Subsidence in coal mines


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subsidence, upsidence, subsidence limits, components of subsidence, prediction of subsidence, analysis of subsidence, coal mines, control of subsidence, subsidence trough, harmonic extraction, abandoned mines subsidence prevention, subsidence prevention in working coal mines, factors affecting subsidence

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Subsidence in coal mines

  1. 1. U.Siva Sankar Sr. Under Manager Project Planning Singareni Collieries Company Ltd E-Mail or Visit at: Subsidence occurs when large areas of coal are mined and the resulting settlement of roof material into the void (the goaf) results in the surface subsiding over the affected area. More commonly there is a gradual lowering of the surface strata which actually bends rather than fractures at the limits of the subsiding area. This bending leads to tensile strains in the surface strata (and possibly in structures on the surface) which may result in the formation of cracks. It is around the edges of the subsiding area where damage may occur. The central area of subsidence usually is subjected to a gradual lowering, possibly suffering some tilt and strain as the workings pass beneath. This may cause damage to such items as roads and pipelines but this is easily repaired and there is little evidence of it being a subsidence area after movement ceases. Structures, including houses, built in mining areas should be designed to accept a small degree of tilt and strain on a short term basis without suffering major damage."Upsidence" is also a surface phenomenon associated with mining andsubsidence and occurs where workings pass beneath a gorge or similarsurface feature causing a concentration of horizontal stress in the stratabetween the bottom of the feature and the top of any goaf cavity. Thisincreased stress may cause strata beds close to the surface to bendupwards and possibly fracture. 1
  2. 2. Figure Schematic developments of mine effects.Subsidence Phenomenon: 2
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  4. 4. Fig. Subsidence from total extraction (long wall) mining of ahorizontal coal seam under a level landscape 4
  5. 5. Mine subsidence can be defined as movement of the ground surface as a result ofreadjustments of the overburden due to collapse or failure of underground mineworkings. Surface subsidence features usually take the form of either sinkholes ortroughs.Sinkhole subsidence is common in areas overlying shallow room-and-pillar mines.Sinkholes occur from the collapse of the mine roof into a mine opening, resulting incaving of the overlying strata and an abrupt depression in the ground surface.Sinkholes are typically associated with abandoned mine workings, since mostactive underground mines operate at depths sufficient to preclude the developmentof sinkhole subsidence.Subsidence troughs induced by room-and-pillar mining can occur over active orabandoned mines. The resultant surface impacts and damages can be similar,however the mechanisms that trigger the subsidence are dramatically different. Inabandoned mines, troughs usually occur when the overburden sags downward dueto the failure of remnant mine pillars, or by punching of the pillars into a soft minefloor or roof.Components of SubsidenceSubsidence consists of five major components, which influence damage to surface structures and renewable resources Vertical displacement (settlement, sinking, or lowering). Horizontal displacement (lateral movement). Slope (or tilt), i.e., the derivative of the vertical displacement with respect to the horizontal. Horizontal strain, i.e., the derivative of the horizontal displacement, with respect Fig. Schematic of ground to the horizontal. movements caused by subsidence Vertical curvature (or flexure), which may be approximated by the derivative of the slope, or the second derivative of the vertical displacement with respect to the horizontal. 5
  6. 6. Terminology of Subsidencelimit angle or angle of draw:the angle of inclination between thevertical at the edge of the workings andthe point of zero vertical displacementat the edge of the trough.angle of break or angle of fracture: The inclination to the vertical of theline connecting the edge of the minedarea with the surface point exhibitingthe maximum tensile strainInflection Point:On the major cross-section of thesubsidence basin, the point dividing theconcave and convex portions of the Fig. Schematic of groundsubsidence profile is called the movements caused by subsidenceinflection point. At the inflection pointthe subsidence is equal to half of themaximum possible subsidence at thecenter, the surface slope is maximumand the curvature is zero.Terminology of SubsidenceAngle of major influenceWhen the opening or gob has reached the critical size the major surfacedeformations occur on both sides of the inflection point within a certain distance.This distance is called the radius of major influence. Beyond this distance surfacedeformations are very small. The angle of major influence is the angle betweenthe horizontal and the line connecting the inflection point and the edge of theradius of major influence. (tanβ =h/r where h is the mining depth).Angle of full subsidenceOn a major cross-section of the subsidence basin under super critical width ofmining the acute angle between the horizontal and the line connecting the edge ofthe flat bottom of the subsidence basin and the edge of the opening is called theangle of full subsidence. It indicates the degree of subsidence development andcan be used to define the area within which subsidence has been fully developed. Fig. Sketch depicting area of influence. Maximum subsidence at P by mining entire area of influence. 6
  7. 7. Terminology of SubsidenceCritical Subsidence: Subsidencereaches the maximum possible valueat the center. This area is obtained ifthe lines of draw plotted from theopposite sides of the excavation meetat the surface. This is also called “Fullarea”.• Subcritical Subsidence: For such acase, no point on the surface showsthe maximum subsidence.• Supercritical Subsidence: The widthand length of the opening continue toincrease even after occurrence ofcritical subsidence. The maximumpossible subsidence doest notincrease, but spread laterally into anarea. NEW- Non Effective Width: of extraction is the underground width of extraction which does not cause practically any movement on the surface. In India NEW varies between 0.3 to 1.17 times the depth of extraction. 7
  8. 8. Factors Affecting Mine SubsidenceSeveral geologic and mining parameters and the nature of the structure affect the magnitude and extent of subsidence that occur due to coal mining Effective Seam Thickness Multiple Seams Seam Depth Dip of Seam – flat, moderately inclined, steeply inclined Competence of Mine Roof and Floor – strong or weak Nature of Overburden Near-surface Geology Geologic Discontinuities – bedding planes, faults, folds, etc Fractures and Lineaments In Situ Stresses- vertical and Horizontal stresses Degree of Extraction Surface Topography – flat, sloping, hilly area Groundwater Water Level Elevation and Fluctuations Mined Area- sub critical, critical, super critical Method of Working – Bord & Pillar , longwall Rate of Face Advance Backfilling of the Gob Time Elapse Structural Characteristics of buildings, monuments etc Fig: Effect of Dip of Coal Seams Fig: Subsidence in Multiple Coal Seams 8
  9. 9. Effect of Sloping Ground on Surface SubsidenceGeneralized surface subsidence Vs time curve obtained from field study 9
  10. 10. SURFACE DAMAGES DUE TO SUBSIDENCE• Surface subsidence manifests itself in three major ways:• Cracks, fissures, or step fractures.• Pits or sinkholes.• Troughs or sags. SURFACE DAMAGES DUE TO SUBSIDENCE 10
  11. 11. Subsidence Measurement Various subsidence monument designs PLAN SHOWING SUBSIDENCE PILLARS OVER BORD AND PILLAR PANEL B A R R I E R B A F Subsidence RR A Pillars IE O R G B A R R I E R 11
  12. 12. PLAN SHOWING SUBSIDENCE PILLARS OVER LONGWALL PANEL 90 M FIRST SUBSIDENCE 5 CRACKS (7CM WIDTH) 500M X 62M M 7.5M CENTRE LINE 11M BM 1 10 MAX.SUBSIDENCE: 2.4M 29 37 100M 10M Direction of 5M Advance 2 LIN E 2A L IN E 3 L IN E 3 A L IN E 4L IN E 4 A L IN E TOTAL NO.OF STATIONS : 812 NO.OF LINES ALONG STRIKE DIRECTION : 19 NO.OF LINES ALONG DIP-RISE DIRECTION : 6 SPACING BETWEEN ” ” :5M SPACING BETWEEN ” ” :10MSubsidence Prediction Methods Theoretical methods: Use of continuum mechanics concepts of elastic, plastic or elastic-plastic material properties of overburden strata Profile function method: Profile functions are developed based on measured subsidence data. There are about 20 profile functions are developed in all over the world. Influence function method: Incorporates the mathematical modeling of influence function Zone Area Method Empirical Modeling: Based on the measured subsidence data empirical models are developed. Physical Modeling: Parametric study of the subsidence prone area Numerical Modeling : The most popular technique and cheaper method for estimating surface subsidence and displacements. It can incorporate any material, bedding plane, anisotropy, etc. 12
  13. 13. Subsidence Prediction- Empirical Methods The relation between maximum subsidence, Non-effective width, depth and height of extraction and other parameters recommended by NIRM is presented below: Longwall method Smax = he*0.6(1+(W/H)/0.754)-12.68) Smax = Maximum subsidence for a given width to depth ratio ‘x’ he = Effective height of extraction (Height of extraction x % of extraction) W = Width of the panel, ‘m’ H = Depth of the panel, ‘m’ Bord & Pillar Method Smax = he*0.65(1+(w/H)/0.75)-8) Smax = Maximum subsidence for a given width to depth ratio ‘x’ he = Effective height of extraction (Height of extraction x % of extraction) W = Width of the panel, ‘m’ H = Depth of the panel, ‘m’ Subsidence Prediction- Empirical MethodsFurther, the equation for slope (G) is G = K1S/HK1 = 2.2 + 24*e-11.8 (X-1)X = Width to Depth ratio/NEWS = Subsidence in ‘mm’H = Depth ‘m’The equation for compressive strain isE(-) = K2 (S/H)K2=1.4+ 24*e-14.3(X-1)The equation for tensile strainE(+) = K3 (S/H)K3=1.35+ 28*e -19.4(X-1) 13
  14. 14. Fig. Damage to structures from subsidence related to strain and structure length (after National Coal Board 1975ab) Subsidence Prediction- Empirical MethodsSheorey et al., 2000, suggested the following equation for predicting the subsidence formultiple seam cases:where,S = Maximum subsidence, mX = Ratio of width to depth ratio and Non Effective WidthSubsidence in case of closely spaced multiple seams could be calculated using thefollowing empirical equation {NIRM, 2001}:where,S = Maximum subsidence, mH = Average of minimum depths of the panelsW = Average width of the panelshe = Total extraction thickness X % of extraction 14
  15. 15. Brief description of Subsidence Prediction Methods Numerical Modelling• The numerical method for prediction of surface subsidence is now gaining popularity over the profile or influence function due to its capability to considered geological complexities, irregular shaped structures, complex constitutive behaviour of coal, coal measure strata, goaf, bed separation and re-contact, roof failure mechanism, goaf behaviour etc. It has a capability to consider sequential excavation process in the simulation. This will give realistic results in terms of subsidence as well as strain. 15
  16. 16. Subsidence Profile over multiple number of Bord & Pillar Panelsof a Coal Mine Subsidence Control Measures 16
  17. 17. Different Methods for Subsidence Control PARTIAL EXTRACTION METHODSPartial extraction methods can be classified in to two groups• Pillars and overburden both are stable Wide stall method Pillar splitting method• Pillars are stable whereas overburden may fail Non-effective width (NEW) extraction Chess board method Goaf pillar method 17
  18. 18. PARTIAL EXTRACTION METHODS• Wide Stall Method It involves the widening of the galleries from its side or sides.• Pillar Splitting Method Pillars are split and form number of stooks. PARTIAL EXTRACTION METHODS Chess board method Goaf and pillar methodPlan of panel designedby extracting diagonalrows of pillars. 18
  19. 19. HYBRID METHOD OF PARTIAL EXTRACTION“It is a combination of Wide stall method and pillar splitting method”Non- Effective Width (NEW) Caving Methods Partial Extraction Methods NEW should be less than Equivalent width for different panel extraction shapes of Caving and partial extraction methods to prevent surface subsidence NEW for different coal seams based on Rockmass factor of the overlying roof rocks is also to be considered 19
  20. 20. Harmonic Method of Extraction Extraction of a panel causes tensional and compressive strain at the surface. The working of two seams should be so advanced simultaneously to cancel out the balance of strain, caused by face by the strain induced by another at a different level. This approach is known as harmonic mining, which however is not simple Because the mine has to be preplanned and also problems due to interaction between faces in different seams have to be encountered. 20