GEO-CON INC.GEOTECHNICAL CONTRACTING

February 20, 1990

9-G073

f €B 2 6Cooper Industries1001 Fannin StreetSuite 4000Houston, TX 77002

FAX: 713-739-5555

Attn: Michael O'Brien

Additional InformationInstallation of Temporary Deep Mine Bulkhead

Proposed Remediation MethodsOsborne Landfill

Grove Citv. Pennsylvania

Gentlemen:

We are pleased to present this additional information on theinstallation of a temporary deep mine bulkhead for the abovecaptioned project. The purpose of this bulkhead is to permitproper installation of a permanent slurry cutoff wall where theslurry wall alignment intersects the existing strip minehighwall.

Background

The technique of installing deep mine bulkheads has been usedon numerous sites throughout Pennsylvania. Primaryapplications are for mine subsidence control and to act as awater barrier within active deep mines.

Enclosed please find an ASCE technical paper on mine groutingfor subsidence control. The end of this paper describes aproject at Grove City College which involved the creation of agravel bulkhead at mine level to act as a "form" to permit thefilling and stabilization of a mine beneath an existingstructure. This particular project was performed by Geo-Con,Inc.

Also enclosed please find a typical profile, pjLan view andisometric view of a mine bulkhead which is useji as a waterbarrier within an active deep mine. •

PQ Box 17380 - P i t t s b u r g h . PA 15235 - T e l (412)856-7' 1357Texas Office (817)383-1400 _ _ _ _ , ,. , _

California Office (406)453-3587 " A R 0 0 H 0 I 7

GEO-CON INC.

February 20, 1990Page 2

We are also in the process of obtaining additional backgroundinformation on the use of deep mine bulkheads from the U.S.Bureau of Mines. See enclosed letter dated February 14, 1990.This information will be forwarded to you once we have receivedsame.

Osborne Site

Based on our considerable mine grouting experience, we feelthat there are two basic technically sound techniques which canbe utilized to construct the mine bulkhead on this site. Theyare as follows:

1. Utilize standard, older bulkhead techniques which requirethe installation of a front and rear gravel bulkhead"form" with a concrete center plug.

2. Create the temporary bulkhead with a single row bulkheadusing a low slump concrete at mine level to create thebarrier. ,

Based on site constraints, our grouting experience, andeconomics, we feel that the second technique would be bestsuited for this project.

The preliminary procedure anticipated to construct this barrieris as follows:

1. After installation of a thirty foot (±) wide work platforminto the highwall for access, a single row of air rotarydrill holes along the proposed centerline of the slurrywall would be drilled to the base of the coal mine. Theinitial spacing would be on five to ten foot centers. Theinitial purpose of these holes is to locate the variousmine corridors at the proposed slurry wall alignment.

2. Additional air rotary holes would then be drilled alongthis alignment to better define the limits of the minecorridors, and to act as additional injection holes.

3. A tremie pipe would then be lowered to the base of themine in drill holes which encountered open voids. Theseholes would be grouted using a stiff, lows slump, concretetype grout to form the bulkhead at mine l,evel.

GEO-CON INC.

February 20, 1990Page 3

4. Drill holes which encountered solid or fractured coalwould be pressure grouted using a fluid slurry type grout.

5. After allowing the grout to sufficiently set in a givenarea, check holes would be drilled to mine level, midwaybetween the various injection points. Based on theconditions at mine level, these holes would be grouted asdescribed in above Item Nos. 3 and 4.

6. Additional intermediate check holes would then be drilledto mine level in required areas, and grouted as necessary,until all check holes are relatively watertight.

7. The slurry wall operation would then proceed at thecompletion of installation of the mine bulkhead.

As always, please let us know should you have any questions orrequire additional information. -

Sincerely,

GEO-CON, INC.

Kenneth B. AndromalosDrilling and GroutingGroup Manager

KBA/gml

Enclosures

Sent via FAX.

AR301*6I9

GEO»CON INC.

SUDSIPENCE CONTROL BY HIGH VOLUME GROUTING

Kenneth B. Andromalosf1), A.M.ASCE, and

Christopher R. Ryan<2>, M.ASCE

ABSTRACT

Various grouting techniques have been used over the yearsto stabilize existing structures and new construction overabandoned coal mines. The purpose of any mine groutingprogram is to provide additional support at nine level tocontrol future mine subsidence. Mine .grouting programsgenerally fall into two general methods: flushing andgrout columns. With the flushing method, the entire mineunder the area to be protected is filled. Grout columnsare used to shorten the free span of a mine roof to thepoint where arching will prevent any s palling fromprogressing to the ground surface. Because of itsfineness and low cost, flyash is commonly used as a bulkfiller in. both methods. Cement and sometimes coarseaggregate are utilized with the flyash to form the minegrout mixture. Based on conditions encountered at themine, the consistency and proportions of flyash-cementgrout can be modified to suit the mine conditions. Thecompressive strengths of various flyash-cement grouts usedfor mine grouting are presented in this paper. The use ofhigh volume grouting techniques utilizing flyash-cementgrout slurries for mine subsidence control are discussed.A recent project is discussed to illustrate thetechniques, equipment and the grout mix that can beutilized.

INTRODUCTION

The activities of the coal mining industry have resultedin different types of structural and environmental damageto the land. Deep coal mines collapse over a period oftime and, under the right conditions, tha collapseprogressively works its way to the bedrock surface, andoverburden soil falls or washes in with consequentsubsidence and damage to structures founded at the

Group Manager, Geo-Con, Inc.; Pittsburgh, PA 15235President, Geo-Con, Inc., Pittsburgh, PA 15235

AR30l»620

GEOCON INC.surface. In general, nine subsidence is inherent in olderand, in particular, abandoned mining operations. Due toincreasing public awareness, better technology andstricter enforcement . of mining laws, newer miningoperations tend not to result in these problems;

The states of Pennsylvania and West Virginia, with heavyconcentrations of mining industry, received aid fromseveral Federal agencies and have taken a leading role insolving the subsidence and pollution problems of coalmines. Recently, the Federal Government's Office ofSurface Mining, a division of the Department of theInterior, has taken the funding lead for these types ofproblems. There has been extensive research on techniquesand there are various methods to combat each of theabove-mentioned problems. This paper concentrates onsolutions which involve high volume grouting with fluidflyash-based slurry grouts. .

Grouting is the injection of a fluidized suspension ofcolloidal particles into the underground soil and rockprofile. Grout holes are drilled to the desired point oftreatment and grouts are pumped through grout pipes intothe treatment zone. Grout mixes can vary in consistencyand strength and may be altered in the field to suit theconditions encountered. Since mine-related problemsrequire a blanket solution on the basis of data gatheredat scattered points, the flexibility of grouting makes itvaluable as a solution. Contracts are normally written asunit-price, with the bids being compared on the basis ofquantities derived from an engineer's estimate. Actualquantities installed may vary significantly as thegrouting program is altered to suit conditions actuallyencountered. <

The types of grouting discussed in this paper usuallyinvolve much greater quantities of materials than othermore sophisticated types of grouting, e.g. dam foundationgrouting and grouted structural underpinning. A typicaljob may involve the injection of thousands of tons of drymaterials. The type of equipment used is suitable forhigh volume materials' handling. Frequently, significanteconomics can be achieved by relaxing some procedural andmaterial specifications from "normal" grouting practice.For example, in most of the areas of Western Pennsylvania,large diameter air-rotary drill rigs can drill grout holesat a fraction of the cost of water-flush drill holes.Some sacrifice may be made in the cleanliness of fissuresin the sides of the drill hole but, in cases where largecracks and voids ..are the major concern, there is littletechnical disadvantage.

. » « iOn many projects, flyash is used as a bulk filler. Flyashwith the most consistent quality is normally delivered dry

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'£ GEO-CON INC.in bags or pneumatic trucks. In the dry form, the ash isdifficult to handle because of dusting problems. It isusually cost effective to use "wet" or "conditioned" ashthat has sufficient moisture to hold together in a piledumped on the ground by conventional trucks and handledwith earth moving equipment. Wet ash that is excavatedfrom flyash holding ponds may contain rock and bottom ashlumps. This loss of uniformity has virtually noimportance when grouting all but the smallest fissures.The use of flyash provides a neat environmental symmetry.The ash is returned to its source, resolving twoenvironmental issues simultaneously: disposal of the ashand solving a coal mine-related problem.

SUBSIDENCE

The causes and effects of subsidence over mined-out areashave been discussed in numerous publications, a few ofwhich are included in the attached reference list. Thetype of mining with which this paper is concerned is where"rooms" and tunnels are cut into the coal seam, leavingcoal in place as pillars and the "walls" of the rooms forsupport. The amount of coal left in place is usuallyenough to sustain the total weight of rock and soiloverhead without crushing. However, the span of mine roofbetween pillars may be quite large and may, with time,begin to spall off. If the pillars are far enough apartand the rock surface is close enough so that arching doesnot control the progressive spalling and collapse of themine roof, it will eventually work its way to the bedrocksurface. Overburden may then wash into the mine, causingsink holes and damage to surface structures. In the bulkof the Western Pennsylvania areas where subsidence hascaused damage, the mines are only 100 feet (30 meters)deep and in many cases, much less. The maximum depth towhich surface subsidence has occurred in the United Statesas a result of mine subsidence is 700 feet (213 meters).This record depth was experienced on a project nearBirmingham, Alabama (Ryan, 1983).

Grouting has been used in cases where subsidence hasalready begun to cause surface damage, and in cases wheresites are treated in advance of construction to counteractpotential subsidence. Designing a solution to minesubsidence problems is usually complicated by the factthat the original mine voids are no longer empty. Oftenthe mine roof may,, be partially collapsed. There may bewater standing or "flowing on the mine floor or the minemay be completely flooded. Frequently, mine voids arepartially backfilled with "gob" which may be rockfragments placed there as a result of later miningoperations or sedimented by the action of water. The gobmay be very soft but. stiff enough that it cannot be easilydisplaced. The actual state of any mine can ba

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GEOCON INC.complicated by any of the above conditions, the extent ofwhich may be difficult to determine through the use of

i , exploratory borings.,

TREATMENT METHODS '

The types of treatment basically fall into two generalmethods: flushing and grout columns. With the.flushingmethod, the entire mine under the area to be protected isfilled. Grout columns are used to shorten the free spanof mine roof to the point where arching will prevent anyspalling from progressing to the ground surface. Detailsof the two methods and their relative technical andeconomic advantages are presented in the paragraphsbelow. In both cases, the zone of treatment is usuallytaken as the area projected downwards at a fifteen-degreeangle of influence outside the structure (Ackenheil andDougherty, 1970).

Flushing consists of backfilling the entire mine void inthe treatment zone with a weak grout. Holes are drilledaround the perimeter, usually on fifteen to twenty-fivefoot (4.5 to 7.5 meter) centers. The perimeter is groutedfirst to contain the grout that will be subsequentlypumped in the central area. A thicker grout or crushedstone may be used on the perimeter to help prevent excessgrout from flowing out of the treatment zone. Once theperimeter barrier has been established, central holes are

V_y drilled, usually on twenty to twenty-five foot (6 to 7.5meter) centers and a weak grout is "flushed" (pumped at afast rate) into the mine until refusal (Figure 1).Usually., grout is allowed to come to the surface so as tofill voids in the rock that may be present due to theearly stage of roof collapse. The grout that is used forthis application should be designed to withstand crushingby the weight of overburden. Typically, flyash-cementgrouts with ratios in the range of 8:1 by dry weight areused. The flushing technique provides the followingtechnical and economic advantages:

Lowest cost per unit volume of materials due tomaximum use of cheap fillers such as flyash.

Absolute lowest requirements for quality controlof materials. If the grout mix can be pumped, itis probably suitable. • .

Lowest reliance on the strength of any gobpresent, since the overburden pressure isdistributed over the widest area possible.

Essentially yields full roof contact; does notrely on arching to prevent progressive collapse.

Androme

GEOCON INC.The flush-grouting technique has disadvantages where theperimeter of the area is difficult to seal or where theheight of the void means that the volume of materialbecomes prohibitive. t In general, this method is bestapplied in situations ' where the mine void is low or whereit is mostly backfilled with gob. It is also useful incases where the void is so close to the surface that pointsupport methods (like grout, columns) cannot rely onarching to develop between supported points.

•ANGLE OF INFLUENCE"6 DIAMETER

GROUTHOLE (TYR)

GROUNDSURFACE

'GROUT ^ GROUTANDGRAVEL

FIGURE 1 - SCHEMATIC OF FLUSH GROUTINGFOR MINE SUBSIDENCE CONTROL

Once the void height increases beyond a few feet, it isusually more economical to consider grout columns (alsocalled gravel or stone columns). With this technique,holes are drilled on grid spacings narrow enough so thatthe mine roof will support itself by arching between

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GEO'CON INC.columns. Some holes on the grid will encounter solid coalpillars; these are backfilled with grout. The balance ofthe holes are first filled with a gravel cone; gravel(3/4-inch [2 cm.] crushed slag or stone) is poured downthe holes and spread by a jet of compressed air at thebottom. The compressed air maximizes the roof contact.

Once this is done, a grout pipe is driven through thegravel and down into any gob below. Grout is injectedinto the gob and gravel in measured amounts at one-foot(30 cm.) intervals with the objective of making a groutedcylinder of sufficient diameter to support the mine roof(Figure 2). The gravel may tend to slump during thegrouting and additional gravel may have to be added.

O15

6"OIAMtTER GROUT **\ ANGLE OF INFLUENCE

PROPOSED STRUCTURt

—* «x *i; i,r jj. j i r \ "; . trJ T

ANDGROUT

FIGURE 2 "SCHEMATIC OF GROUT COLUMNSFOR MINE SUBSIDENCE CONTROL '

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GEO-CON INC.This process is more expensive on a unit-volume basis thanflush-grouting because there is a considerable amount ofwork needed to be done "down-the-hole", i.e., the grout isinjected through grout pipes and measurements of the topof the stone are needed frequently. Also the grout istypically a more expensive 3:1 flyash-cement grout by dryweight.

Good control of material quality is needed since smalllumps can plug the grout pipes or prevent the stone columnfrom being properly grouted. Grout columns do present thefollowing advantages, however:

Lower cost in some cases because only a smallpercentage of the mine void is actually treated.

No reliance on creating barriers to seal off thetreatment zone.

Flexibility in spacing to suit load factors anddiffering degrees of risk.

A major drawback of the technique has always been thatthere is no way to reliably check the roof contact areaupon completion. Adequate roof contact area is difficultto achieve in mines that are completely flooded.Experience on experimental columns in open, dry mines andwhere borehole photography has been used, has shown thatgood technique does result in good contact. Of course, asthe void height increases, the volume of the stone columnsthemselves increase dramatically to the point of becominguneconomical compared to other systems (caissons, etc.)For this technique, as with flush grouting, a reasonablythorough exploration program to lay out the work as wellas on-going engineering control during the groutingprogram is necessary to ensure a good end-product.

FLY AS II BASED GROUT SLURRIES

In the United States, more than 60 million tons (55million metric tons) a year of flyash are produced as aby-product from coal-burning power plants. Presently,only about twenty percent of the higher quality ash isbeing used in concrete (Glogowski and Kelly, 1987). Theremaining eighty percent, although not conforming to allof the ASTH requirements, and 'normally requiring disposal,serves as a readily available, low cost filler for minesubsidence control projects.

ASTM divides flyash into two classifications, Type 7 andType C. Type F flyash is commonly found in the easternstates where local bituminous and anthracite coal isburned in the power plants. Type C flyash is producedfrom the burning of subituminous and lignite coal located

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&GEOCONINC.in the western portion of the United States. Both typesof flyash have pozzolanic properties. ;, Type C fly ashcontains lime which gives it cementious characteristics.When mixed with only water, Type C flyash will seC withoutany addition of ceident. A majority of the writers'experience is with the use of Type F flyash.

Figure 3 presents in graph form the approximaterelationship of compressive strength to grout slurries ofvarious flyash-cement ratios using Class F flyash.Typical water to solids ratios (WSR) by weight for flushgrouting projects vary from 0.4 to 0.6. A 0.4 WSRcorresponds to a water content of about 100 gallons percubic yard (500 liters per cubic meter) of grout.Depending on the amount of overburden and other factors,the flyash-cement ratio of the grout used in flushgrouting typically varies from 7:1 to 11:1 by weight.

10o.

X5000

•z.LU

(OX 10

Q. 0-Q. 5

4000

.3000

< g 1 000

= 0 1 2 3 4 5 6 7 8 9 1 C I Ig FLYASH TO CEMENT RATIO (BY WT.)

FIGURE 3 - RELATIONSHIP OF STRENGTH TOFA/C RATIO FOR CLASS F FLYASH SLURRIES

8 AndromaJ

flR30U627

GEOCON INC.The primary factor in determining the unconfinedcompressive strength of flyash-cement grout mixes is thecement to water (C/W) ratio (Glogowski and Kelly, . 1987).Figure 4 presents a curve which can be initially used whenselecting a C/W ratio to meet a desired strengthrequirement. Because flyash sources vary in quality, ap reconstruction testing program is recommended todetermine the strength of the grout mixes using materialsfrom the proposed sources.

< 8000000CM 7000

** 6000I

'0 5000UJ

UJ 3000

U)en 2000UJcr^ 1000o

-

^s

^

A//

s

//

UJ ° 0.5 |.o 1.5 2.CC CEMENT/WATER BY WEIGHTzo

FIGURE 4 - RELATIONSHIP OF STRENGTH TOC/W RATIO FOR CLASS F FLYASH SLURRIES

* * - I(From Flyash Slurry Testing Programs

Draft Report, GAZ Consultants.) :

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GEO-CON INC.Two advantages that flyash provides in.a grout mix includestrength gain and flowability. strength gains from 28 to365 days can exceed 300% on certain flyash-cement groutmixes. Figure 5 shows effects of curing time on strengthof several Class F flyash mixes. Typical strength gainsfrom 28 to 365 days usually range from .25% to 75% for TypeF flyash-cement grout slurries with cement contents in the5% to 10% range by weight (Glogowski and Kelly, 1987).

10000

VIa.

1000LUo:

10010 100 1000

CURING TIME (days)

FIGURE 5 - EFFECT OF CURING TIME ONSTRENGTH O.F SEVERAL CLASS F FLYASHl MIXES

' • " ' . « ' ' ' ' . • !(From Flyash Slurry Testing Programs

Draft Report, 6AI Consultants.);

10AR30U629

GEp»CON INC.hBecause flyash has a spherical particle shape,flyash-based mixes will flow better with less water thanneat cement grouts. This characteristic provides a groutthat can penetrate fractures, roof fall material and minegob backfill effectively. Figure 6 illustrates theimproved flowability of several grout mixes as theirflyash content is increased and their water to solidsratio is held constant.

ou

50

COo2 4OOOUJtoUJ 30zoo>0 20U.

10

O

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/ — CONEMAUGH,

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^^ ^^ .

WSR = O.I9

i— KEYS10NE . WSR = 0.4

~ ~ ^

•

^ ^

5 IO 15 2O 25FLYASH/CEMENT RATIO BY VOLUME

FIGURE 6 - RELATIONSHIP OF FLOW' TOFLYASH/CEMENT RATIO FOR TWO FLYASH SOURCES

11 Andrc

GEOCONINC.EQUIPMENT f/

The equipment used for both flush grouting and groutcolumn techniques are similar. Large air-rotary rigsdrilling a 6 inch (15 cm.) diameter hole (Figure 7)usually provide the cheapest drill-hole. In cases wheresurface access is obstructed, smaller air-tracks or skidrigs may be used. Grout may be delivered in ready-mixtrucks and held in an agitation tank or grout materialscan be delivered and mixed on site (Figure B). Groutpumps are usually of the positive displacement type,either piston or progressive cavity type pumps. In caseswhere gravel is to be injected, it is placed in a hopperand forced down the hole with compressed air.

FIGURE 7 - AIR ROTARY DRILL

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GEOCON INC.r •/

FIGURE 8 .- ON-SITE MIXING

RECENT PROJECT

Grove City College, in Western Pennsylvania, had plannedthe expansion of the College's recreation hall facility.During the initial investigation for this expansion, itwas determined that several of its existing structureswere heavily undermined. The mined coal seam was locatedapproximately 50 to 65 feet (15 to 20 meters) beneath theexisting structures.

After initial investigation, an emergency two phase minegrouting program "utilizing grout columns was put out forbid. Phase one consisted of the construction of groutcolumns using a thick concrete type grout. Phase twowould then consist of filling the remaining mine spaceswith sand for additional support. A one phase alternativeusing the flush grouting method was submitted andaccepted, which resulted in a lower overall cost and couldbe completed in less time than the original specifiedprogram which utilized grout columns.

The alternative program consisted of initially drillingbarrier holes around .the perimeter of the existingstructures and injecting' a ipeh gravel to firm a barrier.Drilling these holes was accomplished using; truck-mounted

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•GEO'CON INC. ,

air rotary drill rigs. Pea gravel was chosen instead ofcrushed limestone because the rounded pea gravel resultedin flatter side slopes for the barrier which resulted in atighter containment structure. In addition, the peagravel was finer Xfrain'ed which reduced Jbhe amount of groutwhich could penetrate and potentially <^f low through thebarrier. During placement of the gravel down each barrierhole, compressed air was injected into the mine via asteel pipe to aid in spreading the gravel at mine level.

After creation of the gravel barrier, a fluidflyash-cement grout slurry was injected within the barrierto provide the necessary support. This grout was pumpedthrough pipes lowered into angled injection holes whichextended beneath the existing structures. The drilling ofthe angled interior holes was accomplished using track andsmall truck-mounted rotary drill rigs. During grouting,the level of the gravel in the barrier holes was monitoredfor sloughing. "Additional gravel was placed in theseholes as required.

The grout mixture on this project consisted of a 10:1flyash to cement grout by weight with ah average water tosolids ratio of 0.4. This grout was mixed on-site usingtwo 5 cubic yard (4 cubic meter) mixers. As grout wasbeing pumped from one mixer, the other . mixer was mixingthe next batch of grout, thus providing ' a continuoussource of prepared grout. '

Production rates of mixing and placing the grout on thisproject peaked at several hundred cubic yards per day.Pumping was accomplished using a large progressive cavitypump.

Conditioned or damp ash was loaded into the mixers fromstockpiles using a front end loader. .Cement from anoverhead silo was batched into the mixers via a weighscale. Water which was pumped from the flooded mine was'used in grout mixing and was monitored by a flow meter.

During grouting operations, the level of the grout withinthe mine was monitored on a daily basis by soundingvarious boreholes within the work area. On-site testingof the grout mix included density testing using a mudbalance, and testing the grout's viscosity using a flowcone. In addition, water quality tests were performed onthe mine * water, and grout cubes and cylinders wereprepared for compressive strength testing.

* ' ; ' • •• . " ••

The results of the compressive testing yielded an averagegrout strength of 128 p.s.i. (9.0 kg. per sq. cm.) at 28days. The bearing capacity of this grout backfill can beestimated conservatively 'at 'three times it's unconfinedcompressive strength (Czmola and Voytko, 1986). Using the

14 Androma'*

CON INC.23 day strength of 123 p.s.i. yields a bearing capacity ofapproximately 334 p.s.i. (27 kg. per sq. cm.). Since thetotal maximum overburden load above mine level is only inthe range of 70 p.s.jL. (4.9 kg. per sq. cm.), the groutbackfill has more than adequate strength to support theoverburden loads.

CONCLUSION

In this paper, the use of high volume grout techniques areshown to be of significant economic and practical benefitwhen applied to coal mine subsidence problems. The basicadvantages that apply are:

Minimum Disturbance

The problems are treated remotely withoutmajor excavations; in cases where structuresexist on the surface, angle holes can bedrilled beneath the structures.

Flexibility of Technical Solution

A relatively large number of holes provideinformation to the engineer on subsurfaceconditions. Mix ratios, pumping rates andpressures can all- be varied to producedesired effects.

Economical Results

The cruder types of grouting that have beendiscussed in this paper enable large volumesof material to be placed at low cost.

A secondary benefit is the rather neat environmentalsolution that sees the waste product of burning coal,flyash, returned to the mines from where it came to reduceother coal mine related environmental damage. It iscertain that grouting will continue to play a major rolein reducing damage from coal mine pollution and subsidenceproblems.

ACKNOWLEDGMENTS

The authors wish to express their appreciation to Mr.Bohdan Czraola of STS D'Appolonia for his assistance onthis paper. The authors also appreciate the permission ofGAI Consultants and the Electric Power Research Institutefor including the results of some of their testing offlyash based grouts.

15 Andro.

GEOCONINC.REFERENCES

Ackenheil, Alfred C. and Dougherty, Murray T., "RecentDevelopments in Grouting for Deep Mines", Journal of £heSoil Mechanics and Foundations Division. ASCE, Vol. 96,No. SM1, Proce. Paper 7942, January 1970.

Czmola, Bohdan I. and VoytJco, Edward P., STS D'AppoloniaLtd., "Subsidence Potential Minimization Project Report",Grove City College Campus, Grove City, Pennsylvania",September 1986.

Glogowski, P.E., Kelly, J.M., "Flyash Slurry TestingProgram, Draft Report, November 1987", GAI Consultants,Inc. for Electric Power Research Institute.

Ryan, C.R., "Use of Flyash to Solve Coal Mine Subsidenceand Fire Problems", Lehigh Valley Section ASCE, Disposalof Solid and Liquid Wastes, Lehigh University, April 1963.

16 Andromal<

GEO-CON INC.KEY WORDS:

Grouting, coal mines, subsidence, flushing, grout columns,flyash, flyash-cement/ grouts, grout strengths, groutingtechniques.

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GEO-CON INC.GEOTECHNICAL CONTRACTING

February 14, 199 0

U. S. Bureau of MinesPittsburgh Research CenterPost Office Box 13070Pittsburgh, PA 15230

FAX: 892-6614

Attn: Jeffrey S. WalkerRequest for Information

Installation of Dee Mine Bulkheads

Gentlemen:

Pursuant to our telephone conversation of February 12, 1990, 1?am interested in obtaining any technical information available-related to the installation of deep mine bulkheads.I would be interested in stopping in to meet with you to discuss "-— this subject and pickup any written publications 'you may haverelated to mine bulkheads. Please contact me at 856-7700 onceyou have assembled this information.

Thank you for your time concerning this request.

Sincerely,GEO-CON, INC.

Kenneth B. AndromalosDrilling and GroutingGroup Manager

KBA/gml

Sent via FAX.

!

P.O. Box 17380* Pittsburgh. PA 15235 • T»l (4 12)856-7700- FAX (412)373-3357

Tocos Offica (S17J3B3-1400

'AR30li6i»0