Longwall mining at shallow depth

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This Technical paper on “Experience of strata monitoring studies in a shallow depth Long wall extraction by caving in panels 1&1A of PVK-5 Incline” was presented at 3rd National Seminar on Rock …

This Technical paper on “Experience of strata monitoring studies in a shallow depth Long wall extraction by caving in panels 1&1A of PVK-5 Incline” was presented at 3rd National Seminar on Rock Excavation Technique at Nagpur.

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  • 1. EXPERIENCES OF STRATA MONITORING STUDIES IN A SHALLOW DEPTH LONGWALL FACE IN PANELS 1A & 1 OF PVK-5 INCLINE. M.S. Venkata Ramaiah * MD. Suresh Kumar ** Abstract: Success of any Longwall mechanization depends mainly on controlled cavingmechanism of overlying strata, available strata monitoring program and the performanceof power supports.Scientific & sustained strata monitoring studies draws more importance for consistentworking of any Longwall face without landing into major strata control problems.Padmavathikhani Longwalls have been introduced in the year 1995 and the Longwallpackage was imported from CME, China. The mine is situated in Kothagudem Area ofSingareni Collieries company ltd., in Khammam district of Andhra Pradesh state inGodavari valley coal fields. So far 9 Longwall panels have been successfully extracted.Among 9 panels, 8 panels were operated at the face width of 150m at the depths of min.60 m and maximum 203m.The recent panels, panel No.1&1A were initially developed to be extracted at 150m facewidth, at the shallowest depth of 48m minimum and 96m maximum.Keeping the surface subsidence underlying goaves and strata control problems in mind, a 1decision was made to split the panel into two halves (panel 1A & 1) of 62.5m face widtheach.A review of other longwall panels operated at 150m face width shows that impact ofperiodic weightings in the face and influence of stress over power supports and the gateroadways were found high and mostly destructive. No. of roof cavities reported duringthe time of weightings, which in turn affected far “face progress” on several occasions.Keeping the above in view the potential risk associated with such strata control problemsin such a shallow depth Longwall extraction were carefully studied.Accordingly, the high thrust was given to Strata Management program with theexperience gained in other Longwall panels of PVK and also by introducing monitoringprogram of activities in the goaf through a Multi Point Anchor Bore Hole Extensometers(MPBEX) for the first time in Padmavathikhani.The MPBEX has been well utilized for monitoring goaf activities i.e. falls, overhang andcorrelation between the above and the surface subsidence, Such monitoring enhanced“Rate of Retreat” and warranted in tact face and gate supports to promote “saferMining” while working under Shallow cover. Not only that, the MPBEX helps us to assesscaving characteristics of different Roof rocks i.e. main fall, periodic falls and settlementof goaf etc.The present paper deals with Strata Management in shallow depth Longwall extraction ofpanel 1 & 1A of Padmavathikhnai.* Dy.GM(5 Group of Mines), ** Dy. Manager (PVK_5 Inc.), S.C.Co.Ltd., Kothagudem. www.nitropdf.com
  • 2. 1. INTRODUCTION Padmavathi Khani (No.5 Incline) Longwalls have been introduced in the year August 1995. The mine is situated in Kothagudem Area of Singareni Collieries Company Limited in Khammam District of Andhra Pradesh state in Godavari Valley Coal field. So far 10 Long Wall Panels have been successfully extracted in Top Seam and 11 Panel (Panel no.21) is under extraction. So far 4.3 Million Tonnes Coal extracted fromththe Mine by Longwall technology. Out of 11 panels, Eight Panels were operated at the Face width of 150 m at the varying face lengths 600 m to 900m and at the depths of minimum 60 m maximum 203 m. Though the panels 1A & 1 were developed to be extracted with 150 m face width at the shallowest depth of 48 m Min. and 96 m Max. Having The surface subsidence The strata Control problems under shallow cover and Working over caved goaf Kept in mind, a Study was conducted. Based on the study a decision was made to split the panel into two Halves of each 62 m face width leaving barrier pillar of 22.5 m so as to reduce the Max Subsidence. The salient features of Longwall Panels 1A & 1 and other Panels worked in PVK were shown in ANNEXURE I. The long wall faces are operating with Equipments package supplied by M/S. CME, China which includes power supports of 4x760 T chock shields of IFS type. 2. THE POTENTIAL RISK ASSESSMENT While extracting long wall panels under shallow cover may likely to cuase 1) Goaf hang up for longer span and sudden collapse causing subsidence extending to the surface. 2) Geological features like faulting, jointing which may induce sudden collapse of over burden like block Caving. 3) Sudden ingress of water or unconsolidated materials from surface 4) Possibilities ventilating the goaf through cracks which may result into heating3. STRATA MONITORING IN PANEL 1A & 1Success of any longwall mechanization depends mainly on Control caving mechanism of overlying strata, the available strata monitoring program, and the Performance of power supports. www.nitropdf.com
  • 3. Among these, the scientific strata monitoring studies draw more importance. With theexperience gained in other long wall panels, the strata monitoring studies are extensively donein the panels 1A & 1 inGate Road ways Face and Goaf3.1 STRATA MONITORING IN THE GATE ROAD WAYS As the face is retreating, the immediate shale and shaley coal roof of 3 m height abovethe working seam found collapsing regularly behind the face line supports. The immediatesand stone bed in the form of Fixed Beam and Cantilever Beam exerting stress over face,supports and gate roadways. The abutment so influenced over gate roadways generallymanifested in the form of Roof Convergence. The various items monitored in the gateroadways. Convergence stations are installed at every 10 m in the gate road ways. Noabnormal convergence observed in panel 1A.But it was doubled in Panel 1, due to the effectof adjacent active goaf of Panel No 1A (See ANNEXURE II). However it was noted that roof convergence while retreating over Bottom seam caved goaf (both in Panel 1A&1) was higher than that of retreating over Barrier due to goaf 3 compaction. While Panel 1 was retreated over Bottom seam caved goaf with neighboring goaf of panel 1A, the roof convergence found to be still intensive due the combined effect of both the goaves. (See ANNEXURE III )Details of Roof convergence: Panel 1A Panel 1Max. Convergence observed : 37 mm 54mmMax. Convergence in day (24 hrs) during Normal time : 2 to 5mm 10mmMax. Convergence in day (24hrs) during Weighting time : 10mm 27mmMechanical type Load Cells and Vibrating wire Load cells fixed over hydraulic Props within10 from the face where the intensity of abutment likely to be more. The significant increaseof load (3T –10T) was observed during the time of caving (Main weighting and Periodicweightings) www.nitropdf.com
  • 4. Bore hole extensometers (Tell Tale type) were installed in both the gate roads whichhave given more accurate data of bed separation.3.2 STRATA MONITORING IN THE FACEGenerally caving of sand stone bed in the form of cantilever beam enhances concentration ofstresses over face line supports. The load imposed over Power Supports used to be monitoredthrough Leg pressures Leg closures and Face convergenceThe Setting pressure and the pressure increase in legs were monitored through Pressuregauges incorporated in all the leg circuits and Continuous pressure recorders fixed to strategicsupports .The Pump Pressure or Setting Pressure was maintained at 22 MPa (3200 Psi). Seepressure survey graphs in ANNEXURE IV and IV A However, during the time of weighting the pressure imposed over chock shields and Gates observed to be much minimal than that of other panels of 150 m face width. During the main weighting and periodic weightings, there was pressure increase in the 4 chock shields around 30 MPa and only 30 to 40 % of the supports influenced by such weightings. Sparingly around 5 to 10 % of supports reached to bleed pressure of 38.7 MPa , sustained for only few minutes due to the reason that The stress concentration of the sand stone plate (by its own weight) of 62 m X 15 to 25 m experienced to be lying not over the face or face line supports but got shifted away behind the supports. Span of the periodic weighting observed to be 18 to 25 m in Panel 1A and 15 to 20 m in Panel 1. The goaf falls and the related power supports leg pressures, roof convergence, load cell data and tell tale data are summarized in ANNEXURE VI.The support loading characteristics during different face operations were used to becontinuously monitored during normal and weighting period (see ANNEXURE V).However, 1-2 MPa pressure increase observed during face operations at Normal period 6-8 MPa pressure increase in front legs observed during face operations at weighting period. www.nitropdf.com
  • 5. The study of leg closures and face convergence revealed that Around 2mm Leg closure against 1 MPa pressure increase in the legs Around 12mm face convergence recorded during the time of weightingsThe mean load densities during normal and weighting periods areParticulars Mean Load Density (T / Sq.m) Avg. Load on each support (T)During normal period 59.22 393During Main weightings 63.00 412During periodic weightings 68.82 456Geological disturbances in the face i.e., faults, slips, fractures, roof cavities and the formationof breaker line in the roof were being monitored regularly. Additional care taken wheneverslips, fractures encountered while retreating.Roof cavities were not reported during any weighting time and it was managed to have intactroof conditions in panels IA & I by ensuring proper setting of the Power supports during the 5face operation and by intact maintenance of Hydraulics in the face.3.4 STRATA MONITORING BEHIND THE FACE (GOAF) FROM SURFACEAn attempt was made to monitor the caving characteristics from surface by installing MultiPoint Borehole Extensometer. In the permission given by DGMS it was insisted that blastinghas to be done from surface to induce caving. The Multipoint Borehole Extensometer readingswere useful to understand the collapse of different beds which is not other wise available. Onthe basis of the readings it was found that blasting from surface was not necessary. Features ofMPBEx instrument are given ANNEXURE VII.4. THE MULTI POINT BOREHOLE EXTENSOMETERMONITORING.One extensometer installed at the center of the face 50 m from the installation chamber by drilling a bore hole up to the coal seam. The anchors were installed at pre- determined depths 45m, 37.5m 26m and 15m above the coal seam. Four such in the hole anchors fixed to the rock along the axis of the Bore hole. www.nitropdf.com
  • 6. Each anchor connected to a sensor at the collar of the borehole which measures thedisplacement of anchors. Readings were recorded with the help of portable ‘READ OUTUNIT’ regularly. The Borehole data with position of different anchors were shown inANNEXURE VII.5. ASSESSMENT OF CAVING AND FALLS WITH MPBEx DATAGenerally the immediate shale and Shaley coal of 3 m above the working section observed tobe collapsing immediately behind the face line supports. It is understood from the availableMPBEx data given in ANNEXURE VIII that, The anchor I located at 45 m depth has shown deformation up to 241.77mm at the face retreat of 68.3 m. Thus it has given room for the upper layers to deform. The Anchor II located at 37.5 m depth has shown slight movement and deformed up to 250.08mm at the face retreat of 80.0 m. The main fall occurred at the face retreat of 80.0 m, with the area of exposure of 5300 Sq.m. After the Main fall the above Two Anchors I & II dislodged. 6 However there was slight strata movement in Anchor III to the extend of 21 mm. when the face was retreated up to 89.4 m. It was seen to be increasing slowly with the further face advancement and it has gone up to 244 mm when the face was retreated up to 144.9 m.These Chronological events have drawn the following observations. The Anchor I and II fixed in the Grey sand stone of soft and slightly hard beds of 17m was found to be directly responsible for main weighting in a form of simple beam exerting stresses over face line supports a). To the extent that load increased up to 30MPa over only 20 to 30% of power supports b). Also the influence of abutment load over Gate Road Ways, manifested in the form of roof convergence to the extent of 10mm/day when the face was at the distance 22.6 m (Rate of convergence in Main Gate) These Two beds have not caved in simultaneously but one after other. The collapse of these beds as a simple beam transformed to cantilever beam in the goaf.After the main fall the periodic falls are observed to be at regular span of 15 to 20 m interval. www.nitropdf.com
  • 7. The Anchor III fixed to light brown sand stone bed deformed up to 250mm and dislodged at the face retreat of 144.9m, thus these beds were exhibiting “ Slow Sinking or subsiding .” The strata movement in these beds absolutely did not have any influence of load over the face and GatesMonitoring with MPBEx from surface exhibits a conclusive and reliable data regarding theactivities inside the goaf that is• The immediate Grey sand stone strata which readily caved in by exerting load in the form ofabutment to the height of 20 m. This made us to understand that The height of influence of caving is equal to 7t Where t- thickness of extraction• Further it is noted that goaf sufficiently got packed with the above caving.• Though, there is no space for further fall, the overlying beds started sinking slowly indentingcompaction in the goaf.• The influence of load of strata movement above 7 t did not have any impact in the working 7area.• With these, the mechanism of Main Fall and Periodic Falls were understood. Theimmediate sand stone beds up to 7t are only the influencing factor.7. CONCLUSIONS The shallow depth, shorter face panel 1A and 1 have been successfully extracted and .High thrust was given to detailed strata monitoring and MPBEx monitoring from surface to study the caving characteristics of overlying sand stone beds. The Potential Risks involved in extracting longwall under shallow cover were thoroughly studied, understood and Educated to the long wall team of officers , supervisors , technicians, and workmen which gained Dedicated application of working knowledge of the team And improved standards of preventive maintenance of equipment’s, and of course the increased machine availability www.nitropdf.com
  • 8. This ultimately put the average rate of face retreat to the order of 6.0 m /day in Panel 1A 7.5 m/day in Panel 1Such a reasonable Fast Rate of Retreat demonstrated an added advantage to strata controland control caving of over lying strata without creating much strata control problems in theseshallow depth panels. No abnormal influence of abutment loading observed during the time of caving of sand stone beds up to 7t. The face supports at the mid face were loaded partially up to 60 to 70 % of its yield load capacity at the time Main Fall and Periodic Fall, due to the reason that concentration of stresses falls behind and away from the face line supports because of the face geometry and consistent face retreat. Fairly stable roof condition observed in the Gate Road Ways. A complex effect of adjacent goaf, working seam goaf and under lying goaf in the roof convergence while working Panel I was experienced. However the roof made kept intact by additional supports in the gate roads. As the goaf falls being packed regularly there was no abnormal water inflow observed even during the rain events, further the indication of loss of ventilation in underground or breathing of goaf from surface were not 8 found at any point of time. **************************References: 1. A Report on Ground Movement Monitoring – SCCL & NIRM 2. Experience with Shallow cover Long walls, R.Butcher and M.Kirsten. www.nitropdf.com