METHODS FOR SUBSIDENCE PREDICTION AND CONTROL MINE PORTAL TEAM www.MINEPORTAL.in CALL/WHATSAPP-8804777500 www.fb.com/mineportal.in

What is Mine Subsidence Mine subsidence is the collapse of the ground surface over areas where coal or mineral ores were removed.

Modes of Subsidence Trough subsidence Sinkhole subsidence Source: www. aegweb.org, Retrieved 23-08-2011

Source : www.dep.state.pa.us, Retrieved 23-08-2011

Component of Subsidence Vertical Displacement Horizontal displacement Slope (or tilt) Horizontal strain Vertical curvature (or flexure)

Source : www.qjegh.lyellcollection.org, Retrieved 22-08-2011

Development of Dynamic Subsidence Profiles as the Face Advances Source : www. dep.state.pa.us, Retrieved 22-08-2011

Terminology of Subsidence limit angle or angle of draw angle of break or angle of fracture Inflection Point Critical Subsidence Subcritical Subsidence Supercritical Subsidence Non Effective Width

limit angle or angle of draw Source : Geofacts no. 12, Ohio Department of Natural Resources, 2010

Standard Subsidence Curve Source : www.colorado.edu,2011

Critical Subsidence Source : www.dep.state.pa.us,2011

Subcritical Subsidence Source : www.dep.state.pa.us, Retrieved 22-08-2011

Supercritical Subsidence Source : www.dep.state.pa.us, Retrieved 23-08-2011

Subsidence Prediction Methods Theoretical models Elastic Theory Plastic Theory Stochastic Theory Influence function method Profile function method Empirical Methods GIS-Based Subsidence Prediction Method

THEORETICAL MODELS Elastic Theory following equation is used in this method- Zu(x, y) = ultimate subsidence at any point ( x,y ) on the ground surface b = vertical closure of the opening D = depth to the top of the opening

Elastic Theory The notation is used to represent an equation of the form: f (U,V) =f (U2,V2) −f (U2 ,V1) − f (U1 ,V2) +f (U1 ,V1) U1 = Xc – w/2 – x U2= Xc + w/2 – x V1 = Yc – L/2 – y V2 = Yc + L/2 – y where (Xc, Yc) are the coordinates of the center of the underground opening represented by a parallelepiped where L and W are its length and width, respectively

Plastic Theory a = ratio of vertical subsidence to mine closure M = room height H = depth to mine horizon x = distance from the center of subsidence l = opening width Vzm = rate of subsidence & η = kinematic stiffness of the rock body.

Stochastic Theory The stochastic theory is a statistical approach that treats the rock body as a cohesionless medium. A rock mass that contains numerous and pervasive fractures may behave as a material with zero cohesion, The stochastic theory states that when a volume of rock is removed (an extraction element), it is replaced by an equal volume of rock lying above. The resultant particle movement at the ground surface is predicted by a bell-shaped probability curve representing the subsidence profile.

Influence function method Use to describe amount of influence exerted at the surface by infinite small elements of the extraction area. Total subsidence is the sum of the elementary troughs of all extraction elements according to their position. Advantages- Not confined to rectangular extraction. Able to negotiate superposition problems.

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 H = Depth of the panel

Subsidence Prediction- Empirical Methods 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’

Safe limits of subsidence Railways lines Water bodies Buildings Aerial ropeway and electric transmission high tension pylons Roads No movement permitted Maximum tensile strain is 4.5 mm/m Max. total elongation or compression is 60 mm Strain 3mm/m Slope to the extent that the displacement of the top most point should not be more than one third of the radius of the base No stepping due to subsidence

Different Methods for Subsidence Control Working Mine Plane Fitting Trench Around Houses Tension cable Hydraulic Sand Stowing Partial Extraction Method Non-Effective Width Chess Board Goaf Pillar Wide and Stall Splitting of Pillar with Stowing Splitting of Pillar with side bolting Harmonic mining

Different Methods for Subsidence Control Abandoned Mine Point Support Method Gravel Column Flyash Grout Injection Fabric Formed Concrete Areal Backfilling Pumped Slurry Injection Flyash Slurry Injection Pneumatic Flyash Injection

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 extraction Chess board method Goaf pillar method

PARTIAL EXTRACTION METHODS Wide Stall Method It involves the widening of the galleries from its side or sides.

PARTIAL EXTRACTION METHODS Pillar Splitting Method Pillars are split and form number of stooks.

PARTIAL EXTRACTION METHODS Chess board method

PARTIAL EXTRACTION METHODS Goaf and pillar method

PARTIAL EXTRACTION METHODS Plan of panel designed by extracting diagonal rows of pillars.

HYBRID METHOD OF PARTIAL EXTRACTION It is a combination of Wide stall method and pillar splitting method.

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 pre-planned and also problems due to interaction between faces in different seams have to be encountered.

A Case Study: An abandoned mine in Seita Reservoir area, KitaKyushu, JAPAN Source : Synthesis Subsidence Prediction Method, Institute of Environmental Systems, Kyushu University, Fukuoka, Japan

Seita Reservoir area 200 m 1200 m 3D view of spatial distribution of coal seam layers with DEM around Seita reservoir area

observation points P4 P3 P5 P2 P6 P1 P7 P8 Subsidence contour up to 1997 with observation points

Seam Layer 1 Seam Layer 2 Seam Layer 3 Seam Layer 4

Comparison of calculated and measured subsidence value

subsidence value simulation

Maximum subsidence = 3270 mm 3D view of final subsidence model and underground mining layers in GI

References Nelson, Paul E (2010) “Subsidence Modelling Techniques – An Overview of Graphical, Analytical and Numerical Method” , p.12-19. Esaki T, Djamaluddin I, Mitani Y and Zhou G (1997) “Synthesis Subsidence Prediction Method due to Underground Mining integrated with GIS”. www.undergroundcoal.com.au/fundamentals/13_subsidence.html, Retrieved 24-08-2011. http://www.geomechsut.com/ThaiRock2007/PDF_files/08_A_prediction_method_for_time_dependent_land_subsidence_area.pdf, 2010. http://www.dep.state.pa.us/dep/deputate/minres/bmr/act54/sec8.htm , Retrieved 24-08-2011.