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1 U Siva Sankar Email: [email protected] Ground Control : A collective term given to the techniques that are used to regulate and prevent the collapse and failure of mine openings. Ground control is the science that studies the behaviour of rockmass in transition from one state of equilibrium to another. It provides the basis for the design of the support systems to prevent or control the collapse or failure of the roof, floor, and ribs both safely and economically. Ground pressure - The pressure to which a rock formation is subjected by the weight of the superimposed rock and rock material or by diastrophic forces created by movements in the rocks forming the earth's crust. Such pressures may be great enough to cause rocks having a low compressional strength to deform and be squeezed into and close a borehole or other underground opening not adequately strengthened by an artificial support, such as casing or timber. Mining Ground Control
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Ground control in undergound mines

Sep 01, 2014

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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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Page 1: Ground control in undergound mines

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U Siva Sankar

Email: [email protected]

Ground Control : A collective term given to the techniques that are used

to regulate and prevent the collapse and failure of mine openings.Ground control is the science that studies the behaviour of rockmass in

transition from one state of equilibrium to another.

It provides the basis for the design of the support systems to prevent or

control the collapse or failure of the roof, floor, and ribs both safely and economically.

Ground pressure - The pressure to which a rock formation is subjected

by the weight of the superimposed rock and rock material or by

diastrophic forces created by movements in the rocks forming the earth's

crust. Such pressures may be great enough to cause rocks having a low compressional strength to deform and be squeezed into and close a

borehole or other underground opening not adequately strengthened by

an artificial support, such as casing or timber.

Mining Ground Control

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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)

Insitu and Induced stresses and their representatio n on Mohr’s Circle

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THE MINING ENVIRONMENT

IN-SITU STRESSES

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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 stressesvirgin 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:

hz γσ =

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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

vhK σσ=

Horizontal stressLithostatic stress occurs when the stress components at a point are equal in all directions and their magnitude is due to the weight of overburden.

zyx σσσ ==The other assumption is that rock behaves elastically but is constrained from deforming horizontally.

This applies to sedimentary rocks in geologically undisturbed regions where the strata behave linearly elastically and are built up in horizontal layers such that the horizontal dimensions are unchanged. For this case, the lateral stresses σx and σy are equal and are given by:

)1(.

µµσσσ−

== zyxTerzaghi and Richart

(1952)

Later this relation found to be not true as Horizontal stress isalways more than vertical stress

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Vertical and Horizontal stresses

Vertical Stress (after Brown and Hoek, 1978)

Townend and Zoback, (2000)

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

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VERTICAL STRESS CONCENTRATED IN RIBS

HORIZONTAL STRESS CONCENTRATED IN ROOF & FLOOR

Cover - The overburden of any deposit.

Overburden – Layers of soil and rock covering a coal seam.Overburden is removed prior to surface mining and replaced after the coal is taken from the seam.

Lithology - The character of a rock described in terms of its structure, color, mineral composition, grain size, and arrangement of its component parts; all those visible features that in the aggregate impart individuality of the rock. Lithology is the basis of correlation in coal mines and commonly is reliable 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; the overhead surface of a coal working place. Same as "back" or "top."

Characteristics of Coal Measure Roof Strata

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Immediate Roof

The roof strata that is immediately above the coal seam. This is the strata requires support for the mine openings to remain competent. Primary roof - The main roof above the immediate top. Its thickness may vary from a few to several thousand feet.

Secondary roof - The roof strata immediately above the coalbed, requiring support during the excavating of coal.

Competent rock - Rock which, because of its physical and geological characteristics, is capable of sustaining openings without any structural support except pillars and walls left during mining (stalls, light props, and roof bolts are not considered structural support).

Characteristics of Coal Measure Roof Strata

Fissure - An extensive crack, break, or fracture in the rock s.

Fracture - A general term to include any kind of discontinuit y in a body of rock if produced by mechanical failure, whe ther by shearstress or tensile stress. Fractures include faults, shears, joints, and planes of fracture cleavage.

Joint A discontinuity in the rock strata where there is n o sign of relative movement.A divisional plane or surface that divides a rock a nd along whichthere has been no visible movement parallel to the plane or surface.

CleatThe vertical and Parallel cleavage planes or partin gs crossing the bedding. The main set of joints along which the coa l breaks moreeasily than in any other direction.Face cleat - The principal cleavage plane or joint at right ang les to the stratification of the coal seam.

Characteristics of Coal Measure Roof Strata

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Butt cleat - A short, poorly defined vertical cleavage plane in a coal seam, usually at right angles to the long face clea t.

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 earth's 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 faul t zone consists of numerous interlacing small faults or a confused zone of gouge, breccia, or mylonite.

Characteristics of Coal Measure Roof Strata

Fig: Joints exposed in the sandstone roof

Fig: orientation of Cleats and coal seams

Fig: Face and Butt Cleats in the Coal Pillar

Fig.: Influence of Joints

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Reverse Fault

Normal Fault

Strike Slip Fault

Slickensides along the slip plane

Sandy shaleoverShale

StronglyJointed

Sandstoneovershale

Sandy shaleover

Sandstone

Sand stone

Classifications of typical coal measures roof strat a (modified after Peng & Chiang, 1984)

Characteristics of Coal Measure Roof Strata

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

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Stress Distribution Above a Small Mine Opening Pressure arch formation around mine

opening (After Dinsdale, 1937)

PRESSURE ARCH CONCEPTArching - Fracture processes around a mine opening, leading to stabilization by an arching effect.

Abutment PressuresWhen an opening is created in a coal seam, the stress that was present before the opening was created is re-distributed to the adjacent coal pillars that are left. The areas within the remaining coal where the vertical stress is greater than the average are called abutments 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

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�For very wide openings such as those created by longwall mining, major pressure arch formation is likely to create points of excessive pressure in seams above and below

�Arching stresses can either hinder or benefit mining in overlying or underlying seams. �The extradosal ground forms the zone of high compressive stress that can cause ground control problems in the roof, floor and pillars. � The intradosal ground or tension zone is actually a distressed region in relation to the surrounding strata and conceivably the stress encountered in this zone may actually be less than that created by the cover load.

Formation of major pressure arches due to Longwall Mining

(After Stemple, 1956)

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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Idealized Ground Response Curve 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.

Idealized Ground Response Curve (GRC) and support l ine

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Ground Reaction Curve approximation for outby loadin g conditions in a longwall tailgate ( Barczak, et.al;)

Strength = P/A

where, P= Load to break rock

A= Area

Stiffness = Load per unit area(σσσσ) / Strain(εεεε)

Strain = ∆∆∆∆L/ L

This is expressed as the modulus of elasticity or Young’s modulus (E), so,

E = σσσσ/εεεε

As εεεε is dimensionless, E has the same unit as σσσσ. As the number becomes very large, it is usually expressed in Giga-Pascals (GPa)

1 GPa = 1000 MPa

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STRESS AROUND A ROADWAY

HORIZONTAL STRESS LOADS THE ROOF AND FLOOR

AFTER EXCAVATION, HORIZONTAL STRESSES CONCENTRATE INTHE STIFF (BRITTLE) BEDS IN THE ROOF AND FLOOR

PROPAGATION OF FAILURE ABOUT

ROADWAYS

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AS THE LOWER ROOF BEDS SOFTEN, STRESSES ARE REDISTRIBUTED INTO HIGHER STIFF BEDS

UNBOLTED ROOF

THIS FAILURE ZONE WILL CONTINUE TO MIGRATEFURTHER INTO THE ROOF

IF NO REINFORCEMENT IS INSTALLED

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UNBOLTED ROOF

EVENTUALLY A LARGE FAILURE ZONE WILL FORM ABOVE EXCAVATION

UNBOLTED ROOF

IF UNSUPPORTED THIS WILL LEADTO A FALL OF GROUND

FORMING A NATURAL ARCH

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KEY FEATURES OF ROADWAY BEHAVIOUR

• State of stress acting on a roadway is influenced b y– Geological structure– Variation in lithology– Topography– Seam structure (warps/rolls, etc.)– Tectonic setting

• It may be kept in mind that roadways are often deve loped in a modified stress field as a result of adjacent wor kings, overlying/ underlying workings, in abutment areas d ue to pillar/ longwall extraction, etc.– While analyzing a situation, these influences must be given

due importance

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Effect of Horizontal Stress on Stability of Galleri es in Mines

Ground Control Practices and Constraints

To 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 dumps

Constraints: Sometimes rock mechanics principles are need to be completely ignored in normal mining operations such as Coal Extraction, Coal haulage, and Ventilation

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�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 Practices

Some ground control techniques serve more than one of the above functions. For example, a rock bolt may provide for alteration of, and resistance to, ground stress.

Avoidance Stress is avoided in the first place by aligning entries, headings, and boreholes to miss treacherous fault zones, dykes, sills, old workings, and zones of subsidence by a wide margin. When a problem fault must be traversed, the heading is aligned to meet it at near a right angle, rather than obliquely. Stress concentration is avoided by rounding the corners in a rectangular heading. Excavation shape Tensile and bending stresses are altered to compressive stresses when the back of a heading is arched. The same is true of a shaft or raise that is changed from a rectangular to a circular cross-section. Reinforcement The ability of the rock mass to resist shear, tensile and bending stress is reinforced when a cable bolt is tensioned because the friction in joints and fractures is increased.

Ground Control Techniques or Practices

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Reduction The ground stress around one heading arising from its proximity to another opening is reduced by a protective pillar (safe distance) between them. The magnitude of the ring stress is reduced (and displaced) if a circular shaft or raise is advanced by drilling and blasting instead of raiseboring, because the fractured zone “pushes” the peak stress some distance into the solid rock. Controlled (“smooth wall”) blasting techniques are used to minimize overbreak and crack propagation; however, their introduction to highly stressed ground may have another, negative effect (ring stress concentration). To reduce stress in deep shaft sinking, it is typical that smooth wall blasting is abandoned near the horizon where discs were first observed in the pilot hole drill core. Resistance Stresses are resisted with ground support. The support may consist of sets (wood or steel), rock bolts, cable bolts, shotcrete, screen, strapping, or concrete. Ground support is commonly evaluated for comparison purposes by the average pressure that it is calculated to exert against the rock face.Displacement

Ground Control Techniques or Practices

Isolation In deep mining, perimeter headings may first be driven around a stoping block to avoid wrongful stress transfer and minimize stress buildup in stope ends.

e.g.1: At the current South Deep project in South Africa, the shaft pillar at the reef horizon was deliberately mined out before shaft sinking could reach it. e.g.2: It was proposed (W. F. Bawden) that a ring heading around an existing shaft will isolate it from stresses induced by future mining in the near vicinity.

De-stressingDe-stressing displaces stress away from the walls of an entry or heading and into country rock. When properly executed, de-stressing creates a failure envelope that shunts stress away from the excavation.

Ground Control Techniques or Practices

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Various approaches for development of strata contro l techniques

Typical layout of a Conventional depillaring panel with manner of pillar extraction.

Caving Mechanisms – Strata Mechanics – B&P

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Conceptual Models of Loading & Caving of overlying Roof Strata in Bord & Pillar Caving Panel

Caving Mechanisms – Strata Mechanics- B&P

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AMZ includes all of the pillars on the extraction front (or "pillar line"), and extends outby the pillar line a distance of 2.76 times the square root of the depth of cover expressed 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.

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When a gob area is created by full extraction mining (depillaring), abutment loads are transferred to the adjacent pillars or solid coal;

The abutment stresses are greatest near the gob, and decay as the distance from the gob increases;

HD 14.5=From experience and from numerical analysis it is found that the front abutment load reaches to zero at about a distance give by the following equation

Layout of Longwall Workings

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General pattern of Vertical and Horizontal stress redistribution

(Gale 2008)

Forces on supports due to lateral strata movement. (a) Weak roof -- horizontal force acting away from f ace.(b) Strong roof -- horizontal force acting towards f ace. Adapted from Peng et al. [1987].

Vertical stress distributions at seam level around single longwall face (Brady and Brown 1992)

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Three zones in overburden due to longwall mining (Chekan at al., 1993)

Distinct Zones in Overburden of an Longwall Opening

Bending of strata is gradual and distributed over a large horizontal distance, without causing any major cracks.

50HL toSurface

Sagging zone

Strata are broken into blocks by fractures and cracks due to bedseparation; bending is not as abrupt and fractures are less pronounced

20-50HL

Fracturing zone

Strata may separate along planes and fracture or joints may open; individual beds remain intact and displacements are less likely to occur.

12-20HL

Upper limit ofcaving zone

Strata have significant degree of bending, leading to intense fracturing or displacement.

6-12HL

Partial cavingregion

Strata fall onto mine floor, broken into irregular, platy shapes of various sizes, crowded in random manner.

3-6HL

Completecaving region

Caving zone

StrataCharacteristics

Ranges in

Thickness

Zones

Note: HL = The mining height lower seam.

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Bulking factor controlled caving of weak and laminated overlying strata.

Parting plane controlled caving of strong and massive overlying strata.

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.

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.

Caving Mechanisms – Strata Mechanics

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.

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Parameters Influencing CavabilityParameters Influencing Cavability

σσh h (impedes caving)(impedes caving)

σv

J1

J2J2

J1 = SubJ1 = Sub--horizontal joint sets: Essential for cavinghorizontal joint sets: Essential for caving

J2 = SubJ2 = Sub--vertical joint sets: Augments cavingvertical joint sets: Augments caving

Stress relief zoneStress relief zone

ExcavationExcavation

� Tensile strength : Tensile strength : Measured in laboratoryMeasured in laboratory

��Rock densityRock density : Almost constant: Almost constant

��Horizontal stressHorizontal stress : Measured in field: Measured in field

��Identification of layers and their thicknessIdentification of layers and their thickness : : DifficultDifficult

Parameters Influencing Caving SpanParameters Influencing Caving Span

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• Geometry of the discontinuities.

•Shear and tensile strength of the discontinuities,

•Strength of rock materials and in- situ stress field

Natural features that influence cavability of rock are

•Undercut span

•Boundary slots, and

•Mass weakening by creating fractures

Cavability can be enhanced by a set of induced features

High horizontal stresses inhibit the roof caving.

�The caving height is low

�Caving occurs after a long face advance

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�Hydraulic fractures in the roof can stimulate the caving.

�Without creating fractures in the massive roof, the caving height would be low and the caving would occur after longer face advance

� A fracture of large area in massive roof must be created so that its area increases progressively to initiate caving of the roof strata.

Caving Mechanism in B&P Panels

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Caving Mechanism in B&P Panels – Local, Main & Perio dic 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.

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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.

Main Fall

For 150m Longwall face length, Mainfall fall is taki ng 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

TENSILE FRACTURES

CRACKS STARTS TO FORM IN MID SPAN

Periodic Fall

Periodic falls occur at 18 to 25m and 10 to 16m prog ress intervals for coal and sandstone as immediate roof conditions respecti vely