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Subsidence 

 

Summary

This review gives information about mine collapse and subsidence. Several publications on mine subsidence and a list of consultants specializing in subsidence are given. Information about mine subsidence in Canada is also given.

INTRODUCTION

An inherent human assumption is that the ground beneath our feet will always be stable. Earthquakes and subsidence shake and undermine our faith in this belief.

Jack Caldwell recollects "Ground subsidence impinged on my imagination a lot earlier. The final two miles of the ride home from high school took us past the old coal mine workings in Springs. The workings had collapsed long before, and now burnt out of control. There was always smoke rising gently from the area we were forbidden to approach. On a still, dark night there was a faint glow on the horizon that thrilled us as we peered from the back of my father's '49 Mercury traveling back from the town and the glittering shops along the road past the old mines."

"When I worked on the cases described above we had scarcely a written word to go by; we used gut-feel engineering and old-fashioned geotechnical logic. I am happy to report that if you now have to deal with mine workings collapse and surface subsidence, there are nearly 100 books on the topic listed online."


THEORIES

Vertical Theory:
Proposed in 1867, the theory states that the limiting planes of an extracted seam are vertical.

Normal Theory:
This theory assumes that the subsidence is normal to the seam

Between Vertical and Normal:
Based on the observation that the limiting lines bisecting the angle between the vertical and the normal lines when the dip is less than 45 degrees else the line of fracture lies at an angle of 45 degrees minus half of the angle of dip.

Dome Theory:
In 1885, from laboratory observations it was postulated that the ground movement is limited by a kind of dome over the area of extraction. There are only two forces acting over the rocks that are taken into consideration - cohesion and gravity. When the cohesion is greater, the rocks stay intact. When the gravity overpowers cohesion, the roof will fall forming an enlarging arch.

Beam or Plate Theory:
It initially assumed that the immediate roof to be a cantilever beam and considered only the lowest part to be under tension. Later it assumed that the roof comprised of a number of thin beams supported by the one below and gripped at either ends. All the beams bend down in succession with all or most of them breaking off at places where they are gripped.

Trough Theory:
The theory was established in 1907 and distinguishes between the 'main break' and 'after break'. In flat seams, the main break is vertical, and the after break is in a direction bisecting the angle of slide and the vertical. In steeply dipping seams, the angle of draw increases, it is 35.8 degrees from the vertical or for a 40 degrees dip and the main break over the seam at an angle from the vertical equal to the half of dip.

Continuum Theory:
This theory believes that the ground acts as a continuous body bounded by the surface above and excavation below.

Particulate Theory:
A study of subsidence using stochastic equations proposes that the rock to be a stochastic medium and equations determining their behaviour.

Large Deformation Theory:
This paper illustrates in detail the irrationalities of the various subsidence theories and on the other hand professes the large deformation theory.


MECHANISM

It can be assumed that the different layers in the strata of the earth are made up of several layers of different mineralized rocks which are held together by the forces of cohesion between them. There exists a state of equilibrium between the forces of gravity and cohesion in undisturbed strata which means that all the forces negate or balance each other.

However, as soon as the strata are disturbed or an excavation is developed, there is redistribution of forces that takes place and as a result, the existing balance is disturbed. Stresses start to develop because of these circumstances. As the pressure from the strata vertically above (assisted by the force of gravity) starts to build over the roof of such an excavation, thus the role of the immediate roof become crucial. If the roof is plastic as it is generally in case of coal seams, there is a greater likelihood of subsidence while elastic roof can sometimes tolerate excessive amounts of pressure from the overlying strata for surprisingly long period of time delaying subsidence.

It is generally observed that subsidence occurs with the practice of mining methods that involve the caving of the strata or creating larger voids than other methods. For e.g. Longwall method of mining creates substantial voids underground as compared to the room and pillar method of coal mining. Similarly, the block caving method is likely to leave larger voids and thus engender greater risk of subsidence in comparison to other methods such as cut and fill, etc.


THESES

Here are two excellent, readable theses on the topic of mine workings collapse and surface subsidence.

Both these theses describe the theory of mine-induced subsidence in detail sufficient to satisfy the most curious professional working on the most difficult issues. Also the theses provide excellent case histories and discussions of computer codes that may be used to evaluate your conditions. Here is the abstract from Misich’s thesis:

The subsidence characteristics of the Collie Basin sediments have been investigated to provide site specific design criteria for the Wongawilli method of coal extraction. As historical coal extraction (board and pillar) methods did not generally give rise to large scale subsidence, there were very few details on mining subsidence in the Collie Basin available to base any design methodology on. Consequently, the investigation was conducted on a “Green fields” basis. Firstly, the mechanisms involved in the development of mining subsidence needed to be investigated and identified. It was then necessary to determine the effects that mining subsidence would have on mine and ground mass (specifically aquitards) structures and surface features. Once these two areas of work were completed, design criteria were formulated to manage the effects of mining subsidence by controlling the critical mechanisms of subsidence development. The results from this study have greatly enhanced the level of understanding of the subsidence mechanisms involved, and allowed for the development of predictive models which can be used for the design of coal extraction by the panel/pillar mining method in the Collie Basin. Mine planning engineers can now use this design information to derive the most cost effective methods for the extraction of coal within the Collie Basin.

OTHER PUBLICATIONS

The Pennsylvania Department of Environment presents a readable survey of underground mine collapse and the effects of subsidence. Also from this organization is Section VIII of The Effects of Subsidence Resulting from Underground Bituminous Coal Mining on Surface Structures and Features and Water Resources; this is a readable technical review of subsidence and its surface effects.

ESRI tells of grouting old coal mine workings to prevent subsidence. Similarly in Canada near Banff National Park.

Laura Carbognin writes of the activities of the UNESCO Working Group on Land Subsidence in a 2003 publication. If you can get through the never-ending descriptions of meetings in exotic places, this report contains some interesting information of subsidence in unlikely places like Long Beach near my Huntington Beach house.

Don't forget to access the InfoMine library where there are currently over 30 papers on subsidence ranging from case histories to the use of probability theory to predict subsidence potential.

U.S. Department of Transportation, Federal Highway Administration list many publications about and links to sites providing information on subsidence. Here are a few:


CONSULTANTS

InfoMine lists about 450 geotechnical consultants in different parts of the world, however not all companies would be experts in consulting with subsidence. Hence Jack Caldwell lists below some consultants that mention subsidence as one of their work areas and some relevant information about the same.

Agapito Associates describe a case history of mine-induced subsidence thus:

The evolution of surface subsidence is an important focus of study above Molycorp, Inc.'s newest block cave at the Questa molybdenum mine near Taos, New Mexico. The case study compares mature glory hole subsidence over the Goathill Orebody and subsidence emerging in its earliest stage over the new D Orebody block cave. Subsidence above the D Orebody was first detected in April 2003, 30 months after caving was initiated. Caving propagated to surface through 550 m of overburden at an average rate of 0.21 m per day. At the end of 2004, an average of 100 m of draw over a 1.4-hectare (ha) block produced a near-circular subsidence basin 5.8 m deep at its center and 90 m offset from the center of the block. Observations to date indicate a cave ratio of 10:1 and a gross cave bulking factor on the order of 10%. Historically, cave-angle projection models have been used to predict subsidence extents for reclamation planning. In light of evolving regulatory concerns, efforts are underway to develop a more accurate subsidence predictor using a three-dimensional (3D) numerical model. Particle Flow Code (PFC3D), a discontinuum "ball" code, was selected for modeling because of its ability to simulate stress fracturing of the rock mass and large-scale mass flow underground and at the surface, which are believed to be the dominant physical phenomena governing the formation of block cave subsidence. Advances simulating subsidence in the Goathill and D orebodies with PFC3D are discussed.

Western States Mining Consultants describe these two projects:

The Town of Glenrock is underlain by two abandoned underground coal mines which are causing subsidence events on the surface. This project included a report of investigation to determine the most cost-effective approach to subsidence control, the design of a slurry backfill system to pump a sand/water mixture into the mine voids through boreholes, the generation of bid documentation and construction specifications and the construction management of the project for the State of Wyoming. Notable requirements of the project included design of a 3,000 gpm, 4,000 foot long slurry and water pipe line system; design of a 5,000 gpm water supply well field; design of a 200 foot span pipe bridge; roadway, sedimentation pond and drainage control designs and grouting designs for structures and a 1,000 foot unstable segment of a state highway. David H. Scriven was part of the construction management team for this project.

David H. Scriven was the subsidence investigation specialist on the A.K. Kuhn team for the Northside Mine Subsidence Study in Gallup, New Mexico. His assignments included interviews/workshops with citizens; assistance to the State in locating subsurface investigation boreholes; assistance in structure investigations; drilling supervision and borehole logging; data analysis to define subsidence risk areas and subsidence mechanisms; evaluations of remedial actions.

Respec describes their expertise thus:

RESPEC has designed monitoring systems and analyzed subsidence over both dry- and solution-mined excavations in bedded and domal salts and potash deposits. Historically, subsidence data were often merely reported to regulatory agencies. We now use numerical modeling tools and the measured subsidence to monitor mine stability and cavern behavior. Stability interpretations based on subsidence measurements have been used in civil court cases and in testimony before regulatory agencies. We were responsible for monthly reports on dissolution-induced subsidence over the Retsof (New York) salt mine as it filled with fresh water, and presented the local community and regulators with a definitive interpretation of the ultimate subsidence for the mine. We pioneered the use of theGPS to establish reference benchmark elevations for subsidence surveys over salt domes.

Waddington Kay and Associates via Mine Subsidence Engineering claims the following expertise in subsidence in the UK and Australia:

  • Detailed predictions of subsidence parameters at any location over a series of longwalls or isolated single panels.
  • Development of empirical models for the prediction of subsidence parameters in single or multi-seam mining situations.
  • Assessments of the impacts of subsidence on buildings and other structures.
  • Assessments of the impacts of subsidence on services and other items of surface and sub-surface infrastructure including transmission lines, pipelines, roads, railways, bridges, tunnels, canals and culverts.
  • Assessments of the impacts of subsidence on gorges, rivers, creeks, land drainage systems, slopes, escarpments and cliff lines.
  • Analyses of the impacts for alternative mine layouts to assist mine planning.
  • Presentations to Commissions of Inquiry and Land and Environment Court hearings.
  • Design of buildings and structures to resist mine subsidence movements.
  • Preparation of contract documentation and supervision of construction works.

CANADIAN FACTS

Does subsidence affect Canadians? To answer this, see WOM Geological Associates on mine-induced subsidence in Canada. This extract summarizes their findings:

Although rare, subsidence due to mining operations has occurred in various parts of Canada and has been the result of rock mass failure mechanisms such as caving, raveling and strata failures. Notable cases of subsidence caused by rock mass failure have occurred in the municipal areas of Cobalt, Timmins and Windsor in Ontario and the Pictou County area of Nova Scotia. Subsidence, due to the settling of backfill in the former Moneta and Hollinger, produced many problems within the city of Timmins. Damage in mining municipalities has included cracked and distorted homes, and unstable openings near buildings or under roads. Although not yet damaged, many homes and buildings in residential areas have had to be moved or demolished because of unstable ground being detected by geotechnical testing.

I found the paper Mine Subsidence Stabilization In Steeply Dipping Seams In the Canadian Rockies: A Project Overview by Patrick E. Gallagher. He writes thus in the paper's abstract:

The project is located in the Bituminous Coalfields of western Canada near the world famous Banff National Park. A large commercial/residential development has begun on a 5000-acre tract of land where a multiple seam abandoned coal mine is present. The coal was extensively mined from the mid-1800's until 1974 in four separate seams, all of which are steeply dipping (up to 85°). The workings pose a serious subsidence threat to the future buildings to be constructed in the development. Our mine stabilization program was designed to establish priority areas for stabilization and develop grout mixes that could overcome the steep dip of the seams. In this paper we cover topics such as prioritization of the site, explorations of the proposed stabilization areas, and evaluation criteria for the grouting patterns, mix designs, and borehole photography of the underground pumping operations.

The Report Evaluation of Ground Conditions in Canadian Underground Coal Mines by Dr. C. Mark (NIOSH) for Underground Coal Mining Safety Research Collaboration is a treat for the technically oriented. Details are provided of mining practice, pillar design, and the multitude of details needed to prevent collapse and subsidence. Incidentally, I found this publication on the web site of RocDoc a consulting company in Lethbridge, AB. They have a number of other interesting publications available through the site-check them out.


MEASURING SUBSIDENCE

The most popular way of measuring subsidence was land surveys wherein change of level was monitored. These days with greater involvement and widespread applicability of technology, the global positioning system is being employed which has completely revolutionised the subsidence measurement process. The companies also have the option of air surveys which is a very efficient method of measuring subsidence. If conducted in collaboration with other neighbourhood mines, it can be very cost-effective.

SUBSIDENCE IMPACTS

Subsidence can have severe impacts over a variety of aspects of mining.Subsidence can have detrimental impacts on groundwater flow. There have been incidents of perrenial streams, gullies, etc drying up because of sink holes and troughs created in riverbeds. It is all the more serious in areas where the coal seams spread over vast stretches of land. Obviously, there is negative impact of the same on the surrounding wildlife, aquifers, fish, cattle and forests in not only the neighbourhoods but also downstream areas.

It can destabilize the overlying building structures making them inhabitable. As a result of subsidence, cracks may develop into them resulting in degradation of its economic value and an overall slump in the local economy.

There is significant number of videos on YouTube that indicate growing anger and anxiety over Longwall method of coal mining because of the subsidence impacts of the same. This YouTube link displays a string of videos about people and the environment affected by subsidence in United States and Australia.


STREAM UNDERMINING

Subsidence on underground mine workings can disrupt surface water flow and impact groundwater. This simple fact is brought home simply and starkly by a fine brochure from Hatch Mott MacDonald in a brochure Stream Undermining: Technology Update. Obviously the brochure tells you that if you need to avoid impacting surface water and groundwater due to subsidence, you should contact them. And why not? If they can put out this informative brochure, maybe they can help you too.

Read the brochure as I did for technical information, for there is enough there to satisfy the curious, establish the nature of the problem, list the essential steps in dealing with the issues, and be aware and prepared. For example here is their list of stream mitigation measures for subsidence-impacted streams:

  • Seal stream channel to prevent leakage from subsidence fractures.
  • Augment flow in impacted streams using wells or other sources of water.
  • Repair channels to eliminate areas of pooling and ponding.
  • Stabilize upstream areas to control erosion and sediment buildup.

CONTROLLING SUBSIDENCE

There is very limited when it comes to controlling subsidence with caving method of mining. It is not yet established how subsidence can be controlled. However, there have been efforts to develop different approaches towards handling of the same.

Geotechnical knowledge of the strata is extremely crucial in controlling the impacts of subsidence. Trenching is a popular way of controlling ground movement and diverting subsidence away from highways, riverbeds, human settlements, etc.

Jack Caldwell quotes 'In Pennsylvania in a small town to the east of Pittsburgh, I worried over the potential impact of collapse of the 100-ft deep underground mine workings from the early 1920's on ten surface trenches filled with waste from a plant that produced pellets for nuclear submarines. We decided that subsidence would not affect in situ closure of the trenches, and were grateful for the many coal seams between the waste and the mine workings, for they acted as the most effective natural geochemical (carbon) attenuator of seepage from the trenches we could get.'


CONCLUSION

There are sufficient 'economic' excuses for the mining companies world over to practice method of mining which yield maximum revenues; there are enough reasons for provincial and federal governments to make their territories more amenable to industrial development through the mining industry but there is a serious need to address the various negative impacts over society and the genuine concerns of people arising from them.

It is apparent that the mining industry needs to be extremely conscious while selecting the mining methods which involve substantial amount of caving. Lesser recovery or ore/coal can be partially compensated with more efficient milling and marketing processes. The resulting higher prices can be an incentive for more research into the development of newer materials for substituting the existing ones. Otherwise, the breach of trust by mining companies and governments can have widespread and long-lasting political and economic implications over countries around the world.


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