Geotechnical and Geological Engineering, 1995, 13, 63-78
Cuttability a s s e s s m e n t of hard coal seams R. S I N G H * , J . K . S I N G H t , T . N . S I N G H t and B . B . D H A R t *Department of Photogrammetry & Surveying, University College London, Gower Street, London WCIE 6BT, UK and ~CMRI (formerly CMRS), Dhanbad, India Received 9 May 1994 Accepted 20 N o v e m b e r 1994
Summary A number of field and laboratory tests have been carried out on more than 15 coal seams of compressive strengths ranging from 19 MPa to 44 MPa to evolve methods which would help in the selection of suitable coaling machines for hard coal seams. The effect of physico-mechanical properties on cuttability were studied in the laboratory for all these coal seams to identify the relevant parameters affecting the specific energy of coal cuttability. These data were subjected to regression analysis to find the best fit for estimation of laboratory specific energy of coal samples on the basis of simple laboratory and field tests for the strength parameters. Field studies were also conducted over a large number of active mechanized coal faces to study in situ cuttability along with the geo-mining conditions of the site. The field and the laboratory data so generated were correlated and an attempt is made to establish a relationship for estimating the field specific energy for a particular capacity of coaling machines by considering the geo-mining domain of the field in totality. Keywords: Coal seam; strength index; cuttability; specific energy; coaling machine.
Introduction The increasing demand for energy is being met mainly by coal in India. As such, the coal mining trend of the country is different from that of many developed countries. The coal producing target is being increased every year and, to achieve it, the coal mining industry is moving fast towards mechanization. The coal seams belong to the Lower G o n d w a n a stage and are massive with frequent shale and sandstone intrusions (Fig. 1). More than 90% of the underground coal production of the country is achieved through the conventional bord and pillar method of mining, which has adopted explosive energy to break the hard coal during underground working. However, due to the poor production and productivity of this method of mining, mechanical cutting of coal was introduced in the 1960s, when a few shearers and continuous miners were adopted for underground coal cutting. Mechanized longwall faces were introduced after 1978 to increase the underground production of coal and about a dozen such faces are working now in the 0960-3182 9 1995 Chapman & Hall
Singh et al.
64 Table 1. Salient details of the mechanized longwall faces Name of colliery (kW)
Date of commencement
Moonidih ML-V Apr. 1988 Moonidih ML-VI Jan. 1988 East Katras July 1989 Dhemomen Dec. 1989 Seetalpur Oct. 1982 Jhanjra Aug. 1989 Patherkhera Sept. 1982 Singareni Sept. 1983
Extraction height (m)
Production target (tpd)
Actual Power of production (tpd) shearer
1.8 1.2 2.4 3.0 1.8 3.5 1.5 3.2
1150 900 1050 2000 1200 1200 1100 1500
874 220 700 1600 250 1600 600 1600
375 100/200 300 380 200 2 x 200 200 375
tpd = Tonnes per day.
Fig. 1. Small cross section of a massive coal seam with a band of foreign material different coal basins of the country (Table 1). Due to the unavailability of field and laboratory cutting data for these hard coal seams, the present trend of mechanization suffers from a lack of standardization. As a result, coaling machines of either low capacity or over-design are generally produced. This not only hampers the production and productivity of coal mines but also adversely affects the very trend of mechanization. A large amount of research work has been done to visualize the cutting characteristics of coal seams of different countries, and a n u m b e r of testing procedures has been suggested for the selection of coaling machines. Unfortunately, no such detailed investigation has been conducted for the Indian coal seams and a wide variation of geomining conditions restricts direct adoption of the foreign norms and procedures. Therefore an extensive field and laboratory study was undertaken, aimed at evolving an indexing system of coal for the selection of proper coaling machines for the hard coal
Cuttability of hard coal seams Section of coal seam
200
150 E
65 Detail of bands
Rebound Field number drillability _ _ mm/min.
55
9.8
54 55
9.0 10
56
9.6
100
45
10
5O
56
9.4
e-
3
i
0
--
Fig. 2. A typical result of band distribution study (GDK9 incline, III seam) seams of the country. This paper presents the results and analysis of this study, which attempted to correlate the field energy requirement for the operating coaling machines with the geo-mining conditions of the site and the cuttability of the coal seams,
Programme
of work
Coaling machines require energy input depending upon the resistance to the cutting of coal and, as such, the cuttability is influenced by the nature and strength of coal, the presence of geological discontinuities (such as cleats and shale bands) and the stress field. The stress field around an underground excavation varies with the geo-mining conditions of the site and can only be studied in situ, while the other factors can be studied in the laboratory if the samples of coal are large enough. With this presumption, a large number of coal samples of large size were collected from the field and they were subjected to laboratory tests of physico-mechanical properties along with a cuttability study using a drag bit type coal plough rig. All these laboratory data of different coal seams were analysed to understand the influence of different variants of physico-mechanical properties on the cuttability of the coal sample. The in situ cutting data along with the geo-mining conditions of the site were collected from active mechanized coal faces in different collieries to develop a relationship between the laboratory and the field results.
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The coal seams of the country are extremely banded and relatively harder than those reported in the literature from other countries, because of the depositional scenario. The different bands of vitrain, clarain, durain and fusain are likely to influence the cutting energy requirement due to the difference in their physico-mechanical properties and so attention was paid to incorporate a fair representation of their physical distribution. Before any field test or sample collection, the band distribution (Fig. 2) between roof and floor of workable height was studied for each seam. The work described here has two components; laboratory measurement of the physicomechanical properties along with the specific energy for cutting of coal samples and the in situ energy requirement of the operating coaling machines at different coal mines, together with the geo-mining parameters of the site. To carry out the laboratory study, coal samples were collected from 17 different seams at 12 coal mines. Five coal samples of 30 cm3 were collected by pick mining from five different sites of each seam. The observation site was selected in the vicinity of the working area free from the influence of abutment loading. Out of these five samples, four were used for the study of cutting force for different cutting tool parameters, such as depth of cut, rack and pick angles, speed of cut etc. under the controlled sample orientation. The remaining one sample was used for preparing small samples for other tests of engineering properties. Rebound number tests and cleat frequency studies were conducted in the field at each and every site of sample collection. Continuous monitoring of power consumption during different working cycles for operating coaling machines and the in situ drillability tests were done at nine different mechanized coal faces to generate basic field cutting data.
Laboratory study The coal samples from different collieries were subjected to well-defined laboratory tests for their engineering properties such as uniaxial compressive strength (UCS), uniaxial tensile strength (UTS) and other physical properties. All the tests for the physicomechanical properties were conducted at the rock mechanics laboratory of the Central Mining Research Institute (CMRI), Dhanbad. CMRI has been investigating the field and the laboratory properties of coal seams for over three decades. The testing procedure and sample preparation adopted were in accordance with the norms of the International Society of Rock Mechanics (ISRM) (Brown, 1981). A number of indexing systems such as impact strength index (ISI) and Protodyakonov strength index values (PSI) were also determined to furnish information on the overall response of the mass to comminution. The mean value of all the measurements of physico-mechanical parameters of different coal seams are given in Table 2. Tensile strength The operation of a pick in the process of coal cutting acts against the tensile strength of the coal mass and so this parameter is likely to be more relevant in relation to the power of the coaling machines. Unfortunately, recovery of core type samples of coal for tensile strength testing was rather difficult and hence this test could not be done except on four coal seams, where only a few samples were recovered and tested. Due to the poor number of samples, the reliability of these measurements remained poor and were not considered during further analysis of the results.
Cuttability of hard coal seams
67
Table 2. Physico-mechanical properties and laboratory cuttability of coal Name of colliery
Name of seam
PSI
Pathakhera Seetalpur Seetalpur Ramnagore Dhemomain Moonidih Moonidih Moonidih Jamadoba East katras Pootkibalihari Gopalichak Chinakuri Kottadih GDK-9 Sudamdih Jhanjra
Bagdona Hatnal Sanctoria Laikdih Borachak 17th top 16th top 16th bottom XIV X XV X Disergarh Samla III IX/X R-VIII
1.37 1.15 0.93 0.98 1.25 0.91 0.96 0.88 0.95 0.99 0.95 0.90 0.83 1.29 1.35 0.88 1.23
UCS (MPa)
Pd
44.2 43.3 22.7 24.6 32.5 23.0 22.0 19.0 28.0 27.0 26.6 27.7 36.3 31.2 43.5 24.5 35.8
1.36 1.65 1.32 1.30 1.35 1.40 1.38 1.42 1.36 1.34 1.48 1.49 1.40 1.27 1.45 1.48 1.40
RN
UTS (MPa)
ISI
LSE MJ m -3
52 42 32 28 44 21 23 28 25 26 39 28 37 47 56 26 -
4.85 -
84.5 75.7 72.5 78.4 77.0 73.8 74.6 75.1 74.0 72.5 73.5 77.0 72.6 80.0 71.0 72.8
35.0 17.5 14.6 15.3 24.4 10.0 10.6 12.8 11.2 15.8 14.7 11.6 32.2 39.6 13.2 -
(Mg m -3)
2.92 1.86 4.21 -
PSI = Protodyankonov strength index; UCS = uniaxial compressive strength; Pd = bulk density; RN = rebound number (Schmidt hammer); UTS = uniaxial tensile strength; ISI = impact strength index; LSE = laboratory specific energy.
Uniaxial compressive strength U n i a x i a l c o m p r e s s i v e strength reflects resistance of a coal s e a m to t h e field stress a n d the a b u t m e n t loading. T h e e v a l u a t i o n of c o m p r e s s i v e s t r e n g t h was d o n e in the l a b o r a t o r y using cubical s a m p l e s o f 2.5 c m 3 o f different coal seams. U s i n g a universal testing m a c h i n e , t h e s p e c i m e n s w e r e t e s t e d as p e r the g u i d e lines of I S R M . T h e m e a n value of l a b o r a t o r y c o m p r e s s i v e strength, for t h e different coal seams, was f o u n d to v a r y from 19.0 M P a to 44.2 M P a .
Protodyakonov strength index T h e P r o t o d y a k o n o v s t r e n g t h index, t h o u g h well c o r r e l a t e d with the uniaxial c o m p r e s s i v e strength, also includes the influence of the p e t r o f a b r i c a n d the n a t u r e o f the coal. This i n d e x was e v a l u a t e d on a large n u m b e r o f s a m p l e s of the different coal s e a m s according to t h e s t a n d a r d test p r o c e d u r e ( P a n i n a n d T i k h o m i r o v a , 1967) with an a p p a r a t u s built inhouse. T h e d e g r e e of p a c k i n g of 500 Ixm fines in t h e v o l u m m e t e r was s t a n d a r d i z e d d u r i n g m e a s u r e m e n t b y k e e p i n g t h e n u m b e r of t a p s a n d its h e i g h t c o n s t a n t for all the tests. F o r t h e t e s t e d coal samples, the m e a n v a l u e of P r o t o d y a k o n o v s t r e n g t h i n d e x was f o u n d to v a r y f r o m 0.83 to 1.37.
Singh et al.
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Impact strength index Impact strength index is another way of characterizing coal strength, which has immense possibility for practical application in coal cutting and drilling. The value of impact strength index for different coal samples from the different collieries was measured in the laboratory according to the standard procedure and apparatus (Evans and Pomeroy, 1966). The observed mean value of the impact strength index varied from 71 to 84.5 for the different coal seams under consideration.
Bulk density The bulk density (Pd) was evaluated for all the selected coal seams, mainly to visualize the band intrusion and the ash content of the coal seams. The results of this study are shown in Table 2, which shows a variation from 1.27 Mg m -3 to 1.65 Mg m-3 for the different coal seams.
Fig. 3. Laboratory coal cutting study using a drag bit type coal plough rig
Cuttability of hard coal seams
69
Cuttability of coal samples In order to get the combined effect of sample strength and mass discontinuities, a cuttability study was conducted (Singh et al., 1992) on large coal blocks, collected from different bands of a coal seam. This test was performed in a specially designed 50-ton electro-hydraulic coal plough rig (Fig. 3). The cutting test was done in stress-free conditions for different orientations of coal samples with respect to cutting direction. The tests were performed with different cutting-tool parameters and depth of cut. This cutting rig was interfaced with a microprocessor-based data logger system for continuous recording of the normal cutting force, together with the position of the pick in all three directions at various moments in the process of cutting. In these exercises, the amount of coal produced and the force required for cutting were used as basic parameters for the calculation of specific energy of cuttability. The specific energy for coal cutting is defined as the amount of energy required to cut a unit volume of coal (MJ m-3). Using a multiplepick cutting assembly, an extensive study of coal cutting was carried out (CMRS, 1992) to find out the influence of strength and machine parameters on cuttability. For indexing the coal seams, the cutting force and the laboratory specific energy (LSE) were measured, 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 coal sample from different collieries. The specific energy of a large lump of the coal samples so determined under the standard plough set and operational control, was dependent purely on the nature of the coal. The shape of the 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.
Field investigation Laboratory tests, under idealized conditions, gave certain indices indicating the strength and nature of the coal and specific energy of cuttability, without considering the in situ geo-mining conditions in the field. They could serve well in estimating relative cuttability of the different coal seams but cannot give the full picture of the power requirements of the coaling machines. The field studies were therefore undertaken to measure energy consumption by working coaling machines in true field conditions. Geo-mining parameters of the site were also studied to visualize the influence of basic strength and operational factors on the specific energy under different field conditions. The studies conducted in the field can broadly be divided into the following four groups: (1) (2) (3) (4)
cleat study; rebound number study; in situ drillability test; study of energy consumption by operating coaling machines.
Cleat frequency As is well established, the direction of the natural inherent fracture systems of coal seams, such as cleats, plays an important role in establishing a preferred direction for the mine
Singh et al.
70 Table 3. Details of mines and results of the field study Name of colliery
Name of seam
Depth Extraction RN (m) height (m)
PR FSE (mm min 1) (MJ m -3)
Cleat frequency (no. per m)
Dhemomain Seetalpur Eastkatras Pathakhera GDK9 Jamadoba Moonidih Moonidih Moonidih
Borachak Hatnal Xth Bagdona III XIV 17th top 16th top 16th bottom
220 550 120 100 150 450 495 396 343
7.0 11.2 21.0 7.5 9.4 14.0 10.5 15.6
9 8 30 17 15 17 15 16
3.0 1.8 2.4 1.5 3.0 2.4 1.8 2.5 1.8
44 43 26 52 56 25 21 23 28
1.81 3.80 1.40 4.80 1.26 1.98 1.37 0.82 1.13
RN = rebound number (Schmidt hammer); PR = penetration rate (drillability); FSE = field specific energy.
development. The strength of coal is influenced by the frequency and orientation of the cleats. The application of an external force on a coal seam tends to break it along these natural weak planes of the main cleats or the bord cleats. The spacing and orientation of cleats were measured at all five sites of study for each coal seam. A b o u t 100 to 125 cleat observations were taken at each site along the exposed coal surface of the galleries. In general, two sets of cleats were found in all seams and the frequency of both the sets were different. For any single set of cleats of a coal seam, the spacing and orientation were almost constant and the average value of frequency of major set of cleats (CF) was taken into consideration for the analysis of the result. The observed average value of number of cleats (major set) for the coal seams under investigation varied between 8 per m to 30 per m (Table 3).
Rebound number study For the estimation of large scale in situ coal strength, a rebound number study using a Schmidt h a m m e r is an easy and simple method (Sheorey et al., 1984). Using an NR-type Schmidt hammer, rebound number was measured at different horizons between roof and floor of the coal seam at each site. For this study, the exposed coal seam surface was dressed properly to smooth it and remove the loose coal. The test was conducted for different bands of each coal seam at right angles to the main cleat surface. At least 25 readings were taken at each site and the mean value of rebound number (RN) was used for evaluation of a characteristic value for the coal seam. The average value of rebound number for different coal seams varied from 21 to 56 (Table 3).
In situ drillability test In order to get field information on drillability of the coal seams, a specially designed portable drilling rig (Fig. 4) was used. The drilling rig consisted of these main components:
Cuttability of hard coal seams
71
Fig. 4. Sketch of the portable drilling machine used for field testing
(1) drilling machine, 1.5 HP, 300 rpm; (2) drilling mounting frame with: (i) two horizontal chromium plated guide rods of 25 m m diameter and 1.3 m length; (ii) a guide rod holder keeping the guide rods at a fixed interval of 250 m m and with provision for mounting the drilling machine; (iii) a fixing prop of a telescopic nature to control the vertical horizon of the drilling machine with provision for screw tight setting; (3) drill: micro bit of 10 m m diameter and 120 m m length with compatible drill rod; (4) constant thrust control: a pulley and roller mounted assembly holding a constant load of 29 kg.
Table 4. A statistical presentation of observations (Dhemomain colliery)
Parameter
Number of observations
Minimum
Mean
Maximum
RMS value
Standard deviation
PSI UCS (MPa) UTS a (MPa) ISI Pd (Mg m ~) LSE (MJ m -3) RN PR (mm min-1) Cleat frequency (no. m-1) FSE (MJ m -3)
10 19 3 15 5 10 65 10 5 15
1.23 32.0 2.88 76.0 1.32 24.2 40.0 6.5 8.0 1.79
1.25 32.5 2.92 77.0 1.35 24.4 44.0 7.0 9.0 1.81
1.28 33.0 2.96 77.5 1.38 24.6 48.0 7.4 10.0 1.84
1.26 32.6 2.91 76.9 1.36 24.4 44.04 7.2 9.0 1.83
0.012 0.189 0.040 0.381 0.025 0.110 1.237 0.247 0.707 0.017
aNot considered due to poor number of samples. See Tables 3 and 4 for definition of terms.
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Singh et al.
The drill was mounted over the guide rod itself and fixed in between the coal face and the mounting telescopic prop. The penetration rate (PR) was observed at all observation sites of each mine (Table 3). The guide rod was always kept horizontal and the pulleys were well lubricated to ensure constant thrust at all sites although it was very difficult to keep the rolling system free from dust during the drilling.
Study of field specific energy To evaluate specific energy of the operating coaling machines, extensive field monitoring of power consumption was done during different working cycles of these machines at nine different mechanized underground coal mines (CMRS, 1992). To measure the energy consumption rate during coal cutting, the current and voltage of the shearer motor was continuously recorded for different operating cycles of the coaling machines. The corresponding coal cutting rate was calculated by measuring the rate of shearer movement, depth of cut and working height. The power consumption rate of the shearer was also observed during fleeting with a rotating drum. The average value of all these observations were used for the calculation of the field specific energy (FSE) of the coal seam (Table 3). The other geo-mining parameters like seam thickness, working height, web depth, depth cover of the seam, etc., were collected to supplement the specific energy data. Typical statistics of the measurement of different parameters of a coal seam in both laboratory and field are shown in Table 4.
Results and discussion
The ultimate aim of all these tests was to evolve or adopt a simple procedure which could serve as a ready reckoner for coaling machine selection. On the basis of the above studies, two options were available for the system of indexing depending upon; (i) the material property of a coal sample or mass and (ii) the geo-mining domain of the site in totality. Obviously, the second option is the more complete and is preferred in this study. The laboratory cutting data were correlated with physico-mechanical properties of the coal seams to estimate the laboratory specific energy of the coal sample on the basis of easy and conventional field and laboratory measurements. This relation provides information which is completely free from geo-mining constraints of the site. The cuttability study of coal samples in idealized conditions in the laboratory furnished an idea of strength of different coal samples. When we consider the strength of a coal mass in place of a coal sample, the role of geological discontinuities becomes important (Bieniawski and Van Heeden, 1975) along with the shape and size. In particular, to estimate the in situ cutting behaviour of coal mass, the impact of other geo-mining and operational factors like, stress conditions, compactness of deposition, confinement due to surrounding rock strata and machine performance become important and relevant. To consider all these parameters, along with the basic strength information in the form of laboratory specific energy for estimation of the field specific energy of cuttability, the laboratory and field data were correlated to establish a relationship. The efficiency of a coal-cutting machine involves a number of internal parameters of the machine which become very complicated for modelling the actual working condition. However, the mechanism of all the coal-cutting machines, where field studies were
Cuttability of hard coal seams
73
40
[] []
30 ~
[]
~o 20
[]
[]
10
0
i
i
i
i
20
30
40
50
Uniaxial compressivestrength (MPa) Fig. 5. Variation of laboratory specific energy with uniaxial compressive strength of the coal seams conducted, were identical and the surrounding working environments were reasonably similar for these machines. With all these considerations, in this study only the amount of input electrical power was taken into account for the visualization of cutting performance of all these machines of high power and capacity.
Analysis of laboratory data The mechanism of coal cutting is not well understood, mainly due to the inherent complex processes. On the basis of experimental observations, simplified rock cutting models (Evans and Pomeroy, 1966; Nishimatsu, 1972) have been developed to describe the process of chip formation. The exact stress analysis during the chip formation is yet to be developed, but the experimental results of the tool-rock interaction have been used for improving cutting efficiency with pick cutting machines (Roxborough, 1973). There are a number of geometrical and rock parameters which may influence the phenomenon of coal cutting. 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 fracture properties of the coal. The fracture toughness of coal can be correlated with cuttability through specific energy to quantify the cutting performance of a coaling machine. The specific energy of coal, determined in the controlled environment of the laboratory on a large lump of coal with a standard plough set and operational control, was accepted as a standard parameter and was correlated with the other physicomechanical properties of coal. The plot of uniaxial compressive strength versus laboratory specific energy is shown in Fig. 5. Although the relationship between these
Singh et al.
74 40
[]
30 []
[]
[]
[]
>-,
20 H .K
[]
10
0 20
i
i
i
i
30
40
50
60
Rebound number Fig. 6. Variation of laboratory specific energy with rebound number of the coal seams
40-
[]
30
[]
[]
20 r
10 ,-1
0 0.8
i
i
!
i
i
i
0.9
1.0
1.1
1_2
1.3
1.4
Protodyakonov strength index Fig. 7. Variation of laboratory specific energy with Protodyakonov strength index of the coal seams
75
Cuttability o f hard coal seams
two parameters was envisaged to be linear (Singh, 1987), the plot does not corroborate that exactly. The value of laboratory specific energy was found to vary significantly (up to three times) for coal samples from different coal seams having nearly the same uniaxial compressive strength. To identify the main parameters responsible for affecting laboratory cuttability, a number of single variant regression analyses were performed and a relationship for the laboratory specific energy was established with other parameters such as: rebound number; uniaxial compressive strength; impact strength index; bulk density; Protodyakonov strength index; and field drillability. The correlation coefficient of rebound number (Fig. 6) and Protodyakonov strength index (Fig. 7) was found to be very high. The rebound number covers the in situ strength of the coal and Protodyakonov strength index encompasses compressive strength, brittleness and effect of cleats on comminution and so they appeared to give a fair idea of laboratory cuttability. Considering each of these important variables at a time, a computer program for multivariable regression was run a number of times in order to find the solution iteratively. The best fit equation obtained for laboratory specific energy is: S = ie (je+kR)
MJ m -3
(1)
where S = laboratory specific energy (MJ m-3), R = rebound number, P = Protodyakonov strength index, and i, j and k are constants with values 2.061, 1.562 and 0.014 respectively. This relationship has a correlation coefficient of 97%. The value of laboratory specific energy, which represents the strength of a coal sample for cuttability, can be estimated easily by measuring rebound number and Protodyakonov strength index of the coal.
Model o f in situ cuttability
The formation of chips during the cutting process is mainly dependent upon the amount and nature of applied stress and the strength of the coal mass. This basic model of pick cutting has been used in selecting and rejecting different geo-mining parameters during estimation of field cuttability of a coal seam. The in situ behaviour of a coal mass is very different from that of the coal sample in the laboratory. In the field, the coal surface experiences two types of stresses during the cutting process; first, pre-excavation in situ stress and second, stress applied by the pick of the machine. These stresses are matched against a suitable criterion to initiate breakage due to the action of pick. So the nature of the in situ cuttability model should be such so as to reduce the value of field specific energy for an increase in the in situ stress. A detailed field and laboratory study has been conducted by CMRI to establish the effect of depth cover on in situ strength of the coal (Sheorey, 1992) and it has been found that the in situ large-scale strength of coal is more affected by depth cover than laboratory small-specimen strength (Sheorey et al., 1987). For the estimation of in situ coal pillar strength, width/height ratio becomes an important factor, along with the sample strength of coal (Salamon and Munro, 1967). For cuttability, the situation becomes very complex and the triaxial confinement condition hardly exists. The almost one dimensional form of the pillar strength analogy may be useful if the web depth of the coaling machines remains below 1 m. Here confinement due to strata along the roof and floor increases the value of internal friction causing favourable conditions for the stability of the frontal section of the coal seam against the applied force to dislodge it. A lower working height of coal seam may impose higher
Singh et al.
76
resistance on the coaling machine while the higher working height may lower the energy consumption of the coaling machine during cutting. Due to changes in configuration, dimension and nature of action and reaction during cutting, the influence of field stress, size, shape, geological discontinuities and end constraint can play important roles in the cuttability model. On the basis of the above discussion, the characteristic of the formula for field specific energy should have the following characteristics: (1) (2) (3) (4) (5)
it should fit the case studies; increase of in situ stresses should favour the cuttability; the influence of practical ranges of height of extraction should be properly included; increase of field specific energy with increase of laboratory strength; it should exhibit the effect of geological discontinuities and end constraints.
Consideration of field parameters During this study the effect of abutment loading has been ignored and most of the data were collected at the start of a panel, and therefore supposed to be free from the 6.0
5.0 ~ 4.0
Observed values Predicted values
---!
3.0 ~D L.)
I
I ~D
2.0 i
1.0
0.0
iii 0.0
GDK- Seet-
9
alpur
Eastkatras
Dhemomain
Pathakhera
Mooni Mooni Mooni Mooni dih 17 dih 16 dihl6 dihl6 top bottom top A top B
Fig. 8. Comparison of observed and calculated values of field specific energy for different coal seams
Cuttability of hard coal seams
77
abutment loading. An attempt is made to establish a relationship between the field specific energy of the operating longwall faces and the basic strength parameter in the form of laboratory specific energy together with other geo-mining parameters studied in the field and the laboratory using a statistical regression analysis. After a thorough graphical and mathematical examination, it was found that field specific energy did not correlate well with any single factor representing mining parameters, strength of coal or geological discontinuities. Taking all the well-correlated parameters of all the three factors into consideration, the following relation was established to estimate the field specific energy (FSE) of a coal seam: FSE =
0.04S~ + T) TDO,29C0.92 MJ m -3
(2)
where S -- laboratory specific energy (MJ m-3), D = depth cover of the coal seam (m), C = frequency of cleats (number per m), and T = operating seam thickness (m). The index of determination of this relationship is 0.8893. The value of laboratory specific energy represents the sample strength of the coal seam. Increase of depth cover favours field cuttability which encompasses the influence of the in situ stress field on the cuttability. The operating seam thickness exhibits the impact of size and end constraints and the increase in working height reduces the value of field specific energy. It is interesting to note that the statistical technique indicates that T is a factor in the denominator and the numerator of Equation 2. Higher value of T favours cuttability of the coal seam. The incorporation of cleat frequency in the relationship represents the effect of the inherent fracture system of the coal mass on the value of field specific energy. A comparison of the values of field specific energy calculated from the derived equation and observed in the field is shown in Figure 8. In general, the equation appears to be reliable and in reasonable agreement with the limited observations collected in the field.
Conclusion
On the basis of rigorous laboratory and field studies, a relationship is established to calculate laboratory specific energy on the basis of simple tests of physico-mechanical properties of coal seams. Using this value of laboratory specific energy as a basic parameter of strength and considering relevant geo-mining parameters of the site, an expression is derived to estimate the field cuttability of a coal seam. Although there is a good agreement between the calculated and observed values of the field specific energies, the influence of machine parameters has been ignored completely in the study. The incorporation of machine parameters may modify the expression to yield more realistic results. The influence of abutment loading may be another important factor in optimizing the capacity of coaling machines. The formula for field specific energy (FSE) is based on studies for coal seams with working heights between 1.5 m to 3.0 m, depth cover between 100 m and 550 m, cleat frequency (main set) between 8 per m to 30 per m and the laboratory specific energy ranging from 0.82 MJ m-3 to 4.8 MJ m-3. Hence the relationship deduced in this paper may be suitable for the above ranges.
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Acknowledgements The work described herein is based on an S & T project funded by the Department of Coal, Ministry of Energy (Government of India) and supported by Central Mine Planning and Design Institute of Coal India Limited. Mr T.K. Das, Mr B.V.S. Parihar, Mr B.N. Singh and S.K. Singh helped in laboratory and field work of data collection. Dr C. Bandopadhyay and Dr P.R. Sheorey provided appreciable help during data processing and analysis. The co-operation provided by the management of different coal companies during field study and sample collection is thankfully acknowledged.
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