Subsidence in coal mines
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Transcript of Subsidence in coal mines
Subsidence in coal mines
Subsidence can 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.
By: Aakash Deep Singhal (111MN0436)
Theories of subsidence
Vertical and normal theory Dome theory Beam or plate theory Trough theory continum theory Particulate theory
Terminology
Limit angle or angle of draw:the angle of inclination between thevertical at the edge of the workings andthe point of zero vertical displacementat the edge of the trough.Angle of break or angle of fracture:The inclination to the vertical of theline connecting the edge of the minedarea with the surface point exhibitingthe maximum tensile strainInflection point:On the major cross-section of thesubsidence basin, the point dividing theconcave and convex portions of thesubsidence profile is called theinflection point. At the inflection pointthe subsidence is equal to half of themaximum possible subsidence at thecenter, the surface slope is maximumand the curvature is zero.
Several geologic and mining parameters and the nature of the structure affect themagnitude and extent of subsidence that occur due to coal mining
Factors Affecting Mine Subsidence
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 Time Elapse Structural Characteristics of buildings, monuments
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 – Board & Pillar , long
wall Rate of Face Advance Backfilling of the Gob
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 Time Elapse Structural Characteristics of buildings, monuments
etc
Damages due to subsidence
Methods of subsidence control
By suitable design of surface structure
By surface stabilization.
- grout column - grout case
- piers constructed within the mine - deep foundations
Filling methods for void elimination
- hydraulic filling or back filling - dowel process
- pneumatic filling - fly ash injection
- grouting - over excavation and backfilling
- blasting the rock in the roof and floor of the mine
Underground methods of subsidence control 1. Single face advance a. directional control b. rate of advance c. location 2. Harmonious extraction a. multiple seam b. stepped face 3. Partial extraction a. panel and pillar b. yield pillar 4. Back filling
Measurement Techniques
Surface observations
Sub surface measurement
Layout of subsidence stations
Routine measurements by levelling of vertical components
Measurement of magnitude and direction of principal surface strains
Subsurface measurement of subsidence
- wireline method
- Time domain reflectometry (TDR) method
- Mechanical grouting method
Measurement of Subsurface Strata Movement
Mainly consists of monitoring strata separation as a function of face location Can be done by either underground or surface boreholes U/G boreholes are usually drilled in the roof, then roof sag measurements devices are installed
for monitoring strata With the surface borehole technique, NX-sized boreholes are drilled from the surface all the
way down to the coal seam to be mined Movement of strata at different horizons above the seam is then monitored from surface as a
function of face location using any of the following methods
Wireline Method
This technique makes use of electronic logging devices commonly used in oil fields A bullet perforator is lowered into the well, positioned at the desired level & fired into the wall
of the well by means of a surface control A small amount of radioactive material is inserted in the bullet, so when it is shot into the strata
surrounding the borehole & remains there strata movement can be followed Bullets are shot into various strata along the borehole, and their positions are identified with
high peaks of intensity in a radioactive log The change in a bullet’s position indicates the amount & direction of movement of stratum in
which bullet is inserted
TDR Method
TDR or Time Domain Reflectometry, works on the same principle as radar A good cable is grouted in the borehole all the way down to the coal seam Caving or separation of the roof strata create some sort of faults in the cable An ultrafast rise time voltage step is sent down the cable, which is reflected by the
fault A sampler picks it up & superimposes on incoming signal resulting in a step-up or
down which can be seen on CRT The time delay between the initial signal & the arrival multiplied by the travel velocity
indicates the distance where fault occurs.
Subsidence prediction methods
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 anymaterial, bedding plane, anisotropy, etc.
SUBSIDENCE PREDICTION- EMPIRICAL METHODSThe relation between maximum subsidence, Non-effective width, depth andheight of extraction and other parameters recommended by NIRM is presentedbelow:LONGWALL METHODSmax = he*0.6(1+(W/H)/0.754)-12.68)
BORD & PILLAR METHODSmax = 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’
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 in case of closely spaced multiple seams could 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
Numerical ModellingThe 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 behavior of coal, coal measure strata, goaf, bed separation and re-contact, roof failure mechanism, goaf behavior 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.
Subsidence Profile over multiple number of Bord & Pillar Panels of a Coal Mine
References Subsidence in coal mines (pg. no. 448-488) R D SINGH-
New age international ltd. Publishers
http://www.dep.state.pa.us/MSI/WhatIsMS.html
http://www.slideshare.net/sankarsulimella/subsidence-in-coal-mines
en.wikipedia.org/wiki/Tectonic_subsidence
http://en.wikipedia.org/wiki/Subsidence