MINE SUBSIDENCEP.K.BeheraDept. of Mining Engineering Indian School of Mines Dhanbad

Mine subsidence Mine 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 continuous or discontinuous.

ANGLE OF DRAW

(TROUGH)

Mine subsidence The extraction of ore/coal removes support from the overlying strata causing them to sag into the void space created. The sag is propagated upward to the surface; the maximum surface subsidence can be no greater than the thickness of the orebody/coal seam mined. In coal seams/orebodies that dip significantly subsidence can exceed the thickness of the coal seam/ orebodies mined.

Mine subsidence Subsidence usually occurs gradually when it is concurrent with mining. After mining, subsidence may continue to occur in a steady, gradual manner, or it may stop for a period, to be followed by failure at some later date.

Terminology and Definitions Used in Mining Subsidence Angle of break: The angle between the vertical and the point of maximum tensile strain from the edge of the underground working.Angle of dip: Maximum and apparent dip of deposit/ a coal seam.Angle of draw or Limit Angle: The angle between the vertical and the edge of the subsidence trough from the edge of the under ground working.

Terminology and Definitions Used in Mining SubsidenceSubsidence: The vertical movement of any point on surface caused by a underground excavation.Maximum Subsidence: The maximum vertical displacement within a subsidence trough caused by an underground excavation on surface. Maximum Possible Subsidence: The maximum subsidence in a given locality. Subsidence Trough/Profile: The depression formed by strata subsiding into an excavation.Half-subsidence Point: The point in a subsidence profile where the subsidence is half of the maximum amplitude.

Terminology and Definitions Used in Mining SubsidenceCritical Width Depth Ratio: The width depth ratio of extraction under ground which causes complete subsidence (Maximum possible subsidence) at a point on the surface. Non Effective Width Depth Ratio (NEW): The maximum width depth ratio of extraction under ground which does not cause practically any subsidence on the surface.Sub-critical Width Depth Ratio: The width depth ratio of extraction under ground which is more than the non effective width depth ratio but less than the critical width-depth ratio. Extraction of sub critical width depth ratio does not cause complete or maximum possible subsidence over an area on the surface.

Critical, Supercritical, and Subcritical Widths

Terminology and Definitions Used in Mining SubsidenceSupercritical Width Depth Ratio: The width depth ratio of extraction under ground which is more than the critical width-depth ratio. Extraction of super critical width-depth ratio causes complete or maximum possible subsidence over an area on the surface.Curvature: The reciprocal of radius of curvature of any part of subsidence profile.Slope/Tilt of Subsidence Profile: The gradient caused by subsidence between two points on a subsidence profile. It is found by the ratio of difference in subsidence at two points and the distance between two points.Strain: The change per unit length in the distance between any two points on the surface in a specified direction. The tensile strain is considered as positive and compressive strain as negative.

Classification of Mine SubsidenceSubsidence is classified broadly into two types:Discontinuous subsidenceContinuous or trough subsidence Depth of cover (h), working height (m) and percent of extraction of seam/orebody(e) have been identified to kwon the type of subsidence (continuous or discontinuous) likely to take place.

Discontinuous subsidenceSubsidence is termed as discontinuous when large surface displacements over limited surface area occur due to shear action, which forms steps or discontinuities in the surface profile. It may develop suddenly or progressively, and may occur on a range of scales. Some of the types of discontinuous subsidence are shown in the Figure.

Discontinuous subsidence

Types of discontinuous subsidence Types of Discontinuous Subsidence Crown holeChimney caving(sinkhole)Plug cavingSolution cavingBlock caving Progressive caving

Continuous or trough subsidence It involves the formation of a smooth surface subsidence profile that is free of step changes. This type of subsidence is usually associated with the extraction of thin horizontal or flat-dipping orebodies overlain by weak non-brittle sedimentary strata mined by longwall method. This occurs in most longwall coal mining operations and in the metalliferous longwall mining at depth

Rock mass movements associated with longwall mining

Subsidence in Bord and Pillar Mining Two common forms of surface subsidence ariseSubsidence holes and troughs are the main forms of surface instability. The former is defined by the creation of sharply delineated surface sinkholes (cave-ins) formed as a result of upward progressing chimney-shaped failures from mine junctions. These junctions represent the largest mine roof spans

Subsidence in Bord and Pillar MiningSuch failures are common when the roof of the junctions between roadways, in quadrilateral plans (squares, rectangles and rhombohedron) fail to surface. Irregular pillar distribution due to variation in ore quality and thickness, leads to troughs and subsidence hole type failures owing to larger roof spans and pillar failure.

Subsidence in Bord and Pillar MiningThe second form of roof and pillar induced subsidence resembles saucer shaped troughs (mostly having disc-shaped profiles and usually not more than 1 m at its center, extensive in area, up to 300 m, depending on the extent of underground pillar crushing) as a result of multiple pillar failures. It is common with wide room layouts, where narrow rib pillars, will probably fail with time leading to even wider roof failures.

Subsidence in Bord and Pillar MiningUnderdesigning the pillars in terms of strength can lead to gradual or sudden surface subsidence. In both cases these are time-dependent developments which normally occur after the pillars have sufficiently crushed and that the roof control elements have failed. Pillar punching into mine floors also result in trough subsidence.

Subsidence in Bord and Pillar Mining Flooding of coal mines after abandonment can deteriorate clay bearing lithologies (e.g. Shale) which can lead to roof and floor deterioration thereby facilitating closure and rock mass displacements and lead to trough subsidence

Subsidence in Bord and Pillar Mining

Factors Influencing Mine SubsidenceThese include Thickness of extracted materials Overlying mining areasDepth of mining Dip of mining depositCompetence and nature of mined and surrounding strata Near surface geologyGeologic discontinuities Fractures and lineaments

Factors Influencing Mine SubsidenceIn-situ stresses Degree of extraction Surface topography Ground water (including water elevation and fluctuation) Mine areaMethod of mining Rate of advance BackfillingTime

Factors Influencing Mine SubsidenceExtraction Thickness There is a direct relationship between the thickness of the extracted materials/deposit and the amount of surface subsidence that may result, making it an important factor in subsidence predictions. A greater thickness results in a greater amount of surface subsidence. The maximum possible surface subsidence is Smax = ma where m is height of extraction and a is a subsidence factor that ranges from 0.10 to 0.9.

Factors Influencing Mine SubsidenceMultiple Section Working Where multiple mining horizons exist, subsidence which occurs in one area increases the likelihood of similar events in other areas, because the strata have been disturbed.

Subsidence surface profile with multiple seam situation

Factors Influencing Mine SubsidenceWidth and Depth of OpeningThe width and depth of an opening are intimately related as far as surface subsidence is concerned, because together they determine the critical area or area of influence. It is common practice to use the ratio of width to depth.U.K.National Coal Board found the following three fundamental conditions which influence the character of mining subsidence profile for a longwall development at the surface.

Factors Influencing Mine SubsidenceThe width/depth (w /h) ratio < 1.4 for subcritical underground extractionThe width/depth (w /h) ratio = 1.4 for critical underground extractionThe width/depth (w /h) ratio > 1.4 for supercritical underground extraction

Factors Influencing Mine Subsidence Competence of Mine Floor and Roof The mine roof and floor are critical factors in the initiation of subsidence events, since they propagate from these areas. Weak roof materials permit the fall of overlying strata, and compact more easily, resulting in a greater likelihood and severity of subsidence.

Factors Influencing Mine SubsidenceNature of Overburden The strength of the overlying strata above the mining horizon is a factor in the timing and extent of subsidence. Surface and Near-Surface Geology Surface and near-surface soils and unconsolidated materials tend to enhance subsidence effects, because they behave in an inconsistent manner. They are an important factor relative to hydrologic impacts because they affect the exchange of surface water and ground water

Factors Influencing Mine SubsidenceDip of Deposit For inclined seams, the surface trough subsidence is displaced towards deeper edge of the opening and, depending on inclination, may be located outside the dip edge of the opening. Figure shows the variation of angle of draw with the dip angle

Effect of seam inclination on angle of draw

Surface subsidence profiles for dipping seams

Factors Influencing Mine SubsidenceDegree of Extraction The amount of pillar support is directly related to the timing and extent of subsidence. Lower extraction ratios result in greater thicknesses of pillars, which tends to delay and decrease subsidence. As the amount of pillar support is decreased, either by mine design or as a result of pillar extraction, subsidence occurs more rapidly and extensively. Complete removal of pillars is almost always followed by subsidence, with surface manifestations being a function of upward propagation to the surface.

Factors Influencing Mine SubsidenceAngle of draw or Limit Angle The surface position of the boundary between areas of subsidence and no subsidence is defined by the "angle of draw." The angle of draw varies from 8 to 45 degrees (25 to 35 degrees in most instances) depending on the coalfields. The larger the angle of draw the wider will be the zone on the surface in which subsidence should occur. Angle of draw Indian coalfields: 4 - 21

Factors Influencing Mine Subsidence By using the largest of several possible angles of draw a greater margin of safety is established for those areas lying outside the boundary of possible subsidence. There are indications that angle of draw may depend on depth, seam thickness, and local geology, especially major faults or fracture planes or self supporting strata above the coal seam.

Factors Influencing Mine SubsidenceGeologic Discontinuities Faults, folds, and other inconsistencies in the overlying and surrounding strata may increase subsidence potential. The disturbance of equilibrium forces by mining can trigger movement along a fault plane. Faults may also weaken the overlying strata and trigger subsidence in materials that may otherwise show desirable properties. Joints and fissures in the strata also affect subsidence but on a smaller scale.

Effect of faults on subsidence

Factors Influencing Mine SubsidenceSurface Topography Sloping ground like hillsides tends to emphasize the surface manifestation of subsidence, while it is less accentuated on flatter ground and in valleys. Ground Water Drainage gradients may be altered by disturbance of the strata around mine areas. Rocks may become weakened by saturation and erosion patterns could change. Where surface water is present, it may migrate more easily to fractures and fissures in the strata and into the mine area and may induce subsidence. The creation of a cavity as a result of mining results in subsidence.

Migration of water in subsided area

Influence of surface slope on subsidence

Factors Influencing Mine SubsidenceWater Level and Fluctuations The strength and stiffness of the overlying and surrounding rock strata, and any pillars left in the mining area, are significantly reduced by the effects of water.Mining Method The extent and magnitude of subsidence is limited by techniques such as bord-and-pillar mining, which limits the extent of extraction. The timing and extent of subsidence in bord-and-pillar mines is not predictable, and eventual collapse of pillars in bord-and-pillar mining may lead to trenching or sagging of the surface, with considerable displacement and strain over short distances.

Factors Influencing Mine SubsidenceBackfilling Partial or complete mine backfilling reduces subsidence and is dependent upon the type and extent of backfilling. However, it is important to note that backfilling does not eliminate subsidence

Factors Influencing Mine SubsidenceTime Effects The period during which mine subsidence occurs consists of two distinct Active and Residual phases Active subsidence occurs simultaneously with mining, whereas residual subsidence occurs after mining.

The curve indicates maximum subsidence at P as a function of the face locationSubsidence at P begins when the face is 0.7h ahead of P Subsidence accelerates when the face is about 0.3h ahead of point P, reaching 15.5% Subsidence at P reaches 97.5% when the face is 0.7h beyond point P(active subsidence is complete) The remaining 2 to 3% is due to gradual compaction of subsided ground is known as residual subsidence

Factors Influencing Mine SubsidenceThe duration of residual subsidence is important from the standpoint of gauging the duration and extent of environmental impacts, including the extent of liability for post-mining subsidence The actual time involved depends on a number of factors According to some researchers, prediction of when or how much damage may occur becomes difficult. There are documented cases of mine subsidence occurring 100 years after mines were abandoned

Impact of Mine SubsidenceLoss of water in surface water bodies(pond, river, nallah, jores etc)Depletion of water retention capability of sub-surface water tableDepletion of water retention capability of aquifersDamage of buildings on surfaceDamage of rope waysDamage of high tension pylonsDamage of u/g cables

Impact of Mine SubsidenceDamage of u/g pipe linesDamage of overlying virgin seamsDamage of overlying workingsLeakage of air, water and fire into u/g workingPollution of surface atmosphere due to harmful gasses from u/g to surfaceDepletion of water retention capability of sub-soilReduction in agricultural yieldDamage of forest

Mine Subsidence prediction Techniques

For mining engineers the prediction of mine subsidence is very important during the planning and execution stages of any underground mining operation.Considerable research is being carried out for predicting mine subsidence. However, researcher have been successful in developing techniques mostly for continuous mine subsidence rather than discontinuous types.

Mine Subsidence prediction Techniques

Subsidence prediction techniques can be divided into two main types:Empirical and Analytical Empirical techniques are based largely on a combination of experience and the detailed analysis of a large number of observed ground movements. Analytical methods have not found widespread acceptance due to the difficulty of determining the material properties of the overburden rocks and the necessity for making simplifying assumptions to simulate complex field problems

Empirical technique

NCB Graphical Method The most comprehensive and widely used empirical method of predicting subsidence and surface strain profiles is that developed by the National Coal Board (NCB) of UK. NCB graphical method was derived from the analysis of extensive field database collected over the years from a variety of mining conditions.

NCB graphical methodEssentially, the data were summarized the form of a series of non-dimension graphs, where the subsidence is related to ratio of the longwall face width to the depth of working and the subsidence parameter (vertical displacement and horizontal stress) are calculated from this ratio. An example: The figure shows a graph for predicting subsidence in a location where width and height of extraction is known for a given depth of the seam.

NCB graphical method

CMRI Nomogram Method

CMRI has developed a nomogram and the following equations for prediction of subsidence in Indian coal mines:Smax = 500 (1+ M) ( e. h. a. m)(for compressive)( for tensile)

CMRI Nomogram Methodwhere Smax = maximum possible subsidence in mmM = rock mass factor ( see Table 2)e = extraction percentage factor ( see Table 1)a = goaf treatment factor ( see Table 1)h = depth factor ( see Table 1)m = extraction thickness in mGmax = maximum possible slope in mm/mEmax = maximum possible strain in mm/mh = depth of panel from surface in m

Table 1: Subsidence Parameters *0.07 for seams steeper than 1 in 5.

ParameterValueePercentage of extraction 405060708090100e00.10.250.400.600.800.95aGoaf treatmentCavingHydraulic stowinga0.950.07-0.10*hDepth 400 mh1.001.101.15

Table 2: Rock mass factorStrata having predominantly clays, shales, mud-stones, etc.As in case of thick seam mining by descending slicing when the top slice has been extracted with caving and the next slice is under consideration.

Sl. No.Nature of StrataRock mass factor M1.Totally disturbed strata11.02.Partially disturbed strata with the thickness of intact strata being less than five times the seam thickness under extraction0.93.Partially disturbed strata with the thickness of intact strata being more than five times the seam thickness under extraction 0.84.Naturally weak and disturbed strata20.75.Strata having interbedded layers of sandstones and shales 0.66.Strata having predominantly sandstone beds0.5

CMRI NOMOGRAM

Profile Function TechniqueThe profile function technique is based a curve fitting procedure that employ mathematical profile function to match the observed subsidence data. Once a fit is established, through the use of actual field data the function is used to predict subsidence profiles over future areas of mining.

Profile Function Technique A number of different mathematical functions have been developed for critical mining areas by different researchers, but all functions fit the general form.

where s(x) = subsidence at point x, Smax= subsidence at the panel centre (Maximum possible subsidence), B =critical radius or one-half of critical width, x =horizontal distance from the point of half-maximum subsidence to panel centre, d =distance between the point of half-maximum possible subsidence and edge of the opening

Influence Function Technique The influence function method considers an extracted opening to be composed of an infinite number of small openings or elements. Each small element/opening produces an individual surface subsidence profile, and their summation is the total subsidence profile. The influence is maximum when the element is directly below the surface point being considered. It becomes minimum when the element is near the edge.

Influence Function Technique The subsidence of a surface point can therefore be obtained by integrating the influences of all the infinitesimal elements of the excavation. The method is extremely versatile and suffers none of the geometrical restrictions of the profile function In mathematical form

Where, P (r) = Influence function of an element dA on a surface point P r = Radial distance from a surface point where maximum subsidence (Smax) occurs B = Critical radius

Superposition of infinitesimal influences

Analytical Techniques

Some of the commonly used methodsClosed from elastic solutionNumerical methodsMechanistic models

Closed from elastic solution One method of studying subsidence development is to assume that the strata displacement behaves according to one of the constitutive equations of continuum mechanics over most of its range. In this context, the continuum theories were evolved from the analysis of a displacement discontinuity produced by a slit in an infinite half-space elastic media.

Closed from elastic solutionAnalytical procedures were subsequently developed for three types of underground excavation based on elastic ground conditions: (1) non-closure (2) partial closure and (3) complete closure. Additional work extended the closed form solution to transversely isotropic ground conditions in both two and three dimensions.

Numerical Methods Numerical models provide an excellent tool for the quantitative analysis of subsidence and strata mechanics problems and are not subject to the same restrictive assumptions required for the closed form analytical solutions

Numerical MethodsFinite element modelling is commonly applied to subsidence problems since it can readily accommodate non-homogeneous media, non-linear material behaviour and complicated mine geometries. Alternatively, finite difference models can be used for the large strain, non-linear phenomena associated with subsidence development. Other elastic approaches employing numerical techniques include displacement discontinuity, boundary integral and boundary element methods

Mechanistic ModelsThe void-volume model proposes that the actions of discrete deformational and collapse mechanisms are the primary modes influencing subsidence development. However, to facilitate this approach the actual collapse mechanisms must be known and physical models constructed to simulate the process. The effect of scaling factors on this type of model must therefore raise questions concerning the validity of the results, especially since different types of material behaviour are involved in the collapse mechanism.

Control and Prevention Mine Subsidence

Partial extraction Goaf treatmentHarmonic extraction Safety pillarsRapid Mining

Partial extractionPartial extraction involves leaving protective features such as pillars. The pillars are left between panels, has been successfully used to limit maximum subsidence. Depending on the lay out and the extraction ratio, reductions in maximum subsidence in order of 80% may be achieved.

Goaf Treatment Backfilling by hydraulic or pneumatic techniques, using a variety of materials including run of mine waste rock, milled tailings, sand, cemented backfilling material or other materials can reduce the subsidence up to 50% depending on the nature and timing of treatment.

Influence of filling on Subsidence

Borehole Grouting from Surface

Harmonic Extraction

It involves the phased removal of the mineral from a critical area such that the ground surface is smoothly and horizontal strain is minimized. Harmonic extraction requires that the panel be advanced in at least two faces maintained at a carefully calculated distance apart.

Extraction with staggered faces

Safety Pillars Safety pillars are the solid pillars left untouched to protect surface structures located directly above. The size of the safety pillar can be determined by horizontal distance from a surface structure at which the advancing face must stop so that total strain at the site of the surface structure is less than the allowable values. The size of the safety pillar depends on the seam depth from the surface.

Rapid Mining The maximum tensile strain (Et) travelling with an advancing face is generally less than the final obtainable tensile strain, Emax and faster the mining, the less Et is. Furthermore, the maximum compressive strain accompanied with the travelling face is always less than the corresponding maximum tensile strain, Et . Therefore there exists a minimum rate of mining characteristic of each coalfield that will induced maximum tensile traveling stain less than the allowable one

Travelling and final surface strain