Strata Control Monitoring with Convergence, Trend Analysis in Conventional Bord & Plillar Extraction: A Case Study of Underground Coal Mining at SECL Satyabadi Kumar Jena*, Ritesh D Lokhande Department of Mining Engineering, National Institute of Technology, Raipur Raipur, India *
[email protected] R. P. Thakur Director (Tech.) Operations, SECL Bilaspur, India
Abstract—Strata control instrumentation and monitoring aims at evaluation and monitoring the trends of changing rock mechanical parameters (Dilation, Load, Convergence, Stress and axial loading etc.), during mining so that rock mechanical un-eventualities are apprehended well before for effective corrective measures. Host rock geometry in coal mining is represented by stratified rock masses of relative weaker strength. Such stratifications are compound and unite in their virgin state before any kind of mining. Mining underneath (both development and depillaring) tend to develop strain along weaker bedding planes generating bed separation, called as Dilation. By the action of gravity, the dilated mass / stratums are supposed to sag along its span within the mining voids / openings, subsequently reducing the gap between the floor and roof of workings, called as Convergence. The study is pertinent to monitoring and analysis of convergence in an Indian underground coal mines during conventional Bord and Pillar depillaring / final extractions.
I. INTRODUCTION At present, most of Indian coal comes from opencast mines, which involve environmental hazards in terms of air & water pollution and land degradation etc. Apart from this, coal reserve mineable by opencast method is also very limited as per present trend of mining technologies. Coal reserves at depth, can be exploited techno-economically by underground methodologies. In India, such proven reserve for underground mining is about 115 Billion Tons (Anon1, 2015), while present reserve available for open cast mining is about 30 BT only. Due to high growth of
production, deposits, mineable by open cast method are fast depleting. In view of the above facts, the issues relating to success of underground coal mining in India have gained paramount importance, in the arena of National Growth. In absolute terms, success of underground coal mining is subjected to optimization of many parameters, such as production, productivity, safety and technological growth etc. Safety of men and equipment in terms of roof & sides / strata management is one of the pivotal parameters for success of any technology, subsequently the coal mining endeavor. In line with this, the cause wise analysis of disasters in Indian coal mines since the year 1901(Anon5, 2015) says that the principal causes of mine disasters had been Inundation, Fire / Explosions and Ground / Strata movement. As per the analysis, ground movement attributed to 9 disasters involving loss of 114 lives accounting to 5.11% of total fatalities in Indian mine disasters. The study deals with the modern strata management, i.e., strata control monitoring with assessment of roof – floor convergence during final extraction in a Bord & Pillar conventional Indian underground coal mine, ‘Govinda Colliery’, a mine of South Eastern Coal Fields Limited (SECL), Coal India Limited.
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II. STUDY AREA The study pertinent to trend analysis of convergence monitoring during conventional Bord & Pillar depillaring in -1Rise Panel in Govinda Colliery of Jamuna & Kotma (J&K) Area of SECL. There was only one workable coal horizon in the mine, i.e. Middle Kotma seam (MK Seam), with thickness varying from 1.4 to 3 m. Other information are
Dip and direction of seam-N 6 30 W Gradient- 1 in 25 Grade of coal- ROM C Average UHV- 6154 C Compressive strength of sandstone in the immediate roof - 385.67 kg/cm2 Depth of working- 46 m-111 m No of headings- 5-6 Size of pillar- 22 x22 m Gallery width- 4.2 m R M R- 51 Nos. of faults traversing the panel- 11(0.5 to10 m up-throw) A. Details of Adjacent Workings1.North side: Adjoining P 4 Panel of UK Top seam with 47.7 m radial parting and minimum horizontal distance 26 m as per plan. 2.South side: old goaved out panels of same mine situated at higher level. 3.West side: Previously depillared panel(8C Rise) -1 rise panel of MK Seam was being worked through Radha incline and depillaring was started since 1.4.2011. Regarding location, the panel was surrounded by goved out panels of same mine and even adjoining mine. Also a set of 11 nos. of faults had invaded the panel, making it into three parts. Due to such adverse geo-mining situations, extraction in the panel was imposed with two important restrictions by Directorate General of Mines Safety, mentioned as follows.
Pillars in which fault had encountered shall be left fully intact.
No extraction of pillars shall be done within 60 m of water logged working of UK Seam & MK Seam.
Because of the above restrictions, numbers of developed pillars were left un-extracted and at several places the effective extraction width of panel was only about 50 m. Apart from this, presence of 11 numbers of faults with in the panel, made it into three parts / phases for extraction. Presence of such faults has also made some parts of the panel, narrower, i.e. even 2025 m in width. Heeding to above difficulties and
restrictions, it was expected that the caving will be delayed and huge hanging area in goaf will result into overriding of pillars, air-blast and subsequent strata control problems. Generally, cover, above the panel consists of top soil / alluvium of about 6 m and most of the portion with sandstone (fine - medium – course grained) and presence of thin band of shale & mud stone in the midst of the depth at places. Immediate roof consists of fine grained hard sandstone of about 4 m thick throughout, overlain by medium to course grained sandstone, causing caving difficulties during depillaring. Seeing the geological and geo-mining difficulties and anticipated caving constraints, it was decided to go for implementation of Strata Control technique with convergence monitoring in the panel. Objective of the monitoring was to go for safer mining with preapprehensions of strata in-equalities so that safety of persons and equipment can be ensured. III. INSTRUMENTATION & MONITORING During depillaring operation in the -1 Rise Panel, particular attention was being paid to gather detail information regarding roof – floor convergence & goaf fall (Preferably Main Fall) to know the trend and characteristic of caving and its effect on workings. For the same, voracious efforts were paid for installation and monitoring of convergence recorders, either conventional extensible Telescopic Convergence Indicators (TCI) or Locally made Telescopic Convergence Indicators (LTCI) developed at colliery level along-with colour coded scale attached to the bottom end. A. Installation And Use of Convergence Recorders: Convergence stations were fixed at every goaf edge, installing Local Telescopic Convergence Indicators (LTCIs) Strategically, Telescopic Convergence Indicators (TCIs) / Local Telescopic Convergence Indicators (LTCIs) were fixed at every original junction and slice junction to apprehend dynamic / periodic load (initiated from the goaf) with in workings. Convergence monitoring from every station was done in every shift, with documentation of Rate of Daily Convergence, Cumulative Convergence, Distance of goaf edge from the station and Hanging area of goaf roof, on date. Period of monitoring and observation at goaf edges were varying from 3 to 10 days epending upon their sequence and working of slices For the purpose of study, convergence monitoring was taken into trend & profile analysis with respect to main falls only, which can be acknowledged as shown in the figure below
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Fig. 1. Key Plan , Govinda Colliery
Fig. 2. Instrumentation Plan, -1Rise Depillaring Panel, Govinda Colliery a
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(Showing Main Fall, Convergence stations & Faults etc.)
IV. ANALYSIS A. First Part / Phase of the panel (23L-28L) Depillaring in first part / phase of the panel was a spread from 23L to 28 Level and -1 Rise to 1 Rise. In this phase, three main falls were occurred on 10.04.2011, 16.05.2011 and 31.05.2011. Convergence monitoring analysis with respect to each main fall is as follows. 1.First Main Fall: First main fall had occurred on 10.04.2011, covering an area of hanging goaf of 1120 m2 (as shown on the plan). Analysis was done from readings of five convergence stations as 0R/27L, 1R/27L, 0R/26L, -1R/26L and 1R /26L,graphical outcome of which are shown as Fig.3 (a), (b), (c), (d) and (e) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall.
Similarly, convergence profile of first main fall (fig.4) shows that maximum daily convergence of 16 mm / day was experienced at goaf edge at 0 Rise heading (centre of the district span), while minimum daily convergence of 3 mm/day at 1Rise heading, being the panel barrier side and being the convergence monitoring stations at goaf edges. Summarily, it implies that the geo-mining situations were difficult for caving. 2.Second Main Fall : Second main fall had occurred on 16.05.2011, covering an area of hanging goaf of 1200 sq.m (as shown on the plan). Analysis was done from readings of four convergence stations as 1R/25L, -1R/25L, 0R/25L and-1R/24L graphical outcome of which are shown as Fig.5 (a), (b), (c) and (d) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall.
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Summarily, convergence profile of second main fall (fig.6) shows that maximum daily convergence of 15 mm / day was experienced at goaf edge at 0 Rise heading (centre of the district span), while minimum daily convergence of 3 mm/day at -1Rise heading, expressing situations, difficult for caving. 3.Third Main Fall : Third main fall had occurred on 31.05.2011, covering an area of hanging goaf of 760 m2 (as shown on the plan). Analysis was done from readings of two convergence stations as 1R/23 ½ L and 0R/23L graphical outcome of which are shown as Fig.7 (a) and (b) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall. Summarily, convergence profile of third main fall (fig.6) shows that maximum daily convergence of 8 mm / day was experienced at goaf edge at 0 Rise heading (centre of the district span), while minimum daily convergence of 5mm/day at 1 Rise heading. Also it is observed that working span had become narrower because of left ot restricted pillars, summarily, posing difficulty for caving.
4.Heading wise Convergence profile : Fig. 9 (a), (b) and (c) show the heading wise convergence profile during depillaring in the 1st Phase of the panel. Fig. 10 represents the graph of maximum daily convergence vis-à-vis hanging area of goaf before main falls in the 1st Phase of the panel. Summarily, the convergence profiles in the 1 st Phase of the panel shows the folowing. Rise heading experienced maximum daily convergence of 10 mm and cumulative convergence of 46 mm. 0 Rise heading experienced maximum daily convergence of 16 mm and cumulative convergence of 75 mm. -1 Rise heading experienced maximum daily convergence of 10 mm and cumulative convergence of 38 mm. Maximum area of hanging goaf of 1200 m2 was experienced with maximum daily convergence of 15 mm.
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B.
Second Part / Phase of the panel (14L-22L) Depillaring in the second part / phase of the panel is a spread from 14L to 22 Level and -2 Rise to 2 Rise. In this phase, three main falls were occurred on 25.07.2011, 12.08.2011 and 15.09.2011. Convergence monitoring analysis with respect to each main fall is as follows. 1. Fourth Main Fall : Fourth main fall had occurred on 25.07.2011, covering an area of hanging goaf of 2430 m2 (as shown on the plan). Analysis was done from readings of three convergence stations as 2R/19L, 1R/18L and 0R/18L graphical outcome of which are shown as Fig11 (a), (b) and (c) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall. Summarily, convergence profile of fourth main fall (Fig.12) shows that maximum daily convergence of 15 mm / day was experienced at goaf edge at 1 Rise heading (centre of the district span), while minimum daily convergence of 8 mm/day at 0 Rise heading.
Also it is observed that caving has become difficult with such high value of daily convergence due to left out restricted pillars between 0 Rise and -1 Rise. 2. Fifth Main Fall : Fifth main fall had occurred on 12.08.2011, covering an area of hanging goaf of 1810 m2 (as shown on the plan). Analysis was done from readings of three convergence stations as 3R/18L, Fig. 2R/17L and 1R/16L graphical outcome of which are shown as Fig13 (a), (b) and (c) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall. Summarily, convergence profile of fifth main fall (Fig.14) shows that maximum daily convergence of 14 mm / day was experienced at goaf edge at 1 Rise heading (centre of the district span), while minimum daily convergence of 2 mm/day at 3Rise heading (Barrier side). Also it was observed that caving had become difficult with such high value of daily convergence due to left out restricted pillars between 0 Rise and -1 Rise.
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3.Sixth Main Fall: Sixth main fall had occurred on 15.09.2011, covering an area of hanging goaf of 1380 m2 (as shown on the plan). Analysis was done from readings of two convergence stations as 3R/16L and 2R/15 ½ L graphical outcome of which are shown as Fig15 (a) and (b) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall Summarily, convergence profile of sixth main fall (Fig.16) shows that maximum daily convergence of 12 mm / day was experienced at goaf edge at 2 Rise
heading (centre of the district span), while minimum daily convergence of 8 mm/day was experienced at 3 Rise heading (Barrier side). Also it is observed that caving has become difficult with such high value of daily convergence due to left out restricted pillars between 0 Rise and -1 Rise. 4.Heading wise Convergence Profile: Fig. 17 (a), (b) and (c) show the heading wise convergence profile during depillaring in the 2nd Phase of the panel. Fig. 18 represents the graph of maximum daily convergence vis-à-vis hanging area of goaf before main falls in the 2nd Phase of the panel.
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Summarily, the convergence profiles in the 2 nd Phase of the panel shows the folowing. 3 Rise heading experienced maximum daily convergence of 8 mm/day and cumulative convergence of 24 mm. 2 Rise heading experienced maximum daily convergence of 12 mm/day and cumulative convergence of 41 mm. 1 Rise heading experienced maximum daily convergence of 15 mm/day and cumulative convergence of 54 mm. Maximum area of hanging goaf of 2430 m2 was experienced with maximum daily convergence of 15 mm. C. Third Part / Phase of the panel (5L-13L) Depillaring in the third part / phase of the panel was a spread from 5L to 13 Level and -3 Rise to 0
Rise. In this phase, four main falls were occurred on 08.11.2011, 03.12.2011, 19.12.2011 and 14.01.2012. Convergence monitoring analysis with respect to each main fall is as follows. 1. Seventh Main Fall: Seventh main fall had occurred on 08.11.2011, covering an area of hanging goaf of 2860 m2 (as shown on the plan). Analysis was done from readings of three convergence stations as 1R/12L, -2R/11L and -3R/11L graphical outcome of which are shown as Fig19 (a), (b) and (c) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall. Summarily, convergence profile of seventh main fall (Fig.20) shows that maximum daily convergence of 14 mm / day was experienced at goaf edge at -2 Rise heading (centre of the district span), with minimum daily convergence of 6mm/day at -3 Rise heading (Barrier side), expressing sluggish caving situation.
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2. Eighth Main Fall: Eighth main fall had occurred on 03.12.2011, covering an area of hanging goaf of 1850 m2 (as shown on the plan). Analysis was done from readings of three convergence stations as 0R/11L, -1R/10L and -2R/9L graphical outcome of which are shown Fig 21 (a), (b) and (c) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall. Summarily, convergence profile of eighth main fall (Fig.22) shows that maximum daily convergence of 12 mm / day was experienced at goaf edge at -2 Rise heading (centre of the district span), while minimum daily convergence of 4mm/day at 0Rise heading (Barrier side), expressing sluggish caving situation.
3. Ninth Main Fall: Ninth main fall had occurred on 19.12.2011, covering an area of hanging goaf of 960 m2 (as shown on the plan). Analysis was done from readings of two convergence stations as 1R/8 ½ L and -2R/8 ½ L graphical outcome of which are shown as Fig23 (a) and (b) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall. Summarily, convergence profile of ninth main fall (Fig.24) exhibits maximum daily convergence of 12 mm in the centre of the span and minimum daily convergence of 4mm at barrier side, expressing difficult caving situation
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4. Tenth Main Fall: Tenth main fall had occurred on 14.01.2012, covering an area of hanging goaf of 980 m2 (as shown on the plan). Analysis was done from readings of two convergence stations as 0R/7 L and -1R/7 L graphical outcome of which are shown as Fig25 (a) and (b) respectively. Each of the graphs shows the convergence profile and the peak daily & cumulative convergence at the respective station locations, before main fall. Summarily, convergence profile of tenth main fall (Fig.26) shows that maximum daily convergence of 8 mm / day was experienced at goaf edge at -1 Rise heading (centre of the district span), while minimum daily convergence of 4 mm/day at 0Rise heading (Barrier side). Also it is observed that caving has become difficult with such high value of daily convergence due to restricted span as because of left out restricted pillars Fig. 27 (a), (b) and (c) show the heading wise convergence profile during depillaring in the 3 rd Phase
of the panel. Fig. 18 represents the graph of maximum daily convergence vis-à-vis hanging area of goaf before main falls in the 3rd Phase of the panel. Summarily, the convergence profiles in the 3rd Phase of the panel shows the following.
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0 Rise heading experienced maximum daily convergence of 6 mm/day and cumulative convergence of 14 mm. -1 Rise heading experienced maximum daily convergence of 12 mm/day and cumulative convergence of 54 mm. -2 Rise heading experienced maximum daily convergence of 14 mm/day and cumulative convergence of 36 mm. Maximum area of hanging goaf of 2860 m2 was experienced with maximum daily convergence of 14 mm.
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Convergence monitoring along-with using indicator props, while working near slices and ribs to become aware of load piling.
V. DISCUSSION Analysing the study and trend / profile of convergence in the panel, following, facts were concluded.
Stress monitoring on barrier coal pillars to avoid periodic load concentration in the workings.
Including all the main falls, the maximum daily convergence and cumulative convergence had gone upto 16 mm/ day and 75mm respectively.
Roadways convergence monitoring on headings where cumulative convergence exceeds 50mm.
Such high value of convergence, tend to exhibit caving constraints. Such caving constraints, were due to strong immediate roof of fine grained sandstone of about 4m and restriction of pillar extraction as because of presence of water body nearby and presence of geological faults in the property. In the first phase of depillaring, 0 Rise heading had exhibited maximum daily convergence of 16 mm / day and cumulative convergence of 75 mm, which need to be taken into attention along-with preventive measures. In the second phase of depillaring, 1 Rise heading had exhibited maximum daily convergence of 15 mm / day and cumulative convergence of 54 mm, which need to be taken into attention along-with preventive measures. In the third phase of depillaring, -1 Rise heading had exhibited maximum daily convergence of 12 mm / day and cumulative convergence of 54 mm, which need to be taken into attention along-with preventive measures. Following are few precautionary apprehended from the study.
measures
A. Against high rate of change in daily Convergence (more than 6mm /day). Suitably increasing support resistance at goaf edges. Ensuring stability and effectiveness of goaf edge supports with continuous monitoring and maintenance.
To avoid caving constraint due to narrower working span, induced channel caving may be practiced on barrier galleries in advance, restricting loading continuum. Based on general trend of convergence in the panel and physical goaf activity in the mine, the local warning limits for impending roof fall in this panel are indicated as 10mm / day in the central headings and 6 mm / day in the barrier headings, while cumulative convergence exceeds 30 mm. VI. CONCLUSION The study was pertinent to Strata Control Monitoring with trend analysis only. Mentioned strata control outcomes can also be subjected to mathematical and numerical analysis along-with modeling in the future. Future of Indian Coal Mining Industry is mostly dependent on all way success of underground mining. Whereas, roof and side management play pivotal role in any underground technology for its success and sustainability. So, scientific and innovative reformations in roof management, the Strata Control Monitoring have become bare necessity to come out of age old presumptive practices, for survival and growth of coal mining industry. ACKNOLDEGEMENT The views expressed in this paper are those of the authors and not necessarily of the organizations they represent.
Maintaining advance supports, according to peak dynamic loading zone. Suitably increasing positive supports in the advance support zone. B. Against high cumulative Convergence (more than 30 mm). Practicing induced caving at goaf edges. Increasing support resistance suitably, near geological disturbances with in the workings.
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