Report of Investigations 8623

Engineering Properties of CombinedCoarse and Fine Coal Wastes

By Bill M. Stewart and L. A. Atkins

UNITED STATES DEPARTMENT OF THE INTERIORJames G. Watt, Secretary

BUREAU OF MINESRobert C. Horton, Director

This publ i cation has been cataloged as follows:

Stewart, Bill MEngineering properties of combined coarse and fine coal

wastes.

(Report of investigations / united States Department of the Interi-or, Bureau of Mines; 8623)

Includes bibliographical references.

Supt. of Docs. nov: I 28.23:8623.

1. Coal mine waste. I. Atkins, L. A. (Lynn A.) II. Title. III.Series: Report ·of investigations (United States. Bureau of Mines) ;8623.

TN23.U43 [TN803] 622s [662.6'2] 81-607095 AACR2

CONTENTS

Abstract....................................................................... 1Introduction. . ........••...•..... . ............•............ 1Sample collection and test procedures.......................................... 2Material. . .. ..........•..•.. . 3

As received............................................................... 3Varying the grain-size distribution....................................... 4

Test results................................................................... 5Effects on maximum laboratory densities................................... 5Effects on optimum moisture content....................................... 6Moisture-density relation................................................. 6Effect on initial void ratio and soil strength............................ 7Effect on permeability.................................................... 10Additional tests.......................................................... 12

Comparison of coarse and finer samples............................... 12Time factor in testing............................................... 13Alternating coarse and fine layers................................... 13

Conclusions. ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 14ILLUSTRATIONS

1.2.3.

As-received grain sizes of coal waste from mines A and B ••••••••••••••••••As-received grain sizes of coal waste from mines C and D ••••••••••••••••••Effects on maximum laboratory dry density with increasing percent minus

No.4 •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••Effects on optimum moisture content with increasing· percent minus No.4 •••Triaxial shear strength test apparatus ••••••••••••••••••••••••••••••••••••Effects on initial void ratio with increasing percent minus No.4 •••••••••Effects on angle of internal friction with increasing percent minus No.4.Constant head permeability test apparatus •••••••••••••••••••••••••••••••••Effects on permeability with increasing percent minus No.4 •••••••••••••••Grain sizes of samples containing 100 pct minus No. 4 and 100 pct plus

No.4 •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••

4.5.6.7.8.9.

10.

TABLES

1.2.3.4.

As-received materials •••••••••••••••••••••••••••••••••••••••••••••••••••••Minus No.4 material, including minus No. 200 fines, used in tests ••••••••Maximum densities for various mixes •••••••••••••••••••••••••••••••••••••••Changes in volume, specific gravity, and weight as percent minus No.4

changes. •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 95. Differences in physical properties of coal waste samples containing

100 pct minus No.4 and 100 pct plus No.4 ••••••••••••••••••••••••••••••

44

6788

101112

13

355

13

ENGINEERING PROPERTIES OF COMBINED COARSE AND FINE COAL WASTES

by

Bill M. Stewcrt l and L. A. Atkins2

ABSTRACT

The Bureau of Mines conducted labo-ratory tests to determine the effects onthe physical properties of coarse coalwaste of adding large amounts of finecoal waste. Maximum laboratory dry den-sity, optimum moisture content, shearstrength, and permeability tests wereconducted on samples that contained 18 to60 pct minus No. 4 (U.S. Standard sievesize) coal waste. For the nonslaky mate-rials, maximum laboratory densities werehighest and permeabilities lowest whenthe samples contained between 30 and40 pct minus No.4. Shear strength

increased rapidly when the proportion ofminus No. 4 was increased from 20 to40 pct, but then increased only slightlyor leveled off when the minus No. 4 wasincreased to 60 pct. Optimum moisturecontent and permeability were the physi-cal properties most affected by the addi-tion of fines. Optimum moisture contentincreased as much as 3 pct, and permea-bilities decreased up to three orders ofmagnitude. Slaky coal waste material hadphysical property reactions to the addi-tion of fines substantially differentfrom those of nonslaky material.

INTRODUCTION

Space limitations, high costs ofimpoundment construction, and adverseenvironmental impacts associated with im-poundments have caused many coal compa-nies to consider dewatering the fine coalrefuse from the thickener underflow.Fine coal refuse is generally classifiedas minus 28 Tyler mesh (0.699 mm) and iscommonly collected from the preparationplant process water in a stream that in-cludes sizes ranging from 8 Tyler mesh(2.46 mm) to 0.0005 mm. In a typicalpreparation plant, the fine-refuse streamcontains only about 5 pct3 solids and

'Mining engineer.2Engineering technician.Both authors are with the Spokane Re-

search Center, Bureau of Mines, Spo-kane, Wash.

31n this report, "weight-percent" will bedesignated simply as "percent."

therefore requires thickening prior todewatering or other treatment. Feeds tothe thickener are treated with floccu-lants; the solids settle to the bottom(underflow); and the clarified water isskimmed off at the top (overflow). Theunderflow typically contains about 30 pctsolids and the overflow less than 1 pctsolids.

Underflow material can be furtherdewatered by mechanical, thermal, orchemical processes, or by a combinationof these processes. Generally, economicsand the type of material to be dewatereddictate the methods to be used. Dewater-ing allows the operator to dispose of thefine fraction (minus 28 mesh) in a semi-dry state along with the coarse fraction(generally classified as minus 3 inch toplus 28 mesh) produced in the preparationprocess. With this method, only one

2

disposal transportation system is needed,and the surface area required for dispos-al is reduced, compared with that usedfor impoundment. Disposing of coarsewastes and dewatered fines togethershould produce a more stable mass becausethe water used to transport the fineswould be eliminated. This is a definiteadvantage; many past failures of coalwaste embankments were directly relatedto high phreatic surfaces.

However, there are some factors inthe "dry fill" method that may affectstability. One of these factors, thebasis of this research, is the change inphysical properties of total coal wastewhen large amounts of dewatered fines areadded. To investigate these changes, theBureau of Mines Spokane Research Center,under the Bureau's Minerals Health andSafety Technology program, conducted sev-eral physical property tests on mixes ofcoarse and fine coal waste. The onlyvariable imposed was the percentage ofminus No. 4 (U.S. Standard sieve size)material in the test samples.

SAMPLE COLLECTION AND TEST PROCEDURES

Test samples were collected fromfour coal mines in four States (Colorado,Ohio, Pennsylvania, and West Virginia).For the purpose of this report, the Colo-rado mine will be designated mine A, andthe three eastern mines will be desig-nated mines B, C, and D. Mines A and Bare using filters to dewater the fines;mine C is using chemical methods for de-watering; and mine D is using a series offilter presses. All of the mines useeither truck or conveyor transport of thecoarse and fine refuse to the refusepile. Prior to transport of the fines tothe refuse pile, mine C pumps the thick-ener underflow to a series of curingponds where it is chemically treated anddewatered.

Approximately 2 tons of waste werecollected from each mine waste dump. Be-cause it was not the intent of this re-search project to compare field physicalproperties of the material with labora-tory physical properties, in-place testswere not performed during sampling. The

material was placed in sealed drums andshipped via truck to the Spokane ResearchCenter.

The laboratory tests were performedusing the following procedures:

1. Grain-size distribution for theplus No. 200 (U.S. Standard sieve size)fraction was evaluated according to Amer-ican Society for Testing and Materials(ASTM) standards.4 The grain-size dis-tribution of the minus No. 200 fractionwas determined using a particle-size ana-lyzer, which operates on the principle ofStokes' law, using X-ray absorption.

2. The specific gravity of the plusNo. 4 material and of the minus No. 4 ma-terial was determined according to ASTMstandards.5

3. Maximum and minimum densities andoptimum moisture contents of the mixedcoarse and fine material were determinedaccording to ASTM standards.6

4American Society for Testing and Materi-als. Particle-Size Analysis of Soils.0422-63 in 1979 Annual Book of ASTMStandards: Part 19, Natural BuildingStone; Soil and Rock; Peats, Mosses,and Humus. Philadelphia, Pa., 1979,pp. 112-122.

5American Society for Testing and Materi-als. Specific Gravity and Absorptionof Coarse Aggregate. C127-77 in 1979Annual Book of ASTM Standards:Part 14, Concrete and Mineral Aggre-gates (Including Manual of ConcreteTesting). Philadelphia, Pa., 1979,pp. 77-83.

6American Society for Testing and Materi-als. Relative Density of CohesionlessSoils. 02049-69 in 1979 Annual Book ofASTM Standards: Part 19, NaturalBuilding Stone; Soil and Rock; Peats,Mosses, and Humus. Philadelphia, Pa.,1979, pp. 311-319. Moisture-DensityRelations of Soils and Soil-AggregateMixtures Using 5.5-lb Rammer and 12-in. Drop. 0698-78 in 1979 Annual Bookof ASTM Standards: Part 19, NaturalBuilding Stone; Soil and Rock; Peats,Mosses, and Humus. Philadelphia, Pa.,1979, pp. 201-207.

4. Triaxial shear tests ofpIes were conducted accordingdesignation D2850-70 underconditions.7

the sam-to ASTMdrained

MATERIAL

3

5. Permeability of the samples wasdetermined according to the u.s. Bureauof Reclamation Earth Manual designationE-14.8

As Received

The as-received material is de-scribed in table 1. The laboratory testprogram was designed to determine the ef-fects on the maximum density, moisturecontent, and permeability of coal waste

materials when varied amounts of fineswere added. Such would be the case whendewatered fines are combined with coarsewaste in a common dump.

TABLE 1. - As-received materials

Soil Minus Minus Effective Uniformity Specific Description ofSample classifi- No.4, No. 200, size (D10), coefficient gravity material

cation1 pct pct mm (DhO/D10)Mine A GM-SM 54 18 0.03 203.0 1.83 Poorly graded

gravel-sand-siltmixture.

Mine B GP-GM 38 7 .23 44.8 1.80 Poorly gradedgravel, gravel-sand mix withsmall amount ofsilt.

Mine C GP-GC 25 7 .32 33.8 1.92 Poorly gradedgravel, gravel-sand mixture withclay binder.

Mine D GW-GM 38 8 .17 58.8 2.12 Well-graded gravel,gravel-sand-siltmixture.

1American Society for Testing and Materials. Classification of Soils for EngineeringPurposes. D2487-69(1975) in 1979 Annual Book of ASTM Standards: Part 19, NaturalBuilding Stone; Soil and Rock; Peats, MOsses, and Humus. Philadelphia, Pa., 1979,pp, 374-378.

7American Society for Testing and Mate-rials. Unconsolidated, UndrainedStrength of Cohesive Soils in TriaxialCompression. 02850-70 in 1979 AnnualBook of ASTM Standards: Part 19, Natu-ral Building Stone; Soil and Rock;Peats, Mosses, and Humus. Philadel-phia, Pa., 1979, pp. 413-418.

8U•S• Bureau of Reclamation (Departmentof the Interior). Permeability andSettlement of Soil Containing Gravel.

4

To establish a baseline on grain-size distribution, the as-received mate-rial, after thorough drying, was sepa-rated on a shaker into U.S. Standardsieve sizes of plus 1-1/2 inches (plus38mm), linch (25.4mm), 3/4 inch(19.5 mm), 3/8 inch (9.6 mm), No.4(4.9 mm), and minus No.4. The minusNo. 4 material was separated into sievesizes of No. 10 (2.0 mm), No. 16(1.2 mm), No. 30 (0.59 mm) , No. 50(0.31 mm), No. 100 (0.16 mm), No. 200(0.08 mm), and minus No. 200. The minusNo. 200 material was separated on aParticle-size analyzer into clay and siltsizes. Figures 1 and 2 show thegrain-size curves of the as-receivedmaterial for the four samples collected.

Cob- Gravel Sand I Fines

bles Coarse Fine Coarsel Medium I Fine I Silt (ncnolastlc) to clay (plastic)

U.S. Standard sieves Mine A6 3 1-112 3/4 3/8 4 10 16 30 SO 100 200

.......••.•...

1

0\

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"0

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<60 0

U

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GRAIN SIZE, mm0.01

000.001

10

Cob- Gravel I Sand I Fines

bles Coarse Fine lcoarselMediuml Fine JSilt (nonplastic) to clay (plastic)

U.S. Standard sieves

6 3 1-1123/4 318 4 to 16 30 50 100 200 Mine 8

0 r-, ~-0

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GRAIN SIZE, mm

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FIGURE 1. - As-received grain sizes of coal wastefrom mines A (top) and B (bottom).

Varying the Grain-Size Distribution

Once the baseline (as-received)grain-size distributions were establishedfor each mine, minus No. 4 material wasadded or subtracted. The minus No. 4size was selected as the variable becausethere was an abundance of this size mate-rial from each of the mines; it is thesmallest fraction obtainable on the largeparticle shaker; and, most importantly,it includes all the finer coal refusesizes (2.46 mm to 0.0005 mm) commonlycollected in preparation plant processwater. From 18 to 60 pct minus No. 4 wasmixed with the coarser (plus No.4) mate-rial. In this report, 18 to 25 pct minusNo. 4 will be called a low level; 32 to

90

Cob- Gravel Sand I Finesbles

Coarse I Fine canel MedUn I Fne T SIt(nonplastic) 10clay (plastic)

U.S. Standard sieves6 3 1·112 3/4 3/8 4 1018 30 50 100 200 Mine C

I\.1

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~b- Gravel Sand I Finesbias Coarse I Fine !coarse Medium I Fine 1 Silt (nenplastic) to claV (plastic)

6 3 1-1/23/4 3~·s·4Stanpd\r~6sieabes50100 200 Mine 0......., r-t-' r-

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FIGURE 2. - As-received grain sizes of coo] wastefrom mines C (top) and D (bottom).

40 pct, a medium level, and 54 to 60 pct,a high level. Table 2 shows the percent-age of minus No. 4 and minus No. 200 ma-terial used in each sample. After addi-tion of the correct amount of each size

5

material and water to approximate the as-received water contents, the samples werehandmixed for the maximum density, mois-ture contents, shear strength, and per-meability tests.

TABLE 2. - Minus No. 4 material, including minus No. 200 fines,used in tests, percent

Sample Minus Minus Sample Minus MinusNo. 4 No. 200 No. 4 No. 200

Mine A: Mine c:1••••••••••••••••••••• 18 3 ]1 •••••••••••••••••••• 25 72 ••••••••••••••••••••• 32 10 8••••••••••••••••••••• 40 1731 •••••••••••••••••••• 54 18 9 ••••••••••••••••••••• 60 26

Mine B: Mine D:4 ••••••••••••••••••••• 25 5 10 •••••.••.•.••••.•••• 18 451 •••••••••••••••••••• 38 7 111 ••••••••••••••••••• 38 86 ••••••••••••••••••••• 58 11 12 •••••••••••••••••••• 58 12

1As-received samples.

TEST RESULTS

Effects on Maximum Laboratory Densities

Maximum laboratory dry densities(MLD) of the coal waste samples were de-termined by impact (standard compactiveeffort) and vibration methods.9 Maximumparticle size was three-fourths of aninch for the impact method and 3 inchesfor the vibration method. Results of theMLD test can be seen in table 3. Onepoint of interest is the densities of ma-terial from mine C. This material

had high slaking properties; because ofthe cohesive attraction of the clay par-ticles, all three samples showed higherdensities using the impact method thanwere observed using the vibration method.The effects of fines additions on thephysical properties of the slaky mine Cmaterial will be addressed throughoutthis report.

TABLE 3. - Maximum densities for various mixes

Standard effort Vibration Standard effort VibrationSample maximum dry maximum wet Sample maximum dry maximum wet

density, Ib/ft3 density, Ib/ft3 density, Ib/ft3 density, Ib/ft3Mine A: Mine C:

1•••• 88.7 89.1 71••• 99.7 91.22 •••• 96.6 98.7 8 •.•• 98.9 90.631••• 93.1 89.0 9 •••• 98.3 86.1

Mine B: Mine D:4 •••• ND 92.4 10••• 108.9 106.151••• 95.7 95.4 111•• 115.2 118.26 •••• 93.0 93.9 12••• 114.0 109.7.ND Not determ1ned •

1As-received samples.

9Works cited in footnote 6.

6

Figure 3 shows the effects on MLDwhen different amounts of minus No. 4 ma-terial were added to the samples. Mate-rial from mines A, B, and D containing 30to 40 pct minus No. 4 showed the highestMLD. With lower or higher percentages,the MLD declined. Mine C material showeda gradual decline in MLD as the percentminus No. 4 increased, with the highestMLD at only 25 pct minus No.4. However,though 25 pct minus No. 4 was present inthe as-received sample, it is highly pos-sible that the actual amount at the timeof the density test had increased to theoptimum 30- to 40-pct range. This in-crease would be attributed to the slakingnature of the material, which is inducedby the wet, dry, wet preparation preced-ing the density tests, and the compactioneffort during the tests.

Effects on Optimum Moisture Content

Optimum moisture contents are af-fected by the amount of fines added tocoarse coal waste. Figure 4 shows theoptimum moisture content at MLD (standardcompactive effort) versus the percentageof minus No. 4 in each sample. As a gen-eral rule, the optimum moisture contentincreases as the percentage of minusNo. 4 increases. However, as can be seenin figure 4, the increases in optimummoisture content are variable and dependon the type of coal waste being tested.For instance, material from mines Band Cshowed a very slight rise in optimummoisture content as the percent minusNo. 4 was increased from low to medium,but then a much larger rise when the per-cent minus No. 4 increased from medium tohigh. Material from mines A and D actedjust the opposite and actually showed no

120'.------------------------,

KEYl:l Mine AX Mine Bo Mine C• Mine 0

0---_0--~__ ....""""'""'0 =<D

85L-_--..L __ --I.-__ .L-_----'-__ --I.-__ .L-_--'o 10 20 30 40 50 60

MINUS No.4 U.S. STANDARD SIEVE, pet

FIGURE 3. - Effects on maximum laboratory dry den-sity with increasing percent minus No.4.(Mine B data point at 25 pet minus No.4was estimated from data obtained by thevibration wet density method).

70

change in optimum moisturethe percentage of minus No.creased from medium to high.

content when4 was in-

Moisture-Density Relation

The best results from the additionof large amounts of fines to coarse coalwaste would be an increase in density anda decrease in optimum moisture content.The actual effects on MLD and moisturecontent when the percent minus No. 4 wasincreased were quite variable. The ef-fects depended on the gradation of thecoal waste coming from the preparationplant and on the mineral content andgrain size of the original material.

7

14-c KEYQ.

t- 8 Mine AZ 12 X Mine Bw 0 Mine Ct-Z • Mine D00w 10a:::Jt-C/)-0 8::E::E::J::E 6-t-e,0

10 204---------L. --L. ....L.- L...-__ --L ~

o 6030 50 7040MINUS No.4 U.S. STANDARD SIEVE,pet

FIGURE 4. - Effects on optimum moisture content with increasing percent minus No.4.

The highest MLD's were obtained insamples containing 30 to 40 pct minusNo.4 (fig. 3). In three of the foursamples, increasing to a high level ofminus No. 4 had little effect on the MLD(a decrease of less than 2 Ib/ft3). Atthe same time, however, in two of thesesamples, the optimum moisture content in-creased nearly 3 pct (fig. 4). Inwater-scarce areas, this would be an im-portant consideration in water manage-ment. Of the four samples tested, mineD material best satisfied the moisture-density criteria, with an MLD range of108 to 115 Ib/ft3 and optimum moisturecontent under 8 pct.

Effects on Initial Void Ratioand Soil Strength

To determine the effects on soilstrength when fines were added to coarsecoal waste, consolidated-drained triaxialcompression tests were performed. Thesamples were 9 inches in diameter by22.5 inches long and were compacted inthe test chamber (fig. 5) at 95 pct MLD.The samples were compacted with a hand-held, flat-bottomed tamping bar. Sevenlayers, each approximately 3.2 inchesthick, were tamped. The top of each lay-er was scarified prior to adding the nextlayer. The major principal intergranular

8

FIGURE 5. - Triaxial shear strength test apparatus. Left, control unit and data readout; midd Ie,control system for triaxial chamber; right, testing frame with test chamber.

stress (01) was applied at a rate of0.025 in/min. Pore pressure was moni-tored at the top of the samples, and thesamples were allowed to drain at the bot-tom. Most of the samples showed somepore pressure during initial loading.However, at the beginning of each test(after consolidation) and throughout eachtest, no pore pressure developed at thetop of the samples. Because of this,neutral stresses due to pore pressurewere considered negligible. Therefore,the effective lateral pressure (03) isthe applied lateral pressure, and thedeviator stress (01-03) is the effectivenormal stress.

0.45

.40

0f-

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z

.20

.150

KEY[;] Mine AX Mine Be Mine C• Mine D

10 20 30 40 50 60 70MINUS No.4 U.S. STANDARD SIEVE, pet

FIGURE 6. - Effects on initial void ratio with in-creasing percent minus No.4.

As a general rule, the lower theinitial void ratio (IVR) of the materialat placement, the higher the shearstrength. Figure 6 shows that the IVR inmaterials from mines A, B, and D de-creased significantly with an increasefrom low to medium levels of minus No.4.However, the IVR then increased in A andD materials with an increase to the highlevel of minus No.4. This indicates,for these materials, that an "optimum"IVR is found in the 30- to 40-pct rangeof minus No.4.

9

A more detailed analysis of void ra-tios is seen in table 4. With theincrease from low to medium percentagesof fines, material from mines A, B, and Dshowed an increase in volume of solidsand decrease in volume of voids. Thisindicates that the fines filled the orig-inal voids and created less void space.However, in A and D material, increasingto a higher percentage of fines causedthe opposite to occur. The fines re-placed and pushed the plus No. 4 parti-cles away from each other and therebycreated higher void ratios.

TABLE q. - Changes in volume, specific gravity, and weight as percentminus No. 4 changes

Sample Minus No. 4, Initial void Volume, ft3 Specific Weight, lbpct ratio Solids Voids Water Air gravity Solids Voids

Mine A:1••• 18 0.410 0.59 0.242 0.068 0.174 1.91 70.22 4.492 ••• 32 .245 .67 .164 .113 .051 1.87 77.85 7.0131•• 54 .292 .64 .187 .105 .082 1.83 73.37 6.60

Mine B:4 ••• 25 .301 .64 .192 .070 .122 1.83 72.87 6.7051•• 38 .240 .67 .161 .081 .080 1.80 75.20 5.046••• 58 .239 .67 .160 .112 .048 1.77 74.04 6.96

Mine C:71•• 25 .266 .66 .174 .137 .037 1.92 78.61 8.578••• 40 .301 .64 .193 .143 .050 1.96 78.02 8.899 ••• 60 .337 .62 .209 .161 .048 2.00 77.53 10.08

Mine D:10••• 18 .267 .66 .176 .084 .092 2.10 85.91 5.24111•• 38 .178 .70 .125 .123 .002 2.12 93.21 7.7412••• 58 .234 .67 .156 .117 .039 2.14 89.89 7.37

1As-received samples.

10

Mine C material showed a continuousdecrease in volume of solids and increasein volume of voids from low to high per-centage of minus No.4. The volume ofwater is much higher in mine C materialthan in the others, which may be typicalof a highly slakable material.

Figure 7 shows the effects on theangle of internal friction when minusNo. 4 was added to coarser coal waste.To obtain the friction angles, three tri-axial shear tests were run on each mix.As reflected by the decrease in IVR(fig. 6), the friction angle of materialfrom mines A, B, and D increased as thepercent minus No. 4 increased from low tomedium. Increasing the minus No. 4 to60 pct did not have a great effect on thestrength, even though the void ratiostended to increase. The friction anglesof mines A and D material stayed aboutthe same at the high level of minus No. 4as at the medium level. Mine B materialshowed a steady increase of the frictionangle; the strength of this material wasincreasing even at 58 pct minus No.4.This indicates that the shape of the par-ticles was such that they fit very welltogether, similar to the pieces of a puz-zle, with a lot of contact between par-ticles and very little void space.

Mine C material had a much lowerfriction angle than the other materials.Based on two triaxial test sets, thefriction angle decreased from 28.2° to25.7° when the percent minus No.4 wasincreased from 25 to 60. The addition offines and the material's slaking natureresulted in high void volumes at place-ment and, therefore, lower strengths.Much more water was entrapped in thevoids of mine C material than in theothers, first, because there were morevoids, and second, because of the attrac-tion of the water to the clay particles.

Effects on Permeability

Constant head permeability testswere conducted for each mix according tothe U.S. Bureau of Reclamation Earth

40

KEYm Mine A

38 X Mine Bo Mine C• Mine D

Ol 36Ql'tl

Z0I- 34Ua:u...J 32-eza:wI-

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26

24 L..-_---I. __ -L.__ ..L.._---J __ -l.. __ ...l.-_--.J

o 10 20 40 50 706030

MINUS No.4 U.S. STANDARD SIEVE, pel

FIGURE 7.· Effects on angle of internal frictionwith increasing percent minus No.4.

Manual designation E-14.10 The sampleswere placed in the permeability testchamber (fig. 8) at 95 pct MLD. A flat-head tamping bar was used to compact thesamples. The samples were compacted inthree layers, each 3 inches thick. Thetop of each layer was scarified prior toadding the next layer. The samples were1~-1/2 inches in diameter and 9 inchesthick. The samples were tested at100 pct saturation, and the temperatureof the water was the same for all tests.For each mix, permeability readings weretaken several times a day for severaldays until the permeability rate becamenearly constant.

The effects on permeability areshown in figure 9. The permeabilitycurves for mines A, B, and D correspond,as expected, with the IVR curves(fig. 6). The permeabilities decreasedrapidly when the level of minus No. 4 inthe mixes was changed from medium to

10Work cited in foo~note 8.

11

FIGURE 8. - Constant head permeabi I ity test apparatus.

12

10-1.--------------------1

>-I-:::iiii-cUJ::2 -5a:: 10UJQ.

KEYI:J Mine AX Mine Bo Mine C• Mine 0

~- -----

10 20 30 5010-7'--_----'- __ --'----__ '--_--L. __ --'----__ '--_~

o 40 60MINUS No.4 U.S. STANDARD SIEVE, pet

FIGURE 9. - Effects on permeability with in-creasing percent minus No.4.

high. As indicated in table 4, the sol-ids first fill the voids, resulting inlower permeabilities; but then, as moreminus No. 4 is added, the coarse parti-cles are replaced or pushed apart, creat-ing higher permeabilities.

The permeability of mine C materialdid not correspond with its IVR. In the

40- to 60-pct minus No. 4 range, it hadhigher IVR's but much lower permeabil-ities than the other materials. Eventhough the initial void volume of mine Cmaterial continually increased as minusNo. 4 was added, one overriding factorcontributed to its low permeability.When the material was saturated beforethe test, it slaked to clays, fillingthe initial voids with solids,creating smaller pores, and inhibitingwaterflow.

Additional Tests

Three additional tests were per-formed to determine (1) the shearstrengths of samples with 100 pct minusNo. 4 and 100 pct plus No.4, (2) thetime-dependent effects on shear strengthafter water is added to the sample, and(3) the effects on shear strength andpermeability if the sample is in alter-nating layers of coarse and fine, ratherthan a mixture of coarse and fine. Be-cause each of these tests was performedon waste from only one site, the resultscannot be construed as representative ofall sites.

70

Comparison of Coarse and Finer Samples

The first additional test shows theimportance of adding well-graded fines tocoarse waste for stability purposes. Di-rect shear tests were conducted ontwo samples of mine D material, oneconsisting of 100 pct minus No. 4 and theother as 100 pct plus No. 4 coal waste.The gradation curves for each sample areshown in figure 10. Table 5 shows theresults of the tests.

13

TABLE 5. - Differences in physical properties of coal wastesamples containing 100 pct minus No.4

and 100 pct plus No.4

Minus Maximum Angle of InitialNo. 4, laboratory internal Cohesion, void Specificpct density, friction, lb/in2 ratio gravity

lb/ft3 deg0 88.1 29.2 2.8 0.555 2.08

100 107.8 39.3 15.9 .332 2.18The MLD and the angle of internal

friction are much lower in the samplewith 100 pct plus No.4. Two factorscontribute to this. First, the coarsematerial has a limited range of grainsizes (uniformly graded), and, second,because of the absence of fines, thecoarse material has a much higher IVR.The large void spaces between the coarse

.1 Gravel Sand I Finesbles ICoone I Fine Icoarse Medium I Fine I Silt (noncreeuc) 10 clay (Plastic)

U.S. Standard sieves6 3 1-1/2 3/4 3/8 4 1016 30 SO 100 200 ~

1\ t-- 1

\ t--

01\

\r'\

"- "0

0

0 I1

I

100 o

~ 90.,~80

(j 7C-.60

W•••• SO(J)

~ 40

...J 30<o 2o

o .,(f)

20 ••o

30 0

40 gSO W•...60 (J)

<70 ;:

60 ~

90 800

0.001100 10 0.1

GRAIN SIZE. mm

0.01

Icob- Gravel Sand I FinesIblas Coarse Fine !coarse(Mediuml Fine fsm tncncraeuc) to clay (plastic)

U.S. Standard sieves6 3 H/2 3/43/6 4 1016 ·30 50 100200<,

11\\

-- 3

1\ - 1--4

\\ - -5

I-- 6

01\

I-- 7

0\

I-- 8

0 ...' I-- 9... ,." 1

100 o

Q; 90

~ 80

'0 70C-

• 80W:n 50

~ 40

...J 3<02o

~.,o (f)~20 ~

oo

oo C-

O ~(J)

o <o ;:

...J

o ~o 0

000.001

o100 10

FIGURE 10. -

0.1

GRAIN SIZE, mm

Grain sizes of samples containing100pctminusNo.4(top) and 100 pctplus No.4 (bottom).

0.01

particles reduce the sample's resistanceto shear. Because of the large differ-ence in the results, it can be concludedthat, for mine D material, fines are es-sential for strength. For this material,much greater amounts of minus No. 4 thanthe 38 pct in the as-received samplecould be added without detrimental ef-fects to the strength.

Time Factor in Testing

The second additional test was todetermine if the shear strength was af-fected by a delay in time between addi-tion of moisture to a sample and shearstrength testing of the sample. The timebetween adding moisture to the mixedcoarse and fine samples and shear testinghad varied considerably throughout thecourse of the earlier research. Twoidentical samples were prepared with mineD material containing 18 pct minus No. 4and 6.1 pct moisture. Identical directshear tests were performed, 24 hours af-ter mixing with water for one sample, and7 days after mixing with water for theother. The angle of friction was 29.4°for the 24-hour sample and 28.8° for the7-day sample; cohesion was 5.4 lb/in2 forthe 24-hour sample and 5.7 lb/in2 for the7-day sample. These results indicatethat, for mine D material, and probablyfor all nonslaking soils, time betweenadding water to the sample and testing isnot a factor in the shear strength•

Alternating Coarse and Fine Layers

The third additional test was con-ducted to determine the effects on shearstrength and permeability if the samplesare placed in the testing apparatus

14

in alternating coarse and fine layers,rather than mixing the coarse materialand fines together. The as-received sam-ples from mine B, containing 38 pct minusNo.4, were used as the coarse material,and 100 pct minus No. 4 material, whichincluded 24 pct minus No. 200, from thesame mine was used for the fines. Forthe triaxial shear tests, seven alternat-ing layers 3 inches thick (four coarseand three fine) were placed in the testapparatus, and for the permeability test,three alternating layers (two coarse andone fine) were placed. The water con-tent, saturation, and dry density atplacement were 7.7 pct, 50.4 pct, and87.0 Ib/ft3, respectively. The IVR was0.271. The angle of internal frictionfor the layered sample was 35.1°, which

is only a slight reduction from the fric-tion angle of 37.6° for the mixed mine Bsamples containing medium levels of minusNo.4.

Layering of the sample produced 3.2-Ib/in2 cohesion. TIlisis interesting be-cause no cohesion was produced in themixed samples, even with a high level ofminus No.4. The permeability of thelayered sample was 2.25 x 10-5 cm/sec orabout five times lower than the permea-bility of the sample containing 58 pctminus No.4. The lower permeability in-dicates that the layers of fines act asbarriers to the flow of water and couldresult in perched water zones above thefine layers.

CONCLUSIONS

The waste samples used in this re-search were collected from coal mines infour States (Colorado, Ohio, Pennsyl-vania, and West Virginia). The resultsare based on limited data and thereforecannot be construed as representative ofall sites. Each sample tested had adifferent classification according tothe Unified Soil Classification Systemand, therefore, different physicalproperties.

The actual numbers derived fromthese tests are probably not as importantas the trends (increases and decreases)in the physical properties when finewaste is mixed with coarse.

Thereached:

following conclusions were

1. Physical properties of coalwaste change as the minus No. 4 size isincreased from 20 to 60 pct. In the non-slaky samples tested, the MLD and soilstrength are at an optimum high, and theIVR at an optimum low, when the coalwaste contains 30 to 40 pct minus No.4.Somewhere in this range, the fines fillthe voids of the coarse particles but do

not replace or spread out the coarseparticles.

2. The MLD and soil strength ofnonslaky material improve substantiallywhen the minus No. 4 size is increasedfrom 20 to 40 pct. With an increase from40 to 60 pct, the changes in density andstrength are much less.

3. Optimum moisture content andpermeability are the physical propertiesmost affected. A 2- to 3-pct increase inoptimum moisture content was found in allthe samples as minus No. 4 was increasedfrom 20 to 60 pct. In two of the mate-rials, the optimum moisture contentstayed about the same in samples with lowand medium levels of minus No. 4 but in-creased 2 to 3 pct with an increase to ahigh level of minus No.4. In the othertwo samples, the opposite occurred. Thepermeabilities of the nonslaky materialsdecreased one to three orders of magni-tude when the percent minus No. 4 was in-creased from low to medium. The permea-bilities of all the samples were at ornear the lowest points when the samplescontained 30 to 40 pct minus No.4.

4. Highly slakable materials re-acted differently to the addition offines than did nonslaky materials. TheMLD and the angle of inte.rnalfriction(in the highly slakable sample tested)decreased as the minus No. 4 was in-creased from 20 to 60 pct. However, overthe same range, the IVR increased. Over-all, optimum moisture contents were muchhigher and permeabilities much lower forthe highly-slakable material.

5. For mine material D, there wasno effect on material strength as a re-sult of extended periods of time elapsingbetween adding moisture to the sample andtesting the sample.

6. The MLD, angle of internal fric-tion, and cohesion were much higher in asample containing 100 pct minus No. 4than in a sample from the same mine thatcontained 100 pct plus No.4. A well-graded mine D sample containing about40 pct minus No. 4 produced the best

*u.s. GOVERNMENT PRINTING OFFICE: 1981-505-002/129

15

physicalpurposes.

properties for stability

7. Layering the coarse and finewaste decreased the angle of internalfriction only slightly; however, thepermeability of the layered sample wasfive times less than that of a similarsample in which 58 pct minus No. 4 and42 pct plus No. 4 were mixed. The layersof fines seemed to act as barriers to theflow of water and could result in perchedwater tables under field conditions.

8. Mine operators contemplatingdisposal of large amounts of dewateredfines with coarse refuse should conductphysical property tests to determineproper disposal and mixing procedures,mixing proportions, water requirementsfor optimum moisture content, and drain-age and stability requirements. Thetests should be conducted as close toplanned field conditions as possible.

IN T.-BU.O F MIN ES,PGH.,P A. 25847