Subsidence in coal mines

subsidence, upsidence, subsidence limits, components of subsidence, prediction of subsidence, analysis of subsidence, coal mines, control of subsidence, subsidence trough, harmonic extraction, abandoned mines subsidence prevention, subsidence prevention in working coal mines, factors affecting subsidence

1. 1
U.Siva Sankar
Sr. Under Manager
Project Planning
Singareni Collieries Company Ltd
E-Mail :[email protected]
[email protected]
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:

3. 3

4. 4
Fig. Subsidence from total extraction (long wall) mining of a
horizontal coal seam under a level landscape

5. 5
Mine subsidencecan be defined as movement of the ground surface as a result of
readjustments 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 subsidenceis 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 strataand an abrupt depression in the ground surface.
Sinkholesare typically associated with abandoned mine workings, since most
active underground mines operate at depths sufficient to preclude the development
of sinkhole subsidence.
Subsidence troughsinduced 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 Subsidence
Subsidence 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 influence
When 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 subsidence
On 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 Subsidence
Several 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 inclined
Competence of Mine Roof and Floor – strong or weak
Nature of Overburden
Near-surface Geology
Geologic Discontinuities – bedding planes, faults, folds, 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, monuments 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
PILLARPANEL

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2 L IN E 2 A L IN E 3 L IN E 3 A L IN E 4 L I N E 4 A L IN E
CENTRE LINE
29 371 10
90
M
5
M
7.5M
10M
11M
5M
PLAN SHOWING SUBSIDENCE PILLARS OVER LONGWALL
PANEL
BM
TOTAL NO.OF STATIONS : 812 NO.OF LINES ALONG STRIKE DIRECTION : 19
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
S
max
= h
e
*0.6(1+(W/H)/0.754)
-12.68
)
S
max
= Maximum subsidence for a given width to depth ratio ‘x’
h
e
= 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
S
max
= h
e
*0.65(1+(w/H)/0.75)
-8
)
S
max
= Maximum subsidence for a given width to depth ratio ‘x’
h
e
= 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 = K 1S/H
K1 = 2.2 + 24*e
-11.8 (X-1)
X = Width to Depth ratio/NEW
S = Subsidence in ‘mm’
H = Depth ‘m’
The equation for
compressive strainis
E(-) = K
2
(S/H)
K
2
=1.4+ 24*e
-14.3(X-1)
The equation for
tensile strain
E(+) = K
3
(S/H)
K
3
=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 seamscould be calculated using the
following empirical equation {NIRM, 2001}:
where,
S = Maximum subsidence, m
H = Average of minimum depths of the panels
W = Average width of the panels
he = 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, m
X = 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

17. 17
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 Method
It involves the widening
of the galleries from its side or sides.
•Pillar Splitting Method
Pillars are split and form number
of 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 Methods
Non- 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.