Ground control in undergound mines

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ground control in coal mines, stress regime, pressure arch concept, ground reaction curve, mechanics of strata failure, caving mechanism in bord & pillar, longwalls, roof falls, cavability, ground control practices or techniques in coal mines or metal mines

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Ground control in undergound mines

  1. 1. U Siva Sankar Email: uss_7@yahoo.com Mining Ground ControlGround Control : A collective term given to the techniques that are usedto regulate and prevent the collapse and failure of mine openings.Ground control is the science that studies the behaviour of rockmass intransition from one state of equilibrium to another.It provides the basis for the design of the support systems to prevent orcontrol the collapse or failure of the roof, floor, and ribs both safely andeconomically.Ground pressure - The pressure to which a rock formation is subjectedby the weight of the superimposed rock and rock material or bydiastrophic forces created by movements in the rocks forming the earthscrust. Such pressures may be great enough to cause rocks having a lowcompressional strength to deform and be squeezed into and close aborehole or other underground opening not adequately strengthened byan artificial support, such as casing or timber. 1
  2. 2. Rock Stresses Insitu (Virgin) Stresses Induced Stresses Exist in the rock prior to any Occurs after artificial disturbance e.g. disturbance. Mining, Excavation, pumping, Injection, Energy extraction, applied load, swelling etc.Tectonic Stresses Residual Stresses Gravitational Terresterial Stresses •Diagenesis Stresses •Seasonal tpr. variation •Metasomatism (Flat ground surface •Moon pull(tidal Stress) •Metamorphism & topography effect) •Coriolis forces •Magma cooling •Diurmal stresses •Changes in pore pressure Active Tectonic Stresses Remnant Tectonic Stresses Same as residual stresses but tectonic activity is involved such as jointing, faulting, folding and boundinage Broad Scale Local •Shear Traction •Bending •Slab pull •Isostatic compensation •Ridge push •Down Bending of lithosphere •Trench suction •Volcanism and heat flow •Membrane stress Proposed by Bielenstein and Barron (1971) Insitu and Induced stresses and their representation on Mohr’s Circle 2
  3. 3. THE MINING ENVIRONMENT IN-SITU STRESSES 3
  4. 4. 1. Magnitude and orientation of Insitu stresses vary considerably within geological systems.2. The pre-existing stress state changes dramatically due to excavation/construction therefore load must be redistributed.3. Stress is not familiar – it is a tensor quantity and tensors are not encountered in everyday life.4. It is a means to analyze mechanical behaviors of rock.5. It serves as boundary conditions in rock engineering problems as a stress state is applied for analysis and design.6. It helps in understanding groundwater fluid flow.7. At large scale shed some light on the mechanism causing tectonic plates to move or fault to rupture with the added uncertainty in that there is no constraint on the total force, as is the case with gravity loads. Insitu stresses virgin stresses or undisturbed in situ stresses are the natural stresses that exist in the ground prior to any excavation. Their magnitude and orientation are determined by – the weight of the overlying strata, and – the geological history of the rock mass In situ vertical stress For a geologically undisturbed rockmass, gravity provides the vertical component of the rock stresses. In a homogeneous rockmass, when the rock density γ is constant, the vertical stress is the pressure exerted by the mass of column of rock acting over level. The vertical stress due to the overlying rock is then: σz =γ h 4
  5. 5. Insitu horizontal stress The source of horizontal stress is mainly due to the tectonic activities which have resulted in the formation of major geological structures such as faults and folds. Since there are three principal stress directions, there will be two horizontal principal stresses. In an undisturbed rockmass, the two horizontal principal stresses may be equal, but generally the effects of material anisotropy and the geologic history of the rockmass ensure that they are not. The value of K K = σh σv Horizontal stress Lithostatic stress occurs when the stress components at a point are equal in all directions and their magnitude is due to the weight of overburden. σx =σy =σzThe other assumption is that rock behaves elastically but isconstrained from deforming horizontally.This applies to sedimentary rocks in geologically undisturbedregions where the strata behave linearly elastically and are builtup in horizontal layers such that the horizontal dimensions areunchanged. For this case, the lateral stresses σx and σy areequal and are given by: µ σ x = σ y = σ z. Terzaghi and Richart (1 − µ ) (1952)Later this relation found to be not true as Horizontal stress isalways more than vertical stress 5
  6. 6. Vertical and Horizontal stresses Vertical Stress (after Brown Townend and Zoback, (2000) and Hoek, 1978) Ratio of Horizontal to Vertical Stress Sheory,1994  1 K = 0.25 + 7 Ek  0.001 +   zwhere Ek (GPa) is the average deformation modulus of the upper part of theearth’s crust measured in a horizontal direction. 6
  7. 7. VERTICAL STRESS CONCENTRATED IN RIBS HORIZONTAL STRESS CONCENTRATED IN ROOF & FLOOR Characteristics of Coal Measure Roof StrataCover - The overburden of any deposit.Overburden – Layers of soil and rock covering a coal seam.Overburden is removed prior to surface mining and replacedafter the coal is taken from the seam.Lithology - The character of a rock described in terms of itsstructure, color, mineral composition, grain size, andarrangement of its component parts; all those visible featuresthat in the aggregate impart individuality of the rock. Lithologyis the basis of correlation in coal mines and commonly isreliable over a distance of a few miles.Bed - A stratum of coal or other sedimentary deposit.RoofThe stratum of rock or other material above a coal seam; theoverhead surface of a coal working place. Same as "back" or"top." 7
  8. 8. Characteristics of Coal Measure Roof StrataImmediate RoofThe roof strata that is immediately above the coal seam. Thisis the strata requires support for the mine openings to remaincompetent.Primary roof - The main roof above the immediate top. Itsthickness may vary from a few to several thousand feet.Secondary roof - The roof strata immediately above thecoalbed, requiring support during the excavating of coal.Competent rock - Rock which, because of its physical andgeological characteristics, is capable of sustaining openingswithout any structural support except pillars and walls leftduring mining (stalls, light props, and roof bolts are notconsidered structural support). Characteristics of Coal Measure Roof StrataFissure - An extensive crack, break, or fracture in the rocks.Fracture - A general term to include any kind of discontinuity in abody of rock if produced by mechanical failure, whether by shearstress or tensile stress. Fractures include faults, shears, joints, andplanes of fracture cleavage.JointA discontinuity in the rock strata where there is no sign of relativemovement.A divisional plane or surface that divides a rock and along whichthere has been no visible movement parallel to the plane or surface.CleatThe vertical and Parallel cleavage planes or partings crossing thebedding. The main set of joints along which the coal breaks moreeasily than in any other direction.Face cleat - The principal cleavage plane or joint at right angles tothe stratification of the coal seam. 8
  9. 9. Characteristics of Coal Measure Roof Strata Butt cleat - A short, poorly defined vertical cleavage plane in a coal seam, usually at right angles to the long face cleat. Slickenside - A smooth, striated, polished surface produced on rock by friction. Slip - A fault. A smooth joint or crack where the strata have moved oneach other. Fault - A slip-surface between two portions of the earths surface that have moved relative to each other. A fault is a failure surface and is evidence of severe earth stresses. Fault zone - A fault, instead of being a single clean fracture, may be a zone hundreds or thousands of feet wide. The fault zone consists of numerous interlacing small faults or a confused zone of gouge, breccia, or mylonite. Fig.: Influence of JointsFig: Joints exposed in the sandstone roof Fig: orientation of Cleats and coal seams Fig: Face and Butt Cleats in the Coal Pillar 9
  10. 10. Normal FaultSlickensides along the slip plane Strike Slip Fault Reverse Fault Characteristics of Coal Measure Roof Strata Sandy Strongly Sandstone Sandy Sand stone shale Jointed over shale over shale over Shale Sandstone Classifications of typical coal measures roof strata (modified after Peng & Chiang, 1984)Study of characteristics of coal measure strata is important to Determine the stability of Openings Determine Caving Characteristics & proper design of support system Design of Mine layout 10
  11. 11. PRESSURE ARCH CONCEPT Arching - Fracture processes around a mine opening, leading to stabilization by an arching effect. Stress Distribution Above a Small Pressure arch formation around mine Mine Opening opening (After Dinsdale, 1937)Abutment PressuresWhen an opening is created in a coal seam, the stress that was present before theopening was created is re-distributed to the adjacent coal pillars that are left. The areaswithin the remaining coal where the vertical stress is greater than the average are calledabutments and hence the stresses in those areas are called abutment pressures. Minor Pressure Arch Major Pressure Arch Minor pressure arches can form independently from pillar to pillar when the strength of the pillars in situ exceeds that of the abutment pressure, If the pillars yield or fail because of excessive pressure, their load is transferred to neighboring barriers or abutment pillars and a major pressure arch 11
  12. 12. Formation of major pressure arches due to Longwall Mining (After Stemple, 1956) For very wide openings such as those created by longwall mining, majorpressure arch formation is likely to create points of excessive pressure in seamsabove and below Arching stresses can either hinder or benefit mining in overlying or underlyingseams. The extradosal ground forms the zone of high compressive stress that cancause ground control problems in the roof, floor and pillars. The intradosal ground or tension zone is actually a distressed region in relationto the surrounding strata and conceivably the stress encountered in this zone mayactually be less than that created by the cover load. MECHANISM OF STRATA FAILURE • Failure through intact material due to overstressing • Failure along bedding surface due to overstressing • Localized failure of discrete joint bounded blocks • Localized failure of thinly bedded roof sections • In coal measure strata – Bedded, low to moderate strength rock types • Subjected to varying stress levels – Expected behavior of strata • Function of roadway shape, lithology & stresses acting on the roadway 12
  13. 13. Idealized Ground Response Curve and Support line.Idealized Ground Response Curve (GRC) and support line Prior to excavation, the excavation boundaries are subject to pressure equal to the field stresses (point A). After the excavation is created the boundaries converge and the pressure required to prevent further convergence reduces as arching and the self-supporting capacity of the ground develops (point B). A point is reached (point C) where loosening and failure of the rock occurs and the required support resistance begins to increase as self- supporting capacity is lost and support of the dead weight of the failed ground is required (point D). The effect of the support system can also be plotted on the chart. Equilibrium is achieved when the support curve intersects the ground reaction curve (point B). Ideally, support should be designed and installed to operate as close as possible to point C, which allows the available strength of the rock mass to be utilized while minimizing the load carried by the support system. The second support has a higher ultimate capacity (point E) than the first support (point F), but both reach the ground reaction curve at the same spot. This shows that higher capacity does not necessarily ensure better ground control. 13
  14. 14. Ground Reaction Curve approximation for outby loading conditions in alongwall tailgate (Barczak, et.al;) Strength = P/A where, P= Load to break rock A= Area σ Stiffness = Load per unit area(σ) / ε Strain(ε) Strain = ∆L/ L This is expressed as the modulus of elasticity or Young’s modulus (E), so, E = σ/ε ε As ε is dimensionless, E has the same unit as σ. As the number becomes very large, it is usually expressed in Giga- Pascals (GPa) 1 GPa = 1000 MPa 14
  15. 15. STRESS AROUND A ROADWAY HORIZONTAL STRESS LOADS THE ROOF AND FLOOR AFTER EXCAVATION, HORIZONTAL STRESSES CONCENTRATE IN THE STIFF (BRITTLE) BEDS IN THE ROOF AND FLOORPROPAGATION OF FAILURE ABOUT ROADWAYS 15
  16. 16. AS THE LOWER ROOF BEDS SOFTEN, STRESSES ARE REDISTRIBUTED INTO HIGHER STIFF BEDS UNBOLTED ROOF THIS FAILURE ZONE WILL CONTINUE TO MIGRATE FURTHER INTO THE ROOF IF NO REINFORCEMENT IS INSTALLED 16
  17. 17. UNBOLTED ROOFEVENTUALLY A LARGE FAILURE ZONE WILL FORM ABOVE EXCAVATION UNBOLTED ROOF IF UNSUPPORTED THIS WILL LEAD TO A FALL OF GROUND FORMING A NATURAL ARCH 17
  18. 18. KEY FEATURES OF ROADWAY BEHAVIOUR• State of stress acting on a roadway is influenced by – Geological structure – Variation in lithology – Topography – Seam structure (warps/rolls, etc.) – Tectonic setting• It may be kept in mind that roadways are often developed in a modified stress field as a result of adjacent workings, overlying/ underlying workings, in abutment areas due to pillar/ longwall extraction, etc. – While analyzing a situation, these influences must be given due importance 18
  19. 19. Effect of Horizontal Stress on Stability of Galleries in MinesGround Control Practices and ConstraintsTo ensure the stability of UG (Bord & pillar , Longwall, Highwall ) or OC structures, designer must consider principles of rock mechanics to determine Overall Mine layout – the relative location & intersection of entries and pillars, sections, or panels Shape size and number of entries Shape size and number of pillars Optimum support systems for structural stability or controlled failure Overall Mine layout, overall pit slope & dump slope , slope of individual benches and spoil dumps Dimension and number of benches, spoil dumps Shape of overall pit, and spoil dumpsConstraints: Sometimes rock mechanics principles are need to be completely ignored in normal mining operations such as Coal Extraction, Coal haulage, and Ventilation 19
  20. 20. Ground Control Techniques or Practices Ground is controlled in the first instance by proper mine planning. This means controlling the extraction geometry and sequence in such a way that stress levels and failure zones in the surrounding rock are kept below some threshold or potential for failure. It is not always possible to keep stresses low, and in these cases support can be installed to control fractured ground. Support is also used to keep blocky ground from unraveling and resulting in unexpected groundfalls. The following techniques can be used to manage stress and accomplish control. • Avoidance (change heading location and alignment) • Excavation shape (can change stresses from tensile to compressive) • Reinforcement (can provide the rock with additional strength) • Reduction (i.e. leave protective pillars) • Resistance (provide ground support) • Displacement (alter the sequence to “chase it away”) • Isolation (“keep it away”) • De-stressing (actively change the stress by blasting) Ground Control Techniques or PracticesSome ground control techniques serve more than one of the abovefunctions. For example, a rock bolt may provide for alteration of, andresistance to, ground stress.AvoidanceStress is avoided in the first place by aligning entries, headings, andboreholes to miss treacherous fault zones, dykes, sills, old workings, andzones of subsidence by a wide margin. When a problem fault must betraversed, the heading is aligned to meet it at near a right angle, ratherthan obliquely.Stress concentration is avoided by rounding the corners in a rectangularheading.Excavation shapeTensile and bending stresses are altered to compressive stresses whenthe back of a heading is arched. The same is true of a shaft or raise that ischanged from a rectangular to a circular cross-section.ReinforcementThe ability of the rock mass to resist shear, tensile and bending stress isreinforced when a cable bolt is tensioned because the friction in joints andfractures is increased. 20
  21. 21. Ground Control Techniques or PracticesReductionThe ground stress around one heading arising from its proximity to anotheropening is reduced by a protective pillar (safe distance) between them.The magnitude of the ring stress is reduced (and displaced) if a circularshaft or raise is advanced by drilling and blasting instead of raiseboring,because the fractured zone “pushes” the peak stress some distance intothe solid rock.Controlled (“smooth wall”) blasting techniques are used to minimizeoverbreak and crack propagation; however, their introduction to highlystressed ground may have another, negative effect (ring stressconcentration). To reduce stress in deep shaft sinking, it is typical thatsmooth wall blasting is abandoned near the horizon where discs were firstobserved in the pilot hole drill core.ResistanceStresses are resisted with ground support. The support may consist of sets(wood or steel), rock bolts, cable bolts, shotcrete, screen, strapping, orconcrete. Ground support is commonly evaluated for comparison purposesby the average pressure that it is calculated to exert against the rock face.Displacement Ground Control Techniques or PracticesIsolationIn deep mining, perimeter headings may first be driven around astoping block to avoid wrongful stress transfer and minimize stressbuildup in stope ends.e.g.1: At the current South Deep project in South Africa, the shaft pillarat the reef horizon was deliberately mined out before shaft sinkingcould reach it.e.g.2:It was proposed (W. F. Bawden) that a ring heading around anexisting shaft will isolate it from stresses induced by future mining in thenear vicinity.De-stressingDe-stressing displaces stress away from the walls of an entry orheading and into country rock. When properly executed, de-stressingcreates a failure envelope that shunts stress away from the excavation. 21
  22. 22. Various approaches for development of strata control techniquesCaving Mechanisms – Strata Mechanics – B&P Typical layout of a Conventional depillaring panel with manner of pillar extraction. 22
  23. 23. Caving Mechanisms – Strata Mechanics- B&P Conceptual Models of Loading & Caving of overlying Roof Strata in Bord & Pillar Caving PanelAMZ includes all of thepillars on the extractionfront (or "pillar line"), andextends outby the pillarline a distance of 2.76times the square root ofthe depth of coverexpressed in m. Mining depth is the principal factor affecting abutment loads. Cave quality and massive strata in the overburden are also recognized to affect abutment loading. 46 23
  24. 24. When a gob area iscreated by fullextraction mining(depillaring), abutmentloads are transferredto the adjacent pillarsor solid coal;The abutment stressesare greatest near thegob, and decay as thedistance from the gobincreases;From experience and from numerical analysis it isfound that the front abutment load reaches to zeroat about a distance give by the following equation D = 5.14 H Layout of Longwall Workings 24
  25. 25. Forces on supports due to lateral strata movement. General pattern of Vertical and (a) Weak roof -- horizontal force acting away from face. Horizontal stress redistribution (b) Strong roof -- horizontal force acting towards face. (Gale 2008) Adapted from Peng et al. [1987].Vertical stress distributions at seam level around single longwall face (Brady and Brown 1992) 25
  26. 26. Three zones in overburden due to longwall mining (Chekan at al., 1993) Distinct Zones in Overburden of an Longwall Opening Ranges in Strata Zones Thickne Characteristics ss Complete Strata fall onto mine floor, broken into irregular, platy shapes of caving 3-6HL various sizes, crowded in random manner. region Partial Strata have significant degree of bending, leading to intense caving 6-12HL Caving zone fracturing or displacement. region Upper Strata may separate along planes and fracture or joints may limit of 12-20HL open; individual beds remain intact and displacements are less caving likely to occur. zone Strata are broken into blocks by fractures and cracks due to bed Fracturing 20- separation; bending is not as abrupt and fractures are less zone 50HL pronounced Sagging 50HL to Bending of strata is gradual and distributed over a large zone Surface horizontal distance, without causing any major cracks.Note: HL = The mining height lower seam. 26
  27. 27. Caving Mechanisms – Strata Mechanics For an easily caveable roof stratum, the goaf gets packed quite frequently during face advance. Bulking factor of caved material is important and the face is unlikely to experience dynamic loading.Bulking factor controlled caving of weak andlaminated overlying strata. Working face experiences large overhang if the roof strata are strong and massive in nature. Under this condition, stress meters may play important role to visualise the nature and extent of dynamic loading during enmasse movement of the roof strata. Parting plane controlled caving of strong and massive overlying strata. Cavability of a rock formation Quantitatively, it is difficult to define cavability. But, a roof may be considered to be ideally cavable when the roof rocks cave in and fill the goaf as soon as the supports are withdrawn. 27
  28. 28. Parameters Influencing Cavability σv J1 J2 Stress relief zone σh (impedes caving) ExcavationJ1 = Sub-horizontal joint sets: Essential for caving Sub-J2 = Sub-vertical joint sets: Augments caving Sub- Parameters Influencing Caving Span Tensile strength : Measured in laboratory Rock density : Almost constant Horizontal stress: Measured in field stress: Identification of layers and their thickness :Difficult 28
  29. 29. Natural features that influence cavability ofrock are• Geometry of the discontinuities.•Shear and tensile strength of thediscontinuities,•Strength of rock materials and in-situstress fieldCavability can be enhanced by a set of inducedfeatures •Undercut span •Boundary slots, and •Mass weakening by creating fractures High horizontal stresses inhibit the roof caving. The caving height is low Caving occurs after a long face advance 29
  30. 30. Hydraulic fractures in the roof can stimulate thecaving.Without creating fractures in the massive roof, thecaving height would be low and the caving wouldoccur after longer face advance A fracture of large area in massive roof must becreated so that its area increases progressively toinitiate caving of the roof strata. Caving Mechanism in B&P Panels 30
  31. 31. Caving Mechanism in B&P Panels – Local, Main & Periodic Falls Critical conditions of strata behaviour invariably occurred in indian geo-mining conditions after extraction of two rows of pillars with 50 – 60 m span, and at an area of extraction of 4,000 - 6,000 m² including the ribs in the goaf. Longwall Caving Diagram Cut after cut, shear after shear the AFC & subsequently Chock shield supports will be advanced and the immediate roof rock may cave in or not. 31
  32. 32. Main Fall As the retreat further proceeds substantial area of main roof rock forms a plate & caves in by imposing load on supports, known as main weighting. TENSILE FRACTURES CRACKS STARTS TO FORM IN MID SPANFor 150m Longwall face length, Mainfall fall is taking place After an area of exposure of 8000 to 12000 Sq.m for coal as immediate roof and After 7000 to 8000 sq.m for sandstone as immediate roof conditions Periodic FallPeriodic falls occur at 18 to 25m and 10 to 16m progress intervals for coaland sandstone as immediate roof conditions respectively 32

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