外文翻译-岩浆侵入煤层的切割性研究

Cuttability of coal seams with igneous intrusions Rajendra Singh*, A.K. Singh, P.K. MandalAbstract On the basis of physical inspection of exposed area in a gallery, the coal mass near an igneous intrusion band is divided into four zones; called as (1) normal, (2) pulverized, (3) Jhama and (4) mixed zone excluding the dyke/sill. Some simple field and laboratory studies were carried out to visualise the nature and extent of variation of strength and cuttability across these zones of igneous intrusion in a coal seam. Influence of intruded igneous materials over the in situ strength of the coal mass was studied by Schmidt hammer, while the influence over cuttability of the seam was studied through monitoring of current drawn by roadheaders during mechanised gallery drivage in coal seams consisting of igneous intrusions. Both strength and cuttability across the band of igneous intrusion were found to be highly dependent upon the proximity and extent of the intrusion. In fact, the requirement of current for gallery drivage across the coal mass with igneous intrusion was observed to be quite high in comparison to that required for the normal coal mass. To understand this phenomenon in detail, a number of samples were collected from the coal intrusion interface and were subjected to laboratory tests for physico-mechanical properties along with cuttability testing with a drag bit type coal plough rig. The laboratory study showed wide variation in physico-mechanical and cuttability properties of different samples collected from different zones of the affected coal mass surrounding the intrusion band. D 2002 Elsevier Science B.V. All rights reserved.1. IntroductionCoal is a concentrated form of invaluable natural energy. Its proper exploitation is important not only foreconomical development but also for environmental, ecological and conservation points of view. Mecha- nised extraction of coal provides better production, productivity and safety. However, mechanical cutting of coal is difficult because of higher strength coal seams in India (Singh, 1999). This situation becomes even more difficult in a coalfield with large number of igneous intrusions in the coal seams. Jharia coalfield is becomes important if the drivage/working encounters traversed by a number of different igneous intrusions affecting workability of a considerable number of coal seams. Here, a number of coal seams have been baked in absence of oxygen (Jhama formation) during the intrusion of the igneous materials. Depending upon various factors like temperature of the igneous mate- rial, duration of the igneous activity, size of the entry channels in the basin sediment, etc., the composition of jhama varied within wide limits (Chandra, 1992). Currently, powerful coal cutting machines are available in the market to cut harder and harder rock, but a better understanding of progressive change on the behavior of a coal mass around the jhama/dyke contact plane becomes important if the drivage/working encounters an igneous intrusive frequently in the coal mass。2. Literature review Although the first coal retrieval machine was developed and used in a British coal mine more than 130years ago (Walker, 1902), most of the studies on coal cutting were done only within the last five decades. The effect of cutting tool and machine parameters, geo-mining conditions and physic mechanical properties of coal over cutting force was studied by a number of authors and, theoretical aspects of coal cutting were detailed in the late 1950s (Evans, 1958). The exper- imental results of extensive work conducted in this field by Pomeroy (1963, 1964) provided the effects of tool geometry and other operational factors on the cutting force. Initially, Merchants theory (Merchant, 1944) of basic mechanics of the metal cutting process was applied to give an analytical equation of the cutting force for coal/rock cutting (Roxborough and Rispin, 1973). Further research thrust in this field led to construction of a full scale boom tunneling research rig in a laboratory (Speight and Fowell, 1987) to obtain accurate cutting data. On the have been developed to describe the process of chip formation. Experimental results of the tool rock interaction have been used for the improvement in cutting efficiency with the pick cutting machines (Hekimoglu, 1995; Bilgin et al., 1996). An empirical model of in situcuttability of hard coal seams was developed (Singh et al., 1995) to estimate the power of a coal-cutting machine considering the geo-mining domain of the field in totality. However, even with all these studies, the mechanism of the coal cutting is not well under- stood mainly due to the inherent complex processes. The exact stress analysis during the chip formation is yet to be developed. In the process of chip formation, crack propagation is the basic phenomenon for coal disintegration during cutting. This mechanism of fracture shows the relationship between cuttability and physic mechanical properties of the coal. The influence of geological disturbances over the physic mechanical properties of a coal mass is well known. Matsui et al. (1998) attempted to visualise the influence of geological disturbances over the performance of a roadheader during drivage of a coalmine tunnel. However, this study was limited up to the affect of faults only. The influence of thermo-contact transformation of coal mass (Jhama formation) by the igneous intrusion over its physico-mechanical properties (Singh and Singh, 1989) is not well reported. The influence of inhomo geneities due to the igneous intrusion over the strength and cuttability of the coal mass is complex. Presence of large number of ball coal (Chandra, 1992) and frequent bands of igneous intrusions in some of the Indian coal seams makes it very difficult to be worked by a coal-cutting machine. For mechanised gallery drivage in a coal seam containing frequent igneous intrusions, the understanding of affect of these intrusions over its strength and cuttability becomes a subject of interest for the coal mining industry. On the basis of some simple field and laboratory investigations, an attempt is made in this paper to visualise the change in strength and cuttability of the coal mass surrounding and across an igneous intrusion band.3. Study siteThe coal seams belong to Lower Gondwana stage and are massive in nature with frequent stone andigneous intrusions. More than 90% of the under- ground coal production of the country occurs throughconventional bord and pillar method of mining, in which explosive energy is used to break the hard coalduring underground working. However, due to the poor production, productivity and safety of thismethod of mining, different machines are introduced for mechanical cutting of coal. The coaling machinesrequire energy input depending upon the resistance to the cutting of coal and, as such, the cuttability isinfluenced by the nature and strength of coal, the presence of geological discontinuities, and the stressfield. During mechanised gallery drivage, the change in strength domain from coal to jhama near a jhama/ dyke contact adversely affects the rate of advance of the gallery drivage and also the life of the cutting tools(Fig. 1). The magnitude of this problem for a mech- anised gallery drivage is likely to increase further withthe increase of depth of cover of the coal seams as the ratio of coal/jhama decreases considerably (Table 1)for a deeper coal seam. In situ studies of variation of strength of coal/rock mass to power consumption by roadheaders during gallery drivage across the band of igneous intrusion were conducted in XV seam (Pootki-Balihari colliery)and IX/X seam (Sudamdih colliery) of Bharat Cock-ing Coal (BCCL). Gate roads for different proposed longwall panels were in progress at both of these mines for which roadheaders were deployed. A milling type (longitudinal rotating) roadheader was used in XV seam of Pootki-Balihari colliery while, the ripper type (transverse rotating) was used in IX/X seam of Sudamdih colliery. These two seams were consisted of a number of igneous intrusions, which converted a large portion of the coal seam into burnt coal (jhama). In fact, XV seam of Pootki-Balihari colliery was found to consist large amount of ball coal (around 50% of the reserve). Diameter of the ball coal in the seam varied from a fraction of centimeter to 0.5 m. A number of igneous intrusions were encountered during the gallery drivage, but it was not possible to derive a figure of frequency of the intrusions as they were randomly placed and oriented in the coal mass. Although the layout of the longwall roadways was simple, the igneous intrusion intersected these parallelroadways at different angles. For simplicity, only few nearly transverse intersections of dyke and roadways were selected for this study.4. GeologyThe general stratigraphic of Jharia coalfield is given in Table 2. This coalfield produces around 90% of the total coking coal for the country and is traversed by a number of dykes and sills. Two types of igneous intrusive units encountered in this coalfield are: (1) Dolerites and (2) Ultrabasics. These igneous intrusive units have caused partial or total regional burning of a number of valuable coking coal seams, converting more than 1200 million tons of coal into jhama and ball coal. The formation of jhama and ball coal never follows a definite horizon in the coal basin and sometimes intrudes in between the seam or along the roof or floor. Dolerite dykes are encountered on the western part of the coalfield, while central and eastern part of the coalfield is affected by the Ultra- basics. Fig. 2 damage and, in some cases, entire seam section has been transformed into jhama.5. Study programmeThe evaluation of efficiency of a mechanized gallery drivage is achieved by continuous study of the factors which effect on power consumption by the machine. Layout of these galleries was simple as only two parallel gate roads were formed for each longwall panel but the roadways encountered a number of randomly placed and oriented igneous intrusions in the coal mass. One of the main factors, which influences the performance of the machine (roadheader) in geologically disturbed coal seams, is observed to be the abrupt change in strength properties caused by the presence of jhama, dyke, ball coal and the change in the frequency of geological discontinuities. Laboratory and field investigations were undertaken to understand the influence of an igneous intrusion on cuttability and physico-mechanical properties of the surrounding coal mass. A Schmidt hammer (Sheoreyet al., 1984) was used in field to study the change in strength across the band of an igneous intrusion. Continuous field monitoring was carried out at two sites to study the influence of a band of inhomogeneity in the coal seam (caused by the igneous intrusion) over the power of a roadheader for mechanized gallery drivage. In fact, estimation of actual power consumption by different bands around an igneous intrusion was an extremely difficult task. So an approximate indexing of the different bands was done only by current variation monitoring keeping all operational and machine parameters, including rate of advance, nearly constant. Further, the programme of in situ cuttability study was not able to provide enough details to differentiate the characteristics of different zones around an intrusion band in the coal mass due to operational constraints of the field. Therefore, coal/rock samples of larger size, collected separately from different sections around the intrusion band, were subjected to different laboratory tests of physic mechanical properties along with the cuttability study on a drag bit type coal plough rig. Idealised laboratory study was conducted for indepth cutting characteristic visualization of differentsamples of each section surrounding the intrusion band. Cutting test was performed in a speciallydesigned 50-ton electro-hydraulic coal plough rig.This cutting rig was interfaced with a microprocessorbased data logger system for continuous recording of the normal cutting force along with the position of thepick in all three directions at various moments in the process of cutting. In these exercises, the amount ofcoal produced and the force required for cutting were used as basic parameters for the calculation of specificenergy of cuttability. The specific energy for coal cutting is defined as the amount of energy required to cut the unit volume of coal, mega Joule per cubic meter (MJ/m3). The shape of groove formed by the pick during cutting was irregular, so the amount of coal chips produced was used to calculate the volume of these grooves. For indexing different zones near jhama/dyke interface, the cutting force and the laboratory specific energy (LSE) measurement was done, keeping all operational and machine parameters such as, depth of cut, speed of cut, tip and rack angles of the pick, orientation of cut, etc., constant for each and every samples. The specific energy of big lump of the samples so determined under standard plough set and operational control was dependent purely on the nature of the coal/rock.6. Thermo-contact transformation of coal massThe intrusion process itself is quite complex and does not follow any fixed trend everywhere. Also theigneous material of the intrusion varies from place to place. In Indian coalfields, the igneous intrusionscausing regional in situ burning of a coal seam are normally mica-peridotite and dolerite. After tearing the coal seam along the lower resistance path, the intruded igneous materials appear in the form of a dyke and asill in and around a coal seam. Thermo-contact transformation, as a result of the igneous intrusion, resultedin a significant change in physico-mechanical properties of a coal seam. A coking coal seam lost its coking capacity because in situ carbonisation to various degrees was caused by the intrusion. The coal seam was hardened or weakened depending upon the proximity and extent of the intrusion. The rank of coal changed from burnt coal (Jhama) to pulverised soft mass. Metamorphism of coal mass related to theigneous intrusion caused a loss of its natural structural features. Losses of volatile matter from the coal mass by in situ burning caused a sharp increase in the compressive strength (Singh and Singh, 1989) of the jhama (Fig. 3). These characteristic changes in the coal mass and intrusion of foreign igneous materials are the two main responsible factors for the change in the physico-mechanical properties across the band of an igneous intrusion. There is a progressive change in chemical composition of the coal mass around a dyke (Sengupta, 1980). In fact, baking of a surrounding coal mass in absence of oxygen dies out within a distance1 from the dyke as coal is a poor conductor of heat. On the basis of petrographic studies, the affected coal/rock mass between an unaffected igneous intrusion and an unaffected coal mass was divided into seven zones (Chaudhary et al., 1979). However, physico-mechanical properties of the coal mass play an important role during breakage of coal for which the petrographic study does not give much information. Physical inspection of exposed areas of a coal mass near igneous intrusions in different galleries noticed that, generally, the extent of regional burning in the surrounding coal mass is almost proportional to the thickness of the dyke. However, at a few places the extent of regional burning in the surrounding coal mass of a thicker dyke was observed to be less than that of a relatively thin dyke. This anomaly might have large number of studies.7. Field studyUncertainty involved in the occurrence of igneous intrusions ahead of the operating machine made the field study a time consuming process. Only two types of field studies, i.e. strength and current variation across the affected area, could be conducted because of different operational constraints.7.1. Current variation During the study, three dykes were encountered in 26L gallery of XV seam at Pootki-Balihari colliery where a Dosco roadheader (Milling type) was deployed for the roadway drivage work. Continuous working of the machine was resulted approximately 1 m/h advance of the face but the effective working time of the machine was nearly 3 h per shift. Considering the exposed positions and orientations of the dykes in 26L, current drawn by the roadheader during crossing these dykes in 27L was monitored with the help of the readily available arrangement there in the system. The rate of advance was kept same during the observation. properties in that region7.2. Strength variationarge scale in situ strength (Sheorey et al., 1984) of different bands exposed in the gallery around a dyke.Using a NR type Schmidt hammer, the rebound number study was done in different horizons between roof and floor of the influenced area in and around the intrusion, exposed in the gallery. For this study, the exposed surfaces of different bands were dressed properly to smooth it and remove the loose material. Six to eight rows of nearly equally spaced observation points were fixed over all the exposed bands near the intrusion. A closely spaced grid pattern of observation points thus formed over the exposed surfaces of each band is used for a dense measurement in space. The rebound tests were conducted for different bands at right angle to the main cleat/joint surface. The number of observation points in different band varied according to the width of the exposed surface of the band. However, adjusting the spacing of observation points for a thinner band, at least 30 readings were taken for the evaluation of strength characteristic of the band. To have an idea of variati