Mine Subsidence Khondkar A Saleque. PE ( Engineers Australia) FIEB

Mine SubsidenceMine Subsidence

Khondkar A Saleque.Khondkar A Saleque.

PE ( Engineers Australia) FIEB.PE ( Engineers Australia) FIEB.

Why Subsidence?Why Subsidence?

• Subsidence may be caused by Subsidence may be caused by failure of the mine roof, coal failure of the mine roof, coal pillars, or mine floor materials. pillars, or mine floor materials. Subsidence may take form of Subsidence may take form of sinkholes or a chimney –type sinkholes or a chimney –type caving, a gentle sagging or trough caving, a gentle sagging or trough type subsidence, or settlement of type subsidence, or settlement of back fill materials in entry or back fill materials in entry or airshafts.airshafts.

Caving SubsidenceCaving Subsidence

• Caving Subsidence:Caving Subsidence:

Subsidence can produce sinkholes or Subsidence can produce sinkholes or depressions at the ground surface by caving depressions at the ground surface by caving of materials overlying comparatively shallow of materials overlying comparatively shallow mine working. Caving occurs as the roof over mine working. Caving occurs as the roof over opening fractures and collapses into the opening fractures and collapses into the space where extraction has occurred. This space where extraction has occurred. This process continues until the space is either process continues until the space is either occupied by debris or the caving reaches the occupied by debris or the caving reaches the surface surface

Nature Of Caving Nature Of Caving Subsidence.Subsidence.• Caving is common, over room and Caving is common, over room and

pillar operations. The depth of mining pillar operations. The depth of mining and thickness of the bed rock are and thickness of the bed rock are critical in determining whether or not critical in determining whether or not subsidence feature will reach the subsidence feature will reach the surface. The thickness of the extraction surface. The thickness of the extraction and bulking & strength characteristics and bulking & strength characteristics of the overburden rock is also of the overburden rock is also important important

Caving SusbsidenceCaving Susbsidence

• The size of sinkholes caused by caving is The size of sinkholes caused by caving is controlled by the geometry of the mine controlled by the geometry of the mine and properties of overburden. The depth and properties of overburden. The depth of the sinkholes depends on the depth to of the sinkholes depends on the depth to mining, thickness of the extraction, and mining, thickness of the extraction, and bulking properties of the overburden. bulking properties of the overburden.

• The real extent of the surface depressions The real extent of the surface depressions is largely controlled by the size of the mine is largely controlled by the size of the mine opening. Research has found that opening. Research has found that sinkholes typically are circular or elliptical sinkholes typically are circular or elliptical in shape and not larger than the size of in shape and not larger than the size of extraction causing them extraction causing them

Trough SubsidenceTrough Subsidence

• Trough type subsidence is a sagging of Trough type subsidence is a sagging of the overburden triggered where the the overburden triggered where the large extraction ratios and panel sizes large extraction ratios and panel sizes are achieved, both in areas of room are achieved, both in areas of room and pillar mining and long wall mining .and pillar mining and long wall mining .

• This generally occurs as a caving of the This generally occurs as a caving of the immediate roof followed by sagging of immediate roof followed by sagging of overlying strata overlying strata


• The presence of internal barriers and The presence of internal barriers and the low width to depth ratios help the low width to depth ratios help reduce the magnitude of reduce the magnitude of displacement. displacement.

• Trough – type subsidence over room Trough – type subsidence over room and pillar mining will be localised as and pillar mining will be localised as compared to the wide troughs compared to the wide troughs developed by long wall mining .developed by long wall mining .


• As with sink holes, the depth and extent As with sink holes, the depth and extent of troughs will depend on the depth of of troughs will depend on the depth of mining, physical properties of the mining, physical properties of the overburden, and extraction ratios overburden, and extraction ratios achieved. The shape of depression will achieved. The shape of depression will be irregular due to the presence of be irregular due to the presence of remnant pillars. Like longwall mining, remnant pillars. Like longwall mining, subsidence over retreat mining will subsidence over retreat mining will develop rapidly due to high extraction develop rapidly due to high extraction ratios achieved. ratios achieved.

Shaft CollapseShaft Collapse

• The subsidence potential associated The subsidence potential associated with entry or airshafts is high, with entry or airshafts is high, because of the sudden and because of the sudden and catastrophic nature of movement catastrophic nature of movement that can occur. Although small in that can occur. Although small in area, shafts can be dangerous area, shafts can be dangerous because of the haphazard way in because of the haphazard way in which they are backfilled.which they are backfilled.

Evaluation of SubsidenceEvaluation of Subsidence

• The depth of mining is the critical factor used The depth of mining is the critical factor used in evaluating the subsidence potential at a in evaluating the subsidence potential at a given site. The alluvial soils found in given site. The alluvial soils found in Barapukuria may have flown into open holes Barapukuria may have flown into open holes and provided little strength in resisting and provided little strength in resisting subsidence subsidence

• Therefore the thickness of the bedrock over the Therefore the thickness of the bedrock over the mine becomes the critical factor in determining mine becomes the critical factor in determining the subsidence potential in these areas.the subsidence potential in these areas.

Methods to Determine Bed Methods to Determine Bed Rock ThicknessRock Thickness

• There are several methods used to There are several methods used to determine a minimum bedrock thickness determine a minimum bedrock thickness above the coal seam beyond which caving above the coal seam beyond which caving subsidence will not be expected to reach subsidence will not be expected to reach the surface .the surface .

• The methods vary with respect to the The methods vary with respect to the treatment of subsidence mechanisms, treatment of subsidence mechanisms, mine geometry, geology and mining mine geometry, geology and mining method.method.

Mine GeometryMine Geometry::

• With this method, the critical With this method, the critical dimensions affecting subsidence are dimensions affecting subsidence are thickness of cover or overburden thickness of cover or overburden height (H) and extraction thickness (h). height (H) and extraction thickness (h). Piggott and Eyton (1977) suggest Piggott and Eyton (1977) suggest subsidence will not propagate to the subsidence will not propagate to the ground surface over room and pillar ground surface over room and pillar workings where overburden to workings where overburden to extraction thickness ratio (H/h) exceeds extraction thickness ratio (H/h) exceeds 10.10.

Bulking Factor:Bulking Factor:

• Caving of the roof above a mine can Caving of the roof above a mine can continue until the extraction and collapse continue until the extraction and collapse area is filled with broken and bulked rock or area is filled with broken and bulked rock or caving reaches the surface. caving reaches the surface.

• With this method, the height to which caving With this method, the height to which caving can occur is based on the coal seam can occur is based on the coal seam thickness and bulking of the collapsed rock. thickness and bulking of the collapsed rock. The in crease in volume of the collapsed rock The in crease in volume of the collapsed rock is referred to in terms of its “Bulking Factor is referred to in terms of its “Bulking Factor

Bulking FactorBulking Factor

• The bulking factor is calculated in The bulking factor is calculated in terms of thickness and defined as the terms of thickness and defined as the original extraction height minus any original extraction height minus any remaining void divided by the height remaining void divided by the height of the rubble zones above the mine of the rubble zones above the mine roof. roof.

Complementary Influence Complementary Influence Function: Function:

• Complementary influence functions were Complementary influence functions were developed base on two elements: the mined developed base on two elements: the mined element and an unmined element, assigning element and an unmined element, assigning surface response to each. surface response to each.

• The response of the unmined element is based on The response of the unmined element is based on the elastic response of the strata overlying the the elastic response of the strata overlying the structure while the response of the mined structure while the response of the mined element is related to the breaking of the element is related to the breaking of the immediate roof and non- uniform distribution of immediate roof and non- uniform distribution of voids in the rubble. This method is useful in both voids in the rubble. This method is useful in both room and pillar and longwall applications.room and pillar and longwall applications.

Rubble Model:Rubble Model:

• The RUBBLE computer model uses a The RUBBLE computer model uses a finite element method to predict finite element method to predict rubble heights. The RUBBLE model is rubble heights. The RUBBLE model is based on the principles of continuum based on the principles of continuum mechanics and analyses geometrical mechanics and analyses geometrical processes of roof failure and collapse processes of roof failure and collapse caused by underground mining.caused by underground mining.

Subsidence Risk:Subsidence Risk:

• Based on analyses performed, subsidence Based on analyses performed, subsidence risk is typically categorized as either ‘low’ risk is typically categorized as either ‘low’ or ‘significant’. or ‘significant’.

• Areas around shafts or adits are classified Areas around shafts or adits are classified as ‘significant’ due to unreliable method of as ‘significant’ due to unreliable method of abandonment and catastrophic nature of abandonment and catastrophic nature of subsidence in these areas subsidence in these areas

• The remainder of the mine is classified The remainder of the mine is classified base on the critical depth to the coal seam base on the critical depth to the coal seam

Subsidence Risks.Subsidence Risks.

• Areas designated as ‘low’ subsidence Areas designated as ‘low’ subsidence risk are deeper areas and generally risk are deeper areas and generally have no development restrictions. have no development restrictions.

• Areas designated as ‘significant’ are Areas designated as ‘significant’ are the shallower areas and must either the shallower areas and must either be mitigated or avoided.be mitigated or avoided.

Mitigation:Mitigation:

• ““Void fill” grout injection is usually Void fill” grout injection is usually the preferred mitigation technique of the preferred mitigation technique of mine subsidence. mine subsidence.

• The intent of void filling is to reduce The intent of void filling is to reduce the amount of existing void within the amount of existing void within the mine workings, thereby reducing the mine workings, thereby reducing the potential height of caving and the potential height of caving and decreasing the risk of subsidence. decreasing the risk of subsidence.

MitigationMitigation

• The construction process involves The construction process involves drilling small diameter borings into drilling small diameter borings into existing mine voids and rubble zones existing mine voids and rubble zones followed by grout injection. followed by grout injection.

• A pilot program is conducted prior to A pilot program is conducted prior to actual mitigation to determine the actual mitigation to determine the optimal hole spacing to ensure optimal hole spacing to ensure proper grout flow.proper grout flow.

Mitigation.Mitigation.

• Grout: Grout:

A single low strength grout mix A single low strength grout mix comprised of water, cement, fly ash, and comprised of water, cement, fly ash, and aggregate is usually used. The aggregate is usually used. The properties of the injected grout play the properties of the injected grout play the biggest role in the successful mitigation biggest role in the successful mitigation of the subsidence. These properties of the subsidence. These properties include flow ability, aggregate type, set include flow ability, aggregate type, set time and strength.time and strength.

Mitigation.Mitigation.

• Mitigation Effectiveness: Mitigation Effectiveness:

Mitigation of coal mines subsidence is Mitigation of coal mines subsidence is only practical if the effectiveness can only practical if the effectiveness can be verified. During and shortly after be verified. During and shortly after mitigation verification drilling, coring, mitigation verification drilling, coring, geophysical logging, and visual geophysical logging, and visual identification of drill cuttings are identification of drill cuttings are used to confirm presence of grout. used to confirm presence of grout.

Rehabilitation Measures:Rehabilitation Measures:

• Where subsidence is occurring (or is likely Where subsidence is occurring (or is likely to occur) close liaison should be maintained to occur) close liaison should be maintained with the landowner .with the landowner .

• Safety of people and stock need to be Safety of people and stock need to be ensured. ensured.

• It may prove necessary to fence off areas It may prove necessary to fence off areas undergoing active subsidence .undergoing active subsidence .

• Notice should be posted warning the public Notice should be posted warning the public of the potential hazards due to differences of the potential hazards due to differences in level and cracking of the surface.in level and cracking of the surface.

Rehabilitaion:Rehabilitaion:

• . The rehabilitation program must address the . The rehabilitation program must address the direction of mining progress as well as the direction of mining progress as well as the magnitude of subsidence effects .magnitude of subsidence effects .

• The progress of subsidence needs to be The progress of subsidence needs to be monitored as part of rehabilitation program monitored as part of rehabilitation program

• Depending on the nature of the surface features Depending on the nature of the surface features affected, it may prove more cost effective to affected, it may prove more cost effective to discontinue their use temporarily (where discontinue their use temporarily (where appropriate) during the period of active appropriate) during the period of active movement, particularly if the subsidence is of movement, particularly if the subsidence is of limited axial extent and mining progresses fairly limited axial extent and mining progresses fairly rapidly .rapidly .

Rehabilitation :Rehabilitation :

• Where the rural land is affected this may be Where the rural land is affected this may be achieved by appropriate stock management achieved by appropriate stock management

• The alternative may be regular slackening The alternative may be regular slackening and retensioning of fences (perhaps even on and retensioning of fences (perhaps even on a daily basis). Progressive repair or corrective a daily basis). Progressive repair or corrective measures may also be necessary on a regular measures may also be necessary on a regular basis where water courses and utility services basis where water courses and utility services are affected.are affected.

• The effects of subsidence may be minimised The effects of subsidence may be minimised by ensuring that pillars of adequate size by ensuring that pillars of adequate size remain under specific structures.remain under specific structures.

Rehabilitation :Rehabilitation :

• Every effort should be made to minimise Every effort should be made to minimise damage to buildings and structures by damage to buildings and structures by advancer planning both of the mining process advancer planning both of the mining process and appropriate preventive measures .and appropriate preventive measures .

• These may include slackening of bolts and These may include slackening of bolts and other fixings, installations of jacks (to allow other fixings, installations of jacks (to allow progressive levelling)., removals of doors and progressive levelling)., removals of doors and windows, adjustment of tension in restraining windows, adjustment of tension in restraining wires and overhead (electrical) conductors. wires and overhead (electrical) conductors.

Rehabilitation:Rehabilitation:

• Treatment of each structure should be Treatment of each structure should be assessed with regard to best course of assessed with regard to best course of action both during subsidence period action both during subsidence period and for final rehabilitation. and for final rehabilitation.

• Structural damage in buildings and Structural damage in buildings and foundations is usually best left until foundations is usually best left until major movement has ceased, subject major movement has ceased, subject to safety considerations to safety considerations


• Some benefits may be gained by the use Some benefits may be gained by the use of steel straps and bolts to assist retaining of steel straps and bolts to assist retaining the integrity of walls. Repairs may be the integrity of walls. Repairs may be undertaken using standard building and undertaken using standard building and civil engineering procedures.civil engineering procedures.

• Pipelines for water supply, irrigation or Pipelines for water supply, irrigation or sewerage should be repaired, or modified sewerage should be repaired, or modified prior to damage occurring by insertion of prior to damage occurring by insertion of flexible elements to accommodate flexible elements to accommodate predicted movements. Changes of predicted movements. Changes of gradient may then be corrected by gradient may then be corrected by appropriate repositioning.appropriate repositioning.

RehabilitationRehabilitation

• Subsidence movements affecting water Subsidence movements affecting water courses are best addressed by reviewing the courses are best addressed by reviewing the overall drainage pattern in the affected area. overall drainage pattern in the affected area. The establishment of a revised drainage The establishment of a revised drainage pattern to incorporate the new contours may pattern to incorporate the new contours may be the most effective approach. be the most effective approach.

• Reconstruction of the drainage paths by Reconstruction of the drainage paths by excavation/ dredging, or by the construction excavation/ dredging, or by the construction of levees, as appropriate may be feasible. of levees, as appropriate may be feasible.


• Alternately, where water flows are low and Alternately, where water flows are low and precise route of the stream or creek is not precise route of the stream or creek is not critical, the water may be allowed to critical, the water may be allowed to create its own path. create its own path.

• Soil erosion implications, including loading Soil erosion implications, including loading to stream flows, need to be considered .to stream flows, need to be considered .

• Earth dams may be repaired by Earth dams may be repaired by constructing retaining banks and by constructing retaining banks and by resealing cracks with clay. resealing cracks with clay.

RehabilitaionRehabilitaion

• Earth bunds may be necessary to contain Earth bunds may be necessary to contain existing water courses. However existing water courses. However consideration may need to be given to consideration may need to be given to implications of changed water depths .implications of changed water depths .

• Wells may need to be repaired and Wells may need to be repaired and deepened to maintain their utility. deepened to maintain their utility.

• Depressions that create ponds could be Depressions that create ponds could be treated by constructing new drainage treated by constructing new drainage paths and / or using the newly created paths and / or using the newly created ponds as replacements of those lost in the ponds as replacements of those lost in the changed drainage patterns in the areas.changed drainage patterns in the areas.

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Mine Subsidence Khondkar A Saleque. PE ( Engineers Australia) FIEB