Horizonta stress

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REVIEW OF ROLE OF INSITU HORIZONTAL STRESS IN COAL MINES

U.Siva SankarSr. Under ManagerProject Planning

Singareni Collieries Company Ltd

E-Mail :[email protected] or [email protected]

Visit at:www.slideshare.net/sankarsulimella

Rock Stresses

Insitu (Virgin) StressesExist in the rock prior to any disturbance.

Induced Stresses Occurs after artificial disturbance e.g. Mining, Excavation, pumping, Injection, Energy extraction, applied load, swelling etc.

Residual Stresses •Diagenesis•Metasomatism•Metamorphism•Magma cooling•Changes in pore pressure

Tectonic StressesGravitational Stresses(Flat ground surface & topography effect)

Terresterial Stresses•Seasonal tpr. variation•Moon pull(tidal Stress)•Coriolis forces•Diurmal stresses

Active Tectonic StressesRemnant Tectonic Stresses Same as residual stresses but tectonic activity is involved such as jointing, faulting, folding and boundinage

Broad Scale •Shear Traction•Slab pull•Ridge push•Trench suction•Membrane stress

Local •Bending•Isostatic compensation•Down Bending of lithosphere•Volcanism and heat flow

Proposed by Bielenstein and Barron (1971)

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THE MINING ENVIRONMENT

IN-SITU STRESSES

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

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Vertical Stress

Comes from the weight of all the rock above

Increases with depth of cover

At 100m = 2.5MPa At 1000m = 25MPa

Equals depth x 0.025 MPa where depth is in metres

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Rock Stress, Strata and SupportRock stress, strata and support

Strata

Support

Stress Stress

Stress

Vertical and Horizontal stresses

Vertical Stress (after Brown and Hoek, 1978)

Townend and Zoback, (2000)

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Ratio of Horizontal to Vertical Stress

++=z

EK k

1001.0725.0

where Ek (GPa) is the average deformation modulus of the upper part of the earth’s crust measured in a horizontal direction.

Sheory,1994

EARTH’S CRUST

Beneath oceanic abyss : 6 km ThickContinental 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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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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Crustal Tectonic Platesof

Central Europe

Iceland

AtlanticRidge

(20mm/year)

Crustal Tectonic Plates of Central Asia

A f r i c a nP l a t e

I n d i a

E u r a s i a n P l a t e

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Insitu and Induced stresses and their Effects

• Mining operations modify the stresses acting on roc k– 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 corn ers

Rock Stress

HORIZONTAL STRESS LOADS THE ROOF AND FLOORVERTICAL STRESS LOADS THE RIBSHORIZONTAL STRESS LOADS THE ROOF AND FLOORVERTICAL STRESS LOADS THE RIBS

IF THESE INCREASED STRESSES EXCEED THE ROCK STRENGT H THE ROCK WILL FRACTURE AND FAIL

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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 act ing on the roadway

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Fig: Variation of Stresses in Different layers

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 horizontal stress than in entries driven parallel with it

Rock Stress

Floor heaveHORIZONTAL STRESSHORIZONTAL STRESS

Roof shear and bulkingCOAL Roof shear and bulking

Rib squeeze

VERTICALSTRESS

VERTICALSTRESS

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Fig: General Concept of variation in roof condition s with drivage direction in elevated horizontal stress

Effect of Drivage Direction

XXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXX

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Fig: Orientation of Galleries during Development w. r.t Horizontal Stress

Mining Induced Stress

SIDE VIEW

PLAN VIEW

Existing Roadway

Vertical Stress Concentration

Existing Roadway

Horizontal Stress Concentration

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Junction Formation

Junction Formation

xxxx

xxxxxxxxxx

Difficult Direction

Opening out on ‘bad’ side

Turning through maximum stress

xxxx

xxxxxxxxxxxxxxx

Good Direction

Opening out on ‘good’ side

Turning through minimum stress

Stress - Folding

Stress Change Due to Folds or Rolls

Folding can lead to either an increase or a decrease in stress levels depending

on where you are in the rock

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Stress – Effect of Faulting

F

(a) Change in Direction

PLAN VIEW F

Major Horizontal Stress

(b) Stress Concentration

PLAN VIEW

F

F

Major Horizontal Stressconcentration

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

Vertical stress concentrated in front and side abut ments

Vertical stress concentrated in pillar between long walls

Goaf

Goaf

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PATTERN OF STRESS RE-DISTRIBUTION AFTER GALE-2008

Orientation of Longwall Panels With Maximum Horizon tal Stress

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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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Gate Road Stability with respect to Horizontal Stre ss (After Mark)

Fig: Effect of Extraction Sequence w.r.t. Horizontal Stress

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Fig: Horizontal Stress Concentration around Longwal ls

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Use of Sequencing to Stress relieve Entries

Stress - Summary

Understanding stress and its effects is vital for g ood ground control

Horizontal stress effects are just as important as vertical stress effects

Plan ahead to avoid stress concentration effects wh ere possible

Take precautions (e.g. extra supports) where stress concentration effects are expected

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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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)1000(11

+−

+−

= HGE

SS vhav να

νν

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, 0.03 0C/m

γγγγ = Unit rock pressure, 0.025 MPa/m

H = Depth of cover, m

The above formula is useful when there is no influe nce of Topography

Horizontal Stress Estimation In the absence of Insi tu Measurements

Table: Horizontal Stress Recognition Features in Mi nes

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Fig. Summary of “Stress Mapping” features.

Table. Stress Mapping Features

Education

Horizonta stress