Prediction of Surface Subsidence and Its Monitoring


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Prediction of Surface Subsidence and Its Monitoring

  1. 1. Prediction of Surface Subsidence and Its Monitoring Dissertation for the award of Bachelor of Engineering Submitted by M.Venkat Ramana Rao Under the guidance of Prof. B.P.Khare UNIVERSITY COLLEGE OF ENGINEERING KAKATIYA UNIVERSITY KOTHAGUDEM
  2. 2. Abstract The presence of hard and competent strata in the overlying strata at most of the Indian coal mines causes typical subsidence development. The analysis of collected subsidence data revealed that subsidence occurs in two phases. The first phase of subsidence is indicating bending of main roof with very insignificant magnitude of subsidence which is termed in India as non-effective extraction width. The second phase of subsidence follows after initiation of main roof first failure with high magnitude of subsidence within extraction area. The formation of subsidence profiles shows that during first phase, the development of profile is smooth with mild slope and development of steep slopes within extraction area during the second phase. Thus, the developed profile is causing small magnitude of subsidence over the panel edges and there after high magnitude of subsidence with steep flanks within extraction area. The steepness depends on the critical length of the main roof. The observations of limit angle show that it is not the same for all directions of a panel, rather different with highest on the starting side and lowest on the ending side of face advance and in-between on transverse section. To represent this form of subsidence profile, an empherical relation has been developed and correlated with the existing subsidence profiles of already worked out panels. The developed relation is almost showing the correct estimates and deviating at some places. Upon the care observation of the existing geo-mining parameters, it is found that the workings present in the underlying seam are showing its effect and making an abnormal variation in the subsidence profile. In this thesis a careful study was done for establishing the relation between the nature of workings in underlying seam and the amount of change in subsidence value. Further, various conventional and high-tech surveying techniques for monitoring the mining subsidence have been studied, in addition to conducting subsidence survey using total station. For Indian coal mines, it is recommended to use tacheometry survey to monitor vertical and horizontal movements of subsidence monitoring stations and GPS for establishment of control points or bench marks from pit head or nearest national survey grid point as an cost effective approach. Further, remote sensing technique has been advised to monitor the change in the land use pattern.
  3. 3. Acknowledgment It is great pleasure to express my profound gratitude and indebtedness to my Guide, Prof. B.P.Khare, University College of Engineering, Kakatiya University, Kothagudem, for his inspiring guidance, constant encouragement, constructive criticisms and keen interest throughout the progress of thesis. I am deeply grateful to Singareni Collieries Company Limited for allowing me to do the project and for providing lot of information on this subject. My heartfelt thanks are to my Boss Mr.D.Suresh, General Manager, Purchase, SCCL, Kothagudem, for his continuous help and unfading encouragement throughout the preparation of the thesis. My sincere thanks go to my Company colleagues for help and encouraging words. In addition, I thank my friends Mr. Lolla Sudhakar, Additional Manager, Corporate Planning and Mr. M.Venkat Ramana Rao, Under Manager, Corporate Planning, SCCL for their help in reading and preparation of the thesis. I also thank the survey officers of 5 Incline, Mr. M.S.Venkat Ramaiah, Dy. General Manager, 5 Group of Mines, SCCL, Kothagudem and Mr. Manohar, Manager, PVK No.5 Incline, SCCL, Kothagudem for their help and support in collecting subsidence data and conducting field survey. I specially thank the purchase department staff of SCCL, Sri. xxxxxxxx and Sri. xxxxxxxx for their help in typing and preparing the document. I put my special thanks to my wife xxxxxxxx, daughter xxxxxxxx, son xxxxxxxx, and In-laws xxxxxxxx for sharing difficulties and encouragement at every step of my work.
  5. 5. List of figures Fig. 1.1 XXXX Fig. 1.2 XXXX Fig. 1.3 XXXX Fig. 2.1 XXXX List of tables Table 1.1 XXXX Table 1.2 XXXX Table 2.1 XXXX
  6. 6. Symbols xc Critical width to depth ratio of an extraction panel E, Extraction percentage factor Q Goaf treatment factor xn Non-effective width to depth ratio of an extraction panel Rf Rock mass factor indicating the characteristic of the overlying strata of a panel bfl Bulking factor of a stratum 9 Angle of draw P Angle of break a Dip of the seam y Limit angle p Specific density of the sedimentary rocks 6A Unit area a Subsidence factor a! Subsidence coefficient due to bending movement of main roof a2 Subsidence coefficient after the failure of main roof b Bedding plane separation factor B Critical area radius e Influencing factor E Young's modules El Flexural rigidity of the beam h Depth of a panel hc Caving height hf Depth factor hci Thickness of each stratum within caving height gon German unit for angle; 360 degrees - 400 gon L Span length m Extraction thickness Mf Multiple seam extraction factor ; ni Controlling functional parameter n2 Controlling functional parameter P Unit load r Horizontal variable R Radius of influence area^ B+h-cot (y) t Thickness of a beam tp-: Thickness of beds between the competent layer and next parting plane. V8 Void space occupied due to bulking and bed separation Vz-i Subsidence before first break at a point r Vz2 Subsidence after first break at a point r Vzfuii Full subsidence Vzmax Maximum subsidence W Width of an extraction panel w Width of a beam x Width to depth ratio of an extraction panel z Time factor
  7. 7. 1. INTRODUCTION Surface subsidence due to underground mining is an old problem that did not receive due attention in the US until after the mid 1960’s. The increase in use of long wall mining and further housing development in to the abandoned mine lands in the suburban areas further accelerated the public concerns about surface subsidence due to under ground mining. In 1977 the US Congress established the Surface Mining Control and Reclamation Act in which it requires all coal operators to have approved surface subsidence plans. In response to this requirement many research programs were initiated and completed during the past 10 years. When underground mining involves total extraction, it induces overburden strata movements. If not properly planned it causes surface subsidence and affects surface environmental conditions. Total extraction usually refers to long wall mining and bord and pillar mining with pillar extraction. Surface subsidence has long been a subject of intensive research for scientists all over the world and considerable achievements have been obtained. However, due to its difficulties and complicated nature, research into overburden movements has been thus far incomplete as compared to that into surface subsidence. Since surface subsidence is a manifestation of the results of overburden movement, the processes and mechanism of overburden movement must be fully understood in order to establish the mathematical prediction models of surface subsidence. In spite of its brief history, the data obtained from these intensified research programs have demonstrated that surface subsidence due to underground mining is a complicated problem resulting from the interaction between mining operation, Overburden geological condition, and time. As such the exact process and its prediction and prevention tend to be site specific, although there are general trends and principles that are applicable to most subsidence problems. In this Thesis we developed an empherical relation to predict the surface subsidence related to our Indian coal mines particular to Kothagudem area. As all knows that in India there are two major problems what we are facing now. They are 1. Large amount, nearly 3 Billion tonnes of coal is locked-up in the form of standing pillars. 2. Uncontrollable Fires in the seams of Jharia and Raniganj areas due to the extraction of coal in the past by unscientific methods resulted in to surface subsidence which developed cracks in to the goaf causing leakage of air. In India, Jharia coal fields is one of the main sources of Cocking coal having 18 seams with nearly 10m thickness each, has been facing tremendous problem due to subsidence as a result of under ground mining of these coal seams. For the extraction of these seams with out or with minimum amount of subsidence as prescribed requires early prediction.
  8. 8. 1.1 Power Scenario and Coal Demand in India & Reserves: World scenario: It is an accepted fact that Minerals are essential for the development of modern industrial society. Economic growth, the world over is driven by energy, whether in the form of finite resources such as coal, oil and gas or in renewable forms such as hydroelectric, wind, solar and biomass, or its converted form i.e electricity. Coal provides for around 23% of global primary energy needs accounting for 38% of world’s electricity at present. World coal consumption is projected to go up to about 6.4 billion tonnes by 2020. Most of this increase would be primarily in China and India, which are expected to account for about 75% of the increased consumption. Among all the minerals available, Coal is playing a dominant role in world’s energy generation vis-à-vis industrial development with large reserve base. Coal is uniquely placed in respect of all the elements of energy security. As the International Energy Agency has commented: “World reserves of coal are enormous and, compared with oil and natural gas, widely dispersed... The world’s proven reserve base represents about 200 years of production at current rates... Proven coal reserves have increased by over 50% in the past 22 years. The correlation of strong growth of proven coal reserves with robust production growth suggests that additions to proven coal reserves will continue to occur in those regions with strong, competitive coal industries.” A brief analysis of the technology wise coal production reveals that most of the world coal production is coming from opencast mines as the reserves suitable for open pit mining are more compared to underground and also the opencast technology is less complicated. Mechanized longwall contribute about 50% of the total hard coal production from underground mines. Indian scenario: Coal accounts for 63% of our country's energy needs. Commercial energy consumption in India has grown from a level of about 26% to 68% in the last four & half decades. The current per capita primary energy consumption in India is about 248 kgoe/year, which is well below that of developed countries. Driven by the rising population, expanding economy and the quest for improved quality of life, energy usage in India is expected to rise to around 450 kgoe/year by 2010. Considering the limited reserve potentiality of petroleum & natural gas, eco-conservation restriction on Hydel projects and political perception of nuclear power, coal continues to occupy the centre-stage of India's energy scenario. Fuel wise break-up of the primary energy consumption is as under.
  9. 9. Consumption by Fuel India (%) World (%) Oil 32 37 Natural Gas 8 24 Coal 54 27 Nuclear Energy 1 6 Hydro-Electric 5 6 Total 100 100 (Source: BP Statistical Review of World Energy 2005) Coal based thermal power generation capacity presently stands at 61,476MW and a capacity addition of around 60,000 MW has been targeted in next 7 years. This clearly presents high demand for coal in near future. Besides energy generation, the other consumer industries like cements, fertilizers, etc are expanding with increased industrialization creating increased demand for coal. Total annual hard coal production in India is about 373.79 million tonnes (m.t)(2004-05) out of which nearly 80% is from Opencast Mines. Coal India produces about 90% of total Indian coal production and SCCL’s share is about 10%. The expected demand for coal by 2011-12 is about 707 M.T, whereas coal production would be around 550 M.T, leaving a gap of about 157 M.T, which needs to be met by imports/private mining. Coal reserves: India is the third largest coal producer in the world. With hard coal reserves of around 248 billion tonnes, out of which 93 billion tonnes are proven. India holds around 10.2% of the world’s proved hard coal and lignite reserves and produces around 7% of total world’s production. The depth wise coal reserves of India as on January 2005 are as follows: (in Billion Tonnes) TOTAL DEPTH(m) PROVED INDICATED INFERRED (In Bt) (%) 0-300 71 66.5 15 152.5 61.5 300-600 6.5 39.5 17 63 25 0-600 14.5 14 0.5 - 6 (Jharia) 600-1200 1.5 10.5 6 18 7.5 0-1200 93 117 38 248 100 (Source: GSI Report, January 2005) Depth – Wise coal reserves of Andhra Pradesh (Godavari Valley Coal Fields) as on 01.01.2005 in million tonnes is as follows: DEPTH(m) PROVED INDICATED INFERRED TOTAL 0-300 5467 2229 102 7798 300-600 2796 2832 553 6181 600-1200 -- 1018 1929 2947 0-1200 8263 6079 2584 16926
  10. 10. Both the tables clearly indicate that the reserves under command area of SCCL are at greater depth than that of average all India figures. At SCCL most of the existing mines and present projects are for extraction of deposits with in the depth range of 0-300 metres. To have sustained production SCCL has formulated projects for extraction of coal reserves locked within the depth range of 300-600 metres. 1.2 Impact of mining on environment Intensive mining for meeting heavy power demand of the nation creates significantly alarming environmental problems. Transportation of coal to far distances, preparation and burning of coal for power generation produce coal dust, methane, nitrous gases, sulphur dioxide and carbon monoxide. Opencast mining causes land use problems by disturbing the landscapes, forest areas, agricultural lands and reducing ground water etc. Underground mining by intensive mechanization leads to significant disturbance in the strata equilibrium above the extraction panels. Ultimately, it is transmitted to the surface as subsidence causing damages to surface structures and properties. Additionally, exhaustion of grazing and non-arable land which can be undermined without much consequence will make inevitable encroachment of mining operations especially under surface structures such as railways, roads and built-up areas for economic development. Thus, problems associated with subsidence will be further aggravated. The vertical and horizontal movements of ground surface and their derivatives, tilt, curvature, and strains, cause significant damage to the surface and sub-surface properties. Damage to buildings will result due to tilt, curvature and linear deformation of ground built on. Communication networks, rails, roads, pipelines and canalized waterways will be damaged due to alteration in the alignment or deformation in them. Whereas underground pipelines and cables will be damaged by linear deformation Vertical movement of ground surface on its own can cause mining damage to fields, meadows, drainage channels, canals and water courses. It was reported that subsidence damage to buildings and communication installations are more prominent, as compared to damage to ground water level, on hilly terrain. In flat land, the subsidence damage to the natural waterway system may reverse the flow of water course. Till date, the subsidence due to mining of coal has not drawn the attention of mine managers in India. Subsidence studies are being considered only when damage is expected for important structures on the surface. However, the present trend is changing rapidly as environmental issues are cropping up at every stage of mining. 1.3 Geology of Indian coal measures All workable coalfields in India, except those of Assam, belong to the Damudar formation of Gondwana group of Permian age. The formation of thick coal seams is found, in large faulted blocks, along the Damudar, the Mahanadi
  11. 11. and the Godavari valleys. The strata generally dip at low angles, below 100, but may show higher inclination near faults and intrusions. Stratigraphy of Gondwana formation During the Gondwana era, the bulk of strata were laid down as a thick series of fluviatile or lacustrine deposits with intercalated plant remains which ultimately formed as rich coal deposits. Each cycle of deposit started with coarse sandstone and proceeded through shale to coal seams. All Gondwana coals, contain high ash, and even the best seams contain not less than 5% or 6% of ash. The Gondwana group was divided into two major divisions based mainly on palaeontological evidence. The lower division is characterized by Glossopteris flora and the upper division by Ptilophyllum flora. Further the upper and lower Gondwanas have been sub-divided into series of formations. Lower Gondwana which is rich in coal seams has been divided in the ascending order of Talchir, Damudar and Panchet. Upper Gondwana period acquired no importance as coal seams formed during this period were thin and unworkable. In the lower Gondwana, Damudar formation has gained the status of a system because of its most extensive and best developed coal seams with considerable thickness and of great economic importance. The Damuda strata consist of sandstone containing kaolinised feldspars followed by shale and then by coal. The succession repeated many times and during the whole Damuda period there must have been as many as 50 to 60 cycles of sedimentation. The system was further categorised into four measures namely, Karharbari, Barakar, Barren and Raniganj Measures. Out of these, the Barakar and the Raniganj measures are important for the formation of coal seams. The Barakar measures are the chief coal bearing measures, practically in all the lower Gondwana basins in India. It consists of sandstone and grit, with occasional conglomerates and beds of shale in the Jharia coalfield up to a thickness of about 830m. The sandstone often contains more or less decomposed feldspars. In all the areas where the Barakars are exposed, it is seen that sandstone with false bedding, shale and coal seams appear in order and are repeated over and over again. The Barakar seams are best developed in the Jharia Coalfield. The Raniganj measures, with valuable coal seams, were typically developed in the Raniganj coalfield. It consists of sandstone, shale and coal seams. The coal is higher in volatile and moisture than the Barakar coal. There are certain seams which are excellent with long flame and steam coal quality. Special features of Indian coal measures It has been observed that at most of the Indian coal mines the beds of shale and sandstone occur alternatively with coal seams at certain intervals. The coal bearing rocks are traversed by dykes of dolerite and sills of mica-
  12. 12. lamprophyre. In most of the coalfields, there is a strong bed of sandstone varying in structure and form from fine grained to coarse and from bedded to massive, respectively. There is a varying thickness of shale in-between the coal seams and strong beds of sandstone. The percentage of sandstone, in general, varies from 50 to 95 in most of the cases. The sandstone beds are generally stronger as compared to the immediate shale. The average depth of workings is 250 m except in a few cases, with an extraction thickness from 2 to 3 m. The dip of seams (a), in general, is less than 10 gon with multiple seams in close proximity. MAJOR COAL FIELDS IN INDIA • Ranigunj • Jharia • East Bokaro and West Bokaro • Pench-Kanhan, Tawa Valley • Singraul! • Talcher • Chanda-Wardha • Godavari Valley • Asansol • Karanpur Fig. 1.4: Distribution of coalfields in India [13] Distribution of coal deposits The major part of Indian coal deposits comes under the Permian age, popularly known as Lower Gondwana. It is followed by Eocene and Oligocene of the North Eastern Region, lignite deposits of South Arcot and Pleistocene lignite's of Kashmir. In addition to these well known deposits, occurrence of several coal horizons in Eocene sediments in the Northern part of Cambay Basin was found in the sixties while drilling for oil in Kalol and Mehsana. However, these deposits confined to oil bearing formation occurring at depth of 700 m to 1000 m with a thickness of 6 m to 50m. The lower Gondwana, which is confined within
  13. 13. the South - Eastern quadrant, bounded by 78° East Longitude and 24° North Latitude, forms the most important source of coal in India. The above figure shows the distribution of coalfields in India. About 95.5% of Indian total coal reserves occur in 44 coalfields of the Gondwana measures spreading over an area of 14,550 km2. The remaining 4.5% comes under the Tertiary coalfields, covering an area of 1,100 km2. The stratigraphy of coal measures of South Africa, New South Wales of Australia and Northern Appalachian region of USA, shows significant similarities with Indian coal measures. The South African coal measures belong to Karoo sequence of Permian Paleo-age, with thick beds of dolerite and sandstone in the overlying strata of the coal seams. New South Wales coal measures belong to Permian age containing thick beds of conglomerates and sandstone. The Northern Appalachian region coal measures contain, however, bands of hard limestone and sandstone. Furthermore, the similarity of lithology and fossil content of the Gondwana deposits in the southern continents suggest that South Africa, Madagascar, India, Australia, Antarctica and South America formed parts of a continent which lay in the region of the Indian Ocean around what is now South Africa. The Godavari Valley Coal fields of Andhra Pradesh has spread in 4 districts namely Adilabad, Karimnagar, Warangal and Khammam. The Singareni Collieries Company Limited (SCCL) is presently extracting the coal from this coal field by operating 51 undeground and 11 Opencast mines (as on 1.1.2006). The formation of Godavari valley coal fields in Andhra Pradesh is shown in the following figure. Fig 1.4 Coal belts of Godavari Valley Coal fields
  14. 14. 1.4 Methods of extraction In India, coal from underground is being extracted basically by two methods, bord and pillar and longwall. Bord and pillar method In bord and pillar method two sets of galleries, one set normally perpendicular to the other, are driven, forming pillars between them of the size mentioned in the Indian Coal Mines Regulation 1957. In most of the cases, pillars are square shaped. A group of such pillars is formed as a district. Each district is separated from the other by a solid coal barrier in the form of long rectangular pillars. The number of connections from district to district should be minimum, so that each district will be isolated from the rest of workings in case of any fire or after complete extraction of all pillars. Further, coal barriers act as support to the roof to minimize the subsidence damage. Normally, formation of pillars in a district and pillar extraction is two separate activities, one after the other, and a long time may pass between them. Thus, coal pillars may stand for years before they are extracted. This is one of the reasons for not observing subsidence during development of district. Fig. 1.5 shows the development of pillars in a district. Fig. 1.5: Development of pillars in bord and pillar method
  15. 15. During depillaring operation or pillars extraction, they are sliced into small pillars called "stooks" which are then rubbed off one by one. This is a common practice of depillaring, and it is called as the Slice and Rib method. The size of the pillars is reduced in such a manner that the roof strata caves without affecting other mine workings. Generally, a diagonal line of pillar extraction is practiced in most of the depillaring operations. It is considered as the best method for caving the main strata Fig. 1.6 shows the depillaring operation with diagonal face of extraction Fig. 1.6: Depillaring operation in bord and pillar method Longwall method Longwall method of extraction consists of laying out a long face, may be up to 300 m with a set of galleries (gate roadways) on both sides. Thus, a block is developed in a district. Development and extraction can go simultaneously in longwall mining. When this happens, the method is called "longwall advancing". But when extraction starts after development then it is called "longwall retreating". Retreating longwall method of mining is most popular in India. Even though, longwall method of extraction is very common for coal extraction in most of the countries, it was not a successful method till recently in India because of typical stratigraphy of Indian coal measures and lack of proper understanding of the overlying hard strata influence on the extraction face. Now-a-days, the traditional bord and pillar method is getting replaced by longwall method of extraction to achieve higher production.
  16. 16. Fig. 1.7 shows a simple longwall method layout. However, formation of a large void (goaf) due to full extraction of a big block of coal induces severe ground movements and which may cause damages to the surface properties and structures. Hence, a good knowledge of development ground movements due to mining and its pre-calculation are very essential for proper planning of extraction layout. OBJECTIVES: The main objective of this Thesis are: 1) Measurement and collection of data related to subsidence at different horizons within the overburden, which will provide data for predicting surface subsidence for Indian coal measure rocks.
  17. 17. 2) Analysing the collected data and evaluation of different subsidence parameters from field observations. 3) Building a mathematical formula from the results obtained. 4) Study of underlying goafs on subsidence – developing an emphirical relation between the amount of goaf present beneath the present extracting panels and the amount of subsidence by introducing a ‘goaf factor’. 5) Monitoring of subsidence profiles & surface damage to ascertain the conditions and to take remedial actions.
  18. 18. 2. EARLIER THEORIES ON MECHANICS OF SUBSIDENCE 2.1 Earlier theories: a. Vertical Theory: “Schultz” proposed it in 1867. According to this theory whenever a seam is extracted the limiting planes are vertical. b. Normal Theory: Proposed by “Gonut” (Belgium), according to which it was assumed that the strata subsidence normal to the seam. c. Between Vertical and Normal: Proposed by “Jicinsky:. He observed that the limiting lines bisect the angle between the vertical and the normal lines when dip is less than 45 degrees and if the dip exceeds 45 degrees, the line of fracture lies at an angle of 45 degrees minus half the angle of dip. d. Dome Theory: From laboratory observation “Fayol” in 1885 postulated that the movement of ground is limited by a kid of dome over the area of excavation. It is believed that the rocks overlying an excavation are acted on by two forces only cohesion and gravity. If the gravity overcomes cohesion, the roof will fall forming an enlarging arch. e. Beam or Plate Theory: Haulbaum assumed the immediate roof to be a cantilever beam and considered that the lowest part would be under compression and upper part under tension. The fracture often occurs over the waste by causing the lowest portion of fracture along BC as in Fig. Later Eckardt assumed the roof to be composed of many thin beams each one supported by the one below and gripped at the ends. All the beams bend down in succession with all or most of them breaking off at places where they are gripped. The bending yields a positive angle of draw. f. Trough Theory: As early as 1907, Hausse introduced the trough theory. He distinguished between a “main break” and an “after break”. In flat seams, the main break is vertical, and the after break is in a direction bisecting the vertical and the angle of slide. In dipping seams the angle of draw increases, it is 35.8 degrees from the vertical for a 40 degrees a dip, and the main break occurs over the seam at an angle from the vertical equal to half the dip. g. Continuum Theory: In this theory, it is assumed that the ground acts as a continuous body bounded
  19. 19. by the surface above and the excavation below. If the elastic modulo, the initial stress in ground, and the boundary conditions i.e., the distribution of stress on the surface, on the roof or on the floor are given, it is possible to predict stressed and placements at any point of the medium by using the theory of elasticity. h. Particulate Theory: A further study on subsidence trough using stochastic equations has been proposed. The rock medium, for which these equations determine movement, has been called a stochastic medium, such as dry sand. 2.2 The mechanism of subsidence: The weight of overlying rock before mining generally exerts a uniform vertical pressure. The undistributed strata are under the influence of two potential forces. The first force is due to gravity, which acts vertically downwards and may be taken roughly equal to 0.025 MN/cu.m. The second force consists of compressive stresses induced in the earth crust ( due to contraction of the earth’s interior upon cooling ) which acts more or less horizontally. Its magnitude varies from place to place and produces varying effects. So long as the strata is left undisturbed these forces remain potential and in equilibrium. How ever, when excavation commences in seam, these potential forces are liberated (become kinetic) and their joint action is responsible for all the phenomenon of subsidence. The part played by gravitational component are obvious, but the action of the second force is not so evident apart from the “creep” phenomenon is an example of the existence of such stresses. Also the liberation of potential forces stored up in the earth's’ crust due to secular cooling produces lateral movement. The evidence of such stresses can be seen in the walls of a trench made at the surface. The walls because of lateral forces tend to move towards each other. The efficiency of mine timber as a means of supporting the roof also predicates the existence of lateral compressive forces, which help to hold up the roof. A consideration of the enormous weight of strata over head compared with the strength of the timber employed for support is an example of this cage. The lateral forces, which are liberated acts in the opposite direction to that of the advance of the face (towards the goaf). Considering the joint action of vertical and horizontal components their resultant will act obliquely downwards and backwards the goaf. Remembering that in all cases action and reaction are equal and opposite, it is seen that the reaction upon the roof itself is along the line AB, so that the line of strain is projected forward over the coal face. The accounting for “draw” ( the distance which the line of break or strain is in advance of the coal face). 2.3 Types of subsidence: According to Grey (1970, after examining 354 incidents of subsidence above abandoned mines in the Pittsburgh metropolitan area, the subsidence features have a mean diameter (i.e. the average of long and short dimensions) from less than 1 ft. to 1600 ft, with 84% less than or equal to 1.5ft; the subsidence features have depth ranging from less than 1 ft. to 48 ft, with 89% less than 25 ft; 66% of the subsidence features are deeper than they are broad. Nearly 59% of subsidence features occur with over burden less 50 ft. thick and 81% less than 100 ft. No subsidence features occur with over burden thicker than 450 ft. Occurrence of subsidence incidents varies from immediately to more
  20. 20. than 100 years after mining. Accordingly to Grey, the most prevalent subsidence features over abandoned mine lands are sinkholes, with depth of more than 3 ft, and troughs or sags usually less than 3 ft. deep. Sinkholes are steep-sided pits, while troughs are shallow depressions much wider in area than sinkholes. a. Sinkhole type subsidence: A sinkhole is caused by collapse of mine roof that works its way upward. If it is not arrested during the process it will eventually reach the surface and emerge as a sinkhole. The thickness and govern the process characteristics of the over burden, the width and height of the mine openings. In case of Bord and pillar working, the pillars may be experience local failures during mining operations. If pillar is having joint, it edges may fail even under low stresses. This increases the stresses on the remaining part of the pillar causing complete failure. Thus, failure of one pillar may cause other pillar to fail since increased loads are transferred on the remaining pillars and giving rise to circular depression or a sink hole. Even if the pillars are relatively stable and free from joints, the ground surface can be affected by upward wide migration with the laps of time, which may range from a few months to a few years. This happens because the materials which fall out in worked out areas although expands (because of bulk characteristics) but never completely fills the void. Pillars in dipping seams tend to be less stable than those in the horizontal seams. Since over burden above dipping seams produces shear force on the pillars. The sink hole may also be caused while working near the surface. There is a possibility of surface fracture, either before or after the surface has subsided. The roof may caves in a dome shape over the excavation. When the dome of projecting beds have reached a height and width at which it can no longer support the weight of the overlying beds, it caves to the surface. The stresses in the rock are thereby relieved and the surface subsidence in a funnel shaped around the point of rupture. This generally happen when the height of the surface is about 8 times less than extracted seam and the seam under extraction located at a depth less than 5 times the width or 10 times the height of the mine road way. Sink Type subsidence is more abrupt and the profile of sinkhole may resemble a bottle. Soil erosion in to the sinkhole may increase its diameter at the ground surface so that eventually it assumes profile or hourglass. Structure damage caused by sinkhole type subsidence can be costly and dangerous. b. Trough type subsidence: Trough type subsidence, although less prominent, serve damaging effect, both on the environment and structure. Sag or trough subsidence is a gentle depression over a broad area. These depressions are semi-elliptical to circular shaped, partially or fully outlined by tension cracks, and may or may not contain compression ridges. Troughs are caused by the following 3 events roof caving above the opening, crushing of pillars, or punching of the pillars in to the mine floor. Troughs are in the form of vertical subsidence, tilt, curvature, horizontal displacement and strain. Each of these has
  21. 21. different effects on the environment and the structure. For example, in the low-lying area may cause flooding and drainage problems, may upset roads and railway tracks. The differential horizontal structure and building by their compression and extension effects. Ground subsidence could also effect surface topography, damage to sub-surface installations, destruction to wild life and the alteration of flora and fauna. In addition some type of subsidence lead to pollution of ground water supplies. 2.4 Movement in the overlying strata: If the mine excavation is wide so that it cannot be bridged by overlying rock, settlement of the immediate roof over the workings continues in the higher strata and the roof beds begins to collapse. During the settlement, if they are detached from their parent mass with draw their supports from higher beds. The downward movement in the strata spreads very rapidly until it reaches the upper earth surface. In this process, changes in the position of points in the rock mass independent of time takes place as follows. The floor layers arch elastically upwards on the relief of the perpendicular load. a) The seam is compressed by the front abutment pressure ahead of the face and the waste by the back abutment pressure. b) The area over the working detaches it self from the main roof breaks off and falls in the waste. The size of the broken pieces depends on the characteristics of the overlying rocks. c) The main roof settles gradually or breaks off at regular interval leaving slight overhang protruding over the advancing face. In case of pillars working, sags in a wavy outline over rooms and pillars. d) The surface zone of loose over burden behaves plastically and sinks down and form “trough” shaped depression.
  22. 22. 3. LITERATURE REVIEW Subsidence studies in coal mining areas initially originated in Europe in the middle of last century. Since 1870 on wards a number of scientific publications on subsidence studies appeared in Germany and in other European countries. In the beginning it was assumed that full subsidence was equal to seam thickness but also subsidence factor which defines method of goaf treatment as either caving or stowing, and time factor. Further, depth of working and volume of surface subsidence trough, extraction area and relative position of surface points to the working were taken into account. Emergency of subsidence prediction methods started by Keinhorst by using angle of break and limit angles. Bals made a significant contribution to predict subsidence in horizontal strata by modifying the earlier development of Keinhorst. He employed Newton’s law of gravitation. Later Schleider extended Bals work to inclined seams and refined the original function of Bals. Perz considered the dynamic subsidence and included the time factor in prediction of subsidence. A significant development subsidence calculation has been made in European countries after Second World War. Noteworthy contributions are Ehrhardt and Sauer, Brauner and Kratzsch in Germany, Berry, Orchard and Allen and Whetton and King in UK,l Litwiniszyn and Knothe in Poland, Martos in Hungary. The present trend is towards the development of subsidence prediction methods based on measured subsidence data by means of additional functions, local valid parameters and three dimensional Finite Element Method. Even though, various subsidence prediction methods for different coal fields have been developed based on measured subsidence data, the subsidence studies relevant to Indian coal mines briefly mentioned below: Investigations on the nature of subsidence development and strata behavior for the Moonidih block t5-t8 was carried out by A.K. Ghosh and D.Datta (1987). It has been explained that the presence of the stand stone layer in the immediate vicinity of the coal seam caused a small amount of subsidence. It was ascertained by the observation of movement of monitoring stations over time that the failure of the component layer caused shift of its elasto-plastic stage to claustic with significant subssidence on the surface. Further, it was inferred that the distribution of subsidence factor was not continuous, rather discontinues over the extraction block. By conducting regression analysis between cavability index, established by Fuzzy set theory, and first break length observed in the field an empirical equation has been derived to predict first break span for Indian coal mines. But it was assumed that the failure of main roof was occurring only due to its own weight. The dead load coming from the overlying strata was not considered in it.
  23. 23. Surface movements and sub-strata movements using magnetic bore hole anchors for Ratibati Colliery of Ranigunj Coalfield were investigated by Dr. R. Krishna (1989). It was reported that a sudden displacement of discontinuity at the location competent sandstone layer was observed with bore hole anchors when the face was progressed to a certain distance. However, the study was limited to a correlation of sub-strata movement with the bending of self loaded flexural beam. It was not extended to relate the sudden strata movement with the development of subsidence on the surface and its impact to the surface structures. Further, the investigation was confined to a single bord and pillar district. An empirical equation to predict full subsidence was developed by conducting a regression analysis for the subsidence observations of bord and pillar panels of Jharia Coal field by T.K.Mozumdar and B.K.Mozumdar (1989). The width to depth ratio of a panel was considered the only influencing parameter in the estimation of subsidence factor. Similarly by regression analysis, an empirical formula for predicting maximum subsidence was derived for Singareni Collieries by L.A. Kumar (1992). The rock mass factor and Rock quality designation (RQD) index were considered as the major influencing parameters in estimation of it.
  24. 24. 4. PARAMETERS INFLUENCING SUBSIDENCE 4.1 The results of investigations in workings, rates of convergence and roof settlement suggests that strata movement at the mining horizon resembles the behavior of a quasi-elastic beam bedded on a yielding under clay and are chiefly dependent upon the following factors: 1) Depth of workings: The cover-load pressure to be taken up by the roof (immediate roof and main roof) depends on the depth of the workings, the greater the depth; the greater will be the sag in the roof. 2) Nature of the roof: The modulus of elasticity (E) of the roof strata determines the bending resistant (N) which is given by N=E 1 = E bd / 12 (N/cm) where, d is the thickness of rock stratum in cm and b it’s load bearing capacity. The pressure of joints or fissures decreases the bending resistant (they’re by causing more sag). 3) Nature of the floor : The presence of water or reduction in load causes reduction in the height of face and floor heave. 4) The underlay supporting the roof : If the outer edge of the face is not supported in good time, either by fills material or by leaving large pillars, high abutment pressure will be caused giving rise to convergence. The smaller rock particles in the waste (after caving) full the void in a better way and less deformation should be expected. The size of the broken rock will of course depends upon whether the caving zone consists of massive rock or within brittle rock (and the bulk factor). 5) Seam thickness: The roof and the fill material in the process of sagging get compressed and the degree of sag is increase further ( as if a spring being compressed) . The thicker the seam or the fill, the greater will be roof sag. 6) Width of excavation and size of working: The roof has to bridge the face excavation like a cantilever beam. This means an increase in span to be bridged, assuming roof as an elastic beam, will bend at the middle.
  25. 25. 7) Rate of advance of face : After the excavation, the roof can sag only to the extent that it compresses what lies under it. The compression takes place gradually with time. This means with rapidly advancing face, the roof will settle down gradually, both ahead and behind the face. 8) Compressibility of Pillars: The deformation will depend upon the compressibility of the pillar, which is determined by width to height ratio, the load on it, I its flow properties and crushing strength. If blasting in operation fissures may be developed this will affect the stability of pillars. 9) Underlying Goafs and Barriers: The amount of subsidence varies with the presence of goafs either stowed or caved in underlying seams, barriers will also have effect on the ground movement. 4.2 Subsidence & its related parameters; 1. Subsidence (S): On any cross-section, the vertical component of the surface movement vector is called surface subsidence. It generally points downward. But sometimes it points upward in areas ahead of the face line or beyond the edges of the opening. In such case it is a surface heave which is usually less than 6 in. 2. Displacement (U) On any cross-section, the horizontal component of the surface movement vector is called surface horizontal displacement. It generally points to ward the center of the subsidence basin. But in steep terrain, it moves along the down dip direction. 3. Slope (I=ds/dx): On any cross-section, the difference in surface subsidence between the two end points of a line section divided by the horizontal distance between the two points is called the surface slope of the section. 4. Curvature (K=dS2/dx2) On any cross section, the difference in surface slope between two adjacent line section divided by the average length of the two line sections is called the surface curvature of those two line sections. There are two types of curvature convex or positive curvature and concave or negative curvature.
  26. 26. 5. Horizontal strain (E =dU / dx) : On any cross-section, the difference in horizontal displacement between any two points divided by the distance between the two points is called horizontal strain. If the distance between the two points is lengthening. It is tensile strain with positive sign. Conversely, if it is shortening, it is compressive strain with negative sign. 6. Twisting (T = dS/dx.dy) On the surface of the subsidence basin, the difference in slope between two parallel line sections divided by the distance between the two line sections is called twisting. 7. Shear strain (Y = dU/dy) ; Shear strain is the changes in internal angles of a square on the surface of the subsidence basin or on any major cross-section. It is the summation of the differences in incremental (or decremental) lengths between the two opposite sides divided by the original distance between the two opposite sides. 8. Angle of draw (d) : Assuming a rectangular worked out area, the strata affected by subsidence take the form of obtuse pyramid. The angle between the sides of the pyramid and the vertical is called angle of draw or limit-angle or simply as the angle of inclination from the vertical of the line connecting the edge of workings and the edge of the subsidence area. 9. Angle of critical deformation (d): The angle between the vertical line at the opening edge and the line connecting the opening edge and the point of critical deformation on the surface is the angle of critical deformation. After observing 40-long wall subsidence profiles, Peng and Geng (1982) found that the angle of critical deformation is on the average of 10 degree less than the corresponding angle of draw. 10. Angle of Break / Fracture (a) : The angle between the vertical line at the opening edge and the line connecting the opening edge and the point of maximum tensile strain on the surface is called the angle of break. The ground surface at the point of maximum tensile strain is the most likely place where tensile cracks occur. 11. 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
  27. 27. curvature is zero. Karmis (1981) found that distance from the inflection point to the nearest edge of the opening (is the offset d ) = 0.2 h (h = mining depth ). 12 Radius ( r ) and Angle of major influence ( b) : 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. (Tan b =h/r) 13. Angle of full subsidence (f) ; 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. 14. Critical Area: 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”. 15. Sub-Critical Area : If the angle of draw plotted from the edge of excavation area towards the interior of the disturbed zone, on opposite sides, intersect below the surface. In this case no point on the surface will undergo full- subsidence. 16. Super-Critical Area: When the draw lines plotted from opposite sides of excavation intersect above the surface, then it is defined as super-critical area.
  28. 28. 5. CLASSIFICATION OF SUBSIDENCE PREDICTION METHODS Based on physical principles, nearly all the available methods of subsidence prediction can generally be classified as below : I) Empirical and Semi-empirical methods  Graphical method  Profile function method  Influence function method  Zone area method. II Theoretical methods based on continuum mechanics  Elastic analysis  Visco-elastic analysis  Beam theory III Theoretical methods based on idealized mechanistic models  Stochastic model  Void diffusion model (VDM) IV Numerical methods  Finite element method  Boundary element method  Discrete element method 5.1 Description of subsidence prediction methods : A brief description of the above-mentioned methods is given below: 1) Empirical and Semi-empirical methods: *) Graphical methods: These are mainly used in USSR, China and Britain. The method used in the USSR and China is called the “Typical profile method:. This method is based on a dimensionless half subsidence profile, which is derived from a large number of observed profiles. The NCB (1975) method is a little more complex. Since graphical methods have no mathematical errors, high prediction accuracy can be expected. However, these methods do not permit their use in other areas with different mining geological conditions.
  29. 29. *) Profile function method : This method is based on the mathematical description of half subsidence profile over a super critical or a sub critical area. Most of the available profile functions can be standardized in the following form. S(x) = Sm F (x-D)/R,n ) Where Sm is maximum subsidence, R is horizontal development radius, n is a shape parameter which is ignored for symmetrical profile functions, and D is the offset distance from panel edge to half maximum subsidence point which is called inflection point. Several profile functions have been suggested by Avershin (1947), King (1957), Wardell ( 1958), Martos (1958/59), Kolpingkov (1958), Tangshan Coal Institute (1963), Hoffman(1964), and Liu and Liao (1965). Generally, god prediction accuracy for subsidence profile near inflection point can be obtained with this method if proper parameters are given. But most of the profile functions predict a smaller subsidence than observed data over the barrier pillar. In addition, the random behavior of the offset distance of ( quasi-) inflection point is also a key problem affecting the prediction accuracy. *) Influence Function method : This can be described as the following integral : S (x , y) = a M ( F (x,y) dx dy Am Where, ‘a’ is subsidence factor, m is mined height, Am is mined-out area, x and y are co-ordinates of a current point on surface, X and Y are local co- ordinates whose origin is at point (x, y ) and f (X, Y ) is an influence function, which is supposed to be symmetrical. Several forms of the influence function have been obtained by Bals (1932), Beyer (1945), Sann (1949). Knothe (1957), Kochamanski (1957), Ehrhardt and Sauer (1961), and Brauner (1973). The most popular one is the Knothe’s influence function. f (X, Y ) = 1 /R exp ( -II (X+Y)/R ) Where, R is called the main influence radius, which depends on the thickness and mechanical properties of the over burden above the mined-out panel. The influence function method is based on the linear superposition principle. This principle is not precisely correct especially near the panel edges, and leads to what is called the “edge effect”. A simple and widely used correction measure for the “edge effect” is the introduction of the “effective mined out area” which is less than the real area of the panel. With this correction, the influence function method can give a good agreement with the observed data except for the trough edge area. Nevertheless, it is rather difficult to precisely determine the “edge effect” which strongly affects the prediction accuracy. In order to improve this technique, some researches in West Germany and Poland have also suggested some nonlinear principles.
  30. 30. *) Zone area method : An important improvement of the influence function technique is the “Zone area method” presented by Marr (1975) who considered the influence of a zone on surface subsidence as a nonlinear relation rather than a linear relation. The complex geological conditions such as faults and folds can not, however, be taken into account in all these methods. It is suitable for regular band irregular panels, while profile function and graphical methods are only suitable for regular shaped panels. II) Theoretical methods based on continuum mechanics : *) Elastic analysis: Several researchers obtained analytical solutions based on the elastic theory. Hackett (1959) used a two dimensional isotropic elastic model to analyze the subsidence over a thin, horizontally deposed tabular deposit. He considered the problem as that of a horizontal split or cracks in an infinite medium in which the ground was initially subjected to the hydrostatic state of stress. Hackett estimated the influence of a fee surface as increasing the vertical displacement by no more than 10%. However, this was later acknowledged as an error and it should be 100%. Berry and sales (1960, 1961, 1963) considered the ground a thin, tabular, arbitrary oriented opening below a horizontal surface as a homogeneous, elastic medium with an initial hydrostatic state of stress. Emphasis was placed on the subsidence associated with the mining of horizontal deposits. Two dimensional isotropic, two-dimensional transversely isotropic and three – dimensional analysis was presented. The boundary conditions for the opening were supposed to be one of three types: non-closed, partly closed, o completely closed. Approximate solutions for non closure and partial closure states and the exact solution did not coincide with the observed data, while the transversely isotropic solution appeared to be in reasonable agreement with field profiles. Salamon (1963, 64, 65) presented a more general “face element” principle, expressed as below : S(x, y) = f s (X, Y ) F ( r) d A Where a is the area where roof-floor convergence occurs, F ( r ) is an influence function, s (X,Y) is roof-floor convergence distribution which can be computed from a differential equation below : V s = (2/IE) (s-sm) Where V is the laplace operator in the xy plane, s is the induced vertical stress on the seam horizon, Sm is the vertical stress induced by a “mirror image” excavation, E is Young’s modulus, and is related to seam thickness M and Poisson’s ratio as below:
  31. 31. L=M/ (L2 (1-n) Recently, he suggested a more appropriate term “seam element “ to replace the “face element”. In addition to the homogeneous, isotropic models, he also treated a friction less laminated model and a multi-membrance model. An important feature of this analysis is gthat the influence function obtained from the friction less laminated model is the Gaussian curve, which is the famous Knothe’s influence function. In this model, Salamon adopted the empirical relation of horizontal displacement being proportional to slope proposed by Avershin in 1947 as the basis of calculation of horizontal displacement and strain. Another important feature of this model is that it permits the computation of roof-floor convergence. This method can be considered as the advanced form of traditional influence function method. *) Visco-elastic Analysis : Several models have been developed to treat the over burden as a linear visco elastic medium (Astin 1968, Bery, 1964, Imam, 1965, Marshall and Berry, 1966 ). In this case, delayed elastic constants can be employed for estimating the final deformations after creep has ceased. Currently, a general opinion is that the effects of time on surface subsidence as negligible because no evidence supports that more than five percent of total subsidence is due to viscous behavior of the over burden. In long wall mining, this residual subsidence is probably due to the time-dependent compaction of gob. In room and pillar mining, it is mostly due to time-dependent deformation of pillar or weak floor strata. *) Beam Theory : The earliest solution based on beam theory was obtained by Salustowiez (1953), in which Winkle’s hypothesis was utilized. Similar solutions were also adopted by Liu (1983), Bai (1983), and Hao and Ma (1985). Pytel and roof-pillar- weak floor interaction load acts as a uniformly applied load, on a composite beam with step wise varying stiffness and the beam’s reactions are transmitted to the weak floor strata though segmented continuous footings representing panel pillars. The model can consider different size pillars in a panel, different rates of advance and time lag in mining in different parts of a panel and up to 50 pillars across a panel. Not only can it predict surface subsidence but also the pillar settlement and roof – floor convergence. The technique may be applied in virgin areas based on geo technical data obtained during exploration. This is the main advantage of this method. All the above methods based on beam theory predict the surface heave phenomenon, which is commonly observed during the subsidence process. This is a main characteristic of beam theory. The biggest limitation of the beam theory at present is that it can only be used for two- dimensional problems. For thee- dimensional problem, the plate theory must be introduced which will be mush more complex.
  32. 32. III. Idealized Models: *) Stochastic Model : A Stochastic model was developed by Litwiniszyn (1956) on the basis of a hypothesis that movement of a lose medium can be described as a stochastic process defined by a differential equation. dW/dz=d/dx (B11 W)+2d/dxdy (B12 W) +d/dy (B 22 W)+d/dy(A 1 W)+d/dy (A 2 W)+NW Where, B11 , B12, B 22, A1, A 2, And N are real numbers o third order matrices, w is subsidence or third order vector which includes horizontal and vertical displacements. Some simple solutions for homogeneous, simplified non homogeneous and non-linear media have been obtained. Knothe’s method was verified to be a special case of this model. Other little more complex solutions obtained by Litwiniszyn (1974) have not been used yet. This model treated the stochastic and statistical behavior of mine subsidence. *) Void Diffusion Model (VDM) The Void diffusion model was suggested by Hao (1988) and Hao and Ma (1988) based on thee basis principles. A general differential equation was established as below. DS/dz=d/dx( B1 dS/dx)+ d/dy(B2 dS/dy)+d/dx(A1 S)+d/dy (A2 S)+ f(x,y,z,t) Where S is subsidence, B1 and B2 are coefficients of void diffusion, A1 and A2 are coefficients of Void deviation and (x,y,z, t) is the intensity of void sources which can be used to simulate the openings, over burden fractures, compaction of weak strata and activation of adjacent previously mined out panels. This model can consider the influence of non – homogeneity and non – linearity of over burden with finite element technique, and can also take the effect of faults into account. With the concept of void sources, this model is more flexible in the simulation of subsidence process. There fore, a high accuracy of prediction can be achieved. In addition, the distribution of roof features and weak floor deformation, etc. can be estimated from surface subsidence data using reverse analysis techniques. The distribution of void sources can be determined by geo mechanics analysis, physical model tests reverse analysis or combination of all the methods. IV. Numerical Method : *) Finite Element Method : Finite element analysis has been employed in the simulation of mine subsidence by a number of researchers. Research has been expanded from linearity to non linearity, from small deformation to large deformation, from static analysis to interactive analysis and from two-dimensional analysis to three dimensional analysis. This method permits the consideration of complex geological conditions and over burden fractures due to mining, and it is very flexible in the simulation of non- homogenates and discontinuities. Because the problem domain must be discretized into interactive elements, the discretization
  33. 33. errors occur through out the domain and large computer, money and time are needed for analysis. *) Boundary Element Method : In the Boundary element method, only the problem boundary is defined and discretized so that the discretization errors occur only on the boundary and less work is needed for the input data preparation and calculation. Distribution of stress and displacement throughout the domain is continuous and far – field boundary conditions are satisfied correctly. This technique has been extensively used in geomechanics, but its application in subsidence analysis is still very limited. Less flexibility in the simulation of non-homogeneous and and non-linear material behavior may affect the accuracy of predicting mine subsidence. *) Discrete Element Method : Another numerical technique in geomechanics is the discrete element method, which analyzes the discontinuous rock mass as an assembly of quasi- rigid blocks interacting through deformable joints between blocks. The algorithm is based on force-displacement law specifying the interaction between the quasi- rigid block units and a law of motion, which determines the displacements induced by out –of-balance forces. This method had been used in the simulation of coal mine subsidence by a feew people in China, but no published literature exists in western countries. *) For our Indian coal fields CMRI Scientists namely, Kumar, Singh and Sinha (1973) have given the following formulae for the estimation of subsidence, slope and displacement. Maximum subsidence Sm + t a Cos (d) Where t = average or weighted average of the thickness of the seam extracted. d= Dip of the coal seam a= Subsidence factor. Subsidence factor is the ratio of maximum possible subsidence to mining height. It mainly depends on the properties of over burden strata and roof control methods. For strong and hard strata ‘a’ is given by, A = 0.5 (0.9+P) Where, P is the co-efficient of combined strata properties, it is determined by the components of each stratum and their thickness, thus n n P =Σ( hi Qi )/ Σ h i= 1 i=1
  34. 34. Where, h is the thickness of the I th stratum above the roofline in the over burden, and Q is the corresponding co –efficient of stratum property (or) Rock factor. The rock factor Q is an index assigned to different type of rocks according to the hardness and its contribution to the surface subsidence. For the purpose of writing a computer program the strata rocks are classified into five types as sand stone, lime stone, shale, coal and clay. Each type of rock is again divided into seven ranges according to its relative hardness as extremely hard, very hard, hard, regular, soft, very soft and extremely soft. P Q Hard 0.0-0.3 0.45-0.6 Medium hard 0.3-0.7 0.6-0.8 Soft 0.7-1.1 0.8-1.0 *) Subsidence at any point situated at a distance X from the centre of the working is Y= Sm (1-x2 /1 2)2 Where, Sm =Maximum Subsidence I = the distance of zero subsidence from the centre of the working, *) Ground Slope, I I= 4(Sm/1) (x/1-(x/1)3 *) Ground displacement, n n= kh Tan (a) I Where, h= depth of working A= angle of draw K= is a constant and is equal to 3.5 5.2 Subsidence observation from the field study: Subsidence observations are very important for establishing and verification of any subsidence prediction model. Based on the collected subsidence information on Indian Coal fields especially SCCL, the nature of subsidence development at Indian coal mines has been explained and a subsidence prediction relation has been developed. *) Long wall panels of PADMAVATHI COLLIERY (SCCL), India: Subsidence observations conducted for a panel at Padmavathi Colliery of Singareni Collieries Company Limited, Kothagudem, and A.P.. is analysed. Subsidence monitoring lines were laid down in face advance direction and across it with the internal distances between two subsidence monitoring stations specified by the DGMS by circular No. 4/1998. As per the circular, interval between two monitoring stations with in workings is 30m, 10m over coal pillar
  35. 35. barriers and 15m over unmined areas. The panel, namely panel –2 is extracted from the Top seam. The over lying strata of then contained 78% of sand stone. The subsidence observations at this colliery illustrate the typical nature of subsidence development under the influence of massive sand stone in the over lying strata. Further, correlation between the first break and the formation of surface subsidence trough can be seen vividly at these panels. LONGWALL PANEL – 2 ; Panle-2 was extracted between 21-8-1995 and 16-4-1996 from the Top seam. Initial leveling was done prior to mining and subsequent observations were carried out during and after extraction at regular intervals. Initially, however, the observations were restricted to shorter intervals until the development of full subsidence. The panel details including face position in relation to main roof failure and full subsidence are given below. - Panel size : 650m x 150m - Minimum depth : 59m - Maximum depth :110m - Average depth of panel :85m - Dip of the panel :4 gon - Seam thickness ;9.59 m - Extraction thickness : 3.00M - Commencement of extraction :21-4-1995 - Completion of extraction :16-4-1996 - Face position at the time of first break of main roof :70m - Face position in relation to full Subsidence :129m As the extraction of panels proceeds, initial fall of immediate roof was reported when the face position was around 18m from the rear abutment of the panel. The magnitude of subsidence observed on the surface during it was insignificant. However, when the face position from the rear abutment reached about 70m, sudden and abrupt failure of main sand stone roof was observed. The subsidence magnitude on the surface until this time was not very significant. Rapid increase in subsidence magnitude started only after the first break of main roof. Increase in subsidence continued till the face position arrived to 129 m from rear abutment. The subsidence observed indicates very vividly the influence of first break of sand stone layer, which was laying over the extraction panel, on the development of subsidence on surface. Further, from the subsidence verses time observation, it can be implied that the magnitude of subsidence developed with in first 20 days was less than 10% of full subsidence and then followed within 18 days, after the first break, more than 90% of full subsidence. Additionally, it is observed that the cantilever hanging parts of sand stone layer over the abutment were allowing the development of subsidence within extraction area and arresting over the abutments and unmined area. *) Subsidence trough of longitudinal section : The dynamic subsidence profiles of the panel along face advance direction are shown in FIG with panel end positions. It indicated that the locus of first break was followed by a hump in the first break region. Additionally, it shows
  36. 36. non-uniformity of full subsidence with uneven subsidence trough bottom. The reasons for this unevenness at trough bottom can be, first, inconsistent orientation of natural joints and its frequency in the intact superincumbent strata, second, variation of periodic breakage span length of main roof due to variation of its thickness, and third, influence of variation in rate of face advance. Due to unevenness of subsidence trough bottom, the magnitude of full subsidence on longitudinal section can be less than the magnitude of full subsidence on transverse sections or vice versa, depending on the position of transverse section. In case, if transverse section passes over local hump, the full subsidence on transverse section is less than that observed on longitudinal section. The subsidence profiles on both the sides of longitudinal section indicate occurrence of only small magnitude of subsidence over abutments and development of steep flanks within the extraction area. However, the slope of flanks at rear abutment was higher than the front abutment. One of the main attributes for the above is the change of end positions of main roof during the first break and periodic break. During the fir break, the main roof clamped at all the sides whereas during the periodic break it clamped in three sides and fourth side was free, approximately representing a cantilever beam clamped at one side. Because of change in end position of main roof during first and periodic break, the critical span of main roof will not be the same in both the situations; first break span will always be greater than period break span. The full subsidence measured for 3-m extraction thickness was1.52 m, which is approximately 0.51 times of extraction thickness. Therefore, earlier repeated subsidence factor for Indian coal mines 0.5 to 0.6 is once again confirmed with observed subsidence factor at this panel. It must be remembered that this subsidence factor is confined only for panels having no influence due to old workings above or below and overlying strata of which contain high percentage of sand stone. Whenever old workings are present above or below an extraction panel the subsidence factor is shifted from 0.51 to 0.83 and it will be in the range between 0.8 and 0.9. *) Subsidence trough of transverse sections: Subsidence observation lines on transverse sections of panel –2 were laid down as shown in FIG. It shows the subsidence development on line 4 for different dates. It indicates the magnitude of full subsidence more than what was average magnitude of full subsidence on longitudinal section; this is due to occurrence of transverse line over peak point. The shape of trough on this section was almost symmetrical on both sides with full subsidence occurring at the centre of the panel. The slope of profile slightly differed from rise side due to variation in depth; however, it is very significant. Similarly, extension of trough over abutments also differed slightly due to the above reason. *) Limit angles and inflection points: Limit angles for the longitudinal and transverse profiles of panel-2 were calculated and are given in below table. A glance at the table indicates that limit angle at the face starting side was highest with 680 and lowest at the face ending side with 580. Further, it was noticed that the change in the limit angle was high after the failure of main roof and stayed almost the same on both sides after reaching full subsidence (after the
  37. 37. face position of 129 m). The limit angles of the transverse profiles indicate almost same for both sides with a value of around 600. Which is in between the limit angles of starting and ending side of face advance. Hence, it may be inferred that due to presence of hard rock in the overlying strata, the magnitude of limit angle in all the directions is not the same, rather different. Dip side Raise side Traverse Depth Distance Limit Depth Distance Limit angle Line angle 1A 108 78 60 98 70 61 2 103 75 60 93 65 61 2A 101 70 61 86 60 61 3 95 70 60 82 60 60 3A 91 65 60 78 55 61 4 86 61 60 75 55 60 Longitude Face ending side Face starting side Line face position (m) 58 65 22 79 65 10 90 75 70 35 71 65 10 90 85 70 35 71 65 20 81 99 71 41 67 65 20 81 118 72 52 60 65 25 77 129 73 55 59 65 30 73 181 76 49 64 65 30 73 200 77 55 61 65 30 73 350 87 60 62 65 30 73 454 93 65 61 65 30 73 545 99 75 59 65 30 73 596 102 74 60 65 35 69 650 105 80 58 65 35 69 Similarly inflection point, the point with half – maximum subsidence was measured from the edge of the panel in terms of panel depth (h). Below table show the measured values for longitudinal and transverse profiles. Here also, the position of inflection point was not the same in all directions. It was about 0.5h on the starting side and 0.63h on ending side and 0.4h on both the sides of transverse section. Further, it was observed from profiles that inflection point was shifting towards the goaf edge as the magnitude of subsidence was approaching full subsidence. Line Dip side (m) Raise side (m) 1A 0.37h 0.35h 2 0.37h 0.36h 2A 0.36h 0.43h 3 0.36h 0.44h 3A 0.40h 0.36h
  38. 38. 4 0.40h 0.37h Line A 0.5h (starting side) 0.63 (ending side) The observations of limit angles and inflection positions indicate that the shape of subsidence profile is influenced by the failure of main roof and composition of superincumbent strata. Further, the flanks of subsidence profiles, both on longitudinal and transverse section, did not allow the development of subsidence beyond the projected extraction area on the surface. In other words, the maximum portion of subsidence development was allowed within extraction area by hanging cantilever beam of sand stone over the goaf abutment. ** [INCLUDE THE DETAILS AND SUBSIDENCE PROFILES OF OTHER WORKED OUT PANELS]
  39. 39. 6.SURFACE SUBSIDENCE MONITORING Extraction of coal from under ground disturbs the overlying strata equilibrium causing ground movements of the surface. The movements of surface occur along vertical and horizontal planes. The properties of surface falling within the influence area of extraction get affected due to ground movements. The extent of such movements will be defined in terms of the derivatives of vertical and horizontal displacements such as tilt (vertical difference displacement), curvature (differential inclination ) and horizontal strain of both tensile and compressive (horizontal differential displacement) of the ground surface. Monitoring of such ground movements is essential for assessing the damage due to mining subsidence. Further, such subsidence data would help in under standing the pattern of ground movement leading towards establishing a stable prediction model. Subsidence monitoring on the surface is done with reference to permanent control stations, which are established outside the subsidence influence area. Generally, periodic monitoring of ground movements is practiced in most of the coal fields. However, when some important structures such as bridges, big buildings or main highways, etc. are falling within the subsidence influence area, continuous monitoring of ground movements is adopted. For establishing a prediction model and its parameters, measurement of ground surface movements with reference to the area extraction is important. During periodic subsidence monitoring the interval between each set of measurements should be selected in such a way that relevant data regarding the development of surface ground movements such as critical area of extraction for full subsidence, limit angles, inflection points in different directions of the panel and influence of local geology are obtained from such measurements. Under the presence of hard and competent layers in the over lying strata, observation of ground movements at short intervals gives detailed information about hard rock influence on the subsidence specially the first failure of the hard rock. Generally, it is suggested to obtain measurements at shorter intervals tilt the development of full subsidence and wider intervals later while the change in the trough shape after development of full subsidence remains almost the same. Continuous monitoring of ground movements provides information about the object behavior due to mining of deformation. It is used to study the influence of the extraction panel on the object and also to give warning alarm when the monitored value exceeds the specified safety value. However, continuous monitoring method is not a common method. 6.1 Establishments of the subsidence monitoring lay out: In addition to adoption of a suitable monitoring method for ground movements, establishments of a monitoring station layout is equally important. A planned layout of observation stations will provide detailed information about ground movements of the influenced area. Generally, in order to get a subsidence profile along the major and minor axis of an extraction panel, subsidence – monitoring lines are made along the face- advanced direction and across pit. This is the simplest arrangement and it involves less amount of time for measurements. Further, under set of only vertical movements of the observation stations are measured. The main drawback of this sort of layout is that it provides the subsidence profile only for certain sections of a panel. It may not be possible to get the ground movements for the remain area.
  40. 40. In addition to the above approach, a net work can be formed on the panel with equal distance between points. This gives more information about the pattern of subsidence development. However, the number of points for measurement is more compared to the earlier layout. When the distance between two points is less, more precise information about trough formation is obtained. Generally, for measurements of horizontal and vertical movements a 10m distance between two points is adopted. In India, the DGMS as laid down the guidelines for laying out subsidence monitoring stations. According to it that the distance between two monitoring points with in the working area should not be more than 30m and out side the panel less than 15m and above the barrier with in 10m. However, there is no prescribed pattern of subsidence layout in the mine rules. Therefore, at each mine, observation station lines are laid down according to the understanding of the mine surveyor. When objects are structures fall with in the influence area of extraction, individual monitoring stations can be established around the object in addition to identifiable points on the objects for monitoring the deformation or the ground movements of an object. It is summarized that under the existing economic condition, the conventional surveying instruments such as levels or tachometers are more suitable and cost effective in India. However, for establishment of subsidence controlling stations from national survey grid or pithead, Global-positioning system is very much suitable as it takes less time and gives the required accuracy. Aerial photo grammetry is not suitable for monitoring subsidence in India where the subsidence monitoring area is small and hence the cost of flying is very high. The application of remote sensing is limited to monitoring the environmental changes due to mining. At present, subsidence monitoring layout and distance between monitoring stations are made according to the knowledge of mine surveyor. It is suggested that monitoring stations network with equal distance between two stations may be set up so that the data generated can be used for further detailed analysis and for understanding the pattern of subsidence development so as to avoid or reduce subsidence damage through proper planning of extraction panels. Further, it shout be ensured that all the monitoring stations are setup with proper foundation so that there will not be any influence of top soil on the station. 6.2 Various methods of monitoring With available technology today, ground movements can be monitored at short intervals with high accuracy. Various surveying instruments and techniques are available for continuous or periodic monitoring of the ground movements as well as the deformation of the structure. 1. Monitoring with various survey Instruments : Periodic monitoring of ground movements: Generally, periodic monitoring is carried out either with leveling equipment for vertical and tachometer for both horizontal and vertical movements measurement. In addition to these conventional instruments, the Global positioning system receivers, aerial photographs and satellite imagery can be used for periodic monitoring of the ground movements. Modern digital levels and electronic tachometers greatly help in the measurement of ground movements and processing of such digital data, later through the computers reduces the cost and time of surveying and mapping operations.
  41. 41. i) LEVELS ; Precise levels which include digital levels or levels with plan-parallel plate micrometers together with invar staff give an accuracy between 0.4mm to 0.6mm/km (for double measurement) while the engineers levels provide an accuracy of more than 1-2 mm/km. They are being used in mining for establishment of bench marks and for determination of settlements due to mining subsidence. The modern leveling equipment’s are very compact and are easy to handle since they are self- leveling. Accordingly to German mine surveying regulations, the required accuracy for precise leveling is 2x sqrt (s) mm, where ‘s’ in km. ii) TACHEOMETER ; Modern electronic tachometers provide accurate measurements of angles and distance by transmission of a beam of light from the base instrument to a reflective survey point. The modern electronic tachometer consists of digital precise theodolite, electro-optical range finder, micro computer, program module and recording unit. It provides and store the x, y and z co-ordinates of a point immediately after measurements. Further, it also records the zenith angle, slope distance, and horizontal azimuth and then computes and record horizontal distance, direction and deference in elevation. There are tacheometers, which can measure up to 5 km to an accuracy of 1mm + 1ppm. Zeiss Rec Elta total station is one of the latest instruments which work with reflectors and also without. Its power and speed of measurement help additionally in measuring moving tangents. Many tachometers base units can be fitted with servo drives which can speed up setting out operations considerably as the instrument would assume vertical and horizontal settings by it self. An instrument such as the Geotronics Geodimeter system 600 can automatically calculate the bearing and distance of a previously stored point and will sight by itself at that point for setting out. The angle measurement of different points can be easily done by this instrument by sighting in the tangents and then allowing the servo motor to automatically carry out the respective measurements. Geotronics claims that the servo techniques have allowed the development of a new method of sighting for use in conventional surveying. Many electronic tachometers are up graded so those customers can start with a basic instrument and add on later the requisite hardware to provide extra facilities that they needed. iii) GLOBAL POSITIONING SYSTEM RECEIVER ; GPS receiver functions area a space based positioning system, which is a world wide, all weather system to provide three dimensional co-ordinates of a point or position. It has revolutionised the whole surveying system. Its importance has been felt in mining industry in : I) Establishment of baseline near to mine from a national survey grid, II) Preparation of a reliable and accurate local network connecting all the mines of an industry, III) Determination of co-ordinates of control stations required for mapping by remote sensing or aerial photography and
  42. 42. IV) Guidance of mining machines (drill rigs, shovels….) to a precise position. Further, experiments have been carried using GPS receivers for closely monitoring the ground movements due to mining activities. With GPS, there is no need to point the instrument at a target. Thus, surveying is not hampered by poor visibility and obstacles between measuring stations. Further more portable GPS receivers can be back packed or mounted on a vehicle while the hardware in it is electronics and not optical and hence not fragile. With minimum of two GPS receivers, one as base station receiver and another is roving receiver surveying can be done to a higher accuracy with time reduction. The stationery receiver is setup at a known co-ordinates point near the mine pit and the roving receiver is setup at the points whose co-ordinates are desired. Further, surveyed data can be transferred directly in to the computer system for calculation of desired co-ordinates. Since, 1993 instruments based on real time location technique have come into market using radio broadcasts. The stationary carrier phases GPS data is transmitted to a roving GPS receiver. The roving receiver processes the received data from the stationery receiver and its own data to get in real time co-ordinates of its location with centimeter level accuracy. Co-ordinates are displayed on a hand held controller/key pad. This real time kinematics method has been gaining very good applicability in the surface mining industry. The latest developments in it are faster times to the first fix, lighter, and more compacts and portable units with battery power. Tremble Navigation of the US made a break through in the re-initialisation of rover’s receivers. If the signals coming to the receiver fall less than four satellites, due to obstruction, they must return to a known survey point to re-initialisation of the system. Where re-initialisation is a problem, new types of total stations using dual frequency have been developed which enable the surveyors to stop anywhere for a minute or so to re-initialise, or even to re- initialise on move. iv) AERIAL PHOTOGRAPHS ; Aerial photography is another surveying method, which is frequently employed in geotechnical engineering and mining, particularly for periodic measurement of ground movement due to mining. Aerial photos are taken over the survey area with the help of an aircraft flying at a certain altitude to get the required scale of photographs. Since subsidence development is a dynamic process/phenomenon during active mining, information regarding ground movements can be recorded by aerial photographs within a few hours while the conventional ground survey methods would require number of days to gather the same data. By taking photos, surface conditions are frozen in time and the station positions are computed at the instant of the photography. Aerial photography has got definite advantage when compared with conventional survey. It is possible to conduct surveys of inaccessible area. However, a ground survey, with GPS or Tachometer, is required to establish a few control points outside the subsidence area if no control points are available. The aerial photographs provide complete view of the photographed area with selection of any discrete natural object as monitoring point and allow the
  43. 43. interpreter of the photograph further to assess the subsidence impact of the entire environment, including the vegetation in that area. The photographs further permit re-evaluation, re-measurement and acquiring of additional information, which might not have been recognised earlier as important data, at any later time. There are a few constraints, however, in photo grammetric surveying. A clear cloud free atmosphere is a must for aerial photography. The points must be imagined on the photographs with out being hidden by the surrounding objects if they are to be surveyed. The cost aspect of the photo grammetric surveys makes it unsuitable for small areas. The major draw back of aerial photogrammetry is that it is not economically viable if repeated set of measurements is required at shorter intervals. The level of accuracy obtained is less in comparison with modern conventional ground surveying instruments. Since 1980 aerial photogrammetry has been in use in Ruhrkohle AG, Germany for prediction monitoring of subsidence, to fulfill various statutory requirements. It is reported that aerial photogrammetry is an economical method for generation of large amount of data for the company in combination with Geographic information system, particularly in recording ground movements de to mining and for preparation of differential digital terrain models. Further, the RAG is conducted experiments to use digital aerial photogrammetry in combination with remote sensing techniques in order to generate automatic digital terrain models with low cost. The US Bureau of Mines conducted aerial survey for monitoring the ground movements due to mining activities. It was reported that three- dimensional displacements measured with electronic tachometer and aerial photos for a point were almost identical. In general aerial surveying cannot be recommended as the only surveying method to monitor the surface subsidence due to its limitations. v) SATELITE IMAGES; The use of satellite images at present is limited to mapping of the biophysical changes of the ground surface of the subsidence area. The thermal infrared imagery are useful for identifying the under ground coal fire zones. Remote sensing data, however, are only complementary data to the subsidence observation data generated by other surveying methods. Even though it is possible to determine the height of an object using remotely sensed stereo images, the application of it for subsidence measurements is restricted because of the limited spatial resolution and consequent inadequate accuracy. The application of remote sensing for monitoring subsidence damage may be possible, as new interferometric methods become operational. The analysis of the data provides information about the changes that have taken place in flora and fauna, water bodies, moisture pattern and presence of faults and fractures due to subsidence.
  44. 44. vi) CONTINUOUS MONITORING: Continuous monitoring of ground movements is under taken in particular cases where buildings, railway tracks or bridges fall within the influence area of extraction. This required special arrangements and would increase the cost. For monitoring ground movements, selected points are fitted with leveling staves/ reflectors and these will be measured at the specified time intervals. The data are transmitted to a central place for further analysis which will provide warning signals when a measured value exceeds the specified levels. Various companies manufacture the digital levels and electronic tacheometers with a provision for continuous monitoring. Leica has a developed a motorised digital leveling instrument NA 3003.A motorised unit is added to the normal digital level NA 3003. It consists of three parts one for rotation, the second for focusing and the third for controlling and interfacing. All the three parts are responsible for correctly turning the leveling instrument to the staff field and recording the readings. With a standard motor and normal width of staff, distances up to 80m from the instrument can be measured. For a rotation of 100 gon, it takes 40 sec. As soon as the instrument is turned to wards the staff, it automatically focuses the staff and readings are transmitted to central place. For simple measurements it takes about 5 sec. If the instrument needs more than 60 sec for measuring a point, automatic measurement function is stopped and a failure message is transmitted with in 5 sec. In Germany it is being used to monitor the deformation of the bridges and buildings due to mining activities. In addition to digital leveling, Leica has developed the Lieca Automatic Polar System (APS) for continuous deformation monitoring. It consists of motorised Lieca TM 3000 V theodolite and system controller software. The APS is designed to continuously monitor a network of electronic distance measuring reflectors of an area subjected to deformation. A CCD camera integrated in the telescope of the TM 3000 V defects the target. Image processing hard ware and software in the system controller performs automatic target detection and measurements. It claims that 1 mm for 100m (deformation) accuracy has been achieved with the above instrument. vii) SLOPE MEASURING INSTRUMENTS; In addition to the standard surveying instrument which measure direction, distance and elevation difference, there are other special instruments called tilt meters or inclinometers to measure precisely change in slope of the objects which are with in the subsidence area these are portable instruments for measuring the tilt in structures such as buildings, dams and embankments and also for measurements related to the stability of slopes, open pits and walls of excavation. The instrument is attached to the structure to be monitored. Measurements can be made on horizontal or vertical surfaces. Subsequent sets of reading show how the structure is behaving and will give an indication of permanent deformation as time progress.