Ground Subsidence Process Analysis - Coursework Example

Subsidence H.B. 1041, 106-7-103(10 Ground subsidence" means a process characterized by downward displacement of surface material caused by naturalphenomena such as removal of underground fluids, natural consolidation, or dissolution of underground minerals, or by man-made phenomena such as underground mining Characteristics of subsidence Subsidence may occur abruptly-virtually instantly-or gradually over many years. It may occur uniformly over a wide area as local depressions or pits separated by areas which have not visibly subsided. It happens most common in the sedimentary rocks over abandoned coal and clay mines. The crystalline rocks in which most metals are mined have greater strength and are less likely to settle or collapse. Subsidence can also occur where underground water has dissolved subsurface materials or has been withdrawn by wells. Sinking caused by the caving in of underground mine workings. Consequences Subsidence can result in serious structural damage to buildings, roads, irrigation ditches, underground utilities and pipelines. It can disrupt and alter the flow of surface or underground water. Surface depressions created by subsidence may be filled in, only to sink further because the underground void has not been completely closed. Areas may appear to be free of subsidence for many years and then undergo renewed gradual or even drastic subsidence Aggravating Circumstances Weight, including surface developments such as roads, reservoirs, and buildings, and man-made vibrations from such activities as blasting, heavy truck or train traffic can accelerate the natural processes of subsidence. Fluctuations in the level of underground waters caused by pumping or by injecting fluids into the earth can initiate sinking to fill the empty space previously occupied by water or soluble minerals. Descriptive definition In general, the type and severity of surface subsidence is governed by the amount ground surface and the location of removal or compression, and the geologic conditions of a particular site. Withdrawal of pore fluids, usually ground water, is a common cause of ground subsidence. Massive lowering of the groundwater table by "mining" of ground water* in a poorly consolidated aquifer results in subsidence of the ground surface. Hydrocompaction produces ground surface collapse from excessive wetting of certain low-density weak soils. This can occur in two general types of soil, a) wind deposited silts b) predominantly fine-grained colluvial soils. In either case, collapse occurs from excessive wetting of previously dry, collapsible soils. Wetting of these materials weakens the already weak or unstable soil structure, which undergoes internal collapse and densification (reduction of air voids). Densification of the weak soil column produces ground surface collapse and subsidence in the vicinity of excessive wetting. Removal of fine material by piping* is probably an additional factor in some cases of subsidence by wetting. Such excessive wetting can occur from irrigation, broken water lines, surface ponding, or drainage diversions. Dissolution of soluble rock or soil materials also results in ground subsidence. This occurs in areas underlain by highly soluble rock formations-especially gypsum (CaSo4. 2H2O), or halite (NaC1); and to lesser extent in limestone (CaCO3) materials. Removal of earth materials by water solution leads to surface collapse. Hydrologic factors that may cause the solution and removal of material may be natural or man-induced. Natural solution is the result of the normal hydrologic processes of downward percolation of surface water and/or lateral movement of ground water within the water table (either the main ground water table or a perched water table). Man-induced hydrologic changes or activities can have much the same effect on soluble earth materials. Such activities include temporary or permanent stream channel changes, irrigation ditches, land irrigation leaking or broken pipes, temporary or permanent ponding of surface waters, the mining of soluble minerals by means of forced circulation of water within the earth. Removal of support by underground mining is a common cause of ground subsidence. Extensive removal of minerals, mineral fuels, rock aggregate, and other materials results in large underground void spaces. Subsequent natural processes including fracturing, chemical changes, caving, flowage, and other related adjustments often produce surface subsidence, fissures, and tilting of the land surface above and/or adjacent to the surface projection of underground workings. Man-induced changes in the hydrology of the underground workings and/or overlying rock and soil materials can affect subsidence. In addition to actual undermined areas, special hazards are posed by certain appurtenant structures such as air shafts and various other mine workings. Additional problems in identifying and delineating areas of potential subsidence include the presence of faults and other geologic complications, and the fact that "final mine maps" may not show the actual extent of mining. Also, discrepancies in survey ties between the mine maps and surface reference points may be sizeable. http://geosurvey.state.co.us/Default.aspx'tabid=358 example: Shanxi Province is in north of China, it has one of the biggest mining in China. Over half of total mining had been dig out therefore serious subsidence always occur in Shanxi Province. Subsidence will cause a lot of problems such as collapse of buildings, collapse of bridge, flooding and so on. I would like to talk about the effects, precautions and remedial measures for bridge, high way, pipes and electricity Effects of movement of bridges due TO SUBSIDENCE Abutments, piers, adjacent supports can move apart or move together, tilt or twist in the horizontal or vertical planes, and settle relative to each other. The bridge spans will then tend to resist some or all of these support movements according to its design, stiffness and their connection to the supports PRECAUTIONS FOR BRIDGE due TO SUBSIDENCE It is necessary to collect information on: (a) Details of the bridge design and its relation to local topography (given in the diagram) (b) The near-surface soil conditions (given in the question sheet) (c) Levels of local water courses and the possibility of flooding or lack of headroom under bridges over water (given in the question sheet) (d) Details of the mining operations in space and time, including the direction of approach of the working face, the geology, and data on movement already experienced or to be anticipated in the area. By investigating the bridge, it should be concern as to whether greater possible subsidence can be provided for by increasing clearances at abutments, providing for movement at horizontal joints, by cutting gaps and even removing certain redundant members. A bridge which is in need of improvement is likely to become dangerous as the result of mining operations; it might then be a good idea to think of demolishing the bridge before damage occurs and replacing it with a more suitable bridge which incorporates with facilities that deals with subsequent movements. In attempting to appraise the likely distortions of the bridge and its supports it may usually be assumed that the ground supporting the bridge will suffer the same displacements as if the bridge were not there. But, in addition, the subsequent points need careful study. Firstly, many abutments are also retaining walls with a small factor of safety against instability and such stability as exists is often aided by the superstructure though that may not have been the purpose of the designer. the action of mining subsidence may excessively reduce the stability of the abutments and any wing walls. Secondly, where the bridge crosses a deep natural or artificial cutting or a high embankment, the strains of mining subsidence may cause a landslip which may cause danger to the bridge. Natural valleys are sometimes the sites of deep-seated faults at which mining often terminates. In such cases damage at the surface will be much more severe than under normal situation. It may be sensible to enforce a speed restriction on traffic and maybe also a load limit. REMEDIAL MEASURES for Bridge Procedures that are taken to remedy the defects in bridges which are caused by mining subsidence will regularly be of a similar nature to those adopted in bridges subjected to natural settlement or deformation from landslips and war-time damage, and reference may be made on such topics like underpinning and repairs. Damaged foundations can be strengthened by underpinning; the work should be undertaken in short sections, the positions of which must be cautiously chosen, every section being finished before the next is commenced. Where there is a chance of further subsidence taking place and where the monolithic character of a pier or abutment has been or is likely to be affected, continuity between the sections of underpinning should be effected to counteract inequality of settlement. For this it may be compulsory for reinforcing bars or post-tensioning methods to be employed. Foundations which have been seriously disturbed, it may then be sensible to take up bored cast-in-place piles in conjunction with a sequence of needle beams. The excavations essential for remedial works of this kind will facilitate inspection of the existing foundations and enable defective parts to be repaired or replaced as part of the underpinning operation. Consequently, damage to arches may be remedied by centering and rebuilding the defective part of the arch. Alternatively, where circumstances permit, an additional lining in brickwork or reinforced concrete may be constructed. Generally, this will require new skewbacks to be cut in the abutments. Further move'ment where it is particularly is due to draw will be anticipated, consideration may then have been given to have the theory of three-pin structure which use steel or reinforced concrete. In correlation with this work pressure grouting is to be suggested for repairing fractures and ensuring a close contact between the existing arch ring and any relieving arch beneath. Pressure grouting of the fill in a spandrel-filled arch may also be helpful when the subsidence is done, but only certain types of granular fill are suitable for this behavior. EFFECTS OF MOVEMENT OF HIGHWAY due TO SUBSIDENCE Evidence has disclosed that all parts of the highway can be affected by mining subsidence and report illustrate that substantial damage has been caused in the past. The effects of movement can be summarized as follows:- (a) Severe irregularity caused by uneven settlement of carriageways associ'ated with faulting or shallow workings. (b) Distortion in horizontal and vertical alignment of the highway. (c) Fracturing of carriageways leading to deterioration of road foundations and necessitating reconstruction. (d) Corrugations on the running surface. (e) Fissures in the road surface and in the rocks beneath. (f) Damage and displacement of kerbs and channels, flagging to footpaths and boundary fences. (g) Damage and displacement, and alteration of gradients, of the storm water system and consequential flooding of the highway. (h) Consequential damage to the highway following the bursting of water and gas mains, caused by the pulling of joints or the fracturing of pipes, and the failure of underground cables by the pulling of joints. (j) Flooding of highway due to settlement. The most common types of damage reported were undulations and distortions of the carriageways, cracks in carriageways caused by tension, and corrugations in the carriageways caused by compression. In some cases, considerable lengths of roadway have had to be closed and reconstructed due to the distorted conditions of the surface and foundations or due to flooding consequent upon the lowering of the surrounding land. PRECAUTIONARY MEASURES Public utility services should be laid under the verges where it is likely to avoid digging up the carriageways when repairs are needed, where in a mining area it occurs quite frequently, and pipes should have flexible and extensible joints which will withstand subsidence movements. Sometimes it is advisable to wait at least until movement has ceased in order to lay services above the ground. Surface-water drainage systems and culverts should be articulated, and expansion joints should be left round gulley. Ditches filled with coarse granular material have been used instead of pipes. Drainage systems have been laid will increased gradients to ensure flow after subsidence occurs. Where a road is liable to become flooded due to general settlement of the area, the only precaution which can be taken is to raise the road before subsidence occurs. Reflective or illuminated warning signs should be used where subsidence damage has occurred. REMEDIAL MEASURES (a) Temporary repairs are such that they make the road commodious to traffic, and can be carried out quickly and at a minimum cost, bearing in mind that it may be a considerable time before the movement beneath the highway is complete. This movement depends upon many factors, according to the depth of the seam and the position of the workings in relation to the highway. These works should be of a purely temporary nature and the materials used should be as flexible as possible to allow for further movement. (b) Permanent repairs should be carried out only when settlement of the surface is complete. Before carrying out these works the mineral undertakers should be consulted with regard to the possibility of future workings. EFFECTS OF MOVEMENT ON Pipes due TO SUBSIDENCE There are special conditions that are imposed on an underground pipeline in areas which subjects to mining subsidence, these are as follows: (A) Due to the passage of a transverse wave: (i) A slow and more or less uniform longitudinal bending and stretching of the line in the vicinity of the wave. This action travels along the line as the wave advances. (ii) A slow and more or less uniform shortening of the pipe behind the wave as the ground reaches its new level. (b) Due to the passage of a parallel wave: (i) Possible disturbance to the uniformity of the bedding with following local longitudinal bending and local concentra'tions of failure load at the "beam" reaction points. (ii) Severe permanent bending and stretching at the ends of the wave. (iii) Possible permanent lateral displacement with stretching of the line. (c) Due to the passage of a wave inclined to the pipe axis at an angle of less than 90'; (d) In the vicinity of a "pillar" or at a stopped or an abandoned face: (i) In a line at right angles to the face, severe permanent longi'tudinal bending and stretching, with possible concentra'tion of movement at particular joints. (ii) In a line parallel and near to the face of the subsiding side, permanent and possibly uneven lateral and vertical dis'placement with possible disturbance to the uniformity of the bedding and with more severe distortion in both planes at the ends of the pillar or face. Concentration of move'ment at particular joints may occur in either or both planes. (e) Due to geological or induced faults involving abrupt changes of level at the surface: (i) In a pipeline at right angles to the fault line; very severe permanent longitudinal bending and stretching accompanied by local disturbance to the bedding and high shearing forces on the pipe in the vicinity of the fault plane. (ii) In a pipeline parallel to the fault line and on the downthrown side; effects similar but possibly more severe and variable than those described in (b). Summarizing, the pipeline may be called upon to suffer:- (1) Severe longitudinal (axial) bending tending to rupture it transversely. (2) Possibly severe crushing overloads tending to rupture it longitudinally or to cause buckling or even complete collapse. (3) Considerable axial lengthening and shortening tending to cause drawn joints (especially at bends, branches, manholes or valves), or axial buckling or crushing, upheaval or broken sockets; their effects being aggravated by local resistance to sliding due to projections beyond the pipe barrel, or other anchorages. (4) Severe transverse shear forces. PRECAUTIONARY MEASURES for pipe due TO SUBSIDENCE In areas which will subject to mining subsidence careful consideration should be given to the material of the pipes and to their jointing. Pipe behavior and desiderata - The method in which a pipeline responds to the various movements and forces to which it is subjected will depend on:- (a) The strength, thickness, and flexibility of the material used for the pipe wall. (b) Whether the pipe itself is flexible or depends for its flexibility on flexible joints. (c) The spacing of flexible joints where these are used and their ability to accommodate displacements of the pipe in any direction. (d) The degree of axial anchorage, if any, imposed on the pipeline. The alternative material is resolute into a compromise between elastic character'istics, resistance to corrosion and cost, in relation to the desirable life of the pipeline and local conditions. An effort has been made below to categorize the range of possible materials. However, the choice must be governed economically e.g. the size of the pipe, and sound engineering judgment which is based on knowledge and experience of local setting. In order, to meet the special conditions of loading imposed on a pipeline by ground movement, the order of preference of the material is, disregarding corrosion resistance, "Flexible", "Semi-rigid" and "Rigid". In account to their flexibility, steel pipes have the advantage of being able to weld; they are handicapped, however, by the necessity of protection against corrosion. Any coating used for this purpose should be flexible as, if brittle; it may crack or be displaced when the pipeline is deformed. For pipes of brittle materials, suitable joints will then be essential. The smaller the distance between the joints the greater the axial flexibility of the line and the lower the crushing overload which may develop due to irregularities in bedding. This provision may materially increase the cost of the line, longer than those specified in the various British Standards, to cope with the expected large axial movements of individual pipes, failing which joints may "pull" locally with consequent leakage and consequential damage. Joints - (a) Non-pressure pipes - Of the flexible joints which is currently available the simplest and possibly the least expensive way is the single rolling rubber ring which is squeezed between the external surface of the spigot and the interior surface of the socket. This joint has proved suitable for concrete pipe sewers over many years and it is now being used for pressure pipes. Hot poured and cold-caulked compositions have also been used but there is little evidence of their behaviour in service. The necessity for the joint to open and close, possibly several times in the course of the life of the pipeline, places severe limitations on these com'positions. (b) Pressure pipes - There are numerous mechanical flexible joints on the market all depending more or less on rubber sealing rings in various forms. Choice will again depend on the degree of angular movement provided and the ability of the joint to open and close without leakage, and also on cost. Pipe laying and bedding - In view of the probable disturbances to the pipe bed, which may be expected, and the detrimental effects of irregular bedding on the pipe loads, particular attention to uniformity of the bed material over an adequate depth is enviable in order to prevent or reduce the formation of hard spots. Where the trench bottom is of uneven resistance or where the strata in which the pipe is laid are likely to deform unevenly as subsidence occurs, it will be advisable to excavate well below the formation level and re-pack with soft uniform material such as sand, fine gravel, ashes, flue dust, sandy clay or broken shale and surround the pipe with the same material. pipes are bedded in concrete, gaps should be left in the bed at every joint to maintain the potential flexibility of the pipeline. The depth of cover should be kept as small as possible in the interests of maintenance. REMEDIAL MEASURES for pipe due TO SUBSIDENCE The section where the pipes are damaged should be cut out and should be replaced by other pipes of suitable materials, with flexible joints and in short lengths. The damaged section may be under high compressive stress; if so, great care should be taken when cutting it out as the sudden release of stress can give rise to violent results. If joints are rendered permeable they may then need renewing, possibly with flexible couplings. In some cases leak clamps may be used. EFFECTS OF MOVEMENT ON Electricity due TO SUBSIDENCE Underground cables - The effect of ground movement on an under'ground cable system varies with the method used in laying the cable, the type of cable installed, and the type and harshness of the movement. In most of the cases, the effect of movements is shown at cable joints where relative movement between the internal conductors and the external sheathing takes place. Extension which occurs at the core-jointing ferrule may be completely drawn and this will then result in a discontinuity; where in the case of compression, buckling of the conductors may give rise to their earthed by contact with the lead covering over the joint in the sheathing. In harsh cases the lead cover itself may be wrecked. Where lead-cover cables are used, the cover itself may be cracked; this may allow moisture to enter the cable and give the chance for electrical breakdown to be very high. Such breakdown may also occur in cables with paper-insulated conductors where harsh localized deformation causes extensive tearing of the paper. On high-voltage cables the faults usually become obvious by causing pro'tective-gear operation and allow fault location, but on low-voltage cables where ring mains are familiar, open circuits which occur may not be evident for a significant time. A fastidious risk with low-voltage cables is the likelihood of wrecked neutral conductors, but this is usually protected against. However, underground link which disconnect boxes may develop faults; they have some'times been broken by movement of the associated cables. Overhead lines - The effect of ground subsidence on overhead lines is not as serious as the underground cables, since the effect can be seen and precautions can then be taken to lessen such troubles. For wood-pole lines, local subsidence typically results in displacement of poles from the vertical with resultant upset in conductor tensioning. In extreme cases, if not rectified, this may result in either conductor conflicting or over tension and breakage of the conductor, depending upon the nature of the sub'sidence and the type of pole. As for the steel-tower overhead lines the effects are rather more serious for the subsequent reasons:- (a) Steel towers are usually very much higher than wood poles; hence the displacement of the top of the tower will be much greater for the same ground movement. (b) Steel tower lines usually use suspension type insulators. Any displacement of the tower top may result in reduced electrical clear'ances and the likelihood of electrical flashover. (c) The foundations of steel towers are typically of the concrete block type, the foundation of each leg being independent. Any subsidence which is not uniform over the area of the tower base therefore results in deformation of the foundation stubs and the lower members of the tower structure. (d) Longer spans are also used with steel-tower lines comparing it with wood-pole lines (800 to 1,000 ft as against 300 to 400 ft). Unless, the subsidence is extensive, therefore only one steel tower may be affected. With the local subsidence of one tower, conductor uplift conditions may be produced under low-temperature conditions or a reduction of ground clearance beyond the allowable statutory limit. PRECAUTIONARY MEASURES for Electricity due TO SUBSIDENCE Underground cables - Places where subsidence is expected, cables have been flaked from side to side of the ditch and buried in sand or laid in a similar manner in a wide concrete duct filled with sand. The results of such measures have not, however been completely suitable. It has been found that with high voltage cables, it is preferred to use cables with a double layer of steel wire armouring to resist abnormal tension effects in combination with a form of expansion joint which permits horizontal movement of the conductor cores. The other method which is adopted to protect joints against electrical breakdown is to incorporate expansion pits on each side of the joint. The cable is flaked sideways in these pits in order to permit movement without damage to the joint itself. The pits can be made simply handy for inspection purposes and by marking the original position of the cable, the extent of any movement can be measured and the need to excavate and inspect the joint itself assessed. Expansion joints are not used on medium or low voltage cables, the neutral conductor may be provided with an over-length jointing in a normal joint so that the phase conductors will fracture before the neutral, hence avoiding unbalanced voltages on single-phase services. Practicable, cables can be effectively carried over a danger area, supported on cleats mounted on low concrete posts. Overhead lines - usually no special precautions are taken in the erection of an overhead line to guard against subsidence. In the case of steel tower lines but it is usual to investigate the projected route with a view to avoiding unstable ground in the vicinity of tower positions. The working of the deeper coal measures do not usually concern but any unworked measures near the surface should be avoid by arranging some degree of security with the mineral undertaker. Line routes should be frequently patrolled when subsidence is likely to occur so that any movements can be detected at an early stage and appropriate steps taken to minimize damage. REMEDIAL MEASURES for Electricity due TO SUBSIDENCE Underground cables - There are a number of ways which are possible to cause damage:- (a) The affected part may be replaced and provided with expansion joints at each end, but consideration would have to be given to the extent of further possible subsidence. (b) The replacement cable may be laid temporarily above ground and drop'ped into a prepared trench when subsidence is complete. (c) The repaired cable may be diverted to avoid the subsidence area. (d) An overhead line may be erected to replace the damaged cable. When damage is confined to electrical breakdown at joints, repairs may well be affected by lengthening or shortening the conductors and thus need to provide a new lead cover, if required it will be over the outer layer. Overhead Lines - When a wood-pole overhead line has been affected by subsidence, the poles can typically be straightened up to their original vertical position or satisfactorily stayed to avoid further deterioration. The conductors can then be re-tensioned if necessary. As for steel-tower line, the procedures to be adopted will depend on the effects of the ground movement. For example:- (a) When a tower remains vertical after subsidence, provided that the electrical clear'ances are unaffected and ground clearances can then be adjusted by re-tension'ing if necessary, no additional action is required. This may also apply where a tower has tilted initially and returned to a level keel when ground movement has been finished. (b) If a tower develops a permanent tilt it may be re-leveled by jacking. (c) When subsidence is complete, any damaged bracing members should be replaced (d) When extreme tilt is experienced, the line will have to be taken out of com'mission and the tower guyed or stayed until it can be safely re-erect on new foundations in an adjacent position. (e) The overhead line where conductors are overstressed is the result of subsidence damage to towers as it may be necessary to re-'tension them, depending on the harshness of the overstressing. EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Subsidence: Dissolution & Human Related Causes http://www.tulane.edu/'sanelson/geol204/subsidence.pdf SUBSIDENCE OF THE ATLANTIC-TYPE CONTINENTAL MARGIN OFF NEW YORK M.S. STECKLER and A.B. WATTS http://www.earth.ox.ac.uk/'tony/watts/downloads/stecklerwatts78.pdf Land Subsidence in the United States by' Devin Galloway, David R. Jones, and S.E. Ingebritsen http://pubs.usgs.gov/circ/circ1182/ U.S. Department of the Interior, U.S. Geological Survey Persistent URL: http://pubs.water.usgs.gov/cir1182 Page Contact Information: Publishing Service Center Last modified: Thursday, September 01 2005, 01:59:11 PM http://qjmam.oxfordjournals.org/cgi/content/abstract/39/1/85 http://qjmam.oxfordjournals.org/cgi/content/abstract/39/1/85 Read More