Post mining subsidence abatements in Wyoming abandoned coal mines

Mining Science and Technology, 12 (1991) 215-231

215

Elsevier Science Publishers B.V., A m s t e r d a m

Post mining subsidence abatements in Wyoming abandoned coal mines Mario G. Karfakis and Ertugrul Topuz Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA (Received March 9, 1990; accepted September 7, 1990)

ABSTRACT Karfakis, M.G. and Topuz, E., 1991. Post mining subsidence abatements in Wyoming abandoned coal mines. Min. Sci. Technol., 12: 215-231. Coal has been mined continuously in Wyoming since 1865. Nearly all the coal produced in the first 90 years of mining was from underground bituminous mines. Subsidence has been a threat in Wyoming since the beginning of coal mining; constituting an extreme danger to public health, safety and property, As a consequence, Wyoming mine subsidence problems qualify for the highest priority of funding under the Surface Mine Control of Reclamation Act of 1977. Abatement projects have been undertaken by the Wyoming Abandoned Mine Lands (AML) Program to prevent or minimize further subsidence in Wyoming communities. This paper gives a brief historical account of mining activity in Wyoming. Subsidence characteristics and occurrences in various communities are presented. The locations of key abatement projects and the reasons for their selection are then given. The selection criterion for the backfilling and grouting methods and the techniques themselves are presented, and the problems encountered during various projects are discussed. Successful projects are analyzed and recommendations for future projects are given.

Introduction As the Union Pacific railroad moved west through Wyoming, mining for steam coal followed. During the 1860's underground coal mining began along the rail route and continued into the 1950's. From the large-scale mines that were developed to supply the railroad industry, to small, one-man operations for home heating, nearly all the coal produced in the first 90 years of mining came from room-and-pillar mines. This intensive mining left Wyoming with a problem that has occurred in a number of communities since the turn of the century and has no easy solution: mine subsidence. In modern times, what used to be coal mine camps have been replaced by communities, some of which have been located over or are expanding toward abandoned under0167-9031/91/$03.50

ground mines. This has made mine subsidence in Wyoming a major ground movement problem affecting the surface. The following sections give the historical mining background, characterize the subsidence occurrences, identify the various communities with subsidence problems, describe the abatement techniques and key abatement projects, and present conclusions and recommendations.

Historical background Coal-bearing strata underlie about 41% of the total surface area of Wyoming and are located in large sedimentary basins adjacent to major mountain ranges and uplifts [1]. Coal has been mined continuously in Wyoming since production was first reported

© 1991 - Elsevier Science Publishers B.V.

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M.G. KARFAKISAND E. TOPUZ

in 1865. By 1985 a total of over 1.4 billion tons of coal had been mined in the state. In the first 90 years of mining nearly all the coal produced was derived from underground bituminous mines. Although the first coal discovery was reported prior to 1834, it was not until construction of the transcontinental railroad across Wyoming in 1867 that commercial coal mines began operating in Southern Wyoming (Almy, Carbon, Rock Springs, Diamondville, Kemmerer, Hanna). With the move by the late 1880's of the ChicagoNorthwestern and Burlington-Missouri railroads into northeastern Wyoming, coal mining began at Glenrock and was followed by mining near Douglas and Sheridan. Until the early part of the 20th century, when mechanization was introduced, the only underground mining method used was roomand-pillar mining. The mines were developed by inclined shafts or slopes driven down the dip of the coal seam from the outcrop (Fig. 1). The slopes were developed in sets with two or more parallel entry headings spaced about

30-50 ft apart. Generally, one slope was used for haulage and air intake and another for air exhaust. In order to protect the slopes from collapse, large pillars of coal were left on both sides of them. At regular intervals of between 200 and 400 ft, level entries were driven off the main heading along the strike of the seam to facilitate the haulage of coal. Rectangular rooms were developed up-dip from the level entries in order to allow gravity to help pull the coal down from the working face to the haulage way. When these mines began operating, the coal was mined by hand; therefore, it was expedient to orient the long dimensions of the rooms with the dominant joint direction in the coal. After mining of the room was completed in a given area of the mine, as many of the pillars as possible were removed. Pillar robbing was a c o m m o n practice, even while mining was still being carried on in deeper workings. In general, the geologic formations comprising the overburden in the subsidenceprone mining districts are variable in cornSTRIKE

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SUBSIDENCE A B A T E M E N T S IN A B A N D O N E D COAL MINES

position, and can change vertically and laterally over short distances. The overburden is usually of poor quality, often with interbedded shale and claystone being located directly over the mined coal unit. However, in a few areas of the state, a massive and continuous sandstone caps the coal seam.

Mining subsidence problems Generally, there are two types of subsidence, associated with abandoned underground mining, which affect the land surface. These are: (1) troughing, which is a gentle depression of the terrain; and (2) chimney subsidence, where a conical depression, or sinkhole forms at the surface. Although such phenomena as trough and sinkhole subsidence are c o m m o n occurrences, the basic mechanisms associated with the phenomena are not well known [2]. The three basic mechanisms responsible for subsidence over abandoned room-and-pillar mines are: pillar failure, squeezes or punching, and the collapse of roof beds spanning adjacent pillars. Pillar failure and punching of pillars into the mine floor are thought to be the predominant mechanisms of trough subsidence over room-and-pillar mines. Trough subsidence is a gentle, continuous, and shallow dish-shaped depression in the ground surface formed by sagging or downwarping of the overlaying rock formations into a mined-out area. Trough subsidence will cause vertical and horizontal displacements, slope or tilt, and, in smaller size occurrences, curvature at the center. Chimney subsidence results from the intermittent, sequential collapse or unraveling of underground mine roofs in localized areas, whereby caving migrates through the overlying material until the failure zone intercepts the unconsolidated overburden. The surface expression can either be a trough with a dish-

like depression and a continuous and symmetric surface profile, or a sinkhole with an abrupt surface expression, usually in the form of a conical depression with the apex upward. In plane view, they are circular or oblong reflecting the geometry of the mine workings. In all reported studies on chimney subsidence, chimney cavings occur in overburden of less than 300 ft and in zones of rock weakness or areas of extensive vertical rock fracturing [3]. Of all of Wyoming's inventory of problems related to abandoned mines, subsidence in urban areas has had the greatest and most direct effect on h u m a n life. It can cause conditions that threaten life and create significant property damage. The psychological effects may be substantial for people who live in subsidence-prone areas where a home owner must live with the knowledge that subsidence may strike at any time, or in some cases has already begun to experience the effects of subsidence and must watch the continual and progressive deterioration of his property. The following discusses subsidence that has occurred or has the potential to occur in 7 Wyoming communities [4]. These communities are: Evanston, Gillette, Glenrock, Hanna-Elmo, Kemmerer, Rock Springs and Superior (Fig. 2). Evanston

Coal mining began in Evanston in 1869. All mining occurred about 3 miles north of town in an area known as the Almy District. The undermined land covers an area of 2 square miles. The State mine inspector's office described in its reports, during the period of 1886-1895, a number of conditions in the mines that could lead to subsidence [5]. Mine fires, pillar failures, floor heaves and roof cavings were c o m m o n occurrences. Leaving narrow pillars or fenders behind a n d / o r pillar robbing were the general practices in the

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area. Sinkholes are presently the most prevalent subsidence features in the district. They occur in a broad band along the outcrop at an overburden depth of less than 200 ft [6]. A total of 108 sinkholes have been identified from aerial photographs and ground checking. Considering the conditions in the mine, there is a potential for trough subsidence in the future. Gillette

Available reports indicate that some of the first commercial coal mining operations in Gillette began around 1908. Much of the coal in the immediate vicinity was taken from the Felix coal bed. The Felix coal bed is extensively exposed in the hills west and south of Gillette and provided much of the domestic

fuel supply for the community during the first half of this century. Fifteen mines have been identified along the Felix coal bed outcrop. Very little information is available on the size and extent of these mines. The existence of additional, one-man mines is very possible. Based on the information on the mines that have been located, it seems unlikely that any mines extended more than a few hundred feet into the seam from the outcrop. The most extensive Felix seam mine on record extends about 400 ft from the outcrop. It is prudent to assume that, considering the economics, there may have been unrecorded mining in the Felix seam at any location within 400 ft of the coal outcrop. Thus, it is considered that any location within 400 ft of the outcrop is capable of having a mined void beneath it, and hence being liable to subsidence [7]. Since

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SUBSIDENCE A B A T E M E N T S IN A B A N D O N E D COAL MINES

1980, there have been 8 documented sinkhole subsidence occurrences reported at overburden thicknesses of less than 100 ft.

160 ft, subsidence events at overburden thicknesses of 250 ft have also been reported [9].

Kemmerer Glenrock Although small quantities of coal were mined locally as early as 1866, the first significant mine in the area was started in 1884. Approximately 200 acres of land in or adjacent to the town of Glenrock are underlain by abandoned coal mines. Subsidence has occurred in the Glenrock area since at least as early as the 1930's and continues. In excess of 35 documented subsidence events have been reported, with a number of them causing damage to surface structures. Geotechnical investigation has indicated that the coal pillar strength and floor bearing strength provide adequate support in the mines; however, the roof rock is weak [8]. Roof falls have occurred and will occur. The associated subsidence may affect the surface in areas above the mines having limited overburden thickness. It was estimated that, in areas where the overburden thickness is less than 100 ft, there would be a risk of subsidence damage to property.

Coal in the Kemmerer area was first reported in 1843. It is believed that small-scale workings began as early as 1874. Major exploitation of the Kemmerer coals began with the arrival of the Union Pacific and the Oregon Short Line, and has lead to a situation where now essentially all of the towns of Frontier, Diamondville and the northeast part of Kemmerer have been undermined. The State mine inspector's office has reported conditions that can contribute to subsidence, such as mine fires, mine flooding and extensive pillar pulling. Although there are a number of sinkholes along the entries of the old mines, only nine documented cases of subsidence are near the towns. In addition to the documented cases of subsidence, there are several cases of surface deformation in the Kemmerer area that may be attributed to trough subsidence. In general, the greatest future potential for sinkhole subsidence, for the mining heights observed and 20 ° dip of the coal beds, is limited to within 150-250 ft of the coal outcrop [10].

Hanna-Elmo Rock Springs Coal mining began in Hanna in 1889. The area has been extensively mined and most of the town of Hanna is undermined. From 1889 to 1948, the State mine inspector's reports indicated a number of problems in the mines that contributed and will continue to contribute to subsidence. Mine fires, explosions and large roof cavings were common. Pillar robbing was the practice and workings 30 ft high were common. Surface disturbances due to subsidence cover a large area. In excess of 325 subsidence features have been identified with the great majority being sinkhole-type subsidence. Although 95% of the subsidence has occurred where overburden thickness is under

Reports indicate that coal mining began in the Rock Springs area in 1869 in the Union Pacific Coal Company mine No. 1. Demand for coal from the area persisted well into the 20th century, leaving approximately 2,500 acres of undermined property within the Rock Springs city limits. State mine inspector's reports indicate that high pillar extraction and robbing were the practice and that flooding of the mine workings occurred. Subsidence events within the city have been taking place since the turn of the century. Approximately 60 subsidence events have been documented, the large majority being chimney-type subsi-

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dence occurring where the overburden thickness is less than 150 ft. There is evidence that most of the subsidence events occur along a strip delineated by the fluctuation of the mine-water pool [11]. The full potential of mine subsidence in the city of Rock Springs is currently being investigated.

Superior Coal mining in Superior was initiated by the Union Pacific Coal Company in 1906. During the period 1906-1962, more than 42 million tons of coal were produced. Seam thickness in the area ranges from 6 to 12 ft. Coal was extracted by the room-and-pillar method. The ground in the mines was locally unstable and required timbering. After pulling the pillars and the supports, the roof was allowed to cave. Investigations at Superior have determined that the possibility of chimney subsidence exists along a relatively narrow band parallel to the coal outcrop [12]. Approximately 75 past subsidence events were identified from aerial photographs [5]. Subsidence only occurred in areas where overburden thickness was less than 100 ft.

Subsidence abatement techniques The Wyoming Abandoned Mine Lands (AML) Program has implemented a variety of subsidence abatement techniques over the last few years. In all cases, only remote methods to control subsidence have been used, with no actual work taking place underground. Three basic techniques have been employed. These include pumped slurry backfill, grout injection, and water-induced subsidence. Each of these techniques is described in brief detail below.

Pumped slurry backfill Slurry backfill techniques were used in Wyoming in the hope of preventing both

M.G. KARFAKIS

A N D E. T O P U Z

sinkhole and trough subsidence. In the case of sinkhole subsidence, backfilling all or some of the mine voids with granular material will reduce the effective void height, thus substantially reducing the probability that a sinkhole will propagate to the ground surface. Note that, in general, the roof is not directly supported by this technique. The objective for trough subsidence is different from that of backfilling to control sinkhole subsidence. Backfilling all or part of the void reduces the effective height of the pillar by supporting its base; this is equivalent to increasing the width-to-height ratio of the pillar, thus increasing the effective strength of the pillar. Increasing pillar strength can therefore, in some cases, help reduce the potential for trough subsidence [13]. Slurry backfill methods have been used in 3 communities in Wyoming (Rock Springs, Hanna, and Glenrock). Each of the projects will be described later in this paper. However, at all 3 locations the techniques used were basically the same, and were similar to those pioneered by the U.S. Bureau of Mines [14]. In this approach, slurry injection wells are drilled into the voids, and aimed to intersect the blind end of long rooms of bord-and-pillar mines such as those in Hanna and Glenrock, or in central, strategic locations for more conventional room-and-pillar mines such as those in Rock Springs. A homogeneous mixture (slurry) of water and granular material is then pumped from a remote plant site to the injection hole. The free-falling slurry moves through a vertical steel casing to be deposited in subsurface voids. The degree of void filling is a function of the velocity of the slurry flow at the mine level. As the slurry first enters the mine void from the injection borehole, its velocity is greatly reduced due to the increased cross-sectional area of the transporting conduct. This rapid drop in velocity causes the solid particles to be deposited in the form of a doughnut-shaped mound on the mine floor. As the height of the mound approaches

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the mine roof, the velocity of the slurry increases through the narrowing channels, and the solid particles are transported to the outer limits of the mound. At the boundary of the mound, the velocity decreases abruptly and the solids are deposited on the advancing face of the pile. The process continues until the energy required to maintain the flow of the suspended particles becomes greater than the energy supplied by the available gravity head. At that point, the slurry well must be abandoned, and injection continued at another hole. If the injection pressures were allowed to exceed that of the overburden pressure, lifting of the ground may occur, possibly inducing subsidence or disturbing surface facilities. A general illustration of this process is shown in Fig. 3. The general layout of the system used for slurry backfilling is shown in Fig. 4. It comprises the following major elements: (1) Granular material supply area. (2) Crushing and screening of material to proper specifications and stockpiling. (3) Slurry plant, where water is mixed with granular material, to make a dilute slurry. Equipment includes a mixing tank and slurry delivery pumps. (4) Water supply wells, supply water for mixing with the granular material to form slurry. (5) Slurry delivery system, comprising a main line from the plant to the injection locality, movable distribution fines to the individual injection wells, the wells themselves, and a line to purge the system in case of power or system failure.

Grouting Grout injection differs from hydraulic backfill in two basic ways. First, grouting treats a site-specific area, i.e. a house, a highway, etc., whereas hydraulic backfill treats a greater (less site-specific) area. Second, grouting provides more direct support to the over-

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burden and surrounding material than does hydraulic backfill. Two basic grouting techniques have been used in Wyoming. One utilizes a sand-grout mixture which is injected into rubblized zones of roof failure and small voids in order to support the roof and to eliminate the available space for continued downward movement of the overburden material (Fig. 5). Another, used in areas of large void and increased span length, employs gravel-grouted columns to provide direct support to the mine roof. A supporting column consisting of a gravel cone is placed in direct contact with the mine roof, and is subsequently cemented and strengthened by the injection of a cement grout (Fig. 6). The basic procedures for grouting are relatively simple. Injection holes, 6-8 inches in diameter, are drilled as close to the structure to be treated as possible, with surface casing being installed if required. Injection holes are drilled as close to the depth of the known mine floor as possible; however, due to the loose nature of the material, this may not always be very close. Once an injection hole is completed, a rigid 2-3 inch diameter grout tube would be placed down the drill hole. Starting at the bottom of the drill hole, grout is pumped through the tube. The grout tube is withdrawn slowly, filling the rubble and void zones with grout. Grout is placed under gravity or with very minimal pumping pressures in order to avoid overpressuring of the overburden and to prevent grout from progressing beyond the area requiring treatment [15,16]. If, during the course of drilling, large voids are discovered, then gravel-grouted columns are used. After the grout tube is lowered down the drill hole, 1 inch aggregate is poured down the annular space between the grout tube and the drill hole to form the supporting column (cone). Grouting then can progress as described above. In some instances, the grout tube is not used in the procedure. Using this technique, aggregate is first placed down the drill hole to form the cone, then grout is

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poured down the drill hole in a free-flowing manner into the aggregate.

Water-induced subsidence

In this technique, instead of preventing subsidence, subsidence is accelerated by saturating the overburden over the mine workings. The technique requires an area of active subsidence with relatively shallow overburden. This procedure can only be applied to areas where there is no danger of affecting surface structures.

Selection criteria

The following is a general listing of the criteria used in selecting abatement techniques: (1) Economics--the cost-effectiveness of the techniques suited to a particular site. (2) Availability of technology which will solve a given subsidence problem. (3) Mine void condition--this may have a bearing on whether slurry or grout and if grout, which grouting technique, should be used. It is determined by: (a) degree of roof collapse; (b) condition (stability) of overburden; (c) moisture conditions; (d) void sizes. (4) Public opinion, concern, and preference. (5) Fear of possible unwanted induced subsidence. (6) Surface features, including: (a) extent of the area requiring treatment; (b) presence of surface structures. The above factors are listed in no particular order, with each site being evaluated based u p o n its own merits and specific conditions.

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A N D E. T O P U Z

Subsidence abatement projects W y o m i n g m i n e subsidence problems qualify for the highest priority of funding under the Surface Mine Control of Reclamation Act of 1977, in that they constitute an extreme danger to public health, safety and property. F r o m the investigations conducted in the aforementioned communities, mine subsidence is considered a Priority 1 hazard in Hanna-Elmo, Glenrock, Rock Springs, and Superior. Abatement projects were undertaken by the Wyoming A M L Program to prevent or minimize further subsidence in these communities. Furthermore, the active subsidence at the Monarch Mine, northwest of Sheridan, Wyoming, was accelerated by water-induced subsidence. The following section describes the subsidence control projects. Hanna-Elmo subsidence control

The town of Hanna is located in south central Wyoming, 40 miles east of the city of Rawlins. Hanna is a small community with a population of approximately 2000 people. The town continues to rely on the Union Pacific Railroad and local coal mines for its economic base. Coal mining by the Union Pacific Railroad left voids of up to 30 ft under the town. Some of these voids have subsequently failed, leaving subsidence sinkholes and damage from trough subsidence in and around town. The continued risk of subsidence in Hanna has prompted the Wyoming A M L Program to initiate subsidence control measures in Hanna. The selection of subsidence abatement techniques in Hanna was based on three main points identified during the initial design investigations. The area selected to receive remedial action covered about 80 acres. The individual rooms of the mine were relatively free of rubble, with average dimensions of 25 × 25 × 300 ft. All mine voids were inundated with water.

SUBSIDENCE A B A T E M E N T S IN A B A N D O N E D COAL MINES

TABLE 1 Subsidence potential Overburden thickness (ft)

Risk of sinkhole subsidence

Risk of trough subsidence

0-100 100-160 160-250 250-300 300-800 800 +

high medium low very low -

very low very low low low moderate high

Because of the large area requiring treatment, the large size of the voids, and the flooded mine conditions, grouting was determined to be technically and economically unfeasible. It was determined that pumpedslurry backfill was the most applicable technique at Hanna. The selection of the 80 acres of the town which would receive backfill was based on the risk of sinkhole and trough subsidence occurrence. Although detailed geotechnical and subsidence potential studies were performed, the selection of the area to be treated depended upon the overburden thickness (Table 1). Although where the overburden is 800 ft thick the risk of trough subsidence is high, the effective strain that could be noticed at the surface would be low. With this in mind, it was decided to treat areas where there was a risk of sinkhole subsidence, and a moderate risk of trough subsidence. The basic technique of hydraulic backfill at Hanna is very similar to the technique described previously, except for one aspect. The Hanna mine more closely resembles a bordand-pillar system. The drill holes were placed at the blind end of the rooms for backfilling. The success of the Hanna project was excellent [17]. A total of 800 000 tons of material was placed beneath the northwest section of town at a cost of $79611 per acre. Slurry densities as high as 7.2 lb solids/gallon water were used without causing premature refusal at the wellhead.

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Monitoring of backfilled rooms showed that in some cases, nearly 100% of the void was filled with granular backfill material. The filling effectiveness was considered to be due to the large open rooms, slope of the coal seam (25%), and the lack of floor rubble. Few problems were encountered during the backfilling operation. The first problem developed after the initial cone building stage of backfilling was completed. At this stage, if injection was stopped, material that built up at the blind end of the room would backwash up into the drill hole, plugging it off. To prevent continued difficulties with this phenomenon, the water supply system was redesigned to allow pumping of clear water into the injection hole if slurry pumping had to stop, i.e. during shift changes and days off. The other major problem encountered during the project was the presence of lumps or clods in the fill material stockpile. The lumps were formed by belly-dump trucks continually driving over the material as the stockpile was built. The lumps would not completely break down in the mixing tank and would plug suction lines and cause undue wear on pump impellers. Radial stocking of material, or a second screening, could have helped with this problem. Elmo, a small subdivision 2 miles east of the main town of Hanna, was also found to need abatement work. Because of the small area to be treated (10 structures), the rubblized condition of the mine, and the cost which would be incurred by setting up a hydraulic backfill system, it was decided to use the grouting technique. The technique used is similar to the grouting technique described earlier; however, no gravel-grouted columns were used. A total of 2770 tons of grout was used to treat the 10 structures at an average cost of $32000 per structure. No major problems were encountered during the grouting operations, and no indications were present that the abatement was not successful [15].

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Glenrock subsidence control

Glenrock, Wyoming, is located 30 miles east of Casper and was the site of 2 separate underground coal mines. At one time, these mines were on the outskirts of the community but now lie under 200 acres of the town. The Wyoming A M L Program undertook the investigation and evaluation of the subsidence potential of the area in order to determine if subsidence control was necessary. The subsidence problems in Glenrock are mainly associated with sinkhole subsidence. The mining height was generally 6 ft with a bord-and-pillar configuration. Both the mine floor and roof are composed of a moderate to weak sandstone, and in most of the mine areas the sandstone roof is overlain by sand and gravel bedding. The presence of sand and gravel above the sandstone roof creates a problem; once subsidence passes through the sandstone, the gravel bedding no longer provides a competent roof or arch. Consequently, sands and gravels begin to "funnel" into the mine working. This situation was witnessed during the site investigation when a large diameter drill hole initiated a similar occurrence and over 100 yd 3 of sand and gravel were "funneled" into the mine workings. Following the investigation, it was app a r e n t that m o r e t h a n 20 acres of subsidence-prone residential and business property required some type of treatment. Considering the size of area, the availability of fill material, the convenience of surface access and cost effectiveness, a hydraulic slurry method was selected. The slurry plant set up was similar to that previously discussed. The system was nominally designed to deliver a sand slurry comprised of 15% solids by volume at a rate of 286 t o n s / h through a 10 inch diameter pipeline with pumping lengths of up to 5000 ft. U p o n completion of the work, approximately 65000 tons of solids had been injected into the mine workings through 133 boreholes at a

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cost of $252 295 per acre. The total amount injected was less than the projected tonnage. The reduced tonnage can be mainly attributed to the severely collapsed state of the mine workings [18]. The drilling indicated that the mine was in severe state of collapse and long continuous voids or rooms were no longer present, causing the quick rejection of injection holes. This situation required a large number of completed injection holes be available at any given time. Further, a special manifold system had to be developed for the easy and quick switch and hook-up of injection holes. The major problem encountered during the injection process was induced subsidence. In one particular area, numerous subsidence events occurred. The events varied greatly in magnitude; many were in the form of settling of old subsidence pits and in the formation of tension cracks, one of the major events encompassed the entire width of a highway, where the surface dropped about 15 ft. This occurrence was adjacent to a previous event which happened 10 years before the Wyoming A M L operation. At the time of the later event, slurry was being injected up-dip of the highway, in an area with very shallow overburden (as little as 19 ft). In the Glenrock area 2 separate grouting projects were undertaken. A grouting program was designed to stabilize the abovementioned highway, which had not been effectively treated by slurry injection. The grouting technique described previously was used. Modifications were made to the grout mix depending u p o n the mine conditions encountered (i.e. void, tight rubble). Overall, 1000 ft of highway were stabilized using more than 1730 yd 3 of c e m e n t / f l y a s h / s a n d grout. The other grouting program completed in Glenrock was directed towards stabilization of homes and commercial properties. Again, this work was conducted in a similar technical fashion to that previously described. This work included injection of over 7000 y d 3 of

SUBSIDENCE A B A T E M E N T S IN A B A N D O N E D COAL MINES

grout at a cost of $25 548 per structure. During the grouting program, a mine fire was encountered. This created a major problem and required some quick alterations to the grouting technique. When the fire was first detected, two holes down dip of the fire area had already been drilled. The third hole drilled below the structure to be stabilized was up-dip of the area, which created a "ventilation system" for the fire. Following completion of the third hole, the mine fire multiplied in magnitude and the 90 ft string of drill steel became so hot that it required the use of water and pipe wrenches to pull it out. All holes were capped and the structure was evacuated because of the elevated gas levels. Grouting began the next day. By alternating the holes and adjusting the grout mix, the fire was extinguished without allowing the grout to set up in the holes.

Rock Springs subsidence control Almost the entire city of Rock Springs, Wyoming (population 17000) lies over abandoned underground coal mines. During the mining period, over 100 million tons of coal were removed from 3 seams in the Rock Springs area. Consequently, over 2500 acres of developed areas are over old mine workings. Of this, 900 acres have been determined to have a high to moderate subsidence potential. Some of this acreage was addressed in previous projects conducted by the United States Bureau of Mines. The Wyoming A M L Program undertook a comprehensive investigation, design, and construction project intended to further stabilize untreated portions of the city. Based upon the investigation, both hydraulic backfilling and grouting methods of stabilization were selected for various areas. The hydraulic backfill was selected for large areas for three main reasons: (1) proven record of success (USBM placed 933000 tons by this

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method); (2) economic considerations; (3) fill material availability (abandoned surface coal mine spoils). The hydraulic backfill project was broken into two contracts, the northern and the southern projects. The northern project was completed as scheduled. The southern project was terminated prior to completion [16]. During the slurry injection for the southern project, ground movement and associated damage to residential buildings were reported. The damages from the event were severe enough to consider a cause-and-effect relationship between the event and the backfilling project. As a consequence, the project was terminated and an investigation of the subsidence followed. The investigation was inconclusive with respect to a proven correlation between the subsidence event and the backfill project [19,20]. The overall backfilling project resulted in the placement of over 200 000 tons of material into the mine workings under 33.5 acres at an average cost of $136900 per acre. However, this was well below the estimated tonnage for the backfilling. The overestimation of the projected backfill tonnage can be attributed to the advanced state of collapse of the mine workings, which could not be fully taken into account during the design stage. Approximately 17 acres were treated by grouting [16]. The downtown business district of Rock Springs was previously slurry-backfilled by the Bureau of Mines, but isolated areas were left untreated. A grouting program was designed to control subsidence and stabilize caved zones in the downtown area. Grouting and construction of grouted-gravel column techniques were determined to be most appropriate because of the shallow depth of mine workings as well as the previous public sensitivity to pumped-slurry backfilling. Rubblized zones were filled with grout. In areas where large voids existed, large-diameter holes were drilled to the mine floor, gravel was emplaced to the level of the mine

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roof and grout was injected into the gravel (Fig. 5). In addition to the structures in the downtown area, individual structures in areas showing subsidence-related chronic problems were selected for grout treatment. The objective was to stabilize areas in which mine conditions were not appropriate for pumpedslurry backfilling. Stabilization was achieved through the injection of grout into rubblized zones and the placement of grouted-gravel columns or concrete in areas where open voids were present. Two criteria were used for selecting homes to receive a grout treatment. First, the home had to be located in an area with a history of subsidence problems and have less than 120 ft of overburden. Second, the structure had to exhibit some form of structural damage, or be located near a home or series of homes that showed definite subsidence-related damage. When these criteria were met and the home owner expressed interest in having the work done, grouting stabilization was undertaken. Over 250 structures have been stabilized using these grouting techniques. A total of 11000 yd 3 of grout has been placed. The average cost per structure was $9500. Overall, the Rock Springs grouting programs have been very effective. To date, no measurable movement has occurred in any of the structures that received grout treatment.

Superior subsidence control Superior, Wyoming, is a small community 20 miles east of Rock Springs. Following an investigation, it was determined that the area's elementary school had suffered subsidence-related damage [21]. The school is located over an abandoned mine. The local overburden is 300 ft thick. The coal was mined using a high extraction ratio roomand-pillar method where pillar robbing was the practice. As a consequence, the surface is affected by trough subsidence. Architectural

M.G. K A R F A K I S A N D E. T O P U Z

and structural damage to the school building are consistent with trough subsidence. Grouting was chosen as a control measure because only a small area and a single structure were affected by subsidence. The depth of the mine workings required the design of a modified grouting technique to avoid drilling 300 ft and grouting the entire length. A highstrength grout was injected at close spacing (10 ft) above a shallow (60 ft) competent sandstone layer. The grout with the sandstone layer formed a continuous beam or plate under the school building, allowing the structure to move as a single rigid block and eliminating differential movements. Approximately 630 yd 3 of grout were injected to cover an area of 16000 ft z over a thickness of 60 ft. Core drilling after the project indicated that grout completely penetrated the fractured overburden, forming a monolithic block under the school building.

Monarch-induced subsidence The abandoned Monarch underground coal mine is located 9 miles northwest of Sheridan, Wyoming. The active subsidence area associated with the mine had no structures located over it; however, the large sinkholes were considered a hazard to humans and to the livestock which frequented the area. Before surface backfilling of the open subsidence pits, it was thought that, by saturating the active, thin overburden (75 ft) area with water, roof caving would be accelerated. This would allow fill to be placed over the site without worry of fill settlement in the future. The idea of saturating the area came from local residents who reported that every spring, as the area became wet, subsidence occurrences would increase. As a result, a sprinkler irrigation system was set up under the AML Program, in order to pump water from the Tongue River. Nine million gallons of water were distributed over the 15 acre site [22].

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Subsidence was indeed accelerated, and after a resting period of 3 weeks, the site was backfilled. Subsequent monitoring visits to the site have shown that no additional settlement has taken place, a testimonial to the success of the project.

Conclusions

The Wyoming AML Program has completed approximately $25 million of subsidence-related work to date. Of that amount, over $17 million has gone to subsidence control projects in urban areas. These projects have utilized a variety of techniques and have resulted in modifications to existing techniques. Prior to selecting a subsidence control method, the most important item of information is the location of the given mine and its physical condition. Wyoming has mainly relied on old mine maps and drilling to provide this information. However, in many instances drilling was supplemented by using downhole video cameras, geophysical logging or remote void-sensing methods. The key to whether a significant subsidence risk exists in a given location is the presence of underground mine workings. If there are no voids, there is no subsidence risk. It is, therefore, necessary to identify the location and geometry of the underground workings. The method of void detection must be relatively low-cost, should be reliable for depths of up to 200 ft and in dry or flooded conditions. At the present time, the only reliable method appears to be drilling. Nevertheless, this technique is expensive and inefficient, since it only provides information for one point [23]. As long as we continue to have subsidence problems in urban areas, the need for a low-cost, efficient and reliable method of detecting underground voids will exist. It is important to have a good understand-

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ing of a subsidence problem before making a selection of the abatement technique. The cost of obtaining the information may be high but will generally be offset by the saving made in selecting the appropriate abatement technique and designing it properly. The success of the hydraulic slurry injection process is dependent upon many factors. From the Wyoming experience with hydraulic backfilling, several recommendations can be made. Injection into inundated voids (below the water table) showed better success than injection into dry voids. In inundated mine conditions, the actual injected tonnage was as estimated during the design stage. Furthermore, there were no subsidence events related to the injection. The effectiveness of hydraulic slurry injection also greatly depends on the state of collapse of the mine workings. Mine workings with a minimal rubblized zone will accept more fill material by allowing the free, unrestricted movement and escape of the injection water. Overall, Wyoming has seen much better subsidence control success with grouting. This is primarily due to the fact that most of the old mines are severely collapsed. Over 260 individual structures have been grouted with excellent results. Of these structures, only one has shown additional movement. This movement was slight (existing crack widening) and has now stopped. Grout quantities needed for an individual structure have been as great as 200 yd 3. This is rare, however; the average structure required approximately 25 yd 3. One recommendation that is often overlooked is the public relations aspect of any subsidence project. Wyoming tries to utilize public and individual meetings to explain the subsidence process and what takes place during an abatement program. Meetings with residents have helped to show them what equipment will be in the area and what type of disruptions can be expected. Many times, voids under property have been shown to the home owner with the use of the borehole

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camera. Public relations can greatly reduce citizen complaints and problems. Nationwide, considerable underground mine void backfilling has already been undertaken to prevent or minimize the adverse effects of mine subsidence. This is an expensive process and is usually done only where improvements of considerable value to the overlying land surface are likely to occur. Questions still exist on the effectiveness of slurry backfill and grouting operations. Long-term monitoring of backfilled areas is needed in order to evaluate and quantify the merits of these techniques. In Wyoming, the Abandoned Mine Land Program will continue to provide assistance to subsidence victims only over the next biennium. Long-term assistance will be provided by the Wyoming Mine Subsidence Insurance Program. The insurance program applies only to individuals who purchase the insurance, and then only covers the cost of repairs resulting from structural damage. Long-term ground stabilization, will be dealt with by a Mine Subsidence Mitigation Fund, which has been created with money set aside from the state's AML allocations and which will address subsidence problems after the termination of the AML program.

M.G. KARFAKIS

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References 1 Jones, R.W., Overview of Wyoming coal deposits and coal mining. In: M.G. Karfakis (Editor), Proc. Governor's Workshop Mine Subsidence, Univ. Wyoming, Laramie (1986), pp. 1-50 and 1-77. 2 Karfakis, M.G., Subsidence over abandoned coal mines--mechanisms and prediction. In: Proc. Symp. Engineering Geology and Soil Engineering, 23rd (Logan, Utah) (1987). 3 Karfakis, M.G., Mechanism of chimney subsidence over abandoned coal mines. In: S.S. Peng (Editor), Proc. Int. Conf. Ground Control in Mining, 6th (Morgantown, W.Va.) (1987). 4 Karfakis, M.G., Beach, G. and Case, J.C., Subsidence problems in Wyoming and their social impact.

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In: Proc. Nat. Syrup. Mining, Hydrology, Sedimentology and Reclamation (Springfield, Ill.) (1987). Case, J.C., Overview of coal mine subsidence in Wyoming. In: M.G. Karfakis (Editor), Proc. Governor's Workshop on Mine Subsidence. Univ. Wyoming, Laramie, (1986), pp. 1-1 and 1-46. Brown, A., Land use recommendations to minimize subsidence risk near Evanston, W.Y. Evanston subsidence evaluation project final Rep., MidWest Mining Co., Littleton, Colo. (1986). Brown, A., Land use recommendations to minimize subsidence risk in Gillette, Wyoming. Gillette subsidence evaluation project final Rep., MidWest Mining Co., Littleton, Colo. (1986). Gormley, J.T., Glenrock subsidence control project 8A, Converse County, Wyoming. Rep. Gormley Consultants, Englewood, Colo. (1986). Karfakis, M.G., Chimney subsidence over abandoned coal mines. Int. J. Min. Geol. Eng., 5 (1987): 131-141. Brown, A., Kemmerer subsidence risk report. MidWest Mining Co., Littleton, Colo. (1986). Colaizzi, G.J., Land use planning considerations for the city of Rock Springs, Wyoming, Suppl. Proj. 6A Final Rep. Goodsen and Assoc., Denver, Colo. (1987). Brown, A., Land use recommendations to minimize subsidence risk in Superior, Wyoming. MidWest Mining Co., Littleton, Colo. (1986). Brown, A., Hanna subsidence control project phase i report. MidWest Mining Co. Casper, Wyo., Vol. 1 (1985). Whaite, R.H. and A.S. Allen, Pumped-slurry backtilling of inaccessible mine workings for subsidence control, us Bur. Mines, IC8667 (1975). Brown, A., Peacock Mine grouting completion report. MidWest Mining Co., Lander, Wyo. (1987) Colaizzi, G.J., Final report Rock Springs subsidence control project 6A. Goodsen and Assoc., Denver, Colo. (1987). Brown, A., Slurry backfill project completion report. MidWest Mining Co., Lander, Wyo. (1987). STS D'Appolonia Ltd., Summary report glenrock subsidence control project 8A. STS D'Appolonia, Denver, Colo. (1987). Suprenant, B.A., Karfakis, M.G., Edgar, T.V., Basham, K.D. and Abel, J.F., Investigation of residential damage in Rock Springs, WY. DEQ-State of Wyoming (1985). M.G. Karfakis, and Suprenant, B.A., Ground failure investigation over abandoned coal mines: A case study. In: Proc. Int. Conf. Case Histories in Geotechnical Engineering, 2nd (Rolla, Mo.) (1988) pp. 173-176.

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21 Gormley, J.T., Superior subsidence control report of investigation. Gormley Consultants (1986). 22 HKM Engineering, Wyoming AML Project 8, Old Monarch Mine. HKM Engineering, Sheridan, Wyo. (1985). 23 Brown, A., Cost effective location of shallow aban-

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doned underground mines. DEQ-LQD Rep. 1071/6, State of Wyoming (1986). 24 Barnard, S. Key administrative aspects of subsidence abatement projects. Proc. Conf. Coal Mine Subsidence in the Rocky Mountain Region (Colorado Springs, Colo.) (1985), pp. 271-280.