18

The main consideration ínvolves determining whichshales can be placed as rock fitl in thick lifts andwhich shales must be placed as soil and compacted inthin l-ifts. Test pads should be constructed todetermine the required procedures (lift thickness,watering, disking, type of cotnpactor, and number ofcompactor coverages) for each different shale¡naterial.

The comnon persistence and eventual nagnificationof shale-enbankment distrêss suggest the need forearly evaluation and treâtment of embankmentproblems. Existing distressed embankments should beevaluated by performing a systematíc review ofdesign, construction¡ and maíntenance records plus acomprehensive field and laboratory investigation todefine the cause of distress or fail-ure. Theprimary consideration in the re¡nedial treatnent ofshale embank¡nents should be surface and subsurfaceilrainage rneasures. when other remedial Èechniquesare applied' drainage measures are usuall-y anecessary supplenent.

ACKNO¡{I,EDGMENT

The conduct of this investigation required the ex-pertise and efforts of a large group of professionalengineers and scient.ists at the U.S. Arny EngineerWaterways Experiment Station plus a large amount ofvolunteer help.

We would like to acknoerlèdge the valuableassistance of the following rnembers of the lr¡ES

research team: G.H. Bragg, Jr., R.J. Lutton,D.!4. Patrick, J.H. Shamburger, an¿l T.W. ziegler.Many other engi- neers and technicians at WES al-sonade significant contributions. The investigationwas accomplished under the general supervision ofDon c. Banks and James P. SaIe.

Special thankè arê due the ¡nany state highwayagencies for their contributions, partícu1arly thefollowing members of the advisory group: JoeArnstrong (Montana), Roland Bashore (Ohio), HènryMathis (Kentucky) , ceorge l{eadors (Virginia), RodPrysock (California), Dave Royster (Tennessee) , BiIlSisiliano (Indiana) r ând Burke Thornpson (WestVirginia). J.K. Mitchell of the University ofCalifornia and L.E. Wood of Purdue University also

Transportation Research Record 790

participated as advisory-group nembers. C.W. Lovellof Purdue University has also made sígnificantcontributions to this and other studies of shale.

REFERENCES

I. J.H. Shamburger, D.M. Patrick, and R.J. Lutton.Design anil Construction of Conpacte¿l Shale Em-bank¡nents: Volurne l--Survey of Problem Areasan¿l Current Practices. Federal Highway Atlminis-tration, U.S. Department of Transportation,Rept. FHWA-RD-76-6L, Aug. 1975.c.H. Bragg, Jr., and T.w. zeigler. Design andConstruction of Compacted Shale Enbank¡nents:Volu¡ne 2--Evaluation and Remedial Treatnent ofShale Embankments. Federal Highway Ad¡ninistra-tion, U.S. Department of Transportation, Rept.FHI.IA-RD-75-62, Aug. 1975.R.J. Lutton. Design and Construction of com-pacted Shale Embankments: Vol-urne 3--Slaking In-dexes for Design. Federal Highway Ad¡ninistra-tion, U.S. Department of Transportation, Rept.FHWA-RD-77-1, Feb. 1977.w.E. Strohm, Jr. Design and construction ofCompacted shale Embankrnents: Volume 4--Fieldand Laboratory fnvestigations, Phase III.Federal Highway Administration, U.S. Departmentof Transportation, Rept. FHWA-RD-78-140' Oct.1978.W.E. Stroh¡n, JE., G.H. Bragg, JE., and T.w.ziegler. Design and Construction of ConPactedShale Embankments: Volu¡ne 5--Technical Guide-Iínes. Fe¿leral Highvray Adminístration, U.S. De-partment of Transportation, Rept. FHWA-RD-78-I41,, Dec. 1978.J.A. Franklin ând R. Chandra. The SlakeDurability Test,. International Journal of RockMechanics and Mining Science, vo1. 9, No. 3, MayI97 2.W.E. Strohm, Jr. Design ancl Construction ofShale Embankments: Summary. Federal HighwayAd¡ninistration, U.S. Departnent of Transporta-tion, Rept. FHbIA-TS-80-219, April 1980.

Ã

7.

Publication of this paper sponsored by Commîttee on Engineering Geology.

t¡on, maximum credible earthquake, saturation or nonsaturat¡on, and ¡ndexshear-strength parameters. The use of stability charts in design analysis is

frequently lustified by the simplified nature of the l¡m¡t¡ng cond¡t¡ons. Theemphasis in the design of mine-waste embankments ¡s to control the locat¡onof different mater¡al types and dra¡nage more than to control slope inclina-t¡ons,

Surface mining involves the removal and disposal ofl-arge quantities of overburden material, ¡nuch ofwhich is shale. This ¡naterial is often disposed ofin large waste embankments several hundred metershigh that vary in volu¡ne fron severat nillion toseveral hundred million cubic neters. Until re-cèntly, most of these embank¡nents were noÈ engi-neered. These e¡nbank¡nents can be distinguished fronhighway or earth-dam embankments by the varíability

2.

3.

4.

6.

Stability of rilaste-Shale EmbankmentsBRUCE C. VANDRE AND LOREN R. ANDERSON

Research conducted by the U.S, Forest Serv¡ce and Utah State Un¡vers¡ty onthe stabil¡ty of waste-shale embankments ¡s described. M¡ne-waste embank"ments can be distingu¡shed from other engineered fills by their variable andloose nature, by the lack of control of gradat¡on and density dur¡ng construc-t¡on, and by their deformation tolerance. Stability requirements dictated bygovernment regulations generally focus on the protect¡on of adjacent surfaceresources rather than on the ut¡lity of the embankment. Laboratory and field¡nvest¡gations indicate that waste shales in southeast ldaho have high voidratios. moderate permeab¡lity, and low-plasticity fines and are suscept¡ble tocollapse settlement on saturation. Commonly occurring slope movements canbe classif¡ed as slumps, shallow flow slides, and foundation spreading. Fullydeveloped rotational sl¡des are not common in southeast ldaho. The deepslope movements generally result from a reduct¡on ¡n toe support caused bygroundwater, excavation, or weak foundation so¡ls. Shear-strength lest¡ng ofshales at different gradations, durabilities, and mo¡sture conditions indicatesthat ultimate shearing resistance can be differentiated at two levels that can berelated to material conditions. The design of mine-waste embankments shouldbe based on l¡miting cond¡tions that may include max¡mum probable prec¡pita-

Transportation Research Record 790

of the naterial properties and the J-arge void spacewithin the embankment naterial. Because of uncon-trolled placement and the variable nature of thewaste-enbankment naterials, conventional proceduresfor evaluating stability and shear strength are oflimited usefulness.

The purposê of this paper is threefold:

1. To present a discussion of the constructionmethods and resulting characteristics of wasteembanknents,

2. Ib present laboratory and fiel-d test datathat characterize the properties of the waste-shaleembankments constructed in the mountaíns ofsoutheastern ldaho, ând

3. To proposê the use of index shear-strengthdeterminations for the design of waste-shaleembankments.

GENERATION AND DISPOSAL OF WASTE SHATES

Since shal-e is the most abundant rock type occurringat the earthrs surface, it is not surprising thatlarge quantities of mineablê mÍnerals are associatedvríth shal-es. CoaI frequently occurs in a rock unitknown as a cyclothen, vrhich consists of successivebeds of sandstone, shal-e, clay, coal, andIi¡nestone. This successi.on of beds is repeated manytinês in the strata of the coal fields. Frequently,the shale and sandstone nembers for¡n the greaterpart of a cyclothem and coa1, clay, and limestoneare subordinate. Co¡runonly, 30-70 percent of thewaste rock associated with the surface nining ofcoal is shale (!). rn southeast ldaho, 50-70percent of the waste rock associated with thesurface mining of phosphate ore is shalê (2). Theremainder of the waste rock is predominantly chertand linestone.

The disposal of i,raste rock can be a major aspectof the nining operation. Waste-to-ore ratios cantypically vary from 1:I for underground mining to20:1 for the strip mining of coal. The waste-to-oreratio for phosphate is approxinately 6:1. Theallowable economic waste ratio depends on com¡nodityprice, refining cost, ore grade, and mining rnethod.

The optional tocations for the waste fi11s arevalleys' sidehills, and the rnining excavation.Gêneral1y, stability is not a concern in the casè ofwaste materiä1 placed as backfi11. Backfilling isli¡nited by the cost of the doubl-e handling ofmaterials or the presence of future potential ore inthe êxcavation. Backfilling is done in some statesto satisfy reclamation requirenents. Even whenbâckfilling is done, sone surface waste placementcannot be avoided because the volume of excavatedmaterial after fragmentation r¡i1l increase andexceed the size of the excavation.

METHODS OF WASTE-EMBÀNKTIIENT CONSTRUCTION

Methods of constructing waste enbankrnents at thephosphate mines of southeastern fdaho vary depenclingon the steepness of the terrain at the mine and thetype of earthnoving equipment used. The differencein construction methods causes differences in theengineering properties of the embankment rnaterial.

At one mine, where the topography is steep, theembankments are built by dumping waste material overa slope at the end of the embankment (end dumping).The material flows down the slope at an anglê egualto the angle of repose of the ¡naterial. Thevertical heíghts of such e¡nbankments often exceed 30m (100 ft) and have been as high as 90 m (300 ft).The area of the dump is íncreased as naterial íscontinually placed over the edge of the etnbanknent.End dumping resuLts in high void ratios because the

19

bulk of the material is not cornpacted duringplacenent. Segregation of particle size â1soresults. The large ¡naterials ro11 to the bottom ofthe slope, ånd the finer materials remain nèar thetop. If the coarse rnaterial near the base of thedump is durable, it behaves as ã rock fill. Thereis grain-to-grain contact of the coarse particles,and the voids are generally not fil-led with fines.Near the top of â dunp, the fines frequently controlthe permeability and shear strength of the material.

!\laste dumps at a second nine, where thetopography is conparatively flatr are placed inhorizontal J-ayers approximately 0.5 m (1.65 ft)thick. wheel tractor scrapers place the material.Some compaction is achieved by the scraper wheel-s asthey pass dirêct1y over the material. The densitiesvary because portions of the fill do not receivedirect wheel contact. Final- slopes are generallygraded to 3 horizontal to 1 vertical at both ninesfor reclanation purposes.

Other common rneans of waste-èrnbankment plåcement,not used in southeast ldaho, include aerial tramwaysand conveyor be1ts. These free-faIl methods alsoresult in particle segregation and high void ratios.

EMBANKMENT PERFORMÀNCE REQUIREMENTS

The disposal of nine waste can be a liability fromthe perspective of both the nine operator and thesurface resource manager. In ¡nining, overburdendisposal is a nonproductive cost. In addition,waste enbanknents can change or creäte potentialhazards for future land use or result in adverseenvironmental inpacts. Sone federal- regulations(e.9., 36 C.F.R. S 252.1) require waste disposatoperations to be I'conclucted so as, where fe¿sib1e,to ¡nini¡nize adverse environmental impacts" and totake into consideration (a) air and water qualitystandards, (b) protection of fish and wildlife habi-tats' (c) reclamation, and (d) harnony with scenicvalues. Embankment instability rnay adversely affectany or all of these factors. The prine function ofvraste embankments is disposal. Performance require-ments generally consider the protection of other re-sources more than the utility of the embankment.

From an engineering perspective, vrasteembanknents can be distinguishèd from high$¡ayembankments and dams with respect to design 1Ífe,rnaintenance, and settlement tolerance. The designlife or term of service for highway etnbankments ordams is frequently limited by the durability of theappurtenant structures. The appurtenances can alsocontrol the allor¡able defor¡nation limits of thee¡nbank¡nent. Asphalt, concrete, and steel havefinite and sonewhat predictable service 1ives.Waste embankrnents are Iandforns and, geologically,landforms are inherently unstable. Itrs just amatter of tirne, which is uncertain.

Stability depends on future physical conditionssuch as rainfaIl, earthquakes, groundwater develop-ment, or changes in the properties of embankment ma-terials. Theoretically, "worst" conditions such asthe probablê naximu¡n precipitation and the maxi¡numcredible earthguake can be estÍmated. These condi-tíons are the upper physical Iinits extrapolatedfrom existing knowledgè, which is Ii¡nited. The un-certaínty of future conditíons is the primary basisfor the risk of future instabilítíes. Whereas worstconditions are considered in the design of da¡ns be-cause of adverse consequences, such as loss of life,worst conditiÕns are considered in the design ofwaste embanknents because of the ever-present possi-bility of their occurrence. Designing for lesserconditions nay be more acceptable when probability,rather than consequences, is the basis for a worst-conditions design. Acceptance of lesser desígn con-

20

ditions nust consider optirnization (mineral develop-ment versus environmental protection) ' cost-effec-tiveness, and societyis reluctance to consciouslyirnpose risks on future generations. To enabLe gov-ernment agencies to approve "optimum" designs' regu-Iations must permit discretionary acceptance ratherthan promul-gate mandatory design standards. How-ever' discretionary authority may result in ineffi-cient, time-consumíng, or inequitable approval prac-tices.

Magnitudes of settlement that would generally beunacceptablê for roadway or dam embankments areacceptable for vraste embankments. The limitingdeformation for waste embankments is the a¡nount thatwould disrupt surface drainage, create excess porepressure' or creâtê other hazards for stability.

SLIDE CLASSÎFICATION

Slope movenents cornmon to non-water-inpoundingmine-waste enbankments nay be classified as slunps,shal-Iow flow sl-ides' or foundation slides (3). Insoutheast Idaho, slumping has generally been limitedto shallow depths. Edge slumps result fron theoversteeping of the upper portion of the slope.This can be caused by an accunul-ation of fines or bytemporary cohesion associated with noisture (4).Fully developed rotational slides are not common inshales that have a low clay content. Ho\dever,deep-ernbankment sLides can result fro¡n a reductionin toe support caused by groundwater, excavation, orweak foundation soils. If the foundation shearstrength exceeds the enbankment shear strength andis frictional in nature, and if excess porepressures do not occur in the foundation, slidingsurfaces wil-I be confined within the embankment.

Shatlow flow slides are frequently initiated byrain or snowmelt. rnfiltrating water can saturatesurface soils' provided an adequate supply of wateris available to fíIl the air voids and the runoff orrainfall intensity exceeds the infiltration rate.The depth of saturation depends on soil permeabil-ity, porosity, degree of saturation, and the avail-able supply of water. FIow slides occur because ofthe shear failure of the soil or the collapse of thesoil structure. These slides can disrupt reclamâ-tion activities and can affect locations at consid-erable distances below the waste e¡nbanknent.

Foundation sfides invol-ve shearing at the embank-ment-foundation interface or below the embank¡nent.End dumping' a rapid-loading condition, can resultin the developnent of excess pore pressures in thefoundation soils and cause a slope wedge to trans-late laterally. The foundation soíIs can be shovedor pushed ahead of the advancing toe. This type ofsliding is classified as foundation spreading. Thedevelopment of excess pore pressure depends on thedegree of saturation, the permeability of the foun-dation material, and the rate of advancement of thedump s1ope. Íf. the loading exceeds the shearstrength because of pore-pressure buildup, founda-tion spreading wíLl occur. Generällyr the slope¡ri11 stabilize after a period of ti¡ne if the fiIlplacernent is discontinued. To resurne dunping andavoi¿l future novemènts, the dump heíght or advance-¡nent rate would need to be decreased. Îf thedrained resídual foundation strength is unusuallyIo$¡, buttressing and confinenent or translation toflatter terrain nay be needed to stop the slide.For saturated clay foundations, generally it is notfeasible to limit rates of fill placenent to avoidundrained loading conditions. In addition' it isnot common practíce to construct dunp slopes raPídlyenough to develop excess pore pressures in sandfoundations. The rate of advancement of dunps sup-

Transportation Research Record 790

ported by silty rnatería1 can be critical to founda-tion stabiJ.iby.

Embankments dunped on steep slopes may translatealong the contact between the embanknent anil thefoundation. These slides may occur during embank-ment construction because of the steepnèss of thefoundation or can be initiated later by the decay oforganic matter, earthquake forces, the melting ofburied snow, or other occurrences of groundwater.The slope of the natural- ground cletermines both thepotential for and the consequences of sliding. Asthe slope of the foundation increasesr so do thesl-iding potential and the potential area of impactof the sliding.

DESCRIPTION OF THE INVESTIGATION

In 1978 (phase I of the investigation), laboratoryand field testing were undertaken to classify andevaluâte the general engineering Properties of.overburden ¡nateriats lrasted in the nining ofphosphate ore ín southeastern ldaho (5). In thespring of 1980r waste-shaIe materials were shearedât 1ow normal- pressures to evaluate strengthparaneters for the analysis of shallow flow slides(6). In the fall of 1980 (phase 21, waste-shalematerials were tested to develoP index shearstrengths for use in the stability analysis ofmine-waste embankments.

Phase I

CIassi fication

Seven shale samples, weighing approximately 110 kg(50 fb) each and representative of the finerparticle sizes, were obtained from waste embankmentsat two different phosphate mines in southeasternIdaho. Particles larger than 51 nm (2 in) werediscarded. The average and range of grain-size¿listributions are shown in Figure 1. The liquidlirnit and plastic limit, averagecl for theclassification sanples' were 23 and I9 Percent,respectively. Most of these sanples were classifiedas silty gravet (Unified Soil- Classification GM).

conpaction

To proviile some perspective as to the range ofconpaction occurring in layer-placed embankments, l-5density tests were taken at locations benèath orbetvreen the wheels of the hauling equipment.Laboratory compaction curves (AASHTO T99-741 weredeternined for the satnples at each densitylocation. The averaçte degree of laboratorycompaction for locations subjected to wheel trafficwas 90 percenti the avèrage compaction for locationsnot subjected to wheel traffic was less than 80percent. The compaction range was highly variable(standard deviation of 6). The average moisturecontent of the density tests was approxinately 2

percent beJ-ow optirnum. Changes in the Placementmoisture content of waste shales can vary with thetime of year as well as other clinãtic conditions.The fielcl density tests were taken during the sunmer.

In the construction of end-dumped embankments,only the finished areas and haul roads receive wheelcompaction. These areas represent a snall fractionof the entire waste enbankment. Density testing onan angle-of-repose slope is difficult; one test,however, had a relative compaction of 79 Percent.

Permeability

Constant-head perneability têsts were performed onsanples conpacted to densities that varie¿l from 86

Transportation Research Recorcl 790

Figure 1. Particle-size gradat¡on of representative samples of wæte shale,

IJS STANDARD SIEVE SIRÏES

MODEL EO

SCALPED

UPPER RANGE

AVERAGE

LOl¡lER RANGE

.uu/ .+. 075

PARTICLE DIAMETER (nm)

+. / 3 ¿5.412.7

to 95 percent of the laboratory maxinun åt amoisture content 2 percent dry of optimum (AASHTo

T-99). The average permeabílity for four tests was3x10-s cr¡/s (0.9x10-s in,/s). This perneâbilityis believed to be more representative of layer-pLaced embanknents than end-dunped enbanknents andinterior fill rather than exterior fi11. The ef-fects of weathering and siltation fro¡n surface ero-sion could substantially reduce infiltration at thedump surface.

Conpression Tests

Confined compression tests v¿ere run on theclassification samples to evaluate the settlementcharacteristics. sa¡np1es representíng two differentgrain-size distributions were used for testing. Oneset of samples was prepared to represent the coarseshale material that occurs near the botton ofend-dunped embanknents where there is substantialgrain-to-grain contâct of the coarse particles. Asecond set of samples represented the waste-shalematerial that has substantial fines. The coarsesanples vrere prepared by passing crushed chertyshale material through sieves and retaining thefraction passing the 4.75-mn (no.4) sieve butretained on the 0.8-mm (no. 20) sieve. À 6.4-cm(2.5-in) diameter consolidoneter was used to performthe conpression tests. Different noisturetreatments were useil in perforrníng the tests. Tïotests rrrere perforned on samples that vrere subjectedto wetting and drying cycles durÍng the tests.Thrêe tests were performed by increasing the normalload on dry sanples and then saturating the samplesafter they had reached a given stress 1evel. Theresults of these tests on the coarse material areshown in Figure 2. The slope of the strain versusIog of stress curves was nearly tr.rice as steep forsamples subjected to wetting and drying cycles asfor sanpLes that were compressed dry. CompletesaturatÍon of relatively dry sanples caused anim¡nediate increase in conpression (collapse

2L

Figure 2. Strain versus log pressure for coarse waste.shale material show¡ng theeffects of moisture.

- Saturated afterreaching highstress 'ì evel s

Treated with cycìes ofwetting and drying

1 kPa = 0.145 lbf/in2; 1 cm = 0.39 in.

200 500 I 000 I s00

PRESSURE (KPA )

settle¡nent). Hohrever' the magnitude of verticalstrain is about equal to the final nornal pressurefor both the cyclic ¡noisture condition an¿l thecollapse condition.

Si¡nilar tests vrere performed on comPacted samplesof \raste shale that containeil significant finematerial. the saturation collapse settlernent wasnot as great for the fine material, and increasedcompaction decreased the rnagnítude of compression.

Total and Effective Shear-Strength ParametersVersus Conpaction

The effect of compaction on the strength of par-tiall-y and fully saturated waste-shaIe samples r¡asevaluated from the results of 23 consolidated-undrained triaxial shear tests with pore-pressuremeasurements. The triaxial test specimens had a tli-ameter of 3.6 cm (1.4 in) and were prePared by pass-ing one of the classification samples through a2.O-mm (no.10) sieve ancl discarding the P1us-2-mn¡naterial. The scatpe¿l test gradation is shown inFigure 1.

The triaxial shear tests were run on samplesconpacted to 80, 90r anil 100 percent of thelaboratory naxi¡num (AAsHTo T-99) at a moisturecontent approxirnately 2 percent dry of optÍnun. Thetests were run at confining pressures of 69, 207 ,and 345 kPa (10, 30, and 50 Lbf,/in'z). Thestrength envêlope vras based on the maxinum deviatorsÈress reacheil before a strain of I percent. TheMohr-Coulomb strength envelopes had a shear-strengthintercept that curved upward with increasingconfining pressure. Both intercept and curvature

ooo<Nõ+ N

90

BO

70

60

oz;50

,40

è30

E

E l¿

z

ts

r 16z

20

l0 20

RelativeCompactiona(%\

22

Table 1. Friction angle versus oompaction and moisture content for waste-shalematerial from triaxial shear test results.

Transportation Research Record 790

angle and the strength intercept obtained fro¡n thebest-fit strength envelope were 29o and l-.3 kPa(0.18 lbf./ín2 | , respectively. The averagecoefficient of variation (ratio of the standarddeviation to the mean) for the test shear-strengthvalues obtained at three different normal Pressureswas 20 percent.

Phâse 2

Testing samples with scalped gradations will yieldconservative estinates of shear strength for nanyfield conditions. However, the use of this approachhas the following li¡nÍtations for the design of¡nine-Ì{aste embankments: Testing of representativesarnples is not possible, and finejrained materialstrengths may be overly conservative for sone mine-waste-embankment conditions. In the laboratorytesting performed to exa¡nÍne the relationship amongshear strength, gradation, and ilurability, it wasexpected that shear strength could be indexed toparticle-size classification. As testing pro-gressed, the inportance of the íilentification ofshear-strength components becane apparent. The fo1-lowing discussion of the phase 2 investigation con-sists of a review of the mechanics of shear resis-tance of granular nateriâls, test results, conclu-sions, and recommendations for indexing waste-embankment shear strength.

Mechanics of Shearing ResÍstance

Thê drained shearing resistance of granular materialconsists of the fottowing components: slidingfriction, particle rearrangenent, dilaLancy, anclparticle crushing (7'8). The slope of theMohr-Coulomb strength envelope can be explained interms of the different shear-resistance cornponents.At lor,¡er normal pressures, the slope of the envelopeis controlled by resistance to Particlerearrangenent and by díIatancy and slidingfriction. For example, nediun-grained unifor¡nquartz sand experiences very 1íttle crushing below37 kPa (5 Lbf/ín2 ) (9). As the nor¡nal Pressureincreases, the slope of the envelope decreases whencrushing has a net effect of reducing dilatancyeffects. At very high pressures, particle crushingrequires consideräble energy ancl, together with apossible s¡naI1 increase in friction, causes thestrength envelope to steepen.

when the strength envelope is curvedr the fric-tion ångle can be deterníned at various normalprêssures (tan- I shear stress,/normal stress)rather than neasuring the ínclination of thestrength envelope over a range of normal pressures.This "point" friction angte has been referred to asthe "equivalent" fríction angle in a British studyof coarse colliery waste ¡naterials (10).

The friction angle for a best-fít, straight-linestrength envelope will not refLect all of the dila-tancy ând crushing contributions to shear resis-tance, which vary with normal pressure. This ap-proach nay be overLy conservative if it eliminatescrushing contributions. In addition' a straight-tine envelope is conservative if the shear-strengthintercept is eli¡ninated from contribution because itis mistakenly attríbuted to negative pore-pressureeffects. The variation of shear strength with nor-rhal pressure nay also be examined by plotting thepoint frictíon angle as a function of the logarithnof the normal pressure. A linear relation comnonJ-yexists (f!) .

Àt a constant-placetnent relative densityr grada-tion appears to have pronounced influence on theshear strength of rock-fill materials whereas' at aconstant-placernent void ratio, the influence of gra-

VoidRatio

WaterContent(7")

Friction Angle { (o)

Total Effective

8090

t00

8090

r00

0.900.690.52

0.900.690.52

3 1.037.047.5

15.015.037.5

l41414

_b_b_b

29.53 3.039.0

ueAsHTo .fss. bsatu¡ated.

increased with compaction. To indicate the effectsof cornpaction on the partially saturated samples'the friction angl-es, as defined by a straight-lineenvelope through the origin of the axes and tangenÈto Mohrts circle at the 345-kPa confining pressure,are given in Table 1. The conplete results,inctuding the stress-strain curves, have beenpresented elsewhere (å).

Saturation after Plâcement nay be a possiblefuture condition, as a result of extreme flooding orthe nelting of large snow masses that were buried inthe embanknent during construction. Therefore,consolidated-undrained triaxial shear tests withpore-pressure measurements were also run onsaturated samples. Saturation was achieved by usingback pressure. Both total and effective strengthparameters were determined. The results aresu¡nmarized in Table l, along with the partiallysaturated test results. These results are based onbest-fit Linear strength envelopes through theorigin of the axes for shear stress and normalstress.

Direct Shear Tèsts at Low Normal Pressure

Thirty direct shear tests were performed at nornalpressures less than 27.6 kPa 14 tbf,/in2 ) onl-aboratory-nolded samPles of waste shale. Thegradation of these sampfes modeled the averagegradation of the classífication sampÌes presented inFigure J..

The grain-size distribution of the modeled mate-rial was obtained by shifting the graín-size-distri-bution curve for the average' ãpproxinately parallelto itsetf, to the desired maximum particle size forthe taboratory specirnen. The liguid limit and thepIåstic li¡nit of the tested näterial were 27.4 and24.5 percent, respectively. The samples wereloosely placed in the test eguipment and compressedto void ratios that averaged 0.87 with a standarddeviation of 0.03.

The samples were saturated by upward seepage of!¡atèr un¿ler a hydraulic Aradient of one and weremaintained in a saturated condition forapproximately t6 h before testing. The tests wereconducted in accordance with the ¡nethod given byAASHTO T236-72. AII samples were sheared underconsolidated-drained conditions with a constant rateof shear displacement of 0.7 mm,/min (0.03 ín,/nin)and a gap spacing of 6.4 nn (0.25 in). The sa¡npleswere 10.2 c¡n 14 in) in diameter and approximately5.1 cm (2 in) in height. The slow rate of sheardispJ,acement al-loweil sufficient tí¡ne to ensure totaldissipation of pore-water pressure wíthín thesample, and effective stress paraneters grereobtained.

Failure e¡as defined as the ¡naxinun shearingstress or the shearing stress at 10 percent lateralstrain (shear displacement dívided by the samplediarneter), whichever occurred first. The frÍction

Transportation Research Record 790

clation see¡ns to be negligible (1¿). A resultingsimilarity in void ratios appears to be the explana-tion for the acceptable use of modleled graclations inshear-strength testíng. In the shearing of a loosegranular material¡ deforrnation ten¿ls to increase theshear strength by decreasing the void ratio (parti-cle rearrangenent). It is apparent that, when shearstrength is determined by strâin criteriar loose na-terials in which there is a large difference beti{een¡nininun and maximu¡n void ratios will have low shearstrengths conpared with rnaterials that have a smallrange of possible void ratios. The energy requiredfor compression partici.e rearrangement is less thanthat required for dilatancy or crushing.

It is conrnon knowledge that the fines contentIninus 0.075-n¡n (no.200) sieve] affects the shearstrength of granular ¡naterials. The AnericanÀssociation of State Highway and TransportationOfficials (AÀSHTO) and Unified Soil Classificationsystems suggest that approximately 15, 35, and 50percent fines contents have a significant effect onthe engineering properties of soils (13.1. Thernechanics of how fines affect shear strength havenot been explained. Intuitively, the presence offines is expected to decrease resistance to particlerearrangement and dilatancy. Studies that haveexanined the relation betvreen shear strength andfines content in mine-waste Í¡aterials have notconsidered the combined effect of relative densityand gracling in their examinations (11).

Laboratory Testing Program

The U.S. Forest Service perforrned 97 tlirect sheartests and 8 triaxial tests on 6.4-cm (2.5-in)diameter sanples for the phase 2 investigation.Utah State University performed 2I direct sheartests on 10.2-cm (4 ín) diameter sanples. Thedirect shear tests were performed at normalpressures that varied fro¡n 12 to 372 kPa (1.8-53.9lbf/ín2r. The triaxial tests were performed atconfining pressures of 138, 345, or 690 kPa (20, 50,or 100 lbf/in2). The I0.2-c¡n direct shearapparâtus was used prinarily to test the coarsergradations. Triaxial testing was performeil forcomparative purposes and to obtain higher normalpressures than would be practical v¡ith the directshear apparaÈus.

Five gradations (A through E) of durable,.Iow-plasticity shale were tested (see Figure 3). Thesegradations incluiled the significant fines contentrecognized by the engineering soils classificationsystems previously nentioned. The ¡ninus-2-m¡n (no.t0) particle-size content was based on Èhe ratio ofpercentage finer than 2 mm to percentage finer than0.075 mn (no. 200) of the average gradation for theclassification mâteríal (Figure 1). The 2-n¡n parti-cle is the boundary size betrdeen sand and gravel inthe ÀÀSHIo Soil Classification system. It is also areference size in slake-durability testing. Thelinear particle-size distributions between the con-trol sizes--naxinum test particle size 2 nm and0.075 m¡n--are reasonable, considering that placementmethods result in poorly gracled partici-e distribu-tions.

For conparatíve purposes, two other materi-als--quartzite and nondurable shale--were tested atgradation E. The non¿lurable shales degraded ínto apile of flakes after immersion in waterr r.vhereas thedurabl-e shale for¡ned only a few fractures.

The direct shear sarnples were tested by using arate of shear displacement that varíeil fron 0.5 to1.3 mm/¡nin (0.02-0.05 inlnin). The gap spacingequaled 1 cm (0.4 in) for the 1.25-c¡n (0.5-in)maxi¡num particle gradations and Ìvas equal to orgreater than the naxÍnum particle size for the other

23

gradations. Moisture conditions for the tests srereeither air-dried or saturated.

Test Results and Discussion

The shear-strength pararneters and void ratios forthe different gradings and materials are summarizedin Table 2. The void-ratio ranges were estinated byconsidering the range of densities measured for 10loose placernents at each gradíng and randomconpression neasurenents at the varioug normal orconfining pressures. The terms point friction angleand envelope friction angle are defíned in Figure 4.

In the air-dry conclition, the gra¿lations of thedurable and nondurable shales compressed fron 3 to Ipercent at nornal pressures that varied from 99 to371 kPa (14-54 lbf/in2 ) and confining Pressures ashigh as 690 kPa (100 lbf,/in'z). In the saturatedcondition, the durable shale compressed up to L2percent and the nondurable shale compressed rapidlyup to 24 percent at the 37l-kPa loading. ThequarÈzites compressed less than a fraction of 1percent at the 371-kPa normal pressure loadíng forboth the dry and saturated conditions.

All shale gradaÈions except gradation E exhibitedpeak shear strengths (curve type I in Figure 5) atnorrnal pressures fro¡n 99 to 37L kPa. shale atgradation E developed peak shear strengths (curvetype 3 in Figure 5) at normal pressures of less than37 kPa (5 lbf,/in2 ) , at which level dilatancy couldnot be prevented. Type 3 curves were also developedby material tested at gradation A and ât the highertriaxial confining pressures (significant co¡npres-sion) .

The effect of saturation on the durable shaleswas to change the shape of the stress-strain curvefron a flattening curve (type 2) to a continuouslyrisÍng curve (type 4). The shear strength of thedurable saturated shale generally reached the shearstrength of the air-dry shal-e within 15 percent

Figure 3. Gradations for shear-strength testing (phase 2).

US STANDARD SIEVE SERIES

.075

No+

90

80

70

60

50oz

440Fz330

20

t0

2 4.75 12.7

PARTICLE DIAMETER (mn)

24

Table 2. Summary of shear-strength parameters.

Transportation Research Record 790

l

l'

Sample Grading

InitialVoidRatio

NormalPressure(kPa)

Ave¡age Point d C) Envelope O (o)/Intercept (kPa)

Peak Ultimate Peak Ultimate

Durable shale

Nondurable shale

Quartzite

BCDE

0.8-0.95

0.5-0.650.65-0.70l. I 5-1.251.05-1.2

i.0s-1.20.9-1. I

a

1.05-1.2

3sls37130

371036148

"o l^1

42157

35/0371428alß291332sl7s35^ 124

3813394lo

37 llO29alß

4010

12.4-37.t99.4-371.999.4-371.9I I 9.5-352.0I I 9.5-352.0I 19.5-352.012.4-37.t99.4-311 .999.4-37 t.999.4-371,.999.+37 r.9

12.4-37.r99.4-37 t.9

42 4039 36

3la37 3543 3947 40450

393ga

39324

49 3917 0

úe ENVELOPE FRICTION ANGLE

9P POINT FRICTION ANGLE

-/

Note: 1 kPa = 0,145 lbf/in2.aSâturated sample.

Figure 4. Defin¡tions ofenvelope and point fr¡ctionangle.

NORMAL STRESS

Figure 5. Class¡ficat¡on of laboratory stress-stra¡n curyes.

strain deformation. The stress-strain curve for thenondurable shales v¡as the flattening tyPe for boththe saturated an¿l dry conditionsi howeverr thesaturated shear strength was significantLy IoHer.

Figure 6. Comparison of triaxial and d¡rect shear test results (gradation Dl.

o _-__^_

'1350

'1350

N0RMAL STRESS (kPa)

The friction angle for the saturated nondurableshales (32o) ' averaged for the clifferênt nor¡nalpressures' nearly equaled t.he similarly averagedsaturated ulti¡nate friction angle for thefine-grained gradation A (3Io). After saturation'the collapsed void ratio of gradation E nearlyequaled that of gradation A. The nondurable shalescollapsed to such an extent on exposure to e¡ater inthe triaxial apparatus that säturation of the sampleun¿ler confining pressure becarne extrernely difficultbecause of the decreasecl perneability. Saturationanil shear testing ilegracled the nondlurable shalesanplês fron zero percent passing the 2-¡n¡n (no. 10)sieve (gradation E) to 59 percent passing the 2-mnsieve and 1I percent of the total sanple finer thanthe 0.075-nn (no. 200) sieve. The durable shalesa¡nples also degraded during saturated shearing to36 percent passing the 2-nm sieve and 4 peréent ofthe total sa¡nple finer than the 0.075-¡nm sieve.

The Mohr-coulo¡nb strength enveLope for gradationD of the durable shâIes determined by triaxialtesting is shown in Figure 6. The direct shear testresults' also included in this figurer fit on thestrength envelope for the triaxial tests. Thevariation in shear strength between triaxial anddirecÈ shear tests reported for míne-waste mâterialsby others (1,1) coulit be explained by differences innormal pressure ranges. The other triaxial testresults also closely agree¿l grith the averaged directshear results. However, the variation anongreplicated dlirect shear tests suggests thâtrepetitive testing is advisabl-e.

Averaging the test friction angl-es for the shal,esin each colu¡nn in Table 2 has procluced the followingresults: For point friction angle¡ peak = 43o and

ØUdts

úf

Udts

U-Ø

F

DIRECT SHEAR TEST RESULTS

Note: I kPa = 0,1/t5 lbf/¡n2.

STRAI N

Transportation Research Record 790

Figure 7. Fr¡ction angle versus log pressure for differenlparticle gradat¡ons and durability.

ultimate = 37o; for envelope friction angle,peak = 38o and ultimate = 35o. If one consiclers thepreviously described shear-resistance theory forgranular maÈerials, the difference between the pointpeak and point ultirnate friction angles (6") can beattributed to ditatancy. The envelope frictionangle for the peak strengths (38o) also appears toremove dilatancy effecÈs in that its value neârlyequals that of the point ultirnate friction angle(37o). The parâmeters for the ultímate frictionangle envelope cannot be explained in terms ofshearing-resistance conponents.

Point friction angles are plotted in Figure 7 asa function of the logârithn of the failure normalpressure. Steeply stoping plots I through 4 forpeak friction angles indícate the severe effect ofnor¡nal- pressure on dilatancy. The flatter slope forultimate friction angles for the nondurable shale(plot 5) suggests that normal pressure has a lessdetracting effect on crushing contributions to shearresistance. The ultinate shear resistance for thedurable shales at gradations A and E and the quartz-ite vrere not significantly affected by normal pres-sure within the testing load range.

At low norrnal pressures [<37 kPa (<5 lbf,/in'z) ] ,dilatancy appears to be controlled more by graínsize than by måterial type. Ðilâtancy for plot 1(the fine-grained gradation) is lower than that forplot 2, which fits the friction-angle/nornal-pressure points for both the quartzite and the dur-able shale.

The laboratory test results given in Table 2 ex-hibit two levels of ultimate friction angles' ap-proxirnately 31o and 38o. These friction angles re-flect ¡noisture conditions, grain sizes' and stresslevels. Saturated nondurable or fine-grained shalescan be assigned an index shear-strength value of31o. It appears reasonable to assign an indexshear-strength value of 38o to nonsaturated wasteshales irrespective of grading and durability. Therationales for using ultinate strength for indexpurposes are as follows:

t. If the strain at every point along a sliding

100

NORMAL PRESSURE (kPa)

I 000 I 0 .000

surface in an embanknent could be measured, it woul¿lvary from near zero at the nost recently mobilizedpoint to a relatively high value at the initially¡nobilized point. The peak shearing resistanceoccurs at only one value of strain. Eventuallyr theultimate resistance can be available along theentire failure path.

2. Ultimate strength values do not includeditatancy effects, which vary with place¡nentconditions.

STÀBILITY ANALYSIS AND DESIGN

when Ín situ embanknent conditions cannot be relia-bly predicted, design decisions shoulcl be basecl onlimiting conditions. Liniting conditions for nine-waste embankrnents include ¡naximu¡n probable precipi-tation, maximum credíb1e earthquaker index shear-strength paraneters' and såturated or unsâturated¡nater ia1 .

A sliding r.redgè is frequently used to analyze thestability of granular soí1s. The v¡edge is recog-nizecl to be a fundamental configuration for self-weight-type analysis of clastic materials (15). Atheoretical liníting or worst conditíon for a slid-ing wedge would be the fult arching condition (16).Unequal compression of loose embankment material caninitiate arching. For shales, the shearing resis-tance available along the sliding surface will de-pend on the nateriâ1 conditions. Unless positivedrainage is ensured by the embânk¡nent design andconstruction control, saturated conditions should beassu¡ned. Hosrever' the design for saturated condi-tions is often not practical.

The infinite slope nodeJ, can be used to analyzethe potêntiaI for shallow flow sliding. For thistype of sJ.iding to occur, a saturated wêtting frontmust develop. For ¡naterial that has a shear-strength intercept' the stable sloPe angle will beaffected by the depth of saturation. rn ¡nost tem-perate climatesr ¡naxi¡num probable preciPítationevents will supply enough vtater to saturate to sucha depth that the stabilizing effect of the strengthintercept is offset. If the strength intercept is

25

Ê(

50

ooço!

14soz

zoF

ff40

J5

l0

L.

l- Gradation A

2- Gradation E, Durable shale and quartzit3- Gradation D, Durable shale4- Gradation C, Durable shale

1 5- Gradation E, Nondurable shale

HIGH DENSITY, I,IELLGRADED, STRONG

\ PARTICLES (12)

Note: 1 kPa = 0.145 lbl/in2.

LOl.] OENSITY,POORLY GRADTD, NTAK

PARTICLES I I 2)\

26

ignored (1initíng condition), design slopes becomequite flat an¿l stability is controlJ-ed by the occur-rence of saturation. Hoerever, highly permeable ma-terial can be used to prevent the development of awetting front at the final construction-slope toca-tion.

Stability charts can be developed for use in theanalysis of the stability of mine-waste embank-ments. The use of stability charts is often justi-fied by the simplified nature of limiting condi-tions. Charts have been developed for analyzingdeep-seated wedge slides, foundation spreading, anilshallow flow slides (I7). The average nornal pres-sure on a wedge sliding surface has also beencharted as a function of embank¡nent height and foun-datíon inclinatíon.

In deciding on design safety requirements, it isimportant to nake a distinction between constructionslopes and abandoned and/ot reclaimed slopes.Frequently, the safety factor can be lower forconstruction slopes than for the final slopes. Thisreductíon can be justified by a comparativelyshort-term risk and the opportunity for remedÍal orrnitigating measures. In additionr assuming that theupper level of shearing resistance will be availablernay be reasonable for construction evaluations butnot reasonable for long-term conditions.

The nost controversial stability issue associatedwith the regulation of ¡nine-waste ernbankments is theacceptabil-ity of end-dunping construction methodsand the abandorunent of angle-of-repose slopes. Ifone uses the inclex shear-strength parameters andchart analysis, anglê-of-repose slopes appearreasonably safe in relation to deep-seated sliding¡províded the slope of the foundation is less than14o anil saturation can be prevented beneath theslope. The potential for shallow flow slides can bèel-ininated by using durable' highly permeablenaterials in the construction of the final slopes.However' the use of these materials would 1i¡nitrevegetation opportunities. The potential forraveling cannot be eliminated but can be mitigatedby designing benches. ft appears, then, that theconstruction of waste embankments by end dunping naybe acceptable in situations that involve (a)restricted sitê locations, (b) select wastenaterials, and (c) limited revegetation requirements.

ACKNOWLEDGMENT

The phase 1 testing reported in this paper wassupported with funds fro¡n the Surface Environmentand Mining program of the U.S. Forest Service.These funds were ad¡ninistered by the ForestryScíences Laboratory, Interrnountain Research Stationin Iogan, Utahr and by the Intermountain RegíonalOffice in ogilen¡ Ubah. Richard Riker änd Chal-ernSae-Liang performed the testing at Utah StateUniversity.

sanple preparation and triaxial testing for thephase 2 investigation were performed by Gerald C.Wilson of the Geotechnícal and lvlaterials TestingFacility, fnÈernountain Regj.onal office, U.S. ForestService. Utah State University research assistant,fa¡nes Higbee perfor¡ned the 21 direct shear tests onthe 10.2-cm (4-in) samples.

REFERENCES

1. w.E. Davies. Geologic Factors in waste BankStability. Mining Congress Journal, Vol. 59,No. 1, Jan. 1973, pp. 43-46.

2. D.!1. Butner. Phosphate Rock Miníng in South-eastern Ïdaho. Bureau of Mines, U.S. Depart-

Transportâtion Research Record 790

ment of the fnterior, Bureau of Mines CircuLar7529, 1949.

3. B.C. Vandre. The Review and Regulation ofSlope Stability: A Technical Perspective.Proc., 10th Ohio River Valley Soils Seminar,Lexington, KY, Oct. 1979.

4. A.D. Pernichele and M.B. Kahle. Stability ofwaste Du¡nps at Kennecott's Bingham CanyonMine. Trans., society of Mining Engineers,American Institute of Mining and MetallurgicalEngineers, Vol.250, No.4, I971r p.3637.

5. R.E. Riker, L.R. Anderson, and R.W. Jeppson.Engineering Properties and Slope Stability ândSettl-e¡nent Analysis Rel-ated to Phosphate MineSpoil Dumps in Southeastern ldaho. Utah waterResearch Laboratory, College of Engineering,Utah State Univ., Logan' Rept. H-78-001' May1978.

6. C. Sae-Liang. An Investigation of the Effectof Grain Size Distribution and Densíty on theShear Strength of Mine Shale Material. UtahState Univ., Logan, M.Sc.E. thesis, 1980.

7. K.L. Lee and H.B. Seed. Drained Strength Char-acteristics of Santls. Journal of Soil Mechan-ics and Foundations Division' ÀSCE, Vol.93,No. SM6, 1967, pp. 1l-7-141.

8. P.w. Rowe. The Strêss-Dilatancy Relations forStatic Equilibrium of an Assembly of Particlesin contact. Proc., Royal Society, London,Series A, VoI. 169, L962t pp. 500-527.

9. A,s. vesic and G.w. clough. Behavior of Granu-1ar Materials Under High Stresses. Journal ofSoil Mechanics and Foundations Division' ASCE'VoI. 94, No. 5143, 1968, pp. 661-668.

10. Review of Research on Properties of Spoil TipMaterials. Wi¡npey Laboratories, Ltd., ÈliddIe-sex, England, National Coal Board ResearchProject, 1971.

11. T.M. Leps. Review of Shearing Strength ofRockfill. .Tournal of SoiI Mechanics andFoundations Divisionr ASCE, VoI.96' No. Sl,t4'1970, pp. 1159-1170.

L2. E. Becker, C.K. Chan, and H.B. Seed. Strengthand Deformation Characteristics of Rockfill Ma-terials in Plane Strain and Triaxial compres-sion TesÈs. Departnent of Civil Engineering,Univ. of California, Berkeley, Rept. TE-72-3,1972.

l-3. M.M. Al-Hussaini. Contribution to the Engi-neering Soil Classification of CohesionlessSoils. U.S. Army Engineer Waterways ExperinentStation, Vicksburg, MS, Miscellaneous Paper S-77-2t, 1977.

14. M.J. Superfesky and G.P. Willians. ShearStrength of Surface-l.line Spoils Measured byTriaxial ancl Direct Shear Methods. Northeast-ern Forèst Experinent Station, U.S. Forest Ser-vice, Broomall, PA, General Tech. Rept. NE-39,1978.

15. D.H. Trollope and B.C. Burman. Physical andNumerical Experiments with Granular Wedges.ceotechnique, VoI. 30, No. 2, 1980' pp. 137-15?.

16. D.H. Trol-l-ope. The Systenatic Arching TheoryApplied to the Stability Analysis of Embank-ments. Proc., 4th International Conference onSoil Mechanics and Foundation Engineering¡London, Vo]-. 2, 1957, pp. 383-388.

I7. B.C. vandre. Stability of Non-water-hpoundingMine Waste Enbanknents, Rev. Ed. Intermountai.nRegional office, U.S. Forest Service, Ogden,UT, March 1980.

Publícation of this paper sponsored by Commíttee on Engíneering Geology.