Horizonta stress


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insitu stress field in earth crust, stress environment in mines, effects of horizontal stress, control measures of horizontal stress, stress mapping, measurement of insitu stress field

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Horizonta stress

  1. 1. REVIEW OF ROLE OF INSITU HORIZONTAL STRESS IN COAL MINES U.Siva Sankar Sr. Under Manager Project Planning Singareni Collieries Company Ltd E-Mail :ulimella@gmail.com or uss_7@yahoo.com Visit at: www.slideshare.net/sankarsulimella 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) 1
  2. 2. THE MINING ENVIRONMENT Rock stress is a measure of forces in the rock Three components: one vertical, two horizontal Vertical stress is equal to the weight of rock above Horizontal stresses come from movement of the earth’s crustIN-SITU STRESSES 2
  3. 3. Vertical StressComes from the weight of all the rock aboveIncreases with depth of coverEquals depth x 0.025 MPa where depth is in metresAt 100m = 2.5MPa At 1000m = 25MPa 3
  4. 4. Rock Stress, Strata and Support Rock stress, strata and support Strata Stress Support Stress Stress Vertical and Horizontal stressesVertical Stress (after Brown Townend and Zoback, (2000)and Hoek, 1978) 4
  5. 5. 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. EARTH’S CRUST Beneath oceanic abyss : 6 km Thick Continental crust : 35-50 km Thick Oceanic crusts have been formed within past 200 million years, whereas the continents contain rocks which are more than 3,500 million years old. 5
  6. 6. THEORY OF PLATE TECTONICS OR CONTINENTAL DRIFT Earth’s crust is cracked into a series ofplates, which are moving around the earth’ssurface Continents are composed of light materialsand they rest upon the moving plates Plate edges occur along mid-oceanicridges where new crustal rock is being addedas molten material wells up from below EFFECTS OF PLATE MOVEMENT The oceans are widening/spreading atthe rate of 1 to 10 centimeters per year The earth is not expanding Crust is being destroyed at the plateedges ( oceanic trenches) 6
  7. 7. Crustal Tectonic Plates of Central Europe Iceland(20mm/year) Atlantic Ridge Crustal Tectonic Plates of Central Asia Eurasian Plate Ind ia African Plate 7
  8. 8. • Mining operations modify the stresses acting on rock – Mining of a heading • Vertical stress concentration in the sides • Lateral stress concentration in the roof and floor – Mining of longwall • Vertical stress concentration ahead of coal face • Lateral stress can concentrate at the LW panel corners Insitu and Induced stresses and their Effects Rock Stress HORIZONTAL STRESS LOADS THE ROOFRIBS FLOOR VERTICAL STRESS LOADS THE AND VERTICAL STRESS LOADS THE RIBS IF THESE INCREASED STRESSES EXCEED THE ROCK STRENGTH THE ROCK WILL FRACTURE AND FAIL 8
  9. 9. VERTICAL STRESS CONCENTRATED IN RIBS HORIZONTAL STRESS CONCENTRATED IN ROOF & FLOOR 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 9
  10. 10. In virgin ground the ‘excess’ lateral stress is usually of a tectonic origin (Herget, 1988) and proportional to the rock stiffness. Effects of horizontal stresses are; Compressive type roof failures (commonly called cutter roof, guttering, snap top, and pressure cutting) In thinly bedded roof the failure develops as the progressive layer-by-layer crushing of the individual beds Directional effects, because of roof damage is generally much greater in entries oriented parallel to the maximumFig: Variation of Stresses in Different horizontal stress than in entries drivenlayers parallel with it Rock Stress COALVERTICAL Roof shear and bulking VERTICAL STRESS STRESS Rib squeeze Floor heave HORIZONTAL STRESS HORIZONTAL STRESS 10
  11. 11. Fig: General Concept of variation in roof conditions with drivagedirection in elevated horizontal stress Effect of Drivage Direction XX X XX XX XX XX XX XX XX XX XX XX XX XX X XX XX XX XX X XX X XX X XX X 11
  12. 12. Fig: Orientation of Galleries during Development w.r.t Horizontal Stress Mining Induced StressSIDE VIEW Existing Roadway Vertical Stress ConcentrationPLAN VIEW Existing Roadway Horizontal Stress Concentration 12
  13. 13. Junction FormationJunction Formation xxxxxxxxxxxxxxx Difficult Direction xxxx Good Direction xxxx xxxx xxxxxx Opening out on ‘good’ side Opening out on ‘bad’ side Turning through minimum stress Turning through maximum stress Stress - Folding Folding can lead to either an increase or a decrease in stress levels depending on where you are in the rock Stress Change Due to Folds or Rolls 13
  14. 14. Stress – Effect of Faulting Major Horizontal Stress(a) Change in DirectionPLAN VIEW F F F(b) Stress Concentration PLAN VIEW Major Horizontal Stress concentration F Anderson’s (1951) Normal faulting regions, where Sv>SHmax>Shmin Strike slip faulting regions, where SHmax>Sv>Shmin Reverse faulting regions, where SHmax>Shmin>Sv Stress Change Around Longwalls Goaf Goaf Vertical stress concentrated in front and side abutments Vertical stress concentrated in pillar between longwalls 14
  15. 15. PATTERN OF STRESS RE-DISTRIBUTION AFTER GALE-2008 Orientation of Longwall Panels With Maximum Horizontal Stress 15
  16. 16. Orientation of Longwall Panels With Maximum Horizontal StressOrientation of Longwall Panels With Maximum Horizontal Stress 16
  17. 17. Orientation of Longwall Panels With Maximum Horizontal Stress Horizontal Stress - Longwalls• Horizontal stress can not pass through gob area or broken or collapsed roof; therefore zones of stress relief and stress concentration are created• Their location depends on panel orientation, direction of retreat and sequence of extraction 17
  18. 18. Gate Road Stability with respect to Horizontal Stress (After Mark) Fig: Effect of Extraction Sequence w.r.t. Horizontal Stress 18
  19. 19. Fig: Horizontal Stress Concentration around Longwalls 19
  20. 20. Use of Sequencing to Stress relieve Entries Stress - SummaryUnderstanding stress and its effects is vital for good ground controlHorizontal stress effects are just as important as vertical stress effectsPlan ahead to avoid stress concentration effects where possibleTake precautions (e.g. extra supports) where stress concentration effectsare expected 20
  21. 21. Control of Horizontal Stress: Change panel orientationDuring development the galleries should be located in a direction parallel or moderate stress concentration zone w.r.t. Horizontal Stress Change panel extraction sequencePanels extraction sequence should be such as to bring the galleries under stress relief zone Reduce entry width Angled crosscutsAlign crosscuts parallel to Horizontal stress to improve stability Three-way intersections Horizontal stress - MeasurementInsitu Stress Measurement methods.Most widely used methods world over to ascertain “Magnitude andDirection”. Hydro Fracturing Method, and Over Coring Method Field Observations Stress Mapping – only Direction 21
  22. 22. Horizontal Stress Estimation In the absence of InsituMeasurements ν α EG S hav = Sv + ( H + 1000 ) 1 −ν 1 −νShav = Average horizontal in situ stress, MPa V = Poisson’s ratio of coal, varied from 0.19 to 0.23 α = Co-efficient of thermal expansion of rock = 30 x 10-6/ 0CE = Modulus of elasticity of coal, varied from 0.84 to1.70 GPa G = Thermal gradient, 0.030C/m γ = Unit rock pressure, 0.025 MPa/m H = Depth of cover, mThe above formula is useful when there is no influence of Topography Table: Horizontal Stress Recognition Features in Mines 22
  23. 23. Fig. Summary of “Stress Mapping” features.Table. Stress Mapping Features 23