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U.Siva SankarSr. Under ManagerProject Planning

Singareni Collieries Company Ltd

E-Mail :ulimella@gmail.com or uss_7@yahoo.com

Visit at:www.slideshare.net/sankarsulimella

� Subsidence occurs when large areas of coal are mined and the resulting settlement of roof material into the void (the goaf) results in the surface subsiding over the affected area.

� More commonly there is a gradual lowering of the surface strata which actually bends rather than fractures at the limits of the subsiding area.

� This bending leads to tensile strains in the surface strata (and possibly in structures on the surface) which may result in the formation of cracks.

� It is around the edges of the subsiding area where damage may occur. � The central area of subsidence usually is subjected to a gradual lowering,

possibly suffering some tilt and strain as the workings pass beneath. � This may cause damage to such items as roads and pipelines but this is

easily repaired and there is little evidence of it being a subsidence area after movement ceases.

� Structures, including houses, built in mining areas should be designed to accept a small degree of tilt and strain on a short term basis without suffering major damage.

"Upsidence" is also a surface phenomenon associated with mining and subsidence and occurs where workings pass beneath a gorge or similar surface feature causing a concentration of horizontal stress in the strata between the bottom of the feature and the top of any goaf cavity. This increased stress may cause strata beds close to the surface to bend upwards and possibly fracture.

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Figure Schematic developments of mine effects.

Subsidence Phenomenon:

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Fig. Subsidence from total extraction (long wall) mining of a horizontal coal seam under a level landscape

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Mine subsidence can be defined as movement of the ground surface as a result ofreadjustments of the overburden due to collapse or failure of underground mine workings. Surface subsidence features usually take the form of either sinkholes or troughs.

Sinkhole subsidence is common in areas overlying shallow room-and-pillar mines. Sinkholes occur from the collapse of the mine roof into a mine opening, resulting in caving of the overlying strata and an abrupt depression in the ground surface.

Sinkholes are typically associated with abandoned mine workings, since most active underground mines operate at depths sufficient to preclude the development of sinkhole subsidence.

Subsidence troughs induced by room-and-pillar mining can occur over active or abandoned mines. The resultant surface impacts and damages can be similar, however the mechanisms that trigger the subsidence are dramatically different. In abandoned mines, troughs usually occur when the overburden sags downward due to the failure of remnant mine pillars, or by punching of the pillars into a soft mine floor or roof.

Components of SubsidenceSubsidence consists of five major

components, which influence damage to surface structures and renewable resources

�Vertical displacement (settlement, sinking, or lowering).

�Horizontal displacement (lateral movement).

� Slope (or tilt), i.e., the derivative of the vertical displacement with respect to the horizontal.

� Horizontal strain, i.e., the derivative of the horizontal displacement, with respect to the horizontal.

� Vertical curvature (or flexure), which may be approximated by the derivative of the slope, or the second derivative of the vertical displacement with respect to the horizontal.

Fig. Schematic of ground movements caused by subsidence

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limit angle or angle of draw:the angle of inclination between the vertical at the edge of the workings and the point of zero vertical displacement at the edge of the trough.

angle of break or angle of fracture:The inclination to the vertical of the

line connecting the edge of the mined area with the surface point exhibiting the maximum tensile strain

Inflection Point:On the major cross-section of the subsidence basin, the point dividing the concave and convex portions of the subsidence profile is called the inflection point. At the inflection point the subsidence is equal to half of the maximum possible subsidence at the center, the surface slope is maximum and the curvature is zero.

Fig. Schematic of ground movements caused by subsidence

Terminology of Subsidence

Angle of major influenceWhen the opening or gob has reached the critical size the major surface deformations occur on both sides of the inflection point within a certain distance. This distance is called the radius of major influence. Beyond this distance surface deformations are very small. The angle of major influence is the angle between the horizontal and the line connecting the inflection point and the edge of the radius of major influence. (tanβ =h/r where h is the mining depth).

Angle of full subsidenceOn a major cross-section of the subsidence basin under super critical width of mining the acute angle between the horizontal and the line connecting the edge of the flat bottom of the subsidence basin and the edge of the opening is called the angle of full subsidence. It indicates the degree of subsidence development and can be used to define the area within which subsidence has been fully developed.

Fig. Sketch depicting area of influence. Maximum subsidence at P ' by mining entire area of influence.

Terminology of Subsidence

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Critical Subsidence : Subsidence reaches the maximum possible value at the center. This area is obtained if the lines of draw plotted from the opposite sides of the excavation meet at the surface. This is also called “Full area”.

• Subcritical Subsidence: For such a case, no point on the surface shows the maximum subsidence.

• Supercritical Subsidence: The width and length of the opening continue to increase even after occurrence of critical subsidence. The maximum possible subsidence doest not increase, but spread laterally into an area.

Terminology of Subsidence

NEW- Non Effective Width: of extraction is the underground width of extraction which does not cause practically any movement on the surface. In India NEW varies between 0.3 to 1.17 times the depth of extraction.

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Factors Affecting Mine SubsidenceSeveral geologic and mining parameters and the nature of the structure affect the

magnitude and extent of subsidence that occur due to coal mining� Effective Seam Thickness� Multiple Seams� Seam Depth� Dip of Seam – flat, moderately inclined, steeply inc lined� Competence of Mine Roof and Floor – strong or weak� Nature of Overburden� Near-surface Geology� Geologic Discontinuities – bedding planes, faults, f olds, etc� Fractures and Lineaments� In Situ Stresses- vertical and Horizontal stresses� Degree of Extraction� Surface Topography – flat, sloping, hilly area� Groundwater� Water Level Elevation and Fluctuations� Mined Area- sub critical, critical, super critical� Method of Working – Bord & Pillar , longwall� Rate of Face Advance� Backfilling of the Gob� Time Elapse� Structural Characteristics of buildings, monument s etc

Fig: Effect of Dip of Coal Seams

Fig: Subsidence in Multiple Coal Seams

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Effect of Sloping Ground on Surface Subsidence

Generalized surface subsidence Vs time curve obtained from field study

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SURFACE DAMAGES DUE TO SUBSIDENCE

• Surface subsidence manifests itself in three major ways:

• Cracks, fissures, or step fractures.

• Pits or sinkholes.• Troughs or sags.

SURFACE DAMAGES DUE TO SUBSIDENCE

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Various subsidence monument designs

Subsidence Measurement

Subsidence Pillars

A R I ERB R

G

O

A

F

B

A

RR

IE

R

B A R R I E R

PLAN SHOWING SUBSIDENCE PILLARS OVER BORD AND PILLAR PANEL

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2LIN E 2A LIN E 3LIN E 3A LIN E 4LIN E 4A LIN E

CENTRE LINE

29 371 10

90M

5M

7.5M

10M

11M

5M

PLAN SHOWING SUBSIDENCE PILLARS OVER LONGWALLPANEL

BM

TOTAL NO.OF STATIONS : 812 NO.OF LINES ALONG STRIKE DIRECTION : 1 9

NO.OF LINES ALONG DIP-RISE DIRECTION : 6 SPACING BETWEEN ” ” :5M

SPACING BETWEEN ” ” :10M

FIRST SUBSIDENCE CRACKS (7CM WIDTH)

MAX.SUBSIDENCE: 2.4M

500M X 62M

100M

Direction of Advance

� Theoretical methods: Use of continuum mechanics concepts of elastic,

plastic or elastic-plastic material properties of overburden strata

� Profile function method: Profile functions are developed based on

measured subsidence data. There are about 20 profile functions are

developed in all over the world.

� Influence function method: Incorporates the mathematical modeling of

influence function

� Zone Area Method

� Empirical Modeling: Based on the measured subsidence data empirical

models are developed.

� Physical Modeling: Parametric study of the subsidence prone area

� Numerical Modeling : The most popular technique and cheaper method for

estimating surface subsidence and displacements. It can incorporate any

material, bedding plane, anisotropy, etc.

Subsidence Prediction Methods

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Subsidence Prediction- Empirical Methods

The relation between maximum subsidence, Non-effective width, depth and height of extraction and other parameters recommended by NIRM is presented below:

Longwall method

Smax = he*0.6(1+(W/H)/0.754)-12.68)Smax = Maximum subsidence for a given width to depth ratio ‘x’he = Effective height of extraction (Height of extraction x % of extraction)W = Width of the panel, ‘m’H = Depth of the panel, ‘m’

Bord & Pillar Method

Smax = he*0.65(1+(w/H)/0.75)-8)Smax = Maximum subsidence for a given width to depth ratio ‘x’he = Effective height of extraction (Height of extraction x % of extraction)W = Width of the panel, ‘m’H = Depth of the panel, ‘m’

Further, the equation for slope (G) is G = K1S/H

K1 = 2.2 + 24*e-11.8 (X-1)

X = Width to Depth ratio/NEWS = Subsidence in ‘mm’H = Depth ‘m’

The equation for compressive strain isE(-) = K2 (S/H)

K2=1.4+ 24*e-14.3(X-1)

The equation for tensile strainE(+) = K3 (S/H)K3=1.35+ 28*e -19.4(X-1)

Subsidence Prediction- Empirical Methods

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Fig. Damage to structures from subsidence related to strain and structure length (after National Coal Board 1975ab)

Subsidence in case of closely spaced multiple seams could be calculated using the following empirical equation {NIRM, 2001}:

where,S = Maximum subsidence, mH = Average of minimum depths of the panelsW = Average width of the panelshe = Total extraction thickness X % of extraction

Sheorey et al., 2000, suggested the following equation for predicting the subsidence for multiple seam cases :

where,S = Maximum subsidence, mX = Ratio of width to depth ratio and Non Effective Width

Subsidence Prediction- Empirical Methods

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Brief description of Subsidence Prediction Methods

Numerical Modelling

• The numerical method for prediction of surface subsidence is now gaining popularity over the profile or influence function due to its capability to considered geological complexities, irregular shaped structures, complex constitutive behaviour of coal, coal measure strata, goaf, bed separation and re-contact, roof failure mechanism, goaf behaviour etc. It has a capability to consider sequential excavation process in the simulation. This will give realistic results in terms of subsidence as well as strain.

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Subsidence Profile over multiple number of Bord & Pillar Panels of a Coal Mine

Subsidence Control Measures

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Different Methods for Subsidence Control

PARTIAL EXTRACTION METHODS

Partial extraction methods can be classified in to two groups

• Pillars and overburden both are stable� Wide stall method

� Pillar splitting method

• Pillars are stable whereas overburden may fail� Non-effective width (NEW) extraction� Chess board method� Goaf pillar method

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PARTIAL EXTRACTION METHODS

• Wide Stall MethodIt involves the wideningof the galleries from its side or sides.

• Pillar Splitting MethodPillars are split and form numberof stooks.

PARTIAL EXTRACTION METHODS

� Chess board method

� Goaf and pillar method

� Plan of panel designed by extracting diagonal rows of pillars.

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HYBRID METHOD OF PARTIAL EXTRACTION

“It is a combination of Wide stall method and pillar splitting method”

Caving Methods Partial Extraction MethodsNon- Effective Width (NEW)

� NEW should be less than Equivalent width for different panel extraction shapes of Caving and partial extraction methods to prevent surface subsidence

� NEW for different coal seams based on Rockmass factor of the overlying roof rocks is also to be considered

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Harmonic Method of Extraction�Extraction of a panel causes tensional and compressive strain at the surface.

�The working of two seams should be so advanced simultaneously to cancel out the balance of strain, caused by face by the strain induced by another at a different level.

� This approach is known as harmonic mining, which however is not simple

�Because the mine has to be preplanned and also problems due to interaction between faces in different seams have to be encountered.