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Subsidence in coal mines


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Subsidence in coal mines

  1. 1. Subsidence in coal mines Subsidence can be defined as movement of the ground surface as a result of readjustments of the overburden due to collapse or failure of underground mine workings. Surface subsidence features usually take the form of either sinkholes or troughs. By: Aakash Deep Singhal (111MN0436)
  2. 2. Theories of subsidence  Vertical and normal theory  Dome theory  Beam or plate theory  Trough theory  continum theory  Particulate theory
  3. 3. Terminology Limit angle or angle of draw: the angle of inclination between the vertical at the edge of the workings and the point of zero vertical displacement at the edge of the trough. Angle of break or angle of fracture: The inclination to the vertical of the line connecting the edge of the mined area with the surface point exhibiting the maximum tensile strain Inflection point: On the major cross-section of the subsidence basin, the point dividing the concave and convex portions of the subsidence profile is called the inflection point. At the inflection point the subsidence is equal to half of the maximum possible subsidence at the center, the surface slope is maximum and the curvature is zero.
  4. 4. Several geologic and mining parameters and the nature of the structure affect the magnitude and extent of subsidence that occur due to coal mining Factors Affecting Mine Subsidence  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  Time Elapse  Structural Characteristics of buildings, monuments 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 – Board & Pillar , long wall  Rate of Face Advance  Backfilling of the Gob  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  Time Elapse  Structural Characteristics of buildings, monuments etc
  5. 5. Damages due to subsidence
  6. 6. Methods of subsidence control  By suitable design of surface structure  By surface stabilization. - grout column - grout case - piers constructed within the mine - deep foundations  Filling methods for void elimination - hydraulic filling or back filling - dowel process - pneumatic filling - fly ash injection - grouting - over excavation and backfilling - blasting the rock in the roof and floor of the mine
  7. 7.  Underground methods of subsidence control 1. Single face advance a. directional control b. rate of advance c. location 2. Harmonious extraction a. multiple seam b. stepped face 3. Partial extraction a. panel and pillar b. yield pillar 4. Back filling
  8. 8. Measurement Techniques  Surface observations  Sub surface measurement  Layout of subsidence stations  Routine measurements by levelling of vertical components  Measurement of magnitude and direction of principal surface strains  Subsurface measurement of subsidence - wireline method - Time domain reflectometry (TDR) method - Mechanical grouting method
  9. 9. Measurement of Subsurface Strata Movement  Mainly consists of monitoring strata separation as a function of face location  Can be done by either underground or surface boreholes  U/G boreholes are usually drilled in the roof, then roof sag measurements devices are installed for monitoring strata  With the surface borehole technique, NX-sized boreholes are drilled from the surface all the way down to the coal seam to be mined  Movement of strata at different horizons above the seam is then monitored from surface as a function of face location using any of the following methods
  10. 10. Wireline Method  This technique makes use of electronic logging devices commonly used in oil fields  A bullet perforator is lowered into the well, positioned at the desired level & fired into the wall of the well by means of a surface control  A small amount of radioactive material is inserted in the bullet, so when it is shot into the strata surrounding the borehole & remains there strata movement can be followed  Bullets are shot into various strata along the borehole, and their positions are identified with high peaks of intensity in a radioactive log  The change in a bullet’s position indicates the amount & direction of movement of stratum in which bullet is inserted
  11. 11. TDR Method  TDR or Time Domain Reflectometry, works on the same principle as radar  A good cable is grouted in the borehole all the way down to the coal seam  Caving or separation of the roof strata create some sort of faults in the cable  An ultrafast rise time voltage step is sent down the cable, which is reflected by the fault  A sampler picks it up & superimposes on incoming signal resulting in a step-up or down which can be seen on CRT  The time delay between the initial signal & the arrival multiplied by the travel velocity indicates the distance where fault occurs.
  12. 12. Subsidence 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
  13. 13.  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.
  14. 14. 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) 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’
  15. 15. Sheorey et al., 2000, suggested the following equation for predicting the subsidence for multiple seam cases where, S = Maximum subsidence, m X = Ratio of width to depth ratio and Non Effective Width Subsidence in case of closely spaced multiple seams could be calculated using the following empirical equation {NIRM, 2001}: where, S = Maximum subsidence, m H = Average of minimum depths of the panels W = Average width of the panels he = Total extraction thickness X % of extraction
  16. 16. 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 behavior of coal, coal measure strata, goaf, bed separation and re-contact, roof failure mechanism, goaf behavior 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. Subsidence Profile over multiple number of Bord & Pillar Panels of a Coal Mine
  17. 17. References     