Information Circular 9359

State-of-the-Art Techniquesfor Backfilling AbandonedMine Voids

By Jeffrey S. Walker

UNITED STATES DEPARTMENT OF THE INTERIORBruce Babbitt, Secretary

BUREAU OF MINES

Library of Congress Cataloging in Publication Data:

Walker, JetTrey s,State-of-the-art techniques for backfilling abandoned mine voids / by Jeffrey S.

Walker.

p. em. -(Information circular; 9359)

Includes bibliographical references (p. 17).

1. Mine filling. 2. Abandoned coal mines. 3. Mine subsidences. I. Title.II. Series: Information circular (United States. Bureau of Mines); 9359.

TN295.U4 [TN292] 622 s~c20 [622] 91-41440 CIP

CONTENTSPage

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction '.' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Background 2Area-wide techniques 3

Hydraulic flushing 3Pneumatic stowing 7Excavation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Blasting .. . . .'. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Point support techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Grouting 11Piers 13Deep foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Bulkheads 13

Types of backftlling material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Processing wastes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Mine refuse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Flyash............... 14Prepared aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Evaluation of past subsidence abatement projects .......................•.................... 15Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

ILLUSTRATIONS

1. Idealized example of point support method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32. Hydraulic backftlling ..............................•............................... 43. Idealized hydraulic backftlling process 54. Hydraulic backfill placement in model mine 65. Hydraulic backfilling equipment . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . 76. Idealized pneumatic stowing process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87. Remote stowing elbows 98. Pneumatic ejector 99. Support equipment for conventional pneumatic stowing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

10. Typical grout column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211. Pressure grouting to stabilize collapsed overburden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1212. Sodium silicate injector 1313. Idealized grout column using sodium silicate technology 1314. Deep foundations for subsidence control 14

cfm

fps

ft

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cubic foot per minute

foot per second

foot

gpm gallon per minute

in inch

percent

pound per square inch

pet

psi

STATE-OF-THE-ART TECHNIQUES FOR BACKFILLINGABANDONED MINE VOIDS

By Jeffrey S. Walker1

ABSTRACT

Abandoned underground mine openings are susceptible to collapse because of the mining methodsused, the character of the overburden, and the typically large, wide entries with minimal roof support.The fmal effect of the collapse of the underground workings is surface subsidence. To reduce theprobability of subsidence, methods to backfill the mine void with various types of materials have beendeveloped. This U.S. Bureau of Mines report describes the available technologies for subsidenceabatement and discusses their operation and application. The basis of these abatement methods is thereplacement of the mined material with mine waste. Backfilling of mine voids is the most commonmethod of stabilization used to abate subsidence and protect surface structures. Hydraulic flushing andgrouting, using remote methods from single or multiple boreholes, are the most often-used methods forthe placement of backfill material. Other subsidence abatement techniques are available and may bemore appropriate under different conditions. These other techniques include pneumatic stowing, eitherby in-mine or remote methods, and various point support methods that do not completely fill the minevoid and are used for the protection of small areas of the land surface and surface structures.

IMining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA (now with U.S. Department of Energy, Germantown, MD).

2

INTRODUCTION

Subsidence from abandoned underground coal mineshas become an everyday concern of many citizens living incoal-producing regions of the United States. Becausesubsidence causes direct damage to property, with sig-nificant fmancial loss, and more importantly presents anextreme danger to public health, safety, and general wel-fare, subsidence and subsidence-related problems are con-sidered by the Office of Surface Mining Reclamation andEnforcement (OSMRE) and affected States to be thehighest priority issues relating to reclamation of aban-doned mine lands (AML).

The development and improvement of methods to abatesubsidence from the collapse of abandoned underground

mines has been more or less neglected for the last 10 to15 years. The same methods and procedures used in thelate seventies are still used today. The objectives of theU.S. Bureau of Mines Abandoned Mined Land SubsidenceAbatement Research Program are to improve subsidenceabatement practices through the evaluation of currentbackfilling methods, to increase the use of sound geo-technical engineering principles in site investigation andremedial design, and to develop state-of-the-art tools andprocedures for subsidence abatement. This work will pro-vide a source of documented information describing themechanism of subsidence development, applicable abate-ment methods, and remediation procedures.

BACKGROUND

The principal application of mine backfilling in theUnited States today is different from that of 100 years ago.Backfilling was first used in the anthracite coalfields ofnortheastern Pennsylvania to support the surface in activemining operations, to arrest the development of progres-sive pillar failure, to aid in the recovery of pillars, and todispose of mine waste (1).2 Today, backfilling is primarilyused as a method to abate subsidence from the collapse ofabandoned underground coal mines. There have been afew instances in which this technique was applied in activemines. However, with the increased use of high-extractionmining methods, backfilling during the mining operationmay be a future option for subsidence control.

There are a number of differences between backfillingmethods used in active mines to prevent surface subsid-ence and those used in abandoned mines. The greatestdifference is the lack of access to abandoned mine voids.In active mines it is possible to directly observe the back-filling operation and control the location and amount ofthe fill material deposited. Whereas, in abandoned mines,often there is no access to the mine void. All work mustbe done from the surface in a remote fashion. It is thisdifference that separates the techniques used for in-minebackfilling and remote backfilling of abandoned minevoids. The primary concerns of in-mine backfilling pro-grams are the logistics of material transportation throughthe mine and the capacity of the system. Remote backfill-ing operations require consideration of these factors, andalso installation of equipment through boreholes, contend-ing with unknown mine configurations and collapsed zonesand blind monitoring of the fill.

2Italic numbers in parentheses refer to items in the list of referencesat the end of this report.

There has been little change in the backfilling tech-nology used during the past 100 years, and technical infor-mation to support the design of artificial pillars is almostwholly lacking in the literature. The most recent discus-sion of state-of-the-art methods for subsidence control waswritten in 1974 by the Appalachian Regional Commission(2). Since that time, progress has been made in the designof point support methods and delivery equipment, but thebasic concepts and limitations remain the same.

There are two options available for the abatement ofsubsidence: point support and area-wide techniques.Point support techniques are used for situations in whichsubsidence is occurring in a relatively small area as aresult of individual pillar failure. These methods provideadditional support at a single location, such as at the cen-ter of a large mine opening or beneath a single surfacestructure of interest (fig. 1). Area-wide techniques areused where a large portion of the mine is at risk of col-lapse. The area-wide techniques fill large portions of themine with fill material from multiple injection boreholes.

In general, there are four methods for the constructionof artificial supports used in subsidence abatement: hy-draulic, pneumatic, mechanical, and hand. Only hydraulicand pneumatic methods have widespread application tosubsidence control in AML, because these methods can beimplemented from the surface without the need for per-sonnel to enter the mine. This type of backfilling opera-tion is termed "blind" or "remote." It differs from in-minebackfilling techniques in that the remote method is typ-ically performed through a borehole without placing anyequipment into the mine except for the material injectionpipe and nozzles. In-mine techniques utilize operators in-side the mine to direct and control the flow of materialinto the void. Hand and mechanical methods, such as belt

, "

Mine void

Figure 1.-1deallzed example of point support method.

or sling packing machines, are restricted to construction ofthe support from within the mine. These methods havebecome obsolete or uneconomical for large-scale sub-sidence control and have been replaced with the use ofpumped concrete for in-mine construction. Cribbing madefrom timber or concrete blocks remains the only type ofhand-constructed support used today.

Inherent to remote backfill systems is the problem oftracking the placement of the backfill. It is difficult tomonitor the size of the fill area and to determine the di-rection in which the fill area is growing (3). This problemis relevant to both pneumatic and hydraulic methods.However, because larger areas can be backfilled using thehydraulic method, the need to determine the growth of thefill becomes much more important.

Simply stated, hydraulic flushing consists of washingbackfill material into a mine void using large quantities ofwater. The hydraulic flushing method is believed to haveoriginated in the anthracite coalfields of Pennsylvania in1864 in an effort to prevent the destruction of a church

3

from mine subsidence (1). The practice of hydraulic flush-ing was further developed during the late 1800's and early1900's and was used on a regular basis in the anthracitecoalfields to extinguish fires and support areas of ex-tremely high extraction. In 1901 hydraulic flushing wasintroduced into Europe, where it has been used extensivelyto dispose of mine waste into the abandoned sections ofdeep room-and-pillar mines. The practice of hydraulicflushing in the United States decreased with the declineof the anthracite industry in the 1940's. Currently, useof hydraulic flushing in the coal industry is limited tosubsidence abatement in abandoned underground mines.However, in the metal mines of this country, hydraulicmethods are commonly used to remedy ground controlproblems.

Pneumatic stowing was first applied in Germany in1924, and it has become the favored backfilling method,replacing hand packing and hydraulic methods. Pneumaticstowing consists of the transportation of backfill materialinto the mine through a pipeline by flowing air. In Europethe general use of pneumatic stowing is to place minerefuse in the gob areas behind advancing longwall faces.This practice not only provides roof support, but reducesor eliminates the disposal of mine waste on the surface.Pneumatic stowing is not used widely in the United States,and is typically limited to small-scale use for building mineseals or filling behind tunnel liners. In recent years, how-ever, pneumatic stowing has been used in ground controlapplications in the Western United States where there isa lack of abundant water supplies or the mine workingscannot tolerate excessive moisture conditions (4).

"Hand packing" and "mechanical packing" typically referto backfilling of very small mine areas for specific pur-poses, such as permanent support of unstable roof areas orreplacement of barrier pillars during second and thirdmining operations. However, the term "packing" has beenused as general term to describe any type of mine backfill-ing operation.

The type of abatement method to be used for each situ-ation is not easily determined. Each method has its prosand cons. It is safe to say, however, that all of the sub-sidence abatement methods need to be further investigatedto fully document their capabilities and to improve theirapplication.

AREA~WIDE TECHNIQUES

HYDRAULIC FLUSHING

Hydraulic flushing is the practice of filling the minevoid with various types of material (i.e., crushed stone,mine refuse, sand, or fly ash) by washing or pumping the

material into the mine with water (fig. 2). The fill mate-rial is transported into the mine as a slurry and depositedin the void until the void is completely filled or there isa blockage of the injection pipe. Bulkheads and minepillars are often used to limit the flow of the material to

4

.. '.

Figure 2.-Hydraullc backfilling.

a specific section of the mine. Current flushing methodsrequire pumping of large amounts of water and materialwith little control over the direction and compaction of theresultant backfill. The available hydraulic flushing tech-niques include high-volume pumping, low-volume pumping,and gravity feed (5).

In the high- and low-volume pumping methods, granu-lar material is blended with water in a large mixing tankat a processing plant (typically located some distance fromthe injection borehole) and is then pumped to a series ofinjection boreholes through pipelines (6). The energy pro-vided from the pump and the static head of the boreholegives the backfill material the velocity required to keep thefill material in suspension and determines the size of thearea that can be flushed from a single borehole.Normal-ly,many boreholes spaced close to each other are requiredto backfill a large area.

The efficiency of the backfilling process within the mineis a function of the slurry velocity. When the slurry firstenters the open mine void from the borehole, its velocitydrops rapidly, and the solid particles drop out to form adonut-shaped mound on the mine floor (7). As the heightof the mound approaches the mine roof, the velocity of theslurry is maintained by the narrowing of the opening be-tween the roof and the fill material (fig. 3). When theslurry reaches the outer limit of the mound, the velocitydecreases abruptly and solids are deposited in a processthat is similar to the development of an alluvial fan. In anopen void without obstacles, such as pillars or stoppings,the ruling occurs in a circular fashion (7). However, inactual conditions, large mine openings are not usuallyencountered; the honeycomb pattern of room-and-pillarsections is more often typical. In a table-sized mine modelsimulating a typical arrangement of rooms, it was shown

~tInjection borehole

')Pumped slurry~~, ~Coalbed

. :~:~~~"Sf:~~~@;iM":"""'~ '

Rock stratum

Rock stratum Deposited solids

Figure 3.-1deallzed hydraulic backfilling process (7).

that the flow of the slurry usually develops in one directionuntil the resistance to the flow becomes so great thatanother flow path is developed (8). This process is re-peated until nearly all of the mine openings are filled orthe slurry cannot be pumped any farther. Figure 4 rep-resents the deposition pattern of hydraulic backfill in amodel mine (8).

The lateral extent of the fill is determined by the energyof the pumped slurry and the condition of the mine. Asthe mound of fill builds outward into the mine, the flowchannels between the mound and the tiline roof becomelonger and more serpentine; therefore, the resistance toflow is increased. When this resistance becomes so greatthat the slurry can no longer transport the material beyondthe fill, "refusal" or the limit of the backfilling is reached.In practice, it is extremely difficult to estimate the volumeof fill needed for a specific area because of the uncertaintyconcerning the configuration of the mine and the tendencyof the slurry to cut a channel through the existing fill andflow unrestricted into other parts of the mine (9).

The particle size distribution is variable throughout thefill; this is due to the changing velocity of the slurry. Asthe velocity of the slurry is decreased, the heavier particles

5

fall out of suspension. The smaller and lighter pieces offill material are then carried farther from the injectionpoint and deposited. The resultant fill is therefore strati-fied based on the velocity of the slurry.

Where the dip of the coalbed exceeds 40°, a barrier orbulkhead may be needed to prevent large-scale movementof the slurry downdip away from the planned flushing area(5). This technique is often called controlled flushing.The barriers, bulkheads, or cutoff walls constructed acrossthe mine openings are permanent obstructions designed tolimit the flow of backfill material or to contain the mate-rial within a specific area. These obstructions are madefrom grout or gravel and can be installed remotely or fromwithin the mine if access is available. The construction ofthese items is similar to the construction of several of thepoint supports, which will be discussed later in this report.

High-volume pumping is normally used on very largebackfilling projects to place mine refuse or other availablematerial into the mine void and spread it over a largearea. It is typically used in mine areas that are known tobe open and have few obstructions. The high volume ofwater and its velocity at the injection point ensure a widerspread of material than in any of the other methods sincethe energy of the slurry is much greater. High-volumepumping requires a large quantity of water and fill mate-rial. A minimum of 500 gpm is necessary, and as much as8,000 gpm may be required depending on the size of theboreholes and the type of material (5). Fill materials withhigher densities require a greater amount of flow to keepthe solids in suspension in the slurry mixture.

The advantages of high-volume pumping are that alarge amount of material can be placed in a short periodof time and that low-cost, locally available backfill materialcan be used. The disadvantages of the system are thecosts of establishing a central slurry mixing plant, a pipe-line network, and a high-volume water supply. Further-more, the addition of water to a formerly dry mine maycause sloughing and weakening of the pillars, floor, androof.

In low-volume pumping, a combination of slurry andlow-volume pumps is used to carry the material to the in-jection point. The water requirements are 100 to 500 gpm,about 10 times lower than for high-volume pumping (5).The advantage of the system is the typically small, port-able, construction-type pumping equipment, which causeslittle congestion within the project area. The disadvan-tages are longer material placement times; the need foradditional water, leading to adverse effects in the mine;.and the need for additional boreholes, because the mate-rial spreads a limited distance from the injection point.

Gravity feed systems do not use mechanical pumps totransport material. The solids are dumped into a hopper,which feeds to a stream of water entering the mine. The

6

DuuunounnunnnnnIniLtlTU' U

DnnnuuuonooouulO-by 40-by 6-ftpiliar blocks

~ Backfill material filledto roof level

L 5-./ Height of depositedsolids

Figure 4.-Hydraullc backfill placement In model mine (8).

capacity of this type of system is very low and is limited bythe type of material and the size of the borehole. Theadvantages of this system are the low operating costand the small amount of required equipment. The dis-advantages of this system include the following: materialis not transported laterally beyond the injection point,injection rates are very slow, and filling the void withoutblocking the injection pipe is difficult.

An extensive amount of equipment is needed whenpumping methods are used. In addition to a large supplyof water and fill material, a screening and mixing plantand a network of pumps and pipelines to transport theslurry to the injection borehole or supply water to themixing plant are required (fig. 5). All of this equipmentshould be sized in accordance with the rate of backfillingdesired.

Screens and crusher

7

Cement add itlves

····.c "';'.~., .• ' ~ ..,., .1,.',"c, ..,. "".,.-.:~lVj":.

.. '. '.

valve

. .':~:~;.' ..I ' ••. ;' .'

;". .:; ;~' . . . .

" . '

. I: ;

Figure 5.-Hydraullc backfilling equipment.

The greatest difficultywith hydraulic flushing is the con-trol of the material in the mine. Boreholes are typicallyused to monitor the flow of the backfill during the flushingprocess. Downhole video cameras and/or observationwells are used to determine the extent of the backfill andthe head pressure of the backfill at various locationsduring backfilling operations. However, because the con-figuration of the mine void is typically unknown and theopening is often blocked by fallen debris or abandonedequipment, the backfill material frequently flows in un-expected directions. Without knowing where the materialhas been transported in the mine it is impossible todetermine the support capacity of the backfill.

In several cases, an amount of fill material that exceedsthe estimated volume of the mine void has been pumpedinto a borehole without a pressure increase that wouldindicate filling of the void (5,·9-11). A recent study foundthat at some sites where the flushing operation was con-sidered a success the backfill material did not completelyfill the void (12).

PNEUMATIC STOWING

"Pneumatic stowing" refers to a specialized form ofpneumatic conveying in which mine backfill material istransported into a mine and "stowed" or placed into thevoid. At the present time, the technology has not beendeveloped to the point where pneumatic stowing is gen-erally accepted as a viable method of subsidence control.However, the problems are being overcome as its use be-comes more common. The advantage of pneumatic stow-ing over hydraulic techniques is the elimination of water-related control problems. However, the fill material, withcurrent technology, cannot be carried a great distancefrom the nozzle. This limitation prevents pneumatic stow-ing from being used as a truly area-wide technique. Fur-thermore, this method can only be used in dry mines.

The pneumatic stowing system is a more recent devel-opment than hydraulic flushing. Recent research indicatesthat it may be possible to pneumatically project materialup to 80 ft beyond the injection site (4). Material placed

8

using pneumatic methods can achieve a fairly high degreeof compaction and good contact with the mine roof. Thedrawback to the system is the potential for excessiveabrasion of the injection nozzle and elbows, which causesfailure of the equipment after only a relatively smallamount of material has been stowed.

There are two methods for pneumatic conveying (3).One is dense phase, in which the pipeline is nearly filledwith material that is moved as a fluid with low-velocity airpressure in slugs. The other method is dilute phase, inwhich there is less than 5 pct fill material in the pipelineand it is moved at relatively high velocity as a fluid.

In general, the mining industry uses dilute phase con-veying. However, some mines use rock dust systems,which utilize dense phase transport. The two major appli-cations of pneumatics in the U.S. mining industry havehistorically been hoisting cuttings from shaft and tunnelboring operations and stowing.

There are few published cases of remote pneumaticstowing operations. No personnel are present at thedischarge end. of the pipeline during remote stowing

Motor Blower:

operations, and all equipment used in the mine must beinstalled through a borehole (fig. 6). The available infor-mation indicates that the majority of remote pneumaticstowing operations were unsuccessful either because of alack of site-specific information or from the inability todirect the trajectory of fill material (3, 13).

Research is progressing at the Bureau to improve thecapabilities of pneumatic stowing systems. The problemswith the systems include excessive wear on the injectionpipeline, especially at the elbows and transition points, andinability to project the material into the mine void once itis injected down the borehole. Various configurations ofcollapsible elbows with nozzles are under development toredirect the material from the vertical injection pipe outinto the mine void in a nearly horizontal fashion (fig. 7).The function of these elbow-nozzle assemblies is not onlyto redirect the material into the mine but to reentrainthe material into the airstream. Reentrainment ensuresmaximum trajectory by placing the fill material above theairstream exiting the nozzle rather than below the air-stream. It is calculated that if the research is successful

Figure 6.-1deallzed pneumatic stowing process.

'

- in to 12-in - diameterborehole

, ,I

5-in-IDpipe " 08==========~ ~\\ ",\ ,,'<, 1\\\ ,,'\\ ,,\\ ,& ~

Roller

Cable

9

%-in steel cable

Figure 7.-Remote stowing elbows. Left, Short-radius elbow with blind T; right, segmented wide-radius elbow (4). (10 = Insidediameter.)

elbow-nozzle assemblies of this type may be capable ofprojecting the fill material as far as 100 ft in a 6-ft-highmine entry (4).

A different type of elbow-nozzle assembly is also beingresearched. This type is called the pneumatic ejector (14).The pneumatic ejector consists of a high-pressure air noz-zle fitted to an elbow at the end of the injection pipe(fig. 8). The high-pressure nozzle in the elbow creates asupersonic airstream (in excess of 1,600 fps) across thebottom of the injection pipe. As the backfill material fallsthrough the injection pipe into the mine, a transfer ofmomentum occurs. The result is a combined stream of fillmaterial and air moving horizontally at a velocity of 100 to500 fps. This system was tested by the OSMRE and wasfound to be capable of projecting the material as far as50 ft horizontally depending on the height of the mineopening (15). Theoretically, a 2,000-efm airstream at100 psi will project sand material as far as 90 ft in a minevoid 7.5 ft high. The advantage of this system is that airvelocity turns the fill material, thereby eliminating theabrasion on the injection equipment. Research is continu-ing, and it is expected that several different types of elbow-nozzle combinations will be developed.

Feed pipe

Supersonic nozzle

Compressed air

Figure 8.-Pneumatic ejector.

10

The principal equipment needed to perform pneumaticstowing is the same as for any other pneumatic conveyingactivity: a power pack, a compressor or blower, and anair-lock feeder (fig. 9). The differences between variousapplications are the quantity of air required, the pipe size,and the air pressure. These are all functions of the weightof the material to be conveyed and the length and eleva-tion of the delivery pipeline.

The air supply for pneumatic stowing operations is pro-vided by either a positive displacement blower or compres-sor. The majority of the pneumatic backfill equipment inthe United States use rotary lobe positive displacementblowers, usually capable of supplying 3,000 to 5,000 cfmat 14 to 18 psi. Pneumatic stowing machines that are pow-ered by compressed air are available and have been usedextensively in European mines where up to 100 psi of airat large volumes is readily available.

The chamber air lock, screw feeders, and rotary air lockare three types of air-lock feeders that can be used to in-troduce the backfill material into the airstream withoutexcessive loss of air. The rotary air-lock feeder is by far

Power pack

the most popular feeder. The size of the chamber air lockand the lack of positive sealing capability of the screwfeeder make them less desirable. The rotary air-lockfeeder is essentially a drum with screw-shaped pocketsaround the outside. Material is fed into one pocket at atime. As the drum rotates, the pocket is sealed in thehousing and dumps its load into the airstream whileanother pocket is being filled. The rotary air lock is ahigh-maintenance item because of the abrasiveness of thebackfill material.

The pipelines, elbows; and nozzles for pneumatic stow-ing are also high-wear items. In horizontal pipelines, pipewear is fairly rapid and is most often found on the bottomof the pipe. Because of the excessive wear, the pipelinesare made of hardened steel pipe. The pipe is rotated reg-ularly to achieve longer life. Ramps are sometimes builtinto the pipeline to lift the material from the bottom ofthe pipe and reentrain it into the airstream rather thanletting it slide along the bottom of the pipe. The wear onvertical pipelines is much less, usually only 10 pet of thewear found on horizontal piping.

pipe

~~L .rAir'~~·;;Pyy ~:',::' : ....•.:'\:~/~~"": '. : : : : '. '. :: . .. . ~~ ,\4..1.....' , ', : . . , . . . • : . . "::' q\ '"............. , :~."~,,,. . :·.···,,·:"~~VJ

• .' • , • '. '. • 11,• , •••••• I •••••• I •••• 1\

. ' : ......

'. '

Figure 9.-Support equipment for conventional pneumatic stowing.

Elbows are required at places where the pipelinechanges direction. The elbows must be specially designedto maintain the material entrained in the airstream andmust include replaceable wearing portions. In a well-designed elbow, there is an impact section at the entry anda transition piece to accelerate the flow at the discharge.The liners of the elbows are typically manufactured fromtool steel of at least 650 Brinell hardness and sometimeshave a W-shaped cross section to maintain airflow beneaththe entrained material and to reduce the wearing surface(4).

At the discharge end of the pipeline, a nozzle or de-flector plate is usually used to direct the fill material. Thedeflector plate is usually a steel plate or a section of a pipecut lengthwise. Nozzles have been used to concentrate thestream of material coming from the pipeline and direct thetrajectory without moving the pipeline. It has been sug-gested that the nozzles would increase the degree of com-paction achieved by directing all of the force to a smallertarget area (16).

EXCAVATION

Excavation or daylighting of the mine void is a tech-nique that can be used for shallow mines. This type ofremedial action is similar to a surface mining operation.The area affected by subsidence is excavated to mine level,the remaining coal is removed, and the area is then back-filled to the original grade. This technique is suitable onlyfor areas where little surface activity is present and wherethe surface topography would require extensive regradingbecause of the effects of subsidence. The cost of thismethod is extremely high, especially in areas that are notaccessible to heavy surface equipment and are limited byenvironmental constraints.

11

BLASTING

Blasting to reduce the overburden to rubble and to col-lapse the mine roof in a controlled manner has been usedas an area-wide subsidence abatement technique in onedemonstration project. This project was located at theUrlacker Mine site near Dickerson, ND. This demonstra-tion project was performed at a shallow mine site (30 to50 ft of depth) and was considered a success. The blastdid induce caving that extended to the surface, and thearea has had no reported additional settlement or sub-sidence since 1983, when the project was concluded (17).

The concept of blasting to prevent subsidence is basedon the fact that as a rock mass is reduced to rubble, itswells to fill a larger volume. Theoretically, the magnitudeof the swelled volume is calculated to be equal to or slight-ly larger than the volume of the mine void. In deepermines the rubbled zone is formed just above the coalseam, similar to a longwall operation; however, in a shal-low mine the rubbled zone extends to the surface. Therelationships among the open mine voids; the depth to thecoalbed; and blast design considerations, such as chargeweight, spacing, and delay times, are not well understood.In addition, the swell factors and the crater formationprocess are not well known for coal measure rocks.

The concerns with this type of reclamation includeproblems with blast vibrations in urban areas and environ-mental concerns such as damage to ground or surfacewater sources. Additional research is underway to ad-dress these concerns and to provide additional technicalinformation.

POINT SUPPORT TECHNIQUES

Point support methods are used in areas where an anal-ysis of the subsurface conditions indicates that the in situsupport would not be sufficient for long-term stability.These methods are typically used with the intent of sup-porting only a small area such as a structure or a specificsurface feature. These methods are extremely cost effec-tive when applied correctly. However, the ability toaccurately locate the artificial supports is limited bya lack of knowledge of the mine void and overburdencharacteristics.

GROUTING

"Grouting" is a general term that typically refers to theuse of a fly ash-cement mixture as the backfill material.

Generally grout is placed as grout columns, which consistof small cones of cemented backfill material extendingfrom the mine floor to the roof directly beneath the injec-tion borehole. The addition of cement to the backfill in-creases the strength of the material. The support capacityof grout-stabilized backfill is considered to be very good,and grouting is used regularly as a backfill method in smallareas.

Historically, grout columns have been constructed from6-in-diameter boreholes filled with crushed rock or gravelfrom the mine floor to the surface, which has been pres-sure grouted to form a permanent support. Newer meth-ods utilize quick-setting, low-slump grout pumped through2- to 6-in-diameter boreholes to completely fill small ar-eas of open mine voids or to form self-supporting groutcolumns (fig. 10).

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6-in-diamgrout hole

Ground surface

I I r I

...Gravel and grout

Figure 10.-Typical grout column.

Grout can also be used in areas where caving hasalready occurred. In pressure grouting, grout is forcedinto the voids at a pressure on the order of 1/2 psi perfoot of depth. This method not only fills the mine voidbut penetrates and stabilizes the fractures in the over-burden that occurred as a result of subsidence (5). Fig-ure 11 shows an idealized use of pressure grouting to sta-bilize the rubbled material resulting from a roof collapseand the resulting fractures in the overlying rock mass.

Grout mixtures vary according to the requirements ofan individual project. For most applications, mixtures ofportland cement and fly ash are sufficient, and thesemixtures provide inexpensive and reliable fill material.Mixtures of 1 part cement to 3 to 9 parts sand or fly ashare typical (18). Chemical additives are available tomodify the penetration and setting abilities of the groutmixtures. Chemical grouts are typically several times moreexpensive than cement grouts.

Figure 11.-Pressure grouting to stabilize collapsed over-burden.

As with hydraulic flushing, the drawback to grouting isan inability to monitor placement and to determine arealdistribution and percentage of mine roof contact.

A new technique for the construction of grout columnsis the use of sodium silicate additive to the grout mixture(19). In this technique a chemical additive is applied tothe outside of the grout as it is discharged. The sodiumsilicate reacts with the surface of the grout stream, forminga skin of calcium silicate that acts as a barrier. This allowsthe grout to form a self-supporting column. Figure 12illustrates the device for applying the sodium silicate togrout as it is entering the mine. The sodium silicate tech-nology can be used in both dry and flooded conditions. Indry conditions the calcium silicate skin contains the high-slump concrete, and in wet conditions the skin preventswater from penetrating the grout mass and diluting themix. An example of the shape of a grout column usingsodium silicate technology compared with an average groutcolumn is shown in figure 13.

The strength and capability of grout columns made bythis method have been tested by the OSMRE, and the col-umns have been proven to be effective (19). This tech-nology should increase the use of grout pillars as a meansof support, because it is now possible to control the place-ment of the grout.

Grout bags are another way to control the spread ofgrout during the formation of grout columns. The bagsare placed into the mine void through a 6- to 8-in-diameterborehole. The grout bag, once installed in the mine,contains the grout pumped from the surface so that an

Grout deliverytube

Sodium silicate Injectionmanifold

Sodium silicate deliveryin annulus space

Figure 12.-Sodlum silicate Injector.

f Injection borehole

SodiumSilicategrout

column

Mine floor

Figure 13.-1deallzed grout column using sodium silicate tech-nology.

artificial pillar of a specific size and shape can be remotelyconstructed. These bags have had limited application andhave shown great promise (20). Further analysis and test-ing of the concept is required before the bags can be usedon a regular basis.

13

PIERS

In-mine piers are simply in-mine supports to augmentexisting coal pillars. They are made from concrete, ma-sonry, and in some cases timber cribbing. Although thismethod has been used for support in active mines through-out modern mining history, in-mine piers are rarely usedin abandoned mines, since it is necessary to physically en-ter the mine void to install them. The primary applicationof support piers is to support a specific surface structure.Their use is generally limited to room-and-pillar mines,and the piers are usually constructed immediately aftermmmg,

DEEP FOUNDATIONS

Deep foundations or pile foundations are limited to theprotection of individual structures. These foundations arelarge-diameter drilled piers (usually 8- to 24-in diameter),which are drilled through the mine and bear on the firmstrata beneath the mine. Such methods are used to founda structure on the competent rock beneath the mine open-ing (fig, 14). In general, deep foundations are not used atsites where the overburden thickness exceeds 100 ft. Thismethod is limited almost exclusively to new construction.

The piers are steel casings driven from the surface intothe rock beneath the mine and then filled with concrete.The surface structure is then constructed on beams sup-ported by the piers.

BULKHEADS

Bulkheads are not a means of support, but they areused as barriers to control the flow of the backfill materialin the mine. They are built between existing pillars so thatthe area to be backfilled is completely surrounded. Ingeneral, bulkheads are made from grout or gravel. Theyare constructed by drilling injection boreholes closelyspaced across the known mine opening. The spacing ofthe boreholes must be close enough that the cone-shapeddeposition spreads into a solid barrier rather than leavingvoid spaces near the mine roof.

Gravel bulkheads are preferred when used in conjunc-tion with hydraulic flushing operations. The porous natureof the gravel allows the water in the slurry to pass butretains the solids (21).

14

50' >-;', '-

Drilled piers casedthrough mine void

...-- Mine void

Figure 14.-Deep foundations for subsidence control.

TYPES OF BACKFILLING MATERIAL

The material used in a mine stabilization program isdetermined by local availability, transportation costs, anddesired engineering properties. The most common mate-rials used include waste rock; coal processing waste; flyash; and prepared aggregates such as sand, gravel, orcrushed rock. Almost any type of material can be used, aslong as it is environmentally safe and its consistency issuch that it can be easily injected into the mine and yetprovide a positive support.

The engineering characteristics of a backfill material arevery important to the long-term success of a subsidenceabatement project. The material must be durable, in thesense that it will not degrade during handling or pumping;nonabrasive, because the wear on the system must be lim-ited to the extent economically possible; strong, for long-term support; and stable, so that it will not become weakwhen exposed to the mine conditions. Many differenttypes of material have been tried with varying success.However, the following remain the most common backfillmaterials.

PROCESSING WASTES

Processing wastes from local plants where rock is beingcrushed for aggregate can provide inexpensive fill materialin the form of "off-spec" rock. This material often has suf-ficient engineering properties but does not meet highwaysizing requirements. Limestone dust from such plants is

a highly desired mine fill additive because of its pozzolanic(cementing) properties.

MINE REFUSE

Waste product created by coal preparation plants con-sists of coal, clay, shale, and other rock. The bearingstrength is low if the waste product contains a high per-centage of clayparticles. Therefore, this material is mostlyused for stabilization of dry, shallow mines where theweight of the overburden is relatively low and roof fallsare the major failure mode. "Red dog" or burned minerefuse has a greater bearing capacity than unburned refuseand can also be used as backfill. However, red dog is veryabrasive and does not move freely once it is released fromthe end of the injection pipe.

FLY ASH

Fly ash and bottom ash are noncombustible residuesthat result when coal is burned. Fly ash is a fine-grained,lightweight aggregate composed of small globules of glass-like material. Fly ash is easy to transport pneumaticallyand in slurry form can be transported long distances inmine voids. Though fly ash often has natural pozzolanicproperties, it is necessary to mix it with cement to developsignificant bearing capacity (17). Bottom ash is coarse andheavier than fly ash. Bottom ash is better than fly ash formine stabilization but is normally more expensive.

PREPARED AGGREGATES

Prepared aggregates are the. preferred types of backfillmaterial to use in a flushing or pneumatic stowing opera-tion because the engineering properties of sand, gravel,and crushed stone are well documented by the roadwayand heavy construction industries. The drawback to thesematerials is their cost. Depending on the location, pre-pared aggregates can cost up to four times as much asother backfill materials.

The environmental impact of backfilling abandonedmines with waste materials is not well understood. Themost obvious danger is the contamination of ground watersupplies that come in contact with backfill material. Forexample, coal refuse and fly ash may contain high levels ofcontaminants such as aluminum, arsenic, boron, cadmium,

15

chromium, copper, iron, lead, manganese, mercury, nickel,selenium, vanadium, and zinc. Research indicates thatwhen this material comes in contact with water these ele-ments go into solution, thus contaminating the water.There is the possibility that other backfill materials mayalso include one or more of these contaminants. The ex-tent of the assimilation of these pollutants into the natu-ral environment is a function of many variables that areunique to each site. To date, the documented researchsupports the belief that backfilling is not hazardous tothe environment. However, the Environmental ProtectionAgency (EPA) regulates and classifies backfilling materialusing criteria stated in the Resource Recovery and Con-servation Act and other health-related criteria. Researchis currently under way at the EPA to identify the environ-mental impacts of backfilling.

EVALUATION OF PAST SUBSIDENCE ABATEMENT PROJECTS

Investigations have been performed to measure theresults of a number of area-wide mine backfill projects.The majority of these projects, however, were conductedas a method to confirm the location of the fill material,and few or no tests were performed to evaluate the sup-porting capability of the backfill. One recent report byOSMRE does in fact evaluate the performance of fourpast subsidence abatement projects that used hydraulicbackfilling in terms of lateral and vertical extent of the filland the engineering parameters of the fill, i.e., grain sizedistribution, strength, and in situ bearing capacity (12).The sites evaluated were Green Ridge DemonstrationProject, Scranton, PA; Farmington Subsidence AbatementProject, Farmington, WV; Watson Hill Subsidence Abate-ment Project, Fairmont, WV; and 70th Street SubsidenceProject, Belleville, IL. All of the projects were demon-stration projects sponsored by the Bureau in the 1970's.The methods and results of the first two were documentedin Bureau reports.

The testing conducted at these sites was standardengineering practice for geotechnical investigations ofsubsidence-prone land, with the exception of the use of aspecialized. tool for the determination of the in situ bearingcapacity of the backfilled material. Core samples weretaken at various locations above the study sites, and split-spoon samples of the fill material were recovered. Thegeneral findings of the OSMRE report are as follows:

Backfilling of mine voids was very dependent on theconfiguration of the mine. Some portions of the mine voidreceived very little fill material. This was presumed to be

due to roof falls, stoppings, or other debris that may haveblocked the flow path of the.backfill material into the void.

In mines that were considered to be successfully back-filled because of the significant amounts of material thatwere deposited, a void space of more than 1.0 ft was leftbetween the roof of the mine and the top of the fill.

The flow pattern was not the radial distribution thatWhaite and Allen (6) and Carlson (7-8) had described inreports on modeling hydraulic backfilling. Sieve analysisand observations of the bedded character of the backfillindicated that the slurry flowing from the injection pointsettled in the same manner as sediment in stream chan-nels, forming bars and graded deposits. The fine materialin the slurry was segregated from the coarse material andwas transported farther from the injection point.

Large concentrations of water-saturated fine materialshowed very low compressive strengths and the materialwas determined to be plastic enough to yield when subject-ed to lateral or horizontal pressures.

In mines where coarse coal refuse was used as thebackfilling material, degradation of the clays and shaleswas apparent. This suggests that backfill that containslarge portions of these materials will randomly break downinto finer fragments in a saturated environment and will beinsufficient to support the overburden.

These fmdings cannot be applied universally to all re-mote hydraulic backfilling projects, but the problems foundat these sites are consistent with the concerns of thoseworking in the field. Discussions with individuals whohave drilled into backfilled underground mines for reasonsother than evaluation of the fill indicate that voids andvery soft fill material are not uncommon.

16

SUMMARY AND DISCUSSION

Backfilling of mine voids is the most common methodof stabilization used to abate subsidence and protect sur-face structures. The state of the art of mine backfilling ismuch the same as it was 10 to 15 years ago. In-minemethods are the most well developed and are the mosteffective means of constructing support. However, inabandoned mine areas, access to the mine void is rarelypossible. Therefore, it is necessary to use remote or blindbackfilling methods.

The most common method for the placement of backfillmaterial has historically been hydraulic flushing, either byin-mine or remote methods. These have been area-wideprojects in which several acres or more have been stabi-lized. Pneumatic stowing appears to be increasing inpopularity as the technology advances. Other subsidenceabatement techniques are available and may be more ap-propriate under different conditions. These techniquesinclude the use of point support methods, which do notcompletely fill the mine void, but offer protection to smallsurface areas and surface structures.

At any location where a potential subsidence problemmay exist, the primary task is a thorough investigation todetermine the nature and extent of the subsidence prob-lem. It is necessary to gather information on the conditionof the overburden, the mine roof, the mine floor, and themine pillars. Knowledge of the size and condition of themine void is necessary to effectively design remedial meas-ures to protect the surface. Historically, large area-widehydraulic flushing methods have been considered the onlyway to fill the mine void with material to arrest the col-lapse of the mine opening. However, the development ofimproved methods for site investigations, the improvementin pneumatic and grouting technologies, the increase in theuse of point support methods in emergency subsidenceactions, and the increase in the cost of subsidence abate-ment actions has forced the industry to consider new orimproved support methods.

Hydraulic flushing remains the only cost-effectivemethod for backfilling a large area of unstable under-ground mine void. The reasons for using the area-wideapproach are related to the type of subsidence, the size ofthe area affected, and more importantly, the knowledge ofthe site conditions. In areas where much is known aboutthe geology, the mine configuration, and the condition ofthe mine, a specific remedial design can be made. Inareas where little is known, area-wide methods are theonly choice.

In areas where the cause of the subsidence can be pin-pointed, a point source method can be applied. In some

cases, a number of point support structures may be nec-essary to strengthen a large area. Using point sourcemethods is thought to be more cost effective than usingarea-wide methods if there is sufficient information toprovide a sound remedial design.

The point source method most often used is the grout-ing technique. The grout mix used varies depending onthe intent of the project. Where very good subsurfaceinformation is available, pinpoint drilling can be performedand low- or no-slump grout can be pumped into the voidto provide the support needed in a specific area .. As theuncertainty of the subsurface information increases, thesize of the area into which the grout is pumped is in-creased. Grout additives such as foaming agents, plasti-cizers, or surfactants are used to meet a specific need,such as a slow setup time or increased strength. Usingthese additives significantly increases the cost of theproject.

Pneumatic stowing may be an inexpensive altemative togrout in a dry mine. Pinpoint drilling as used for groutingcould be utilized to locate the injection pipe near the col-lapsing section of the void. Ground support could then bebuilt by directing the trajectory of the fill material.

Other methods are inappropriate for the majority ofAML situations. Methods such as deep foundations ordrilled piers are more suited to new construction than themitigation of damage in a subsidence-prone area. Sim-ilarly, the blasting method may not be acceptable in urbanareas. However, explosives are used regularly in urbanareas for other purposes, and if the blasting method isproven to be a viable method for the abatement of sub-sidence, blasting could possibly be used in urban areaswith proper care.

The development of methods for the abatement ofAML subsidence has progressed only as fast as the needfor new methods. Until recently the need was not a pri-ority of many city planners. As the frequency of subsid-ence events has increased, so has the awareness of the lackof effective methods to abate subsidence. Parallel to thishas been the development of techniques to evaluate thesubsurface condition at an AML site. As the capabilitiesto determine the location and the condition of the minesite become more precise, remedial designs will require avariety of backfilling methods. It is clearly demonstratedfrom past work that it is possible to backfill a mine to anextent that will lessen or prevent subsidence-related dam-ages. Much more research is needed, however, to providesimple, cost-effective solutions.

17

REFERENCES.

1. Ashmead, D. C. Mining of Thin Coal Beds in the AnthraciteRegion of Pennsylvania. BuMines B 245, 1927, 133 pp.

2. Gray, RE., J. C. Gamble, R J. McLaren, and D. J. Rogers. Stateof the Art of Subsidence Control. Appalachian Reg. Comm., ARC-73-111-2550, Dec. 1974, p. 264.

3. Thrill, R E., P. J. Huck, and B. G. Stegman. Monitoring BlindBackfilling in Abandoned Mines. Min. Eng., v. 35, No. 12, Dec. 1983,pp.1625-163O.

4. Roberts, J. M., F. W. Tobias, A. L. Masullo, and J. E. Powell.Report on the State-of-the-Art of Pneumatic Stowing Techniques. Off.Surf. Min. Reclam. and Enforcement unpublished report, 105 pp.; avail-able upon request from J. L. Craft, Off. Surf. Min. Reclam. and Enforce-ment, Greentree, PA.

5. Staff, Office of Surface Mining Reclamation and Enforcement.Abandoned Mined Lands Reclamation Control Technology Handbook.1981, 270 pp.

6. Whaite, R H., and A. S. Allen. Pumped-Sluny Backftlling ofInaccessible Mine Workings for Subsidence Control. BuMines IC 8667,1975,83 pp.

7. Carlson, E. J. Hydraulic Model Studies for Backfilling MineCavities (First in a Series of Tests). Bur. Reclam., ERC-73-19, 1973,32pp.

8. __ • Hydraulic Model Studies for Backfilling Mine Cavities(Second in a Series of Tests). Bur. Reclam., ERC-75-3, 1975, 38 pp.

9. Faddick, R R, and W. R Eby. Sluny Backfilling of Mine Voidsin Hanna, Wyoming. Proceedings of the 1986 Symposium on SurfaceMining, Hydrology, Sedimentology, and Reclamation (Lexington, KY,Dec. 8-11, 1986). Univ. KY, Lexington, KY, 1986, pp. 209-214.

10. Craft, J. L. Office of Surface Mining Reclamation and Enforce-ment. Private communication, 1991; available upon request from R C.Dyni, BuMines, Pittsburgh, PA.

11. Candeub, Flesissing, and Assoc. (Newark, NJ) ..Report on Demon-stration of a Technique for Limiting Subsidence of Land Over Aban-doned Mines. June 1971, 65 pp.

12. Elder, C. H. Evaluation of Procedures Used in Four MineSubsidence Control Projects. Off. Surf. Min. Reclam. and Enforcement

unpublished report, 59 pp.; available upon request from J. L. Craft, Off.Surf. Min. Reclam. and Enforcement, Greentree, PA.

13. Juntunen, R, M. Heil, and B. Mundie. Reclaiming OrphanedLands Using a Pneumatic Backfill Process. Am. Soc. Agric. Eng., Pap.ASAE 83-2035, 1983, 11 pp.

14. Burnett, M., and J. L. Craft. A Pneumatic Ejector for BackfillingUnderground Mines Through Boreholes. Proceedings of the 10th An-nual AML Conference, Association of Abandoned Mine Land Programs(Wilkes-Barre, PA, Oct.1Q-14, 1988). PA Dep. Environ. Resour., 1988,153 pp.

15. Burnett, M. Final Contract Report, Development and Testing ofa DIY Feed Ejector for Backfilling Mine Voids (contract HQ51-GR87-10008, Off. Surf. Min. Reclam. and Enforcement); for inf., contact J. L.Craft, Off. Surf. Min., Reclam. and Enforcement, Greentree, PA.

16. Soderberg, R L., and D. R Corson. Support Capabilities ofPneumatically Stowed Materials. BuMines RI 8202, 1976, 48 pp.

17. Head, W. J., and E. E. Saadeh. Assessments of Fly-Ash CementGrouts for Use in Abandoned Mines. Proceedings of the InternationalSymposium on Environmental Geotechnology, v. 1 (Allentown, PA,Apr. 21-23, 1986). ENVO Publ. Co., 1986, pp. 576-585.

18. Royse, K. W. Underground Mine Reclamation Via Blasting. Pro-ceedings of the 1983 Symposium on Mining, Hydrology, Sedimentol-ogy and Reclamation (Lexington, KY, Nov. 27-Dec. 2, 1983). Univ. KY,Lexington, KY, 1983, pp. 247-254.

19. Reifsnyder, R H., RA. Brennan, and J. F. Peters. Sodium Sili-cate Grout Technology for Effective Stabilization of Abandoned FloodedMines. Paper in Mine Drainage and Surface Mine Reclamation.BuMines IC 9184, v, II, 1988, pp. 390-398.

20. Michael, P. R, A. S. Lees, T. M. Crandall, and J. L. Craft. APoint-Support Technique for Mine Subsidence Abatement. Paper inMine Drainage and Surface Mine Reclamation. BuMines IC 9184,v. II,1988, 401 pp.

21. Vandyke, M. W. Gravel Bulkheads for Confining Hydraulic Back-filling of Abandoned Underground Coal Mines. Soc. Min. Eng. AIMEpreprint 85-363, 1985, 2 pp.

INT.BU.OF MINES,PGH.,PA 29796

'U.S.G.P.O.: 1993--709-008/80030