Caving behaviour in Longwall

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Caving behaviour in Longwall

  1. 1. CAVING BEHAVIOUR AND ESTIMATION OF LOAD IN LONGWALL FACES OF SCCL – A CASE STUDY ABSTRACT With the ever-increasing demand of power, it is expected that the demand for coal is to increase proportionately. It is envisaged to double the present installed coal based power generation capacity of 60,000 MW by the end of XII Five-Year Plan. To have 6% growth rate in the GDP it is required to increase the installed capacity by a rate of 7%. These above figures call for increase in the installed capacity of coal based power plants by about 5000 MW per year, in turn the coal demand will increase by about 20 Million Tonnes per year(1). With the fast liquidation of shallow coal reserves by opencast methods, it is forced on the Indian mining industry to extract coal reserves from deeper horizons. Further, with ever increasing environmental awareness, it is becoming increasingly difficult to envisage opencast projects. Longwall mining technology is an established technology world over for extraction of coal from deeper horizons and it is only alternative for safer bulk production. Especially in case of the Singareni Collieries Company Limited, with limited shallow reserves, longwall mining appears the only alternative to mine coal from depths beyond 300m. Though Longwall Technology is not new to Singareni Collieries Company most ambitious longwall projects were shelved because of severe reservation expressed by policy makers on the basis of evidences of failures. Most of failures in longwalls are attributed to failure of supports selected. Hence, the estimation of support capacity become a topic of serious concern and requires in depth study. An attempt is made to correctly understand the caving behaviour of roof rock in longwall faces for selection of powered roof support capacity. Plate theory of failure of roof rock is adopted to estimate the caving behaviour of roof rock over longwall faces and caving distances are calculated. Principles of bending of plates and beams is utilised to evaluate the effect of interaction of beds over the longwall face. Three case studies comprising of three longwall mines of SCCL are selected and the methodology is applied for estimation of required support capacities and a brief estimate of support load with face advance is also presented. This will benefit the mining industry at large for mining of deep coal deposits with high capital intensive coal mining projects.
  2. 2. 1.0 INTRODUCTION Longwall mining is an established underground coal mining system worldwide. The current world wide trends proved that longwall mining offers the most promising and profitable technological option for under ground coal exploitation. It is proved beyond doubt that longwall mining is the best option for mining coal which occur at greater depth. According to coal statistics about 50% of total coal production in the world accounted from longwall mining technology. Introduction of longwall mining dates back to 1906-07. But only in late sixties there was longwall mining with coal cutting machines. In between 1970-82 around 38 longwall faces were worked in Jharia and Raniganj Coal fieds. In 1978 first mechanised longwall face was introduced in Moonidih Colliery. In between 1978 and 1985 many longwall faces were introduced in both Coal India Limited and Singareni Collieries. Presently lthe latest type of longwall supports and shearers are available in Kottadih and Jhanjra of ECL and at GDK-10A and Padmavathikhani of SCCL. The success of any longwall depends on the proper selection of powered roof support to suit to the caving conditions available. It is clear from the experience of past longwall faces that the failure of any longwall project invariably is linked to the improper selection of powered roof support or failure of support system. The history of Longwall faces of Churcha Colliery of Coal India Limited and GDK-9 incline and GDK-11A incline of Singareni Collieries stand as witness to the above fact. Hence, it is can be concluded that support system selected on the basis of reliable theories supported by field investigations would go a long way in smooth running of longwall faces. 2.0 LONGWALL MINING METHOD Longwall mining method includes drivage of two long roadways of about 1.0 km length in coal and joining them by 150m long drivage perpendicular to them at the end of the above drivages. Hydraulically powered chock shield supports are installed in 150m connecting roadway, called face (Ref.Fig.No.1). Powered shearers are deployed for coal cutting and the cut coal is transported over Armoured face conveyor (AFC) in face and series of belt conveyors to surface. Cut after cut, shear after shear the AFC and subsequently chock shields supports will be advanced. As soon as these supports are advanced immediate roof rock above caves in. As the retreat further proceeds substantial area of main roof rock forms a plate and caves in. During this caving, load is imposed on the supports which is known as main weighting. Once the main roof breaks in, the roof rock does not behave like a plate. It behaves like a cantilever-imposing load on the supports with the deflection of beam. With the further advance of the face, the rock cantilever hanging over the rear of the chock shield support caves in imposing load of the support which is known as periodic weighting. All falls subsequent to main fall are less in magnitude and impact with regard to loading on the support. This is because of filling of goaf after the main fall and load sharing by the goaf waste. Unlike in tunneling and other mining excavations, caving of roof rocks is an essential requirement for the smooth running of longwall face without any strata related problems. A correct estimation of caving behaviour is very much essential for calculating the load being imposed on the chock shields supports. The paper deals with one such method of estimating the caving behaviour and the load that is being imposed on the supports. The process of first weighting and subsequent periodic weightings during longwall retreat and the mechanism of imposing load are presented in Fig. No.2. 3.0 THE PLATE THEORY Estimation of caving and support load in longwall faces is an interesting topic of study at present and it is one of the most seriously studied topic for the survival of mining industry in long run. In the past many theories were developed to estimate the support load. The support resistance required depends largely on the cavability of strata above the longwall face. La basse's approach, Wilson’s approach, American approach and Peng and Chiang approach are few approaches (2) to mention. These methods range from purely empirical to theoretical approaches.
  3. 3. As explained above, in the past many theories were developed to explain the caving behaviour and to estimate the load on support in longwall faces. These models have their limitations too. Qian Ming – GAO(3) et al, after careful study of insitu conditions and analysis on physical models in laboratory concluded that the main roof above the extracted area can be treated as a plate supported and clamped by the elastic foundation in different boundary conditions. Mechanical model can be formed for analysing the main roof weighting span, and fracture capacity. It is assumed that the breaking of plate structure is due to the bending stress in plate attaining the tensile strength of bed. The breaking process of the plate is given by M = f(a/b) x w x b2 ………….. (1) where, f(a/b) = coefficient of bending moment for the face advance of ‘a’ w = weight per unit thickness of bed b = face length Qian Ming and others after study gave a graph from which f(a/b) can be obtained for different a/b ratios, where ‘a’ is the face advance. Hence from the equation 1, bending moment can be calculated. And knowing the cross section of bed, the bending stress developed can be calculated. If the difference of this bending stress and calculated horizontal stress is greater than the tensile strength, then the bed is expected to fail 4.0 CASE STUDIES Singareni Collieries Company is one of the leading coal companies of India and longwall mining was accepted method of mining and at present there are six longwall faces at VK-7 inc., Padmavathikhani, JK-5 inc., GDK-9inc and GDK-10Ainc. Singareni is planning in a big way to start longwall faces in Deep shaft blocks of Godavari coal fields. The above theory of failure is utilised to calculate the caving distances and the load to which the supports are subjected to is evaluated for three cases(4). In two cases the data is compared with the available field data and in the third case a support capacity suitable to the caving conditions is suggested. 3.1 CASE STUDY - I The following Table No.1 will give the calculated values for Padmavathikhani mine(4). Refer figure :fig (3.1,3.2, & 3.3) TABLE No 1 Sl. Parameter Calculated Remarks No. value 1 Caving height above coal seam (m) 13.305 Bulking factor of individual bed is taken into consideration 2 Number of Beds identified within the 3 caving height 3 Details of Beds a) Thickness of Immediate roof (m) 3.48 b) Thickness of Bed 1 & 2 (m) 7.43 & 2.40 c) Density of Bed 1 & 2 (T/M3) 2.12 & 2.14 d) Depth of Bed 1 & 2 (m) 85.78 & 78.35 e) Density of roof above Bed 2 (T/M3) 2.02
  4. 4. Sl. Parameter Calculated Remarks No. value f) Dept of Roof Bed 2 (m) 75.95 g) Young's Modulus of Bed (kg/cm2) 0.44 x 105 & 0.51 x 105 h) Tensile Strength of Bed 1 & 2 (MPA) 1.04 & 1.30 i) Face length & working section (m) 150 & 3.0 4 Caving Distances (1), (3) a) Theoretical Main Fall i) Bed 1 (m) 55 ii) Bed 2 (m) 30 b) Theoretical Periodical Fall i) Bed 1 (m) 11 ii) Bed 2 (m) 7 5 Interaction of Beds A) For Main fall a) After comparing deflections of both beds and equating deflections it was found that an additional load of 4.4 T/Sq.m. is being imposed by Bed 2 on Bed 1. b) With revised values of load on beds the revised values of main fall of Bed 1 is 45 meters c) Further, whenever Bed 1 collapses Bed 2 will also cave in. B) For Periodic Fall a) Analysing the failure of Bed 1 & 2 as cantilever, due to effect of failure of Bed 2 on Bed 1, the periodic failure distance of Bed 1 reduces to 9 mtrs. b) Further, whenever the Bed 1 fails periodically Bed 2 will also fail along with it. 6 Minimum setting load required to induce 307 T& 69 T The setting load of support is 620 caving at rear of support for Bed 1 & Bed T. Hence, breakage will occur at 2 rear of support 7. Estimation of support resistance required A) At Main Fall i) At the time of failure 2013 T (*) ii) When the secondary break occurs 771 T (*) iii) When the Block rests on goaf waste 300 T B) At Periodical Fall i) At the time of failure 700 T (*) ii) When the rear of cantilever beam rests 183 T (@) on goaf (*) Taking moments about face line, Momentary load (@) Taking moments about goaf line 8 Comparison Calculated Data from field value i) First fall 45m 45m ii) Periodic Fall 9m 6 to 7m iii) Bleeding of supports at weighting Very less Very less
  5. 5. 3.2 CASE STUDY – 2 The following table no. 2 gives the calculated values for GDK-10A incline(4). Refer figure :fig (3.4 ,3.5 & 3.6) TABLE No 2 Sl. Parameter Calculated value No. 1 Caving Height (m) 19.6 2 Number of beds identified with in the caving height 4 3 Details of Beds a) Thickness of Immediate roof (m) 3.08 b) Thickness of Bed 1, 1.08 Bed 2 and 7.00 Bed 3 (m) 8.50 c) Density of Bed 1, 2.03 Bed 2 and 2.03 Bed 3 (T/m3) 2.16 d) Depth of Bed 1, 157.08 Bed 2 and 156.00 Bed 3 (m) 149.00 e) Depth of roof above Bed 3 (m) 140.50 f) Density of roof above Bed (T/m3) 2.02 g) Young’s Modulus of Bed 1 0.27x105 Bed 2 0.27x105 Bed 3 (Kg/cm2) 0.197x105 h) Tensile strength of Bed 1 0.704 Bed 2 0.704 Bed 3 (MPa) 1.475 i) Face length & working section (m) 150 & 3.3 4 Caving distances a) Theoretical Main Fall i) Bed 1 Distance (m) 25 ii) Bed 2 Distance (m) 50 iii) Bed 3 Distance (m) 60 b) Theoretical Periodic Fall i) Bed 1 Distance (m) 3.5 ii) Bed 2 Distance (m) 9.0 iii) Bed 3 Distance (m) 13.5 5 Interaction of Beds A) For Main Fall Nil B) For Periodic Fall a) After comparing the defection of beams at periodic weighting of bed 1 and 2, it can be concluded that bed 2 will impose a load of 0.417 T on beam of bed 1 hence when ever beam of bed 2 collapses bed 1 also fails and comes down along with bed 2. b) After comparing the deflection of beams of periodic weighting at bed 2 and 3, it can be concluded that bed 2 offers a resistive force of 270 T on bed 3 and it can be concluded that bed 2 offers a resistive force of 270 T on bed 3 and it reduces bending moment in bed 3 and whenever bed 3 fails bed 2 will also fail
  6. 6. Sl. Parameter Calculated value No. 6 Minimum support resistance to induce breaking at 18.25 T, 235.5T & rear of support for bed 1, bed 2 and bed 3. 428.0T (The setting load lof support is 640 T hence the break will occur at rear of support 7 Estimation of support resistance required A) At Main Fall I) For Bed 1 i) At time of failure 81 T II) For Bed 2 i) At time of failure 1409T (*) ii) When the secondary break occurs 600T (*) iii) When the block rests on goaf 211T (@) III) For Bed 3 i) At time of failure (no secondary break) 2895 T (*) ii) When the block rests on goaf 691 T (@) B) At periodic weighting I) For Bed 1 i) At time of failure 50T (*) II) For Bed 2 i) At time of failure 478T (*) ii) When the block rests on goaf 131T (@) III) For Bed 3 i) At time of failure 1255T (*) ii) When the block rests on goaf 290T (@) (*) Taking moments about face line (Momentary load) (@) Taking moments about goaf line 8 Comparison Parameter Calculated Value Data from field i) Local fall 25m 30m ii) Major fall 50m 50-55m iii) Periodic fall 13.5m 15m iv) Bleeding of support 4 x less less 800T 3.3 CASE STUDY – 3 The following table no. 3 gives the calculated values for 11A Incline (4). Refer figure :fig (3.7 & 3.8)
  7. 7. TABLE No 3 Sl. Parameter Calculated value No. 1 Caving Height (m) 18.26 2 Number of beds identified with in the caving height 4 3 Details of Beds a) Thickness of Immediate roof (m) 1.00 b) Thickness of Bed 1, 5.06 Bed 2 and 9.62 Bed 3 (m) 2.58 c) Density of Bed 1, 2.56 Bed 2 and 2.33 Bed 3 (T/m3) 2.43 d) Depth of Bed 1, 204.76 Bed 2 and 199.70 Bed 3 (m) 190.08 e) Depth of Bed 1, Bed 2 & Bed 3 (m) 187.50 f) Density of roof above Bed (T/m3) 2.22 g) Young' s Modulus of Bed 1 0.27x105 Bed 2 0.32x105 2 Bed 3 (Kg/cm ) 0.2x105 h) Tensile strength of Bed 1 0.616 Bed 2 0.780 Bed 3 (MPa) 0.846 i) Face length & working section (m) 150 & 3.0 4 Caving distances a) Theoretical Main Fall i) Bed 1 Distance (m) 45 ii) Bed 2 Distance (m) 65 iii) Bed 3 Distance (m) 30 b) Theoretical Periodic Fall i) Bed 1 Distance (m) 6 ii) Bed 2 Distance (m) 10 iii) Bed 3 Distance (m) 5.5 5 Interaction of Beds A) For Main Fall Nil a) Bed 3 is caving at a distance shorter than Bed 2. After comparing deflection, it can be concluded that Bed 3 imposes additional load lof 6T.Sq.m. With this additional load, Bed 2 will cave at a caving distance of 60m and whenever Bed 2 fails Bed 3 will also fail. B) For Periodic Fall a) Bed 3 which failing periodically at a distance of 5.5 imposes additional load on Bed 2 and this load reduces periodic failure of Bed 2 to 8 m. b) After comparing the deflection of beams of Bed 1 & Bed 2 a, it can be concluded that bed 1 offers a resistive force of 61 T on bed 2. This will not be able to prevent breakage in Bed 2. While calculating load complex situation is considered when the three beds collapse at once. 6 Minimum support resistance to induce breaking at 160, 412 & 73T rear of support for bed 1, bed 2 and bed 3.
  8. 8. Sl. Parameter Calculated value No. 7 Estimation of support resistance required A) At Main Fall I) For Bed 1 i) At time of failure 1106 T(*) ii) When secondary break occurs 437 T (*) iii) When the block rests on goaf 227 T(@) II) For Bed 2 & 3 i) At time of failure 4470T (*) ii) When the secondary break occurs 1674T (*) iii) When the block rests on goaf 704T (@) At periodic weighting I) For Bed 1 i) At time of failure 234T (*) II) For Bed 2 & 3 i) At time of failure 830T (*) ii) When the block rests on goaf 179T (@) (*) Taking moments about face line (Momentary load) (@) Taking moments about goaf line 8 Support Capacity Required A close look at load to which the supports are subjected to clearly indicate that the load is around 704T. With a support of around 700T yielding load, the longwall face can be run with slight difficulty. But if support of around 800T is selected, better control of roof can be achieved. With such capacity support periodic weighting load can also be handled easily. 5.0 CONCLUSION In this paper a study has been done to estimate caving and support load to which the supports are subjected. This gives a methodology for selection of longwall powered roof supports. In the first two cases studied , a caparison is made on the theoretical estimate and actual practical situation and in the third case an estimate is made . It is clear from first two case studies that the supports selected were slightly over rated but as the coal companies need high reliability and ease at operation, it is logical to go for a slightly higher capacity though it costs a bit more. This methodology will help in deciding support capacities for future longwall projects. A complete computer programme is developed by the Centre for longwall mine mechanisation ,Indian School of Mines ,Dhanbad, to meet the needs of the users. It is clear from the results obtained through calculations that horizontal stress plays a significant role in calculation of caving distances, and hence determination of horizontal stress for different coalfields is required to be done. 6.0 ACKNOWLEDGEMENTS The authors express their sincere thanks to Sri P. Vasudeva Rao Director( operations) Sri G N Sarma Director (P P) and G M (HRD) S C CL for their permission to present this paper and for their constant encouragement. Our thanks are due to Prof S N Mukherjee Head ,Centre for longwall mine mechanisation , I S M , Dhandbad and Dr D N Sarma Supdt Geologist , Exploration Department ,S C CL . The views expressed by the authors are of their own, and thus does need not necessarily depict their official position.
  9. 9. 7.0 REFERENCES 1) "Challenges of mining Industry in Globalised economy " R N Singh , E D, I I C M, pp 17-19,C M T M ,Vol 7 , No 7, July 2002. 2) Longwall Machinery and Mechanisation Vol.I, Powered Support, Prof. S.N. Mukherjee, A.M. Publishers, Dhanbad, 1993, pp. 13-80 & 301-334 3) "The behaviour of the main roof in longwall mining weighting span, fracture and disturbances". Ming Gao Q and H. Fullian, Journal of Mines, Metals & Fuels, July, 1989, pp.240 – 244 4) Estimation of caving and support load in Longwall faces with special relevance to SCCL, LOLLA. Sudhakar, M.Tech, Thesis, Indian School of Mines, Dhanbad, 1996, pp 63-222. *****

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