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