior to undertaking any production programme. However, one of the advantages of the m e t h o d is its capability to provide information about the state of stress in the interior of a working face as mentioned earlier; although most of the observations m a y not give useful information; nevertheless some conspicuous values are b o u n d to be observed during the course of measurement. F o r the present case the incidence of single b u m p fully justifies the observations of the volume eject which is erratic in nature.

Acknowledgements The author is grateful to the authorities of the E C L and in particular to the Officers of Chinakuri

Chouhan, R.K.S. and Zorychta, A., 1985. Combating microearthquake hazards in Mining. Syrup. Mining--Present and Future, January 1985, Banaras Hindu University, Varanasi. Crouch, S.L, Hardy, M.P. and Christranson, M., 1977. Hybrid computer system for optimisation of extraction procedure in tabular coal bodies. U.S. Bur. Mines; Rep. No. Ho 252035, p. 48. Holland, C.T. and Thomas, E., 1954. Coal mine bumps: some aspects of occurrence, cause and control, U.S. Bur. Mines: Bull 535, p. 37. Kuszalir, N.J. and A1 Saigh, N.H., 1982. Some observations on the influence of pillar in mining induced seismicity. Symp. Seismicity of Mines, ISMR South Africa. Neyman, B., Szecowka, Z., and Zuberek, W., 1972. Effective methods for fighting rockbursts in Polish collieries. Proc. Fifth Int. Str. Con. Conf., Paper No. 23. Rice, G.S., 1934. Bumps in coal mines of the Crumberland field, Kentucky, Virginia--causes and remedy, U.S. Bur. Mines; RI 3267, pp. 1-36. Salamon, M.G.D., 1974. Rock mechanics of underground excavation. Proc. 3rd Congr. Inst. SOc. Rock Mech., Denver, Colorado, Vol. 1, Part B, pp. 951-1099. Salamon, M.G.D., 1983. Rock burst hazard and the fight for its alleviation in South African Gold Mines. Syrup. Rock Bursts: Prediction and C?ntrol, IMM London, pp. 11-36. Sharma, N.L., 1953-1954. The geological formations and the economic minerals of the Dhanbad sub-division of the Manbhum district. J. ISMAG, 11: 1-10. Singh, R.D., 1980. Investigations into problems of rockbursts in deep coal mines. R&D coal Int. Rept. No. 27106113/E, Indian School of Mines, Dhanbad. Szecowka, Z., DOmT~l~J. and Ozana, P., 19731Energy index of coal's natural liability to rockbursts. Proc. GIG, Komrr, 59A Kotawice. Will, M., 1982. Seismic observations during test drilling and destressing operations in German coal mines. Syrup. Seismicity of mines, ISRM, South Africa. Wilson, A.H. and Ashwin, D.P., 1972. Research into the determination of pillar size, Part 1, A hypothesis concerning pillar stability. Min. Eng., BI: 409-417.82