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

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)
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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 crust
IN-SITU STRESSES
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3. Vertical Stress
Comes from the weight of all the rock above
Increases with depth of cover
Equals depth x 0.025 MPa where depth is in metres
At 100m = 2.5MPa At 1000m = 25MPa
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4. Rock Stress, Strata and Support
Rock stress, strata and support
Strata
Stress
Support
Stress Stress
Vertical and Horizontal stresses
Vertical Stress (after Brown Townend and Zoback, (2000)
and Hoek, 1978)
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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 the
earth’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.
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6. THEORY OF PLATE TECTONICS OR
CONTINENTAL DRIFT
Earth’s crust is cracked into a series of
plates, which are moving around the earth’s
surface
Continents are composed of light materials
and they rest upon the moving plates
Plate edges occur along mid-oceanic
ridges where new crustal rock is being added
as molten material wells up from below
EFFECTS OF PLATE MOVEMENT
The oceans are widening/spreading at
the rate of 1 to 10 centimeters per year
The earth is not expanding
Crust is being destroyed at the plate
edges ( oceanic trenches)
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7. Crustal Tectonic Plates
of
Central Europe
Iceland
(20mm/year)
Atlantic
Ridge
Crustal Tectonic Plates of Central Asia
Eurasian Plate
Ind
ia
African
Plate
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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
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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
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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 maximum
Fig: Variation of Stresses in Different horizontal stress than in entries driven
layers parallel with it
Rock Stress
COALVERTICAL Roof shear and bulking VERTICAL
STRESS STRESS
Rib squeeze
Floor heave
HORIZONTAL STRESS HORIZONTAL STRESS
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11. Fig: General Concept of variation in roof conditions with drivage
direction in elevated horizontal stress
Effect of Drivage Direction
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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
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12. Fig: Orientation of Galleries during Development w.r.t Horizontal
Stress
Mining Induced Stress
SIDE VIEW
Existing
Roadway
Vertical Stress Concentration
PLAN VIEW
Existing
Roadway
Horizontal Stress Concentration
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13. Junction Formation
Junction 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
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14. Stress – Effect of Faulting
Major Horizontal Stress
(a) Change in Direction
PLAN 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
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15. PATTERN OF STRESS RE-DISTRIBUTION AFTER GALE-2008
Orientation of Longwall Panels With Maximum Horizontal Stress
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16. Orientation of Longwall Panels With Maximum Horizontal Stress
Orientation of Longwall Panels With Maximum Horizontal Stress
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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
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18. Gate Road Stability with respect to Horizontal Stress
(After Mark)
Fig: Effect of Extraction
Sequence w.r.t.
Horizontal Stress
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19. Fig: Horizontal Stress Concentration around Longwalls
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20. Use of Sequencing to Stress relieve Entries
Stress - Summary
Understanding stress and its effects is vital for good ground control
Horizontal stress effects are just as important as vertical stress effects
Plan ahead to avoid stress concentration effects where possible
Take precautions (e.g. extra supports) where stress concentration effects
are expected
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21. Control of Horizontal Stress:
Change panel orientation
During development the galleries should be located in a direction parallel or
moderate stress concentration zone w.r.t. Horizontal Stress
Change panel extraction sequence
Panels extraction sequence should be such as to bring the galleries under
stress relief zone
Reduce entry width
Angled crosscuts
Align crosscuts parallel to Horizontal stress to improve stability
Three-way intersections
Horizontal stress - Measurement
Insitu Stress Measurement methods.
Most widely used methods world over to ascertain “Magnitude and
Direction”.
Hydro Fracturing Method, and
Over Coring Method
Field Observations
Stress Mapping – only Direction
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22. Horizontal Stress Estimation In the absence of Insitu
Measurements
ν α EG
S hav = Sv + ( H + 1000 )
1 −ν 1 −ν
Shav = 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/ 0C
E = 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, m
The above formula is useful when there is no influence of Topography
Table: Horizontal Stress Recognition Features in Mines
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23. Fig. Summary of “Stress Mapping” features.
Table. Stress Mapping Features
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