2 Transportation Research Record 790

A Shale Rating System and Tentative Applicationsto Shale Performance

JOHN A. FRANKLIN

A "shale rating system" based on three properties-durability, strength, andplast¡c¡ty-is proposed. A shale sample is assigned a rating value by first mea-suring its second-cycle slake-durab¡lity ¡ndex. Rockl¡ke shales that have dura-bility values greater than 80 percent for th¡s index are further character¡zed bymeasuring the¡r po¡nt load strength. Soillike shales that have durability valuesof less than 80 percent are further character¡zed by measuring the¡r plast¡c¡tyindex. The shale rat¡ng, derived from these lest results by using a rat¡ng chart,¡s a cont¡nuously variable number ¡n the 0,G9.0 range. Tentative correlat¡ons(trend lines) are proposed that link this rat¡ng with aspects of engineering per-

formance such as excavating methods (e.g., whether tg d¡g or to blast), founda-t¡on properties (e.g,, bearing capacities and foundation moduli), embankmentconstruction (e.g., lift th¡cknesses and compaction methods), and slope stabil-ity (e,g., relat¡ons between slope he¡ght and angle and failure mechanisms).

Shales constitute about one-third of the rocks inthe land surface of the earth and about one-half byvolume of all sedimentary rocks (1). Not sur-prÍsingly, they are common in engineering projectseither in their excavated form as construction mate-rials for shale embanknents or in their natural andundisturbed state--for exanple¡ in foundations, cutslopês, and underground works.

In spite of its abundance, this important rocktype has until recently received littIe attention.In some ways, it is an unattractive and difficultmaterial to study becâuse it is easily disturbedduring driIling, sampling, and specimen prepara-tion. The strength, deformabilityr and other char-acteristics of a l-aboratory test specimen can changeby orders of magnitude if the rock is allowed to dryout, shrink, or svrelI. A further experimental prob-lem is that, whereas the minerals and microtextureof most rocks can be studied easily by using stan-dard opticâI methocls, extremêIy fine-grained clayminerals require techniques such as electron mi-croscopy or X-ray diffraction.

Shâtes also vary greatly ín their proPerties andbehavior. At sorne locations' shale slopes stand fornany years at near-vertical angles, whereas atothers even 10-20o slopes suffer from continualerosion and creep. This has led to a distinctionbetween "clay shales", the softer and more soil"liketypes, and "indurated shales", which, because oftheir greater cenentation and compâction, behavemore Iike harder rocks. The practice of treatingshates ås either a soillike or rocklike nateriaL hasbeen carried into construction specifications, wherepayment has often been based on a distinctionbetween soil and rock. Problems have occurred withshales of intermedÍate quality that behave neitheras soíl nor as hard rock and require specialtreatment.

There is a clear need for a shale classificationsystern that is capable of distinguishing a1l gradesand qualities of shale and allows a correlationbetwêen the type of shale and its performance onengineering projects. In a three-year researchprogram sponsored by the Ontario Ministry ofTransportation and Communications (MTC), a shalerating system has been developed for this purpose(21. A rating nu¡nber R is assigned to a shaleaccording to measurements of the three propertiesconsidered fundamental to distinguish one shale fromanother: durability, strength, and plasticity.Tentative correlations have been developed betweenthe rating number and aspects of engineering perfor-

mance such as excavating methods (e.9., whether todig or to blast), foundation properties (e.9.' bear-ing capacities and settlenents), embankment con-struction methods (e.9., Iife thicknesses and choiceof co¡npaction equipment), and slope sLability (e.9.,relations between slope height and angle and mech-anisns of failure in different Èypes of shale). Thêsuggested correlations are based on linited data,and their value and accuracy will improve with useand experience. Neverthelessr it is believed thatin their present form they serve to illustratetrends of behavior and will stimulate furtherresearch into the performance of bhis inportantgroup of materials.

HISTORIC BACKGROUND

S ize-Strength Classif ication

Before considering the subject of shale charac-terization, it may be helpful to discuss briefly theclassifícation of rocks in general. Of the manycharacteristics of a rock mass, two in particularappear to be important j.n determining rock-nass be-havior in engineering works: (a) the size of blocksinto which the rock mass is divided by intersectingsets of joints and other discontinuities and (b) theintrinsic strength of these blocks. This "size-strength" classificat.ion has been applied, for ex-ample, to the design of rock tunnels (4'5) as abasis for predictíng excavation and supportr equi r ements .

The size-strength classification systern is shownin Figure I. Strong, nassive rocks plot to the topright of the diagrarn, whereas weak, broken rocksplot to the lower left. The diagran can be con-toured to show classes of rock quality. Evidently,the high classes to the top right represent rock-¡nass conditions that require ¡ninimal support yet aredifficul-t to excavatei i.e., they may require blast-in9. The lower-quality naterials towartl the lowerleft can, conversely, be excavated by rippers,shovels, or front-end l-oaders, but slopes or tunnelsin these materials tend to be less stable.

This sínple, two-para¡neter classífication systemcan be criticized because it ignores a number ofproperties that have an inportant influence onrock-mass behavior--for exanple, the frictionalcharacteristics of rock joints. Sone classifica-tions, such as that published by Bieniawski (6) andBarton (7), include a greater number of classifica-tion parameters and as a result are somewhat moredifficult to apply. The two-pararneÈer approach hasbeen found to be a useful starting point and onethat is readily conprehended and usecl.

The size-strength classification is insufficient,howêver, when applied to shales or other rocks ofli¡nited durability. A sa¡nple of shale excavatedfrom the rock mass initially plots at a single loca-tion on the cliagran¡ this location depends on thesize and strength of rock fragnents. When the shaleis exposed to weathering, however, it becones weakeror breaks down to snaller-sized fragments. The ef-fects of short-term weathering processes can berecorded on the diagram in the form of vectors thatrepresent weakening, disintegration' or a

Transportation Research Record 790

Figure 1. S¡ze-strength classificat¡on for rock masses'

conbination of the t$'o processes. Different shalesvary in their susceptibility to short-termweathering agencies, and a neasure of thissusceptibitity is essential in characterizing shalenaterials for engineering projects.

Tests for Shale

Some shales can withstand many cycles of wetting,drying, or frost; others soften or break down afteronly a short perio¿l of exposure. Much research hasbeen devoted to rnethods of assessíng the durabilityof a given shale (8-10). Early tests were qualita-tive, relying on the irnmersion of a sample of shalein water and on visual descriptions of the resultingbreakdown. Attempted quantitative testing nethodswere generally nore complex, reguiring many cyclesof freezing or immersion in water or saltsolutíons. Attenpts by Franklin and Chandra (1I) todevelop a simpler, yet meaníngfu1 and reproducibletest ultirnately led to the development of a l-o-minslake test ín water, the "slake-durability" têst.This test relies on a cornparison of dry weightstaken before and after slaking in a rotatingopen-mesh drun. fn the slaking process,disintegrated fragments are allowed to pass throughthe sieve ¡nesh of the drum. In spitè of theäpparent crudeness of the testing procedure,reproducibility is typically !2 percent foridenticat tesU samples. As a result of extensiveresearch using the slake-durability testr Ganble(I2) recomrnended that the second-cycleslake-durability index be used as a standard forclassification purposes. This proposal has beenincorporated in a "suggested method" by the

International Society of Rock Mechanics (ISRItl).Gamble (ff) also proposed a shale classification

based on a co¡nbination of slake-durabil-ity andplasticity indices. This classification can becriticized, howeverr in that plasticity is only a

relevant property for the nore soillike shales andis difficult or impossibJ.e to measure when the shalehas a rocklike consistency. Furthermore' Ganbl-ersclassífication based on slake durability andplasticity is subdivided into classes of material byway of discrete but arbitrary boundaries, wherèas ã

classification or nrating" in the form of a

continuously variable number woultl seem to be morearnenable to correlations with fiel-cl performance.These limitations led me to develop the alternativeshale rating systetn describecl below.

SHALE RÂTING SYSTEM

The proposed shale rating system is shown in Figure2, the "shale rating chart". A sample of shale isgiven a rating number on thê basis of (a) its slakedurability an¿l strèngth if the shale is rocklíke andhas a slake-durability index greater than 80 percentor (b) its slake durability and plasticity if theshale is soitlike and has a slake-durability indexless than 80 percent.

The ratíng chart is subdivided by lines thatracliate at 2o intervals fro¡n the top center of thediagran to give rating values in the 0.0-9.0 range.By interpolating between the lines, a shale can berated to one (and, if necessary, two) deci¡naIplaces, which permits a continuous and quantitativec lassi fication.

Samples are initially subjected to the slake-dur-

o

HNH

oo¡m

UNIAXIÀL COMPRESSIVE STR¡NGTH (MPa)

4

Figure 2. Shale rating chart,

PLASTICITYINDEX

Ip Ilercent

Transportation Research Record 790

POINT LOAD

STRENGTHa - (MPa)

s5U

1.0

0.8

0.6

10

I

SLAKE-DURABILITY INDEX Id2 (percênt)

ability test to assess their second-cycle slake-durability index, Id2 percent, in accordance i.¡ithISRM recomnended procedures. If this index is foundto exceed 80 percent, the sample is further testedto measure the point-load-strength index (-IL-l_t).If the index is less than 80 percent, the fractionpassing the slake-durability test drum is subjectedto conventional Atterberg-limits determinations toevaluate plasticity index.

The point-load-strength test has been found to beconvenient for strength classificatíon of rocks ingeneral and of shales in particular. It requires nospecimen preparation or nachining and can be con-ducted in the field before the rock has had a chanceto dry or break up. The index used for rating pur-poses is the strength obtained r¡¡hen the loäd is ap-plied perpendicular to the bedding planes--i.e., thestrongest direction. Supplenentary measurements canbe made with the load appliecl parallel to the bed-ding planes to measure strength anisotropy and "fis-sility". Samples are tested at their natural, nois-ture content. Point-load-strength values have beenfound to correlate closel-y with those obtained inthe uniaxial compressive strêngth test. For clas-sification purposes' uniaxial strengths can be ob-tained by applying a factor of 24 to the point-1oåd-strength values.

Figure 3 shows the test results obtained forsamples of 'shales collected in Ontario as part ofthe current research program. The results have beensubdivided according to the geologic age of thêfornation tested. It can be seen thât olderformations, as expècted, are general-Iy stronger andrnore durable and have higher rating values. Perhapsthe ¡nost chåracteristic feature of thís diagrarn,however, is the consíderable scatter in durability,strength, and rating values for the najority offormâtions. The scatter reflects real differencesin shale properties as a result of differing degrees

of Iithificâtion and of in-situ weathering.Evidently the character of these materials differssignificantly from place to place throughout theprovince and even from bed to bed within a singlefornation. The index test results therefore giveimportant additional ínformationr and thecharacteristics of these shal-es cannot be inferredfro¡n rock or formation names a1one.

It may be noted that ontario shales are generall-ymore durable and stronger than âverage shaleselsewhere. This is clearly related to geologic ageas the data assembled by Patrick and Snethen Gg)show (see Figure 4). À review of the percentage ofexpansive cJ-ay present in rocks of various agesclearly shows a marked increase in the expansiveclay nineral content of rocks younger than Devoníanage. onl-y the older shales outcroP in ontario,typically with contents of montrnorillonite and otherswelling clays in the 0-5 percent range. To find"worse" shal-es in canada, one has to go west to thepraírie provinces, where Cretaceous or youngershales with swelling mineral contents in the 20-40percent range are conmon. Even higher percentagesof such swelling ninerals are found further south orwest--for example, in the Oligocene and Mioceneclaystones of Texas or the Miocene-Eocene claystonesof the Pacific coast of Cal-ifornia.

CORREI,ATIONS IIITH FIELD PERTORMANCE

GenerâI Corilnents

To be of value, a classífication system should bereadily correlateil with the behavior of rockmateriâIs observed in construction projects. Un-fortunatelyr if a classification systen is new' sucha capability for correlation with field perfornancewill be limited by users' Iack of exPerience vriththe systern. This is true in the present case, where

20 40 60

SHALE RATING CHART

Note: t MPa= 145lbllin2.

Trânsportation Research Record 790

Figure 3, Relation between shale age,rat¡ng, and ¡ndex properties.

Figure 4, Est¡mates of percentage ofexpans¡ve clay present in Precambrianthrough Pl¡ocene-age rocks.

AGÊ

Pl iocene

Mi ocene

0l j gocene

Eoc e ne

Upper Cretaceous

Lower Cretaceous

Jurassic

TriassicPermian

Pennsyìvanìan

Upper lljssissjppianLor,/er l4i ssi ssi ppian

Devon i an

Si'lurì an

0rdovician

Camb ri an

Precambni an

PT RCENT

65

65

50

40

40

20

20USASHAL ES

the atternpted correlations take the form of rrtrend

lines" only and rely on inferred as well as actualdata. Seldon were the properties of durability'strength, and plasticity found to have been reportedsimultaneously for a particular shale. Gaps in thedata were filled by a subjective assessment based onpublished descriptions of shale character and indexproperties. The proPose¿l trend lines should there-fore be taken onfy as approximate indications ofshale behavior and should not be used for designhrithout further cross-checking. APplication of therating system to three areas of rock engineer-ing--embanknents, slopesr anil foundations--ís dís-cussed below.

Shale Enbankments

MTc specifications define nearth embankments" asbeing constructed in Layers of loose lift thickness'usually 200 mm (8 in) ' compacted to 95 percent ofASII4 D698 maxi¡num dry densíty. rRock embanknentsn,on the other han¿I, are placed by end dumping ín nuchthicker lifts and with only nominal conpaction.

CANADIANSHAL T5

This distinction between nearth" and "rock" canlead tó serious constructíon difficulties and todefects in the completed e¡nbank¡nents. The successof attempts to achieve a specified conpacte¿l densitydepenils on the character of the shale, the selectionof compaction equip¡nent and techníquesr anél theappropriate natching of equipnent ancl techniques tocharacteristics. The objective is to achieve the¡naxi¡num shale breakatown cluring construction so as tominimize breakdown, or "degradation" ' during thesubsequent service life of the embankment.End-result specifications have generally been foundto be inappropriate for the construction of shaleembank¡nents' and the trend is to replace these byprocedural, specifications relaÈed to shalecharacter. fn Ottawa, for example, the specialprovisions of a recent contracÈ called for the useof sÈatic conpactors with tamping- or peg-foot dru¡nsto be followed by steel ¿lrum units, a conbinationthat v¡as found to be most effectíve for the harderand more clurable shale rnaterials encounterecl on thatproject.

Table f gives a tentâtive correlation between the

20

40

30

40

5

;lr5 ¡.5 I

"5J

ONTARI O

SHALES

I4AP REFEREIICE

NUMBER ANDGEOLOGICAL AGE

SLAKE OUIìASILITYINDEX PERCENT

1d2

0 20 40 50 B0 100

Pt)INT LOAD STRTNGTH

INDEX (I4PA) IS.^f,U

2 .4 .6.41 2 4 6 Blo

SHALT RATINGR

1357,ì

I 4.20 DEVONIAN

ll-.]3 slLUnlntt

ORDOVICIAN( Queen s ton )

ORDOVICIAN(Georgìan Bay)

ORDOVICIAN(l"lh i rby )

t)RDOVICIAI.I(ctrazv )

PRECAI'1BR I ANl-4

a

.t . .

.3.L.

a a aa a aa

a

....1aaaaaoa

\.r. ..f.\..

+r

Method orEquipment

6

Table 1. Excavation capab¡l¡ties of varíous methods and types of equ¡pment as

a function oÍ the characteÌ of an interbedded shale'and hard-rock sequence.

Thickness of Limestone Bed(mm)

Transportation Research Record 790

Trends in e¡nbankment side slopes, as a functionof embanknent height and the quality (rating) ofshale construction materíals, are shown in Figure6. A general increase in side-slope angle isapparent wiÈh increasing shale rating, to a naxinu¡nof approximately 35o (I.5:1) for shal-e rock filIsthat, have ratings in the 8.0-9.0 range. side-slopeangle is also affectecl by enbankment height. Smallembãnknents ftypicalty 5-10 m (17-33 ft) in heightlgenerally have flatter slopes for ease ofmaint,enance. As embanknent height íncreases to15-20 n (50-65 ft), it becomes uneconomical" todesign an e¡nbankment with flat slopes, and t,heslopes are generally steepened to the maxirnun thatcan be tolerated safely, based on geotechnicalconsiderations. For high enbankments t20-30 m

(65-100 ft) L the side-slope angl-e progressivelydecreases; this reflects the growing inportance ofernbankment stability and the need to rnaintainacceptâble safety factors.

E'nbankments of significant height are designed byusing the standard soil-mechanics method of Iimitingequilibrium. Calculations require an estimate ofshear-strength parameters for the compacted shalematerial. Figure 7, which is based substantial-Iy ondata by Strohm and others (17), indicates that asshale rating increases the shale fitl becomesprogressively more frictional until, at high ratingvalues (R > 8.0), the shale behaves essentially asa granular fill with limited cohesion and with anangle of internal friction of. >25o.

Embankment permeability is another inportantparâmeter to be estimãted for design. A review ofpublished field-test data is surnmarized below:

fype of MaterialSha1e rock fill-Durable shale

fi 11Moderately durabl-e

shal-e fillweII-conpacted

clay shales

Rating Permeability (m,/s)8-9 r0-3-10-5

7-g ro-s-10-65-7 10-6 -10-74-5 r0-7-10-8

0_4 10-8_10-¡ 2

Permeability values of the conpacted fill range frornI0-3 to 10-s m/s (300-3 fE/daVl for a shalerock fill to as lov¡ as 10-8 to I0-r 2 m/s(3x10-2 to 3xI0-6 ft,z¿lay) for well-compactedshales. Enbankment permeability wilI control theacceptable rate of embankment construction if thedevelopment of excess pore-water pressures is to beavoided. It will also govern lateral drainagethrough the embankment after construction iscomplete. Perneability will generally decreaseduring the life of the embanknent as a result ofshale degradation and the filling of void space.

Cut Slopes in Shales

The long-term stable angle of a slope in shale canvary from about 80 to almost vertícal depending onthe durability of the shale material. Differentsl-ope-failure mechanisms occur in shaLes that havedifferent rating values.

In shales of low durability (ft = 1.0-5.0) ¡

mechanisns of slaking, erosion, and surface creeppredoninate as they do in clãy enbankments.Unprotected steep slopes exposed to continualerosíon by surface runoff r,rater devefop a pattern oferosion gulleys. The sur'face layer slakes, and thedebris is renoved by erosion as fast 'as it isproducêd. Although there is usually no safetyhazard associated with this mechanism, periodiccleaning of ditches is required, and the appearanceof the exposed eroded rock can be unattractive.Slopes that are protected from continuous erosion

Shale LimestoneRatine (%) Average Maximum

Backhoe or scraper 0.0-5.5Shovel 0.G5.5Medium ripper 3.0-6.0Heavy rìpper 3.0-7.0Blasting 6.0-9.0

Note: I mm = O.O39 in.

character of a shale-lirnestone fornation and thelike1y excavation requirenents for borrowmaterials. Ease of èxcavation is governed by alinited number of geologic characteristics. Whenthe borrow is entirely of shaler the key propertiesare likely to be the strength of the shale(reflected by its rating) and its natural "bl-ocksizer' (governed by the spacing of joints and beddingplanes). Iilhen, as is often the case, the shale isinterbedcled with a harder rock such as limestone,the ease of excavation will be greatLy affected bythe percentages of hard rock in the total rock to beexcavated' by the strength of the hard rock, and bythe average ancl maxi¡num thicknesses of the hard-rockbed. Table 1 dravrs on experiênce in southernOntario' where the shales are co¡nmonly interbedded$rith dolomite or limestone that has a uniaxialconpressive strength of 150-200 It{Pa (20 000 to30 000 Lbf/in2l. The table gives â generalindication of the performance of various classes ofexcavating equipment ancl draws attention to theimportance of quantifying the percentages andthicknesses of hard-rock inclusions in a nixed-rockfor¡nation. Additional variables shoulal beconsidered--for exanple¡ the depth of excavation andthe dip of bedding planes. IdeaIIy¿ the limitationsof each make and model of excavator should bedefined in relation to the controlling rockcharacteristics. Inclirect nethods of predictingease of excavation--for example, the use of seismicvelocities--are unlikely to be as reliable as directobservation of key properties such as those note¿l inTab1e 1.

Figure 5 shows trends in optinun lift thicknessand compacted fíeld density as a function of shalerating conpiled from data by Lutten (Iq). GreaterIift thicknesses can generally be allowed for shalesthat have a higher rating. Shales that have ratingvalues in the range of 5.0-8.0 (slake durabilitygreater than 80 percent) can be effectiveJ-ycompacted in lifts of 500-800 rnm (20-30 in) ifappropriate compaction nethods are used. Theseshales behave substantially as rock fill, retaininga percentage of interfragment void space even aftercompaction. Shales that have rating values of lessthan 5.0 require a reduced lift thickness tofacilitate cornplete breakdown of these less durablenaterials. The degree of breakdown achieved ínpractíce can be assessed from the lower half ofFigure 5.

Low values of compacteal density¡ in the range of1.8-2.0 t4g/m' (IL2-I25 lb/ft3 r, are rypical forplastic cl"ay-shales that retain water between claynineral grains. The highest densities, in the rangeof 2.0-2.2 Mglms (I25-I37 1b,/ft3 ) , are achievedwith internediate-rating shales that are relativel-yeasy to break down and cornpact. Fíeld densitiesagain fall to 1o9¡er values for the ¡nore rocklikeshales with a rating of 6.0-9.0 because of theretention of sígnificant void space between shaleblocks in the fill.

<5<10<20<30

<20 <50<50 < 100<75 <125

< 100 < 150No limitations

Transportation Research Record 790

devel-op a vreathering profile. The thickness of thevreathered shale l-ayer may reach 9-13 m (30-43 ft)Ifor exanple, in london Clay (England)], althoughthicknesses of I-2 n (3.3-6.5 ft) are more conmon inshales of higher durability. The rnantle of

Figure 5. Tentative correlat¡ons between shale quality, liftthicknesses, and compacted densities.

LtrFTTHICKNESS(m)

Figure 6. Trends in embankment slope angle as a function ofembankment he¡ght and quality of shale fill.

7

weathered shale tends to be unstable and to creepdownhill or slide along the contact with the freshshale. Shallow slab slides typically occur atintervals of 5-10 years when the slopes are steep,exposing fresh shale to further weathering and

4.O 5-O 6.0SHÄ.LE RATING

Enbankment HeighÈ (n)15 20

900

800

700

600

500

400

300

Eúbankrent Height (ft)

I

Figure 7. Trends in shear-strength parameters of compactedshale fills as a function of shale quality.

repetition of the cycle. Instability of the surfacelayer is encouragêd if undercutting occurs at thetoe of the slope--for example, in river embank-nents. It is also accentuãted by water percolationand frost action along the contact betvrêen weatheredand unweathered naterials. The surface layer isusually more clayey and less permeable than the un-derlying rocks and thus traps water. Freezing ofthe 1ayêr adds to this damning effect. Eventually,a clay slope will reach a stable angle equal to ap-proximately half the residual angle of shearing re-sistance of the material (I8). Since, in engi-neering projects, it is seldorn practical to designcut slopes this fÌat (e.9., 10o), one nust rely tosome extent on cohesion and cementation of theshales to ¡naintain steeper angles over at leastdecades. In addition, slope stabilization measuresare used to improve and maintain stability.

Superficial instability occurs as a result ofdifferent nechanisms in shales that have a mediun tohigh rating, such as those of northern Ontario.Wetting, drying, and particularly frost action re-sult in the fragmentation and loosening of cut-slopefaces so that large blocks break down into s¡naIlerfragments. For exanple, the Manitoba Department ofHighways reports that the Odanah shales of thatprovince are capable of standing vertically torelatively great heights but are cut back to slopesof less than 1.5:1 because, e¡ith steeper slopes,blocks of hard shale continually break off. Itappears that a recent project that used I:1 backslopes will require an annual ditch-clearing program.

The breaking action of frost results partly frornthermal contraction and expansion, accelerated bythe wedging action of ice in microfissures andjoints. AIso contributíng are "fossilizecl" stressesin the rock, which typically reach magnitudes of6-15 MPa (1000-2000 lbf,/in2 ) in the near-surfacerocks of Ontario, Quebeç, and northern New YorkState.

Deep-seated slope failures are generally morecomnon in shales that have lower ratings. In theseshales, the sliding surface nây pass through intactshale material and there may be only li¡nitedinfluence from preexisting bedding and jointing. In

Transportat.ion Research Record

SHAI I RATING R

790

l5 20 25

ANGLT OF INTERNAL FRICTION Ó, (DËGRTES)

the harder' more durable shales, slope failures areinvariably controlled by the orientations ofpreexistíng discontinuity sets. wedge or planarslides are bounded by slicling surfaces coincidentwith preexisting joints and bedding planes.

Figure 8 shows a relation between the stableangle of a slope cut in shale and thê quality of theshale material. If the bedding and jointingorientations are favorable (i.e., they dip into theslope face and therefore have Iitt1e influence onstability), the upper-bound curve of the shaded areain Figure 8 applies. It represents the probabJ.emaximum stable angle where failure must occurthrough intact shale. Near-vertical angles can bereached for a rocklike shale that has a rating inthe 8.0-9.0 range. There is likely to be apronounced increase in the gradient of this curve inthe 7.0-9.0 range to accommodäte the very steepslopes that are possible in rocklike shales. Thelower-bound curve of the shaded area in Figure Irepresents stable slope angles where slope stabilihyis governed by joints "daylightingÍ in the slopefacê. It has been assu¡ned that the slope will standstable at an angle close to the friction angle ofthe joint or bedding plane. When the joint is tightand clean, its friction angle depends on thestrength of the intact shale of the joint walLs andso increases as a function of shale råting. Theconvergence of the upper- and lorl'er-bound curves ofthe shaded area toward the left side of the figurerefl-ects the comparatively ninor êffects of jointingin weak and plastic shale ¡naterials.

The line .ät a constant angle of approximately8-10o in Figure 8 illustrates the potential- effecton sLope stabíJ-ity of the presence of joints filledwith soft and plastic clay. When these joints arepresent at adverse orientations, they govern thestability of the slope irrespective of how strongand durable the shale elsewhere within the slope naybe. It is therefore inportant to identify the lreak"clay rnylonite" sheared horizons that are oftenpresent in shale formations. These are difficult toobserve, since they are often thin and similar incolor to the host rock.

d

u 100

o

-o

Figure 8.shale.

Transportation Research Record 790

Trends in stable cut-slope angle as a function ofthe character of

Shale Foundations

The canâdian Foundation Engineering Manual (19) de-fines rock as a rnaterial that has uniaxial compres-sive strength greater than I MPa (145 lbf/inz) andcannot be dug by hand with a shovel or pneunatícspade. Shales clearly straddle this arbitraryboundary between soil and rock. A principal featureof foundation shales is their variability in uni-axial compressive strength' which ranges from Iessthan I MPa for shãIes with the consistency of stiffto hard clays to as high as 10-100 MPa (1450-14 500I|>f/!nz ) for the high-rated shales and ar-gillites. The susceptibility of shales to weather-ing is usually manifested as an increase of strengthwith depth. This may be gradual or nay occur as anabrupt contrast betvreen the soft' discolored, weath-ered-shale horizon and the unweathered or ufresh"underlying strata. The softening of shale towardthe surface is further aggravated, from the point ofview of foundation behavior, by a decrease in blocksize and bedding-plane spacing. In addition tosoftening, the shale weathers by splitting and byfragmentation. Modulus variations with depth areclearl-y illustratecl by the results of pressuremetertesting (see Figure 9). In shales, a pronouncedanisotropy ís ä1so evident, and this leads todeformability normal to bedding bèing much greaterthan defor¡nability in the bedding direction.

Al-lowable beâring pressure is generallycontrolled by, and can be estimated from, the intactrock strength and the intensity of jointing orbedding (the size-strength parameters discussedearlier). An empiricãl coefficient that reÌates theallowable bearíng pressure to uniaxial compressivestrength is defined in the Canadían FoundationEngineering Manual (19) in terns of ratios offracture spacing to footing width and of jointaperture to joint spacing. rn view of dífficultiesin making field measurements of joint aperture' thecanadian nanual defines three values for theernpirical coefficient that depend only on majorvariations in the spacing of discontinuities: verywide, wide, and moderately close.

These recommendations have been

SHALE RATI NG

graphícally in Figure I0 (19). The contours in theupper right of the diagram apply to the lessfractured and stronger shales and illustrate anexpected reduction in allowable bearing pressure asthe shale becones \4¡eaker, nore thinty bedded, ormore closely jointed. The Canadian manual issomewhat ambiguous in its treatment of the ¡¡eakershales, since recommended bearing pressures forshales with widely spaced joints compute to lowervalues than those recommended for clays with similarstrengths. It might be more realistic if thecontours reflected a continuous trend from shalethrough to clay and there vrerè a gradual decrease incurvature as the nåterial became softer and lessinfluenced by the presence of joints and fissures.The recommendations of the Canadian manual include asafety factor of 3. However, a much greater de-gree of conservatism is likely. Experimental valuesof foundation strength often exceed normally usedval-ues of foundation bearing pressure by factorsfrom 5 to 50.

Foundation ¡nodulus is generally only relevant tothe design of heavíIy loaded structures such as dansand high-rise buildings on shale foundatíons.Figure 1I (20) shows that the foun¿lation modulus ofargillaceous rocks generally increases fron l-0 to10 000 MPa (1450-1.4 nillion lbflinz ) as thecharacter of the ¡naterial inproves fron a normallyconsolidated clay to an indurated' high-durabilityshale. The ratio of modulus to compressivestrength, however, is approxirnately constant in the5O-200 range, typically I00. Foundation modulus,like bearing capacity' is influenceal not only by thestrength of the rock materíal but also by theintensity of jointing in Èhe foundatíon. A rrnass

factor" (J) has been defined that refates intensityof jointing to the ratiÕ between field andlaboratory deformability values. By using J and a¡nodulus ratio of L00r one can construct Figure 12,which relates field defornability moilulus to thesize-strength rock classification. Às jointingbecornes more intense, the field modulus is reclucedby joint compressibility until' for very closelyspaced joints, the modulus appears to approach a

limiting value. The effect of jointing on ¡nodulusplotted

t0

Figure 9. Menard pressuremeter test results in shale showing DEPïHprogressive increase in modulus of deformab¡lity w¡th increasing (m)

depth below surface.

Figure 10. Trends in allowable bearing pressure (shallowfoundationsl as a funct¡on of rock strength and discontinuityspacing.

Transportation Research Record 790

PRESSUREMETER ¡g¡tJ¡I.J5 E (I.îPA)

ALI,OWABLE BEARING PRESST]RE (MP8)

0.6 1.0 2,0 4,o 7,0 10,0

o.2 0.4STIFF h/ crlFFl pÀÞn v r.ôH I rôw I MEnTnM I HIGH

CI,AY ROCK

STRENGTII (MPa)

Note: 1 cm = 0.39 ¡n: 1 MPa = 145 thf/in2

frequency, and distribution of linestone strata (21).

ACKNOWLEDGMENT

I would like to thank the Ontario lvlinistry ofTransportation and Communications, Downsview, forfunding this project and for their active help andcriticisrn during the course of the investigations.The project r¡¡as carried out by Franklin Trow andAssociates, Ltd., of Rexdale' Ontario' Staffmembers who assisteil with the studies included O.Hungr, A. Pâvon, and E. I"fagni. The assístance andconnents received from geotechnical departments ofprovincial transportation authorities throughoutCanada are also grateful-Iy acknowledgeil.

DISCON -TINUITYSPACING(cm)

VTRY I,IIDE

l,llIDE

I'IODERATE LYCLOsE

_30

CLOSE

_5

VERY

CLOST

is nost pronouncêil in the block-size range of t0-I00clt (4-40 in) (cLose to moderately close jointspacing). The resul-ts may be translated intomodulus-depth variations by careful borehole orcaisson logging to measure joint spacings. Forexanple, in rock that has a laboratory strength of17 MPa (1465 lbfrzinz ) , when bedding is spaced at50 cm (19.5 in) near the surface and 150 cm (58.5in) at depth, one woul¿l expect the fíeld rnodulus tovary from 500 to 2000 MPa 172 500 to 290 000Lbt/ín21. These values for rock conditions andfox modulus are similar to those found in thefoundation of the Canadian National Tohrer inToronto, where settLenents were predicted on thebasis of an assurned nodulus of 3700 !1Pa (0.5 nillionlbf/in2 ) to take into account the presence,

Note: 1 m = 3.3ft; 1 MPa= 145 lbf/¡n2,

Transportation Research Record 790

Figure 11. Ratio of deformabil¡ry modulus to compresivestrength for clays, shales, and related materials.

Figure 12. Contours of field modulus of deformability (E¡lof shales as a function of uniaxial compress¡ve strength anddiscontinu¡ty spacing (assuming modulus ratio = 100),

REFERENCES

1. F.,1. Pettijohn. Sedimentary Rocks.Brothers, Nêw York, 1967.

11

Concrete

dÀ=

--oE-z

Arenaceous (coarsegraì ned) SedimentaryRocks

gilìaceous (fine graìned)Sedìmentary Rocks

2. Evaluation ol Sha1es for ConstructionProjêcts. Ontarío Ministry of TransportatiÕnand Communications, Downsview, R&D ProjectRept. 22303 (in preparation).

3. J.A. Franklin' E. Broch, an¿l G. walton.Logging the Mechaniial Charãcter of Rock.

l,leathered Argi ì laceous Rocxs

Overconsol idated Clays

Norr,ra ìly Consol idated Clays

Note: 1 MPa= 145lblliñ2,

't0

COMPRESSIVE STRENGTH, (MPA)

STRENGÎH (MPA)

Note: 'l cm = 0.39 in; 'l [,lPa = 145 lbf/in2.

Trans. r Instítute of Mining and Metallurgy(Great Britain), Vol. 80, l-971, pp. A1-49.J.A. Franklin. Safety and Economy inTunneling. Proc., 10th Canaclian Rock MechanicsSymposiurn, Kingston, ontario, vol. 1, 1975.J.A. Franklin. An Observation Approach to theSelection an¿l control of Rock Tunnel Linings.Proc., Conference on Shotcrete fot GroundSupport, Easton, MD, 1976, pp. 556-596.Z.I. Bièniâwski. Gèomêchânics Classification

DISCONTINUITYSPACING (cn)

VERY I,IIDE

.-5

VERY

CLOSE

Harper 4.

t

6.

Technical Guidelines for the Design and Constructionof Shale Embankments

ALBERT F. DiMI LLIO AND WI LLIAM E. STROHM, JB.

ln 1974, the Office of Research of the Federal Highway AdminisrÌar¡on ¡n¡t¡-ated a comprehensive research study to ¡nvest¡gate the causes of numerous,large-scale failures of shale embankments on major lnterstate routes in severaleastern states during the early 1970s and to develop appropriate remedies. TheU.S. Army Engineer Waterways Experiment Station was to conduct a threephase, five-year ¡nvest¡gat¡on of the shale problem and provide the necessaryguidelines to build safe and functional shale embankments at a reasonable cost.Phases 1 and 2 were to be completed ¡n one year and provide interim guidelinesfor the pract¡cing engineer until the comprehensive guidelines could be deúel-oped. Phase 1 ¡nvolved a state-of-the.artôurvey of des¡gn and construct¡onpract¡ces in use at that time as well as a survey of existing problem areas. Phase2 involved a similar survey of evaluation and remedial treatment techniques forex¡st¡ng d¡stressed shale embankments. Accomplishments from Phases 1 and 2prov¡ded the necessary foundation for the development (under Phase 3) of im-proved design criteria and construct¡on control techniques for both new con-struct¡on and existing problem areas. The development of the improved guide-lines is described, and the h¡ghl¡ghts of the major research results are presented.

The Federal Highway Adninistration (FHIVA) recentlypublished a conprehensive engineering nanual thatprovides technicãI guidelines for the design andconstruction of shale embankments. These guideJ.ines

were developed for FHWA by the U.S. Army EngineerI{aterways Experiment Station (t{ES) at vicksburg,Mississippi. This paper presents the salient pointsof the ¡nanual- and also highlights some of theprominent events that preceded the investigation bythe WES researchers. So¡ne of the proninent findingsthat guided the researchers during the early stagesof the investigation are also discusseil in order todelineate the basis for sone of the gui¿lelines thatwere devel-opeil. Many of these guidelínes were takenfrorn other federal agencies ancl so¡ne state highwayagencies.

The research study was initiated in 1974 as athree-phase investígation. Phases I and 2 v¡ereconducted concurrently during the first year of thestudy to províde preLiminary guidance to statês thatwere sÈruggling with inadequate guidelines foreorrêcting existing failures, evaluating potentialfailures, and constructing new shale embankrnents.Phase 3 involved the evaluation of existing guide-lines and the development of improveil guidetines for

Rock

I2

of Rock Masses and Its Application inTunneling. Proc., 3rd Internâtional Congresson Rock Mechanics, Denver, VoI.2a,1974, pp.27-32.

7. N. Barton, R. Lien, and J. Lunde. EngineeringClassification of Rock Masses for the Design ofTunnel Support. Rock Mechanics, Vo1. 5, No. 4,1974, pp. 189-236.

8. G.w. DePuy. Petrographic fnvestigations ofRock Durability and Comparison of Various TestProcedures. Journal of American Assn. ofEngineering Geology¡ VoI. 2, 1965, pp. 31-46.

9. N.R. Morgenstern and K.D. Eigenbrod.Classification of Argillaceous Soils andRocks. Journal of Geotechnical EngineeringDivision, A¡nerican Society of Civil- Engineers,New York, Vol. 100, No. GTI-O., L974, pp.1I37-l-156.

I0. J.H. Shamburgèr, D.M. Patrick, and R.J.Lutten. Design and Construction of ConpactedShale Enbankments: volume l--Survey of proble¡nAreas and Current Practices. Federal HighwayAd¡ninistration, U.S. Department of Transporta-tion, Rept. FHWA-RD-75-6I, 1975.

1I. J.A. Franklin and R. Chandra. The Slake-Dur-ability Test. International Journal of RockMechanics and Mining Sciences, Vo1. 9t 1972,pp.325-341.

12.,I.C. camble. Durabitity-Plasticity Classifica-tion of Shales and Other Argíllaceous Rocks.Univ. of l11Ínois, Urbana-Champaign, ph.D.thesisr 1975.

13. E. Broch and J.A. Franklin. The point-LoadStrength Test. International Journal of RockMechanics and Mining Sciencesr Vol. .9, 1972,pp. 669-697.

14. Suggested Method for thê Poiñt Loäd Strength

Transportation Research Recor¿l 790

Test, rev. International Society ofMechanics, Lisbon, Portugal, 1977.

15. D.M. Patrick and D.R. Snethen. An occurrenceand Distribution Survey of Expansive Materiatsin the United States by Physiographic Areas.Federal Highway Administration, U.S. Departmentof Transportation, Rept. FHWA-RD-76-82, L975.

l-6. R.J. Luttên. Design and Construction of Co¡n-pacted Shale Embankments: Volume 3--SlakingIndexes for Design. Federal HÍghway Adminis-tration, U.S. Departnent of Transportation,Rept. FHWA-RD-77-I, 1977.

I7 . W.E. Strohn, G.H. Bragg, and T.¡rl. Ziegler.Design and Construction of Cornpacted Shale Em-bankments: Vol-ume s--Technical Guidelines.Federal Highway Administration, U.S. Departmentof Transportation, Rept. FHWA-RD-78-l4, 1978.

18. L. Bjerrum. Progressíve Failure in Slopes ofOverconsol-idated Plastic Clay and Clay Shales.Proc., Journal- of Soil Mechanics and Founda-tions Division, American Society of Civil Engi-neers, Nêh' York, VoI. 93, No. SM5, I976, pp.1-49.

I9. N.B. Hobbs. Settlemênt Õf Foundations onRock: ceneral Report. proc., BritishGeotechnical Society Conference on Settlementof Structurês, Cambridge, Engl_and, 1974,

20. E.K. Robinsky and J.Ð. Morton. FoundationInvestigation for C.N. Tower, Toronto.Presented at 26th Canadian Soil MechanicsConference, Toronto, I973.

2I. Canadian Founilat.ion Engineering ManuaI: partst-4. Cãnadian ceotechnicâI Society, ¡4ontrêal-,1978.

Publication of thìs paper sponsored by Commíttee on Engineeríng Geology.