Indraratna,B_Large-scale Triaxial Testing of Greywacke Rockfill_1993

8/10/2019 Indraratna,B_Large-scale Triaxial Testing of Greywacke Rockfill_1993

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Indraratna, B., Wijewardena, L. S. S. Balasubramaniam, A. S. (1993) GLotechnique 43, No. 1, 37-51

Large-scale triaxial testing of greywacke rockfill

B. INDRARATNA,* L. S. S. WI J EWAR DENAT and A. S. BALASUBRAM ANIAMI

This Paper describes the results of a series oflarge-scale triaxial tests conducted on grey-wacke rockfill, used in dam construction inSouthern Thailand. The tests are conducted atlow to moderate confining stresses to relatetheir findings to the stability of rockfill dams.Considering the current test results in conjunc-tion with previous laboratory data, revisedfailure criteria for rockHI are proposed in non-dimensional form. For both low and hieh con-

fining stresses, lower and upper bounds ofstrength envelopes have been established, basedon a wide array of granular materials. Theinfluence of the coofining stress on the shearstrength of rockfill is studied in depth, and theimplications of a non-linear envelope at lownormal stress levels on the stability of rockfilldams are discussed. Although two parallel rock-fill gradations for specimens compacted tosimilar porosities are considered, the exact roleof particle size effect on shear strength is notexamined in detail, as the difference inmaximum particle sizes tested in this study isnot sufficiently large.

KEYWORDS: dams; deformation; failure; laboratorytests; shear strength.

Cet article d&it les rbultats dune suite dessaisau triaxial zi grande &helle rCli&s sur du remblairocheux (greywacke) &iii& pour la construction debarrages dans le sud de la Thailande. Ces essaissont ex&cuti?s P des pressions de cellule faibles $mod&es pour rapprocher les rbultats de la stabili-ti de barrages P enrochement. En considitrant lesrbultats existant dessais, en conjonction avec lesdon&es de laboratoire pr&&dentes, des crit&es de

rupture de lenrochement sont proposb dans uneforme sans dimensions. Pour des pressions decellule faibles et ClevCes, les enveloppes basses ethautes de r&stance ont Bt6 btablies, ba&s sur unvaste ensemble de mati?riaux granuleux.Linflwnce de la pression de cellule sur la r(?sis-tance au cisaillement est Ctudi&e en profondeur etles con&quences dune enveloppe de rupture nonlinkaire d containtes normales faibles sur la stabili-ti de barrages P enrochement sont disc&es. Bienque deux gradations parallkles denrochement pourdes 6chantillons compact& i des porositb sem-

blables aient Cti consid&es, le r61e exact de I effetde la taille des grains sur la r&stance au cis-aillement nest pas examine en d&tails car la diff&rence entre les tailles maxis des grains test& pourcette etude n&ait pas suffisante.

INTRODUCTION

Greyw acke rockfill has been used in the construc-tion of several d ams in Thaila nd, including therecently completed Chiew Larn Dam . The Elec-tricity Gene rating Autho rity of Thaila nd has pro-posed the construction of similar dams in severalparts of Tha iland in the future. D etailed testing ofgreywac ke rockfill has not been carried out in thepast: proper u nderstanding of the shear strengthproperties of this material should enable betterdesign of large rockfill dams in the future. Grey-wacke found in Southern Thailand is dark grey toblack in colour, and can be classified as a toughsedimentary rock, formed under unstable geo-logical environments. It is generally foundwithout internal stratification or parting. A studyof thin sections indicated that qu artz is its domi-

Discussion on this Paper closes 1 July 1993; for furtherdetails see p. ii.* University of Wollongong, Australia.t Asian Institute of Technology, Bangkok.

37

nant mineral, and feldspar and mica are the othermain constituents. The mean uniaxial compres-sive strength determined using eight NX coresamples was found to be of the order of 13.5 MPafor saturated specimens.

In the past, emph asis has been placed on tri-axial tests conducted at high pressures, whereconfining stress levels as high as 2.5-4.5 M Pahave been applied (M arsal, 1973; Marachi, Chan& Seed, 1 972 ). In most realistic situations, theshear strength of rockfill must be related to muchlower confining stresses. Charles & Watts (1 980 )have reported that the maximum possible normalstress on a critical failure surface of a 50 m highrockfill dam is unlikely to exceed 400 kPa. Evenin the case of the highest rockfill dams in South-East Asia, the maximum normal stresses areunlikely to exceed 1 MP a (Lee, 1986 ). Therefore,the scope of the current investigation has been tostudy the strength and deformation behaviou r oflarge-scale greywa cke rockfill specimens at low tomedium confining pressures, and to relate these


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TRIAXIAL TESTING OF GREYWACKE ROCKFILL 39

strain behav iour of rockfill, specimens of tworockfill gradations with maxim um particle sizes of38 mm and 25 mm were tested. Taking account ofthe principles of similitude (Fum agalli, 196 9;Lowe, 1964 ), it may be postulated that realisticresults c an be obtained only if the gradations atleast show

(a) similar grain size distribution curves(b) comparable field and laboratory compaction(c) similar angularity of particles (conforming

shapes)(d) similar sample size ratios.

With regard to condition (a), the grain size curveof the laboratory rockfill mus t be parallel to thatof the prototype; to satisfy condition (b) the poro-sity of the laboratory specimens must be close tothat of the compacted rockfill in the field. Condi-tion (c) is difficult to fulfil exactly, but the labor-atory particles must be similar in shape to theactual rockfill, although smaller in size. Fig. 2compares the parallel gradations A and B used inthe testing program me with the grain size dis-tribution of rockfill us ed in the Chiew Larn Dam .The difference between the gradation curvesbefore and after testing (at u3 = 600 kPa) indi-cates the extent of particle breakage during shear-ing. A reduction in d,, of at least 30 is observedfor both gradations A and B at the maxim um

confining pressure of 600 kPa. At small cell pres-sures (less than 200 kPa), the degree of graincrushing is insignificant.

Table 1 summ arizes the characteristics of the

laboratory rockfill, including the uniformity coef-ficient C, and the coefficient of curvature C,,before testing. The sample size ratio is defined bythe diameter of the triaxial specimen (300 mm)divided by the mean diameter of the maxim umparticle size. The effect of size ratio on the behav-iour of rockfill specimens in triaxial testing hasbeen discussed in depth by several investigators.Fagnou l & Bonnechere (196 9) and Nitchiporo-vitch (196 9) proposed a minimum sample sizeratio of 5. Marac hi (196 9) concluded that a sizeratio of at least 6 must be employed in order tominimize size effects for rockfill specimens withless than 30 of particles in the maxim um sievesize range. In this study, size ratios of 8 and 12were adopted for rocklill gradations A and Brespectively.

The test specimens were compacted within theprotective mem brane in several layers, each50-60 mm thick, using a hand vibrator. For bothgradations, an initial compacted porosity of theorder of 30 was achieved. The initial watercontent of the rockfill was -5 ; after compac-tion the mean dry density of the specimens wasdetermined to be - 18.5 kN/m 3. During the con-struction of the Chiew Larn Dam , each compactedrockfill layer varied in thickness from -0.6 to1.0 m in the field, producing a dry density ofgreater than 18.0 kN/m3, achieved by heavy

vibratory rollers (10 t), with a compacted porositysimilar to that obtained in the laboratory.

Terzaghi (196 0) noted that dam settlementsafter impounding can be significant for soft rock-

US standard sieve number

200 100 52 25 14 7 4 316in 1/2in 1 m 1.5in 3in 20in100 I I I I I I I I /I-I, I

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Diameter: mm

Fig. 2. Particle size distribution curves of greywacke rocktill


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INDRARATNA WIJEWARDENA AND BALASUBRAMANIAM

Table 1. Grain size characteristics of greywacke rockfill: C,, and C,are determined from the grain size distribution carve

Gradation d,,, : mm d,,: mm C, C, Size ratio

A 38.1 4.9 6.0 0.9 8B 25.4 3.6 6.0 0.9 12

hll (e.g. schist), whereas for harder granites suchsettlements are generally small. He related thisphenomenon to the reduction in strength uponsaturation. In the field, wetting occurs afterimpounding, and the resulting settlements dependon the initial water content of the rockfill. It maybe anticipated that the greater the placementwater content is, the smaller will be the settle-ments after impound ing. In the construction ofthe Chiew Larn Dam, although greywacke rock-fill was placed relatively dry with a mean mois-ture content


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TRIAXIAL TESTING OF GREYW ACKE ROCKFILL 41

the rocktill samples subjected to very small con-fining pressures (~2 00 kPa ) dilation is pro-nounced, whereas at higher confining pressures(a, > 300 kPa), dilation is suppressed even ataxial strains exceeding 15 . At low confining

stresses, gradation A shows more dilation thangradation B; this can be attributed to its largerparticle sizes. How ever, at higher rr3, only a slightdifference in the volumetric strain response isobserved between the two gradations, suggestingthat the particle size effect may be of secondaryimportance as compared with the influence ofconfining pressure on the volume tric strains.How ever, it is important to note that quantifica-tion of the role of particle size would requirefurther testing, w ith a much g reater difference ingrain size.

The stress-strain response of gradations A andB shows that the initial tangen t stiffness increaseswith increasing confining pressure. T he actualmagn itudes of the initial deformation modu lus asa function of the confining stress are shown inFig. 5 (each data point represents the mean ofthree tests). The difference between the initialmod uli of any two corresponding samples at thesame confining pressure is relatively small, anddiminishes with increasing oj. Furtherm ore,comparison of Figs 3 and 4 shows that the peakdeviator stresses of the corresponding samples a re

almost the same. These results sugg est that forparallel gradations with adequa te sample sizeratios, the influence of particle size may diminishif the initial porosities of the test specimens aresimilar. If the compacted field porosity is signifi-cantly different from that of the test specimens,the actual deformation response cannot be simu-lated, even if parallel gradations are used in the

4oooo

c

200 400Effective confining stress: kPa

600

Fig. 5. Variation of initial deformation modulus witheffective cordiniog stress

laboratory. Therefore, the relative densities of thecompacted laboratory specimens mu st reflect thecompaction requirements in the field.

The volumetric strains measured at the peakdeviator stress not only increase almost linearly

with the confining pressure, but are also indepen-dent of the particle gradation (Fig. 6). Lee (198 6)has made a similar observation, even for a greaterparticle size difference. Da ta reported by Ma rachiet al. (197 2) for other types of rockfill indicatethat the volumetric strains at failure increase withincreasing a3 at a gradua lly decreasing rate,approaching a constant value at high c3 (2.5MP a). W hile the results obtained in this study formuch lower confining stresses (100-6 00 kPa) arenot strictly in conflict with M arachi et al. (19 72)they cannot be extrapolated to predict be haviour

at very high stress levels. The axia l and radia lstrains at (ui - 03)_ increase at a diminishingrate as 6s is increased. It is interesting to notethat the radial strains at (a, - Q ~),_ approach aconstant value (6 ) at confining pressuresbeyond 300 kPa. On the basis of the triaxialstrain response discussed above, it may be con-cluded that the confining stress (r3 is the domi-nant factor controlling the deformationbehaviou r of rockfill, while the particle size effectseems to be of secondary importance, unless thedifference between the maxim um particle sizes is

considerable.

Shear strength andfailure criteriaWell-docum ented studies have indicated that

the friction angle of sands decreases with increas-ing cell pressure in drained triaxial tests (Vesic &Clough, 1968 ; Bishop, 1966; Ponce & Bell, 1971).A comprehensive series of large-scale triaxial tests(1 m dia.) conducted on many types of rockfillwas described by Marsal (1967) and Marachi(196 9). They found th at the principal stress ratiofor drained tests (~i/o~)r was considerablyincreased at low confining stresses. Charles &Watts (198 0) verified this phenomenon withsmaller rockfill specimens (0.23 m dia.) in testsconducted on sandstone, basalt and slate. Theexperimental data are summarized in Fig. 7,which also shows results obtained for the grey-wacke rocklill samples (0.3 m dia.) for compari-son.

Although these specimens are weak er than anumb er of other m aterials reported in the liter-ature, greywacke is accepted in Thailand, as asatisfactory rocktill. In contrast, the soft, low-grade slate was considered an unsuitable rockfillby Charles & Watts (1980). The relatively lowprincipal stress ratio of the greywac ke specimensmay be attributed to their lower initial densityand the relatively low uniaxia l compressive


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42 INDRARATNA WIJEWARDENA AND BALASUBRAMANIAM

25 r o--O Axial strain (aradation A)O----II Volumetric $Gain (gradation A)M Radial strain (aradation AI

c Axial strain (gradation B)H Volumetric strain (gradation B)g *O * Radial strain (qradation B)

10

; _I +-v_Gu) h

g E5 0 0I I 1 I I I I

5 0 100 200 300 400 500 600 700> Effective confining stress: kPa

Fig. 6. J nfluence of effective confining stress on sample strains at peakdeviator stress

z 5-C

: 4-;-

g 3-

Charles 8 Watts (1960)

Low grade slate(Charles 8 Watts, 1980)

2 I I I I I I10 30 100 300 1000 3000 10 000

Conftning stress 03: kPa

Fig. 7. Variation of effective principal stress ratio at failure with effective confin-ing stress for various rocktills


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strength of greyw acke as comp ared with highlycompacted, harder rock fragments. The degree ofcompaction and hence the initial porosity canhave a major effect on the shear strength. At aconfining stress of 20 0 kPa , an increase in poro-

sity of 1 can reduce the angle of friction by atleast 0.5 (Marachi, 1969 ).The strength envelopes for the greyw acke gra-

dations A and B considered in this study areshown in Fig. 8(a) and (b). Both envelopes corre-sponding to the low confining stress regionclearly reveal non-linearity, and pass through theorigin, indicating zero cohesion as expected of agranular material. De Mello (1977 ) proposeda non-linear failure criterion for typical rockfill

r, = Uffb (I)

where the constants a and b are considered to becharacteristic parame ters obtained by curvefitting. The physical significance of these con-stants is not clear, because the value of a not o nlydepends on the system of units used, but itsdimensions vary according to the value of b. Inthis respect it seems that the introduction of anon-dimen sional failure criterion is probablyuseful, as a wide array of materials can then becompared directly within the framew ork of simili-tude (Indraratna, 1990 ).

The significance of the uniaxia l compressivestrength cr, as a norm alizing param eter was rec-ognized by Hoek & Brown (1980 ) for rocksamples tested in triaxial compression, andallowed evaluation of the characteristic constants

approp riate for both intact and fractured rocks.Rockfill can be regarded as intensely fracturedrock, and further breakag e of individual frag-ments during shearing is a function of particleangular ity, confining pressure and the point loadindex related to aE of the parent rock.

Marsal (1973 ) introduced the particle breakageindex B8 to characterize rockfill behaviou r bycomp arison of the grain size distribution curvesbefore and after testing. Ho wever, the magn itudeof B, is sensitive to the applied confining stress,irrespective of rock type. Consequ ently the value

of B, cannot be interpreted as a material pro-perty. In this study, the uniax ial compressivestrength has been incorporated in defining thefailure envelope of rockfill. In reality, cc can beestimated more reliably than B , for any rockfillby conducting basic index tests on even an irregu-lar lump . It is proposed that the failure envelopein a non-dimen sional form can be expressed as amod ification of the de Mello (19 77 ) criterion by

00 400 600 1200 1600 2OaJ 2400 2600

Normal stress: kPa

(b)

2)

Fig. 8. Mobr-Coolomb failure envelopes of greywacke rockfill for: (a) gradation A;(b) gradation B


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TRIAXIAL TESTING OF GREYWAC KE ROCKFILL 45

normal stress values, i.e. the effect of particle sizesis minimized at higher no rmal stresses, wheredilation is inhibited. It is significant that all therockfill samples shown in Fig. 9(a) have compara-ble particle sizes and initial porosities. In this

stress range, the non-linearity of the failureenvelopes is pronounced, and a clear distinctionbetween the strong (hard) and wea k (soft) rockfillcan be observed. Fig. 9(b) and (c) illustrates thebehaviour of rockfill a t higher stress levels, usingdata from M arsal (1973) and Marachi et al. (1972)respectively. Note that at elevated effectivenormal stresses, the failure envelopes approachlinearity, hence the conventional Mohr-C oulombanalysis is sufficient to represent the failure ofrockfill.

Figure 10 plots the normalized shear strength

against normal stress relationships on log scales.Together with the present test results, experimen-tal data are presented for a wide range of effectivenormal stresses from 100 kPa-8 MPa. The cor-responding data points for each rockhll specimencan be joined by a straight line (omitted for

clarity) with a regression coefficient r2 > 0.95.The associated constants a and b (equation (2))are given in Table 2 for all these rockfillmaterials. For this wide range of rockfill, thevalues of a and b are less than 0.6 and 0.9 respec-

tively, except for Malpas o conglomerate. For verylow confining stresses (< 100 kPa), b tends to beless than 0.8; at high confining stress levels (> 1.5MP a) it approaches 0.9. Irrespective of the com-pressive strength of rock, particle sizes, angu-larity, initial porosity and initial water content,the above exp erimental data fall within a narrowband defined by the boundaries given in Table 3.

Alternatively, the failure of rockfill specimensmay be represented by the major and minor prin-cipal stresses at failure crir and oSf: this pro-cedure is often adopted in rock m echanics.

Incorporating the uniaxial compressive strengthcc, the following normalized expression is sug-gested to represent failure

Greywacke (gradation A) (present study)

Greywacke (gradation B) (present study)

Malpaso conglomerate (Marsal. 1973)San Francisco basalt (Marsal. 1973)

Mica granitic gneiss (Marsal. 1973)El lnfiernillo diorite (Marsal, 1973)El Granero slate (gradation A) (Marsal, 1973)

1 0.001 0.01 0.1U/U~

Sandstone (Charles & Watts, 1980)Slate (high grade) (Charles 8 Watts, 1980)

Slate (low grade) (Charles 8 Watts, 1980)

Basalt (Charles & Watts, 1980)Crushed basalt (Marachi et a/., 1972)

Argillite (Pyramid Dam) (Marachi et al.. 1972)

Amphibolite (Oroville Dam) (Malachi et al., 1972)

Fig. 10. Normalized shear strengthormal stress relationship for various rockfills


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Table 3. Boundaries of experimental data applicable tonormal stress ranges

This approach can be regarded as an extension ofthe empirical method discussed by Hoek &Brown (1980), who proposed a square-rootrelationship between the principal stresses for thefailure of both inta ct and jointed rocks. M arsal(1973) and Indraratna (1990) have also discussedthe relevance of representing failure criteria interms of principal stresses at failure, with particu-lar reference to triaxial testing. Equatio n (3)emphasizes that in the absence of any confiningpressure, the strength of the rockfill sam ple is

O.

0.

d_-r5

0.0

Table 4. Upper and lower boundaries of peak (failure)

r-m

Greywacke (gradation A) (present study)Greywacke (gradation B) (present study)

Malpaso conglomerate (Marsal. 1973)

San Francisco basalt (Marsal. 1973)Mica granihc gneiss (Marsal. 1973)

El lnfiemillo diorite (Marsal. 1973)l- El Granero slate (Marsal, 1973)

negligible. The relationships between the majorand minor principal stresses at failure for variousrockfill materials normalized by their uniaxia lcompressive strength are plotted in Fig. 11. Not-withstanding the differences in this wide array ofgranulated materials, their peak (failure) responsecan be represented by a narrow band widthdefined by the upper and lower bounds of CI nd /Igiven in Table 4.

The coefficients a and fi determined for individ-ual rockfills are given in Table 2. For most rock-

j-

Sandstone (Charles & Watts, 1980)Slate (high grade) (Charles 8 Watts, 1980)

Slate (low grade) (Charles & Watts, 1980)

Basalt (Charles & Watts, 1980)Crushed basalt (Marachi et a/., 1972)

Argillite (Pyramid Dam) (Marachl et al., 1972)

Amphibolite (Oroville Dam) (Marachi et al., 1972)

Fig. 11. Normal d relationships of major and minor principal stresses at failure for varioustypes of rocklill


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8 INDRARATNA WIJEWARDENA AND BALASUBRAMANIAM

fills, although the values of b and /? are quitesimilar, the value of CI is generally greater thanunity. F or very low confining stresses (< 100 kPa)the value of /I is less than 0.75; at high stresslevels (> 1.5 MP a) it is often greater than 0.85. Asa preliminary design tool in stability analysis, ifthe uniaxial compressive strength of the parentrock is known or determined by basic rocktesting, then the failure envelope of the corre-sponding rockfill can be estimated from one ofthe proposed failure criteria. At high normalstresses (a, > 1 MP a), the values of b and /lapproach unity as the failure envelope becomeslinear. Under these circumstances, the magnitudesof a and tl represent tan & and the effective prin-cipal stress ratio respectively for a given confiningstress.

Influence of confining stress on friction angleThe variation of the drained friction angle 4 of

greywacke is plotted against the effective no rmalstress in Fig. 12, where each point represents theaverage of three independent tests. As the confin-ing pressure and hence the normal stress isincreased, the drained friction ang le for rockfillgradations A and B decreases from 45 to 32 andfrom 43 to 33 respectively. Although the frictionangle of gradation A drops faster than that ofgradation B, as the normal stress is increased to 1MP a, 4 for both gradations attains the samevalue irrespective of the particle sizes. The sur-prisingly large reduction in the angle of shearingresistance at high confining pressures is probablyassociated with the significant increase in crush-ing of angula r particles. At low stress levels, parti-

45

t

M Gradation A: d = 39.1 mm

AM Gradation B: d = 25.4 mm

- Predictions(I%%% % Kjaernsli. 19 81)

311 I I I I 1100 300 500 700 900 1100

Effective normal stress: kPa

Figure 12 also compares the test results ofgreywacke rockfill with the values predicted bythe equivalent roughness method. In determiningthe angle of friction using this empirical method,the equivalent roughness R was taken as 6.5 forquarried (sharp to angular) greywacke rockfill ofcompacted porosity 30 . The mean uniaxialcompressive strength and the basic friction anglefor greywacke from southern Thaila nd have beengiven by Cheng (1986) as 136 MPa and 25. Theexperimental results indicate good agreementwith the predicted values at low normal stresses.At high normal stress levels, the laboratory fric-tion angle decreases faster than is predicted bythe equivalent roughness method. It is importantto realize that the structural component of the

frictional resistance is not intended to model theparticle size effect or the potential breakage ofangula r grains. It may be argued that due to thedependence of the parame ter s on the uniaxialcompressive strength uc, the term R log (s/a,)would not decay as rapidly as the trend shown bythe current test results. On the basis of theseobservations, it may be concluded that theBarton & Kjaernsli (198 1) approach should not beextended to extrapolate the frictional behaviourof rockfill at high effective normal stress levels,particularly beyond 1 MP a. Nevertheless, the

equivalent roughness method is adequate to esti-mate 4 at low normal pressures for the testedgreywacke rockfill.

Fig. 12. Comparison of current test results with predic- Figure 13 compares the findings of the presenttions from the equivalent roughness method study w ith the summ ary of triaxial test data for

cle crushing is small for most rockfill (Leps, 1988 ).While the data obtained by Mara chi et al (1972)indicated a slight reduction of 9 with increasingparticle sizes, a contradicting trend was reportedby Tombs (1969) for maximum grain sizes

varying from 10 to 75 mm. The effect of particlesizes on 4 remains a more complex phenomenonthan the marke d influence of confining stress. Therole of the grain size distribution of the angle offriction cannot be verified purely on the basis ofthe current test results. Barton & Kjaernsli (198 1)proposed that the drained friction ang le 4 ofrockfill can be evaluated from the empiricalexpression

where R is the equivalent roughness of rockfill,related to initial porosity of rockfill a nd genesis,angularity and surface roughness of particles; s isthe equivalent strength of rockfill particlesexpressed as a fraction of the uniaxial compres-sive strength of the parent rock; u,, is the effectivenormal stress and & is the basic friction angle ofsmooth, planar unweathered rock surfaces.


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49RIAXIAL TESTING OF GREYWACKE ROCKFILL

Isabella aranite 100 mm 0 lnfiernillo diorite 3

Cachuma gravel 19mm 0 lnfiernillo conglo m.Cachuma gravel 1 8 Malpaso conglom.

200 mm

Cachuma quarry 75 mm A Pinzandaran gravel

Oroville tailings lnfiernillo basalt

Soledad gravel 100 mm X lnfiernillo gneiss X0 lnfiernillo gneiss Y

1

175 mm

V Contreras gravel0

W Santa Fe rock JGreywacke (A) 36 mm (present study)

0 Greywacke (6) 25 mm (present study)

40 -Low-strength rockfill(Leps. 1970)

35 -

proposed by Authors

Normal pressure o,,: kPa

Fig. 13. Comparison of current test data with previous studies

other rockfill presented by Leps (1970). The datafor greyw acke rockfill fall between this lowerboun d and the line of average rockfill, exceptwhere the effective no rmal stresses exceed 700kPa . Considering the test results of Con trerasgravel and Santa Fe rock together with thecurrent data on greyw acke rockfill, it seems thatthe lower boundary proposed by Leps (197 0) for

low density rockfill is slightly overestimated. Amore conservative lower bound (3 less) as shownby the hatched line in Fig. 13 is suggested by theAuthors. Note also that no experimental data areavailable for confining pressures below 40 kPa,indicating the difftculty of testing ro ckflll samplesat such low confining stresses due to the lack ofcohesion. In the present study, a minim um con-fining pressure of N 50 kPa was required in orderto prevent the samples from bulging d uring thesaturation stage.

In ju ence of rel at i ve densit y and degree of compac-t i o n

The initial porosities of the test specimens weremaintained at 30-32 , in order to relate to theactual field conditions of the Chiew Larn D am.

For both gradations A and B of this greywackerockfill, the corresponding relative densities werebetween 62-6 5 , as determined from several testspecimens. The effect of varying the initial degreeof compaction on the angle of shearing resistancewas not investigated in detail, because theprima ry objective was to evaluate the effect ofconfining pressure (norm al stress) on the shear

strength of greyw acke rocktill. Nevertheless,Brown (198 8) has shown that excessive compac-tion encourages particle breakage, although theinitial friction angle may be enhanced for thesame confining pressure.

Excessive comp action of wet rockfill may notonly induce crushing of angular fragments, butalso cause excess po re pressures if the per-meab ility is too low . In this respect, it is alwaysgood laboratory practice to saturate the rockfillspecimens before drained loading, so that the fric-tion angles obtained in this mann er are more rea-

listic (conservative) than the higher value sencountered for dry specimens. As the down-stream shell of a dam would usually be unsatu-rated, the stability of the down stream slope maybe enhanced due to the additional shear resist-


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50 INDRARATNA WIJEWARDENA

ante provided by suction, depending on the parti-cle size distribution and the relative density.

APPLICATIONS IN PRACTICE

The friction angle corresponding to the failure

envelope is the most important param eterrequired in design for the slope stability analysisof rockfill dams. It takes its maximum value atthe least norma l stress; at extremely high stresslevels it may even approach values close to 30 .The non-linear strength envelope quan tifies thisphenomenon adequate ly. In terms of the angle offriction of comm on rockfill materials, currentrockfill dams are constructed at much flatterslopes, in spite of the capabilities of modernvibratory rollers in compac ting rockfill to achievefield porosities lower than those obtained in the

laboratory. One reason for this, of course, is thepresence of the central clay core, which influencesdesign leading to reduced upstream slope angles.If the appropriate friction angles are not carefullyselected according to the effective norm al stress,the prediction of critical slip surfaces or slidingwedges becomes questionable.

Table 5 summarizes some important character-istics of rockfill dams in Thaila nd. It is quite clearthat the smaller dams (~40 m high) are oftendesigned at steeper slopes than their largercounterparts. For such small dams, the critical

failure surface may be predicted reasonably wellby using a constant friction angle and still main-tain an acceptable factor of safety, without havingto provide berms or external support at the toe.For instance, the Ubol R atana dam has anaverage downstream slope of almost 40, whereasthe slopes of the highest Sri Nag arind dam a reless than 30.

Some research has been conducted on particlecrushing during compression. Although p articlebreakag e has been quantified in the laboratory byMa rsal (19 73) under high confining stresses, in

practice crushing of particle edges may occurduring com paction of highly angula r fragments.For less angu lar rockfill, crushing may not be a

AND BALASUBRAMANIAM

serious consideration. Nevertheless, significantcrushing of naturally soft or weathered rockduring field compaction may cause non-compliance with the permeab ility requirements(free draining), and enhance the risk of developing

construction pore pressures. It has been shown byBalasubramaniam, Lee & Wijeyakulasuriya(198 7) that any significant development of smallpore pressures can affect the effective shearstrength of rockfill. Therefore, good engineeringpractice should not allow any excess pore pres-sure development in the rockfill. In this respect,the placement moisture content, grain size dis-tribution and degree of compaction in the fieldhave an important role to play. Penman (1978)has proposed that the average perm eability ofrockfill should not be less than 0.00 1 cm/s, so

that the amount of fines can be limited to mini-mize the risk of excess pore pressure develop-ment.

CONCLUSIONSGreyw acke rockfill tested in this study shows a

similar engineering behaviou r to many othertypical rockfills. A lthough the particle size andangula rity of the rockfill also influence the stress-strain behaviour, including dilation, the effect ofconfining stress on the shear strength is ofprimary importance. While the degree of grain

crushing may be enhanced at elevated confiningpressures, for competent rockfill subjected to rea-listic norma l stresses particle breakag e may notbe a serious influential factor. T he initial densityof rockfill, however, is important as it is linkeddirectly to the degree of compac tion. The labor-atory d ata on shear strength depend on the initialporosity and relative density of the compactedtest specimens. Therefore, while excessive fieldcompac tion is not justifiable, sufficient compac-tion with conventional machinery must beensured, so that the laboratory and field poro-

sities are similar.The angle of shearing resistance and the associ-

ated failure envelope of rockfill a re directly related

Table 5. Details of some rockiill dams in Thailand (Thai National Committee on Dams, 1977)

Sri Nagarind 140

Chiew Lam 90Chulabhom 70Pattani 85Nam Pung 40Ubol Ratana 32Sirindhom 42

Dam height: m Crest length: m

610 1 : 2 0

700 1 :2 0700 1 : 1 7422 1 :2 0

1120 1 : 2 0800 1 :1 3940 1 : 1 8

r SlopeUpstream Downstream

1 : 1 8

1 : 1 81 : 1 61 : 1 81 : 1 71 : 1 31 : 1 6

r Type of rockfillLimestone and quartzite

GreywackeSandstoneGreywackeConglomerateSandstoneSiltstone


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TRI AXIAL TESTING OF GREYWACKE ROCKFILL 51

to the magn itude of confining stress. At low con-fining stresses (< 500 kPa ), the non-linearity ofthe failure envelope is pronounced. At muchhigher confining stress levels (> 1.5 MP a), theassum ption of the linear Moh r-Coulom b cri-

terion is quite acceptable. Considering the currenttest results of greywa cke rockfill together withprevious ex perimental findings, two modifiedfailure criteria for rockfill have been proposed innon-dimensional form, incorporating the uncon-fined compressive strength of the rock type. Thecharacteristic coefficients of these criteria areindependent of the system of units, and theirvalues for a variety of rockfill types have beendetermined from low to very high stress ranges.The upper and lower bounds of these coefficientsproposed by the Authors provide the engineer

with preliminary design guidelines, in the absenceof detailed laboratory testing of a given rockfill.Using this approach, if the unconfined compres-sive strength of the parent rock is known , theshear strength envelope of the quarried rockfillcan be estimated.

The effect of confining pressure on angle ofinternal friction is very important in the stabilityanalysis of rockfill slopes. A conventional analysisthat employs a constant friction ang le (average)provides an over-conservative factor of safety forshallow slip surfaces. If the actua l va riation of 4

with 6, is incorporated in the design, most roc k-fill emban kments can be raised with steeperslopes while maintaining an adequ ate factor ofsafety greater than 1.5 (Wijewardena, 1991). Theuse of a constant mean 4 for deep-seated slipsmay overestimate the factor of safety, but the dis-parity may no t be substantial.

REFERENCESBalasubramaniam, A. S., Lee Y. H. Wijeyakulasuriya,

V. (1987). Stress-strain behaviour of rockfill. Proc.8th Asi an Reg. Con Soi l M ech. Fdn Engng, Kyoto,Japan 1,21-25.

Barton, N. Kjaernsli, B. (1981). Shear strength ofrockfill. J. G eot ech. Engng D i v. Am . Sot . Ci o. Engrs

107, GT7,873-891.Bishop, A. W. (1966). The strength of soils as engineer-

ini materials. G technique 16, No. 2,91-128.Brown. A. J. (1988). Use of soft rockfill at Evretou Dam,

Cyprus. oteihnique 38, No. 3,333-354.Charles, J. A. Watts, K. S. (1980). The influence of

confining pressure on the shear strength of com-pacted rockfill. Gho t echni que 30, N o. 4,353-367.

Cheng, H. (1986). Shear behaoi our of greyw acke rock@material. MEng thesis, Asian Institute of Tech-nology, Bangkok.

de Mello, V. F. B. (1977). Reflections on design deci-sions of practical signifiance to embankment dams.Gt ot echni que 27, N o. 3, 279-355.

Fagnoul, A. . Bonnechere, F. (1969). Shear strength ofporphiry materials. Proc. 7th Int . Con Soi l M ech.

Fdn Engng, Mexico, 61-65.Fumagalli, E. (1969). Tests on cohesionless materials for

rockfill dams. J. Soil M ech. Fdn Engng Di v. Am . Ci v.

Engrs 95, SMI, 313-330.Hoek, E. . Brown, E. T. (1980). Underground excava-

tions in rock. London: Institute of Mining andMetallurgy.

Indraratna, B. (1990). Development and applications ofa synthetic material to simulate soft sedimentaryrocks. Gi ot echni que 40, N o. 2, 189-200.

Indraratna, B. Kaiser, P. K. (1990). Design forgrouted rock bolts based on the convergence controlmethod. I nt . J. Rock M ech. Sci. Geomech. Abstr . 27,N o. 4,269-281.

Ito, Y. (1983). Design and construction by NATMthrough Chogiezawa Fault Zone for Enasan Tunnelon Central Motorway. (In Japanese.) Tunnels

Underground 14,7-14.

Lee, Y. H. (1986). St rength and deform at i on character-

i sti cs of rock@. PhD thesis, Asian Institute of Tech-

nology, Bangkok.

Leps, T. M. (1970). Review of shearing strength of rock-fill. J. Soi l Mech. Fdn Engng D i v. Am. Sot. Ciu.

Engrs 96, SM4, 1159-l 170.Leps, T. M. (1988). Rockfill dam design and analysis.

Adv anced dam engineeri ng for design, constru cti on

and rehabi l i ta t ion (ed. R. B. Jansen), pp. 368-387.New York: Van Nostrand Reinhold.

Lowe, J. (1964). Shear strength of coarse embankmentdam materials. Proc. 8th Int . Congr. Large D ams 3,

745-761.Marachi, N. D. (1969). Strength and deformation charac-

teristi cs of rock@ material s. PhD thesis, Universityof California.

Marachi, N. D., Chan, C. K. Seed, H. B. (1972).Evaluation of properties of rocktil materials. J. Soi l .M ech. Fdn Engng D i a. Am . Sot . Ci v. Engrs 98, SMl,

95-l 14.Marsal, R. J. (1967). Large scale testing of rockfill

materials. J. Soil Mech. Fdn Engng D i u. Am. Sot.Civ. Engrs 93, SM2, 27-43.

Marsal, R. J. (1973). Mechanical properties of rockfill.Embankment dam engineeri ng, Casagrande v ol ume,

pp. 109-200. New York: Wiley.Nitchiporovitch, A. A. (1969). Shearing strength of

coarse shell materials. Proc. 7th I nt. Conf Soi l Mech.Fdn Engng, M exi co Cit y, 211-216.

Penman, A. D. M. (1978). The behaviour of rockfill

dams. Geotechni cal Engineeri ng (Journal of theSoutheast Asian Geotechnical Society) 9, 105-122.

Ponce, V. M. Bell, J. M. (1971). Shear strength of

sand at extremely low pressures. J. Soil M ech. FdnEngng D i v. Am . Sot . Ci v. Engrs 97, SM 4,625-637.

Terzaghi, K. (1960). Discussion on Salt Springs and

Lower Bear River Dams. Trans. Am. Sot. Ciu. Engrs125, 139-148.

Thai National Committee on Large Dams (1977). Largedams n Thail and. Bangkok, Thailand: Samarn.

Tombs, S. G. (1969). Str ength and deform at i on character-is t i cs ofrockf i l l . PhD thesis, University of London.

Vesic, A. S. Clough, G. W. (1968). Behaviour of

granular materials under high stresses. J. Soi l M ech.Fdn Engng D i v. Am . Sot. Civ . Engrs 94, SM3, 661-668.

Wiiewardena, L. S. S. (1991). Engineeri ng pro pert ies of

-greyw acke rockJil l and their effect on the dynami c sta-

bi l i t y analysis of the Chiew Larn Dam. MEng thesis,Asian Institute of Technology, Bangkok.

Documents

Indraratna,B_Large-scale Triaxial Testing of Greywacke Rockfill_1993