International Journal of Mining and Geological Engineering 1990, 8, 400-408

SHORT C O M M U N I C A T I O N

Mining subsidence- past, present, future

R.E. GRAY GAI Consultants, Inc., Monroeville, PA 15146, USA

Introduction

Mining's origins are ancient and unknown. The earliest mines probably consisted of workings to recover chert for stone tools, or pigment and gems for ornamentation. As these workings extended underground, eventually a void became large enough to cause the first mine subsidence. Although this event was not documented, mine subsidence has been a problem for many years. This paper briefly reviews the extent of mine subsidence in 1990, ideas prior to 1890, the status in 1890, and significant advances between 1890 and 1990, and speculates on advances in the 21st century.

Extent of problem - 1990

When we think of underground mining we often think of metal ores, coals, salts, and aggregates. However, coal mining impacts a vastly larger amount of land than all other types of mining combined. For example, in the United States coal is found in 37 states and mined underground in 22 states (HRB-Singer, Inc., 1980). Underground coal mining is estimated to eventually cover 40 million acres with 8 million already undermined (HRB-Singer, Inc., 1977). Johnson and Miller (1979) report that subsidence due to mining had affected more than 2 million acres in 30 states. Coal mining caused over 99% of the subsidence, since all other metal and non-metal mines affected only 17 000 acres (Johnson and Miller, 1979). Underground coal mining is estimated to cause surface subsidence damage costs in excess of $1 billion from 1973 to the year 2000, with $30 million of damage to structures annually (US Government Accounting Office, 1979).

With the preponderance of subsidence being due to coal mining, it is understandable that most subsidence research and theories are related to coal mining.

Subsidence experience and theories prior to 1890

Early in the 15th century court records from the County of Durham in England indicate a jury awarded £200 for repair of a house damaged by coal mining (Young and Stock, 1916). In

Keywords: Subsidence, coal mining, history of mining.

0269-0136/90 $03.00+.12 © 1990 Chapman and Hall Ltd

Minin 9 subsidence past, present,future 401

Cheshire, England, shafts were sunk shortly after 1670 to mine a shallow salt bed. Serious surface breaks occurred in 1750 (Young and Stoek, 1916).

Belgian engineers were among the first to make a scientific study of subsidence due to mining. In 1825 a commission investigating the cause of surface cracks in the city of Liege concluded that an interval of 300 feet between the mine workings and the ground surface was more than sufficient to prevent subsidence (Young and Stoek, 1916).

J. Gonot, a Belgian engineer, is usually given credit for formulating the first theory of subsidence during a study at Liege in 1839. However, he did not publish it until 1858 and a similar idea was presented by a French Engineer, Toillez, in 1838. Gonot claimed that following extraction of coal the overlying strata would sink and the angle of fracture would be perpendicular to the plane of the coal bed. He also indicated that the break extends through to the surface, irrespective of the depth of mining (Young and Stoek, 1916).

As early as 1859 there were regulations in Austria controlling the mining of coal under railways (Young and Stoek, 1916). In 1867, A. Schulz, a German engineer, published his ideas on the angle of fracture and the size of pillars necessary to protect the ground surface (Young and Stoek, 1916).

The Prussian government appointed a commission in 1868 to collect information from other countries on the influence that mine workings may have on surface buildings. They found that the majority of Belgian engineers believed that when the coal is entirely removed, the most careful packing gives no guarantee against damage to surface buildings, that the packing only lessens the sinking; and that the surface may be protected by leaving pillars equivalent to half the area of the coal seam. In England the commission found the following opinions:

1. The working of coal at every known depth may affect the surface, but at depths greater than 400 metres (1300 feet) it can cause damage only to certain buildings.

2. In the case of complete extraction, filling may be a means of effective protection. 3. Leaving pillars constitutes an efficient protection (Young and Stoek, 1916).

In another study of subsidence at Liege, not surprising in light of the first commission's finding that mining below 300 feet would not damage the surface, G. Dumont in 1871 recognized that drainage of old workings or the flooding of a mine may reinitiate subsidence many years after the initial movements (Young and Stoek, 1916).

In 1884 Jicinsky summarized Austrian experience and postulated the 'harmless depth' concept. With caving of the rock strata overlying the mine workings the rock fractures and increases in volume. For mine voids that are small relative to the mine depth, the likelihood of subsidence decreases wth increasing depth of mining. However, extending this concept to areas of nearly complete extraction with dimensions significantly larger than the depth of mining is erroneous.

Hydraulic mine filling originated in the Pennsylvania anthracite mines in 1884 (Zwartendyk, 1971). Visiting German engineers observed the method in the early 1890s and quickly developed it for use in European longwall mines.

In 1885 H. Fayol, a French mine director, summarized the contradictory opinions of the time concerning mine subsidence as follows (Zwartendyk, 1971):

1. (a) Subsidence movements reach the surface irrespective of the depth of mining. (b) Subsidence movements do not reach the surface when the workings exceed a

certain depth.

402

2. (a) (b)

3. (a) (b)

(c)

4. (a) (b) (c)

Gray

Subsidence extends to the surface without diminution. Subsidence decreases upward. Subsidence takes place vertically above the cavity. Subsidence takes place in an area perpendicular to the bedding of the mined-out seam. Neither (a) nor (b) is valid; subsidence takes place limited by lines drawn at an angle of 45 degrees to the horizon, by the angle of repose of the ground, or by some other similar angle. Packing* prevents surface subsidence. Packing only reduces surface subsidence. Subsidence is greater with stowing than without it.

Fayol's investigation backed by model studies led him to propose that subsidence is limited to a dome over the mined area and that surface subsidence diminishes in proportion to the depth of the workings (Young and Stoek, 1916).

Status in 1890

As indicated by Fayol's summary of opinions, over 60 years of subsidence investigations did not produce a uniform theory by 1890. Fayol's work on dome theory and the harmless depth postulate had great influence in 1890 and for many years after.

Progress 1890-1990

In 1900, a modern view of subsidence (Fig. 1) was published by Wachsmann who indicated that when a coal bed is mined, the lowermost strata collapse, the next higher strata sink and crack, while the uppermost strata sink without breaking or cracking (Young and Stoek, 1916).

In 1909 subsidence seriously damaged a school building in Scranton, Pennsylvania, and led to a study which resulted in Bulletin No. 25, US Bureau of Mines in 1912 which reviews existing mining conditions and discusses methods of supporting the surface (Griffith and Connor, 1912). Considerable attention was given to hydraulic backfilling. It also reported on the compressive strength of anthracite coal.

In 1913, Goldreich, from a study of railway subsidence in the Ostrau-Karwin Coal District (Austria), recognized subsidence troughs and objected to the idea of a harmless depth as 'the overlying rock seldom breaks up enough to cause a volume increase equal to the mined out area' (Zwartendyk, 1971).

Pneumatic stowing was reported to have been successfully used in the Lake Superior copper district in 1914 (Young and Stoek, 1916). Young and Stoek's (1916) literature review which collected much valuable information on subsidence resulting from mining was part of the work initiated by the 1909 subsidence in Scranton.

Lehmann (1919) presented the trough theory of subsidence which has been in general use since. He described the horizontal and vertical ground movements, zones of compression and extension and other concepts.

* Packing = stowing = backfilling: filling the mined-out area with some material, usually rock waste.

Minin9 subsidence - past, present , fu ture 403

B A

- - - - - ~ C o _ c e oF Coa__/z2te_Q~u~e.~ . . . . 2_--

Fig. 1. Angles of fracture in rock and of subsidence in Marl (Source: Wachsmann 1900)

Rice (1923), Chief Mining Engineer of the US Bureau of Mines, commented on the widespread lack of understanding of subsidence among mine managers and indicated some still believed in Fayol's harmless depth. Rice believed all empirical formulas to be of local applicability and urged the systematic measurement of subsidence.

A Committee on Ground Movement and Subsidence of the American Institute of Mining, Metallurgical and Petroleum Engineers (AIME, 1926) collected US subsidence data through a questionnaire. Total subsidence, as a percentage of extracted seam in room and pillar mining, varied widely. Subsidence took place where less than about half of the coal had been left as pillars. With full extraction, subsidence was almost invariably reported only above the mined area. These negative or zero angles of draw were probably due to a failure to extend the surveys outside the mined areas.

In Europe it was recognized by the 1920s that it was often less costly for a mine to contribute to the design of new buildings so as to make them less sensitive to subsidence damage, than it was to pay for repair of damage (Zwartendyk, 1971). Imaginative designs were introduced both to prevent damage and to minimize repair costs where damage could not be avoided (Lfitkens, 1932).

In 1927 Herbert and Rutledge reported on a field study carried out at four coal mines, one of which used advancing longwall and the other three room and pillar. Unfortunately, lateral movements of the survey monuments were not recorded. Their time-subsidence curves showed a slight rise of the ground surface prior to subsidence.

Briggs (1929) contributed greatly to an overall understanding of subsidence by viewing the

404 Gray

problem in wide perspective. His summation of European experience brought this important data to US attention (Zwartendyk, 1971).

The theoretical basis of pre-subsidence calculation methods was enunciated by Keinhorst in 1928. In 1932 R. Bals, using Keinhorst's basic assumptions, developed a formula for pre- calculation of subsidence on a mathematical rather than on a practical experience basis (Sinclair, 1963). These pre-calculation techniques were further developed in the 1930s and 1940s by German and Austrian researchers (Zwartendyk, 1971). Pre-calculation was adopted in Britain by the National Coal Board in about 1950.

From 1931 to 1934 subsidence monitoring in western Pennsylvania included observations on damage to structures (Newhall and Plein, 1936). In 1935 Greenwald and others began a series of systematic measurements of subsidence above mines in the Pittsburgh Coal (Greenwald et al., 1937; Maize and Greenwald, 1939; Maize et al., 1941).

Harmonious mining was first proposed by K. Lehmann in 1938. This procedure involves mining in two seams or by a stepped face system in one seam to reduce ground strains by balancing extension with compression (Sinclair, 1963).

During 194648 Orchard and Wardell studied experience of mine subsidence in the Limburg Coalfield of Holland and the Ruhr Coalfield of Germany, respectively. Both of them noted the generally larger and more uniform angles of draw observed in those coalfields and the more rigorously accurate the instruments and methods of observation used. As a consequence, a more accurate method was proposed and adopted for mine subsidence investigation in British coalfields. In addition, the extensive observation of horizontal ground movements was also undertaken (Wardell, 1950, 1952).

Niemczyk (1949), using the Lehmann trough theory as his base, distinguished five basic aspects of movement. These were: vertical component of subsidence, inclination, curvature, displacement (i.e. horizontal component), and strain (tension or compression). Niemczyk (1949) pointed out that inclination and curvature are functions of the subsidence profile expressed as a mathematical function. Inclination (ground slope) is its first derivative, while the curvature is the second derivative. Ground strain is a function of the horizontal component of ground movement (Zwartendyk, 1971).

In Britain, although horizontal movements were measured from 1930, they were little used until 1950 when their importance in relation to surface damage was realized (Sinclair, 1963). In 1949 the Turner Committee on Mining Subsidence recommended 'that every surface interest should be compensated for subsidence damage caused by the working of coal'. In 1950 the Coal Mining (Subsidence) Act provided compensation for structural damage to small houses. Another act in 1957 removed inequalities in the right of support and required the National Coal Board to undertake remedial work necessary to make subsidence damaged property reasonably fit for use.

K. Wardell, in addition to better measurements of subsidence, made many significant contributions to our understanding of subsidence (Wardell, 1953, 1969).

Br/iuner's (1969) summary of subsidence engineering based on European experience in a US Bureau of Mines publication made this information accessible to US subsidence specialists.

In the US subsidence due to active coal mining is regulated by the Federal Surface Mining Control and Reclamation Act of 1977 (Public Law 95-87). This act requires coal mine operators to submit a Subsidence Control Plan as part of their permit application. In the plan the operator must identify the mineral extraction methods to be used, plans for subsidence control, or methods to be used to prevent material damage resulting from

Minin9 subsidence - past, present, future 405

subsidence. The plan must spell out the measures to be taken for reducing the probability of subsidence, such as backfilling, stowing or supports, as well as measures to be taken on the surface to prevent material damage to structures, or reductions in land values or foreseeable land use. The specific mitigation measures, several of which are identified in the regulations, are left to the discretion of the operator.

In addition to the Subsidence Control Plan, the operator must meet certain performance standards. Again several methods are suggested in the regulations; for example, restoring surface structures, purchasing structures or land or compensating the owner, and developing buffer zones where underground mining is prohibited. To ensure compliance with these regulations, operators are required to post a bond.

Leaving unmined pillars suitable for long term support of the overlying strata wastes the resource being mined. In addition, if the long term support needs are underdesigned, subsidence will eventually occur. Thus, full extraction mining with planned subsidence is permitted by Public Law 95-87.

In addition, a tax on each ton of coal mined is used for reclaiming abandoned coal mine lands including stabilization of subsidence prone undermined areas.

Status in 1990

With a long period of technical investigations, it is appropriate to reflect on our current understanding of subsidence due to coal mining.

Active Mines- with current mining pillars of coal can be left in place for long term support. Pillar strength and overburden loads are well enough understood that subsidence can be deferred for many years, probably centuries. However, pillar deterioration due to stress, weathering, and erosion has not been quantified.

With full extraction (longwall or room and pillar mining) subsidence occurs contempor- aneously with mining. The resulting ground movements can be predicted within limits.

The regulations associated with the Surface Mining Control and Reclamation Act require a subsidence control plan as part of a mining permit application (Von Schonfeldt et al., 1979; Riddle, 1980). A number of subsidence prediction models exist. This need to reliably predict subsidence and its impact on surface structures is resulting in the collection of field data required to develop adequate subsidence prediction models for US coal mining regions.

The response of structures to subsidence ground movements has not been studied adequately. Fortunately, more researchers are recognizing the need for ground/structure interaction studies. Such work may provide the confidence for rational design of structures to resist subsidence damage.

Prediction of subsidence over abandoned mines is not well developed. Zones of high risk can be identified but the time, amount, and extent of subsidence and the resulting strains cannot be predicted for abandoned mines.

Mine stabilization can be done effectively; however, it is expensive. Subsidence resistant designs have not been widely used. Insurance programs to provide assistance, if and when subsidence occurs, appear reasonable and have been implemented in at least six states having significant subsidence problems.

Land use controls such as zoning of areas subject to subsidence for limited use or requiring stabilization or special designs for structures have not been used in the United States.

406 Gray

Historic summary

Figure 2 shows the author's judgement of the development of mine subsidence understanding separately for Europe and the United States. From the previous text and Fig. 2 it is obvious that much of our understanding is due to European work and occasional reviews of

2000 --

80

60

40

20

1900

80

60

40

20

1800

National Coal Board Handbook

Paris Conference. Leeds Conference

rmonious Mining

Prediction

Trough Theory

4odern View

Harmless Depth Concept

Control of Mining Under Railroads (Austria)

First Subsidence Theory

First Scientific Study (Belgium)

EUROPE

Fig. 2. Growth of knowledge in mine subsidence

U.S,

Public Law 95-87 / IJ.-SIB,~I"~ and D,O.E.

Monitoring

Brauner Report

P Systematic Monitoring

A.I,M,E. Study- Herbert and Rutledge

Young and Stoek Review

U.S.B.M. Bulletin 25 Pneumatic Stowing

Hydraulic Mine Filling

European technology, such as by Young and Stoek in 1916 and Br/iuner in 1969. The horizontal scale, which is a qualitative assessment of understanding, is less for the United States in 1990 than for Europe since we have not developed regional models for all coalfields and poorly understand ground-structure interaction for the magnitude of ground strains which occur in United States coalfields. Provisions of Public Law 96-87 requiring subsidence prediction and protection of structures should result in improved understanding.

Needs for the 21st century

Although our understanding of subsidence has improved significantly in the 20th century, there is room for much improvement. Coal mining will be with us far into the future,

Minin9 subsidence past, present,future 407

particularly if we are to remain re~tsonably self sufficient in energy utilization. Listed below are perceived research needs and suggested alternatives to wide-spread stabilization programmes:

1. Collect detailed information on a regional basis for subsidence parameters to develop better predictive models.

2. Determine the importance of rock overburden type, per cent extraction, and angle of draw.

3. Determine the rate of pillar deterioration. 4. Evaluate the long-term effectiveness of void backfilling methods. 5. Determine the effectiveness of partial vs. total void filling. 6. Study ground subsidence-structure interaction. This will improve our ability to design

subsidence resistant structures. 7. Evaluate techniques and costs of subsidence resistant designs against post-subsidence

damage repair. 8. Evaluate use of geofabrics and earth reinforcement for construction of slabs, roads, and

dikes with more resistance to subsidence damage. 9. Consider alternatives to stabilization programmes which are expensive:

(a) Wider use of subsidence insurance. (b) Land use controls - uses areas of high subsidence risk for open space or requires

special designs for structures.

References

American Institute of Mining, Metallurgical and Petroleum Engineers (1926) Report of subcommittee on coal mining to committee on ground movement and subsidence, AIME Transactions, 74, 734-809.

Bals, R. (1932) Beitrag zur Frage der Vorausberechnung bergbaulicher Senkungen, Mitt. Markschei- dew, 42/43, 98/111.

Brfiuner, G. (1969) Subsidence due to underground mining (in two parts). US Bureau of Mines, Information Circulars 8571, 8572.

Briggs, H. (1929) Minin9 Subsidence, E. Arnold and Company, London. Greenwald, H.P. et al. (1937) Studies of roof movement in coal mines; Montour 10 Mine of the

Pittsburgh Coal Company, US Bureau of Mines Report of Investigations 3355, US Bureau of Mines, Washington DC.

Griffith, W. and Conner, E.T. (1912) Mining conditions under the City of Scranton, PA, US Bureau of Mines Bulletin No. 25, US Government Printing Office, Washington DC.

Herbert, C.A. and Rutledge, J.J. (1927) Subsidence due to coal mining in Illinois, US Bureau of Mines Bulletin 238, US Government Printing Office, Washington DC.

HRB-Singer, Inc. (1977) Nature and Distribution of Subsidence Problems Affectin9 HUD and Urban Areas, US Department of Housing and Urban Development.

HRB-Singer, Inc. (1980) Technical and economic evaluation of underground disposal of coal mining waste. Report prepared for US Department of the Interior, Bureau of Mines, Contract No. J0285008.

Jicinsky, W. et al. (1884) Die Einwirkungen des Kohlenabbaues aufdie Tagesoberfl/iche (Monographie des Ostrau-Karwiner Steinkohlenreviers).

Johnson, W. and MILLER, G.C. (1979) Abandoned coal-mined lands; nature, extent, and cost of reclamation. U.S. Department of the Interior, Bureau of Mines.

408 Gray

Keinhorst, H. (1928) Bei Bodensenkungen auftretende Bodenverschiebungen und Bodenspannungen, Glfickauf (Essen), 64, 1141-5.

Lehmann, K. (1919) Bewegungsvorgange bei der Bildung von Pingen und Trogen, Gl~ickauf (Essen), 55 (48), 933-42.

Lehmann, K. (1938) PlanmaBige Abbaufuhrung, Gluckauf, 74, 321-32. Lfitkens, O. (1932) Sicherung von Neubauten gegen Bergschaden, Gluckauf (Essen), 68 (32), 705-11. Maize, E.R. and Greenwald, H.P. (1939) Studies of roof movements in coal mines, 2. Crucible Mine of

the Crucible Fuel Company, US Bureau of Mines Report of Investigations 3452. Maize, E.R., Thomas, E. and Greenwald, H.P. (1941) Studies of roof movement in coal mines; 4. Study

of subsidence of a highway caused by mining coal beneath, US Bureau Report of Investigations 3562.

Newhall, F.N. and Plein, L.N. (1936) Subsidence at Merrittstown air shaft near Brownsville, Pennsylvania, AIME Transactions, 119, 58-94.

Niemczyk, O. (1949) Bergschadenkunde. Essen, Verlag Gluckauf, GmbH, p. 27. Rice, G.S. (1923) Tests of strength of roof supports used in anthracite mines of Pennsylvania, US

Bureau of Mines Bulletin 303. Riddle, J.M. (1980) Dealing with subsidence and SMCRA, Mining Engineering, 32, 1702-4. Sinclair, J. (1963) Ground Movement and Control at Collieries. Sir Isaac Pitman and Sons, London,

England. U.S. Government Accounting Office (1979) Report by Comptroller General: Alternatives to protect

property owners from damage caused by mine subsidence, Report CED-79-25. Von Schonfeldt, H. et al. (1979) Subsidence and its effect on longwall mine design, Paper presented at

Annual American Mining Congress Coal Convention, St Louis, 20-23 May. Wardell, K. (1950) A comparison between British and German experience of mining subsidence, Trans.

Inst. Min, Surv., 30, 51. Wardell0 K. (1952) The surveying observations required for the determination of ground movements

caused by mining, Trans. Inst. Min. Surv., 32, 12. Wardell, K. (1953) Some observations on the relationship between time and mining subsidence,

Institution of Mining Engineers Transactions, 113, 471-483. Wardell, K. (1969) Ground subsidence and control, Mining Congress Journal, 55 (1), 35-42. Young, L.E. and STOEK, H.H. (1916) Subsidence resulting from mining, University of Illinois Bulletin

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exploitation (PhD thesis), University Park, Pennsylvania, Pennsylvania State University.