Frossard, E. et al. Geotechnique [http://dx.doi.org/10.1680/geot.10.P.079]

1

Rockfill shear strength evaluation: a rational method based on size effects

E. FROSSARD, W. HU, C. DANO and P. -Y. HICHER

The shear strength of coarse granular materials, widelyused in civil works such as rockfill in dam construction,is seldom measured because of severe practical experi-mental limitations. This paper presents an originalmethod for evaluating the shear strength of such mater-ials, based on size effects in granular materials affectedby grain breakage according to fracture mechanics.Through a general size effect relation operating on shearstrength envelopes, this method makes it possible todetermine the shear strength of a coarse-grained granu-lar material from the measured properties of a finer-grained granular material made of the same mineral. Inthe paper, the method is explicitly proved, taking intoaccount the statistical distribution of breakage resistancesof particles with different sizes, within materials that canbe considered as physically similar, particularly havingparallel grain size distributions and the same compact-ness. A wide set of independent experimental results isshown to validate the method consistently.

KEYWORDS: dams; particle crushing/crushability; shearstrength

La resistance au cisaillement des materiaux granulaires a`gros grains largement utilises en genie civil, tels lesenrochements dans la construction de barrages, fait rare-ment lobjet de mesures du fait de limitations pratiqueslourdes qui pe`sent sur lexperimentation. Cet articlepresente une methode originale pour evaluer la resistanceau cisaillement de tels materiaux, fondee sur les effetsdechelle survenant dans les materiaux granulaires af-fectes par la rupture de granulats suivant la Mecaniquede la Rupture. Au moyen dune Relation dEffetdEchelle assez generale, qui ope`re sur les courbes intrin-se`ques, cette methode permet de determiner la resistanceau cisaillement dun materiau granulaire a` gros grains, a`partir de mesures effectuees sur un materiau granulaireplus fin, constitue de la meme matie`re minerale. Danslarticle, cette methode est explicitement prouvee, entenant compte des distributions statistiques des resis-tances a` la rupture des granulats de differentes dimen-sions, au sein de materiaux granulaires qui secorrespondent en similitude physique, avec en particulierdes granulometries semblables et la meme compacite. Lamethode est enfin validee sur un large ensemble deresultats experimentaux independants.

BACKGROUNDThe construction of large civil works involving coarse-grained materials, such as rockfill dams (Fig. 1), is increasingworldwide (ICOLD, 2002). Traditionally, rockfill materialshave seldom been tested for shear resistance, because of thesize of the apparatus required, and the corresponding costsand delays for testing, even for small-sized rockfill. Forexample, for a 0250 mm rockfill, a representative cylindri-cal probe for a triaxial test should measure about 1.5 m indiameter by 3 m high, and weigh more than 10 t. However,the trend towards larger works, such as higher rockfill damswith steeper slopes, should be associated with improvedknowledge of these materials for stability assessment, andparticularly knowledge of their shear resistance. Significantincidents have recently marked the commissioning of variouslarge rockfill dams (CBDB-ICOLD, 2007), attributed toexcessive unforeseen deformations required to mobilise theresisting forces necessary for static equilibrium, under nor-mal service conditions.Concern is rising within the profession to return to more

rational approaches in their design, and particularly engi-neering approaches through structural analysis and relevantmaterial testing, as should be done for any large civilengineering structure. This concern also underlines a grow-ing need for progress in the knowledge and rational masteryof the behaviour of rockfill and large embankment works,and is shared by other sectors of construction, such as large

linear embankment works in highways, railways and marineworks.Pioneering experimental work in large-scale testing of a

wide range of different materials was developed in the

Manuscript received 5 August 2010; revised manuscript accepted 13April 2011.Discussion on this paper is welcomed by the editor. Coyne et Bellier Consulting Engineers, Gennevilliers, France. Research Institute in Civil and Mechanical Engineering, EcoleCentrale de Nantes, University of Nantes, France.

Fig. 1. Typical rockfills used in dam construction

atzenkovRectangle

atzenkovRectangle

1960s, together with the development of large rockfill damconstruction in the Americas, by Marsal in Mexico (Marsal,1967, 1972), and at approximately the same time in Cali-fornia by Marachi and colleagues (Marachi et al., 1969,1972; Becker et al., 1972). Table 1 summarises the mainfeatures of the triaxial apparatus developed by these inves-tigators.Other large testing equipment with comparable features

was later implemented worldwide, but its operation remainsoutside the scope of current geotechnical laboratories, andthe data available are quite scarce (Duncan, 2004).Smaller devices have also been used to provide interesting

results concerning the behaviour of rockfills (Charles &Watts, 1980; Barton & Kjaernsli, 1981; Bolton, 1986;Indraratna et al., 1998), of unsaturated gravels and rockfills(Chavez & Alonso, 2003), and of the dynamic behaviour ofballasts (Indraratna et al., 1998; Lackenby et al., 2007).However, the ordinary rockfills commonly used in large

dam construction, with maximum grain sizes typically be-tween 250 mm and 800 mm (and sometimes larger), remainout of range of the largest apparatuses mentioned above.This background justifies efforts to find a method to evaluatethe shear strength of very coarse granular materials frommeasurements performed on finer gradations of the samemineral constituents. The original method detailed herebelow provides a response to this need. It is based upon thekey concept that a characteristic gradation dimension (e.g.the mean grain diameter or maximum grain diameter) affectsthe mechanical behaviour of the granular mass, by control-ling the amount of grain breakage under loading. Thismethod (Frossard, 2006, 2008, 2009a, 2009c) results from anin-depth review of numerous past experimental results onrockfill in the light of current knowledge of the mechanicsof materials, and particularly fracture mechanics, duringrecent research projects on granular fill behaviour (Frossard,2005a).

SIZE EFFECT IN GRANULAR MATERIALS USED INCIVIL WORKS: PREVIOUS WORKThe size effect can generally be described as the

dependence of a material intrinsic property on a character-istic sample dimension (e.g. volume or diameter). Such aphenomenon is encountered in many engineering fields,including quasi-brittle geomaterials in civil engineering(concrete, rock and soil), and was investigated long ago ingeomechanics for its incidence in similarity relations (We-ber, 1971). Here, as a result from fracture mechanics, thesize effect can be expressed by the fact that, statistically,the intrinsic strength of a specimen decreases when itssize increases (Bazant & Planas, 1998). Furthermore, asthis essential result is related to the development of pre-existing flaws and microcracks within the material of thespecimen, the way these flaws and cracks are distributedwill affect the variability of the strength of a brittlematerial. On the basis of data on brittle materials, astatistical approach was proposed by Weibull (1951) to

describe brittle material strength and its variability, aspecific parameter being attached to the variability inspecimen strength in the resulting Weibull distribution. Ingranular materials the size effect affects the crushingresistance of individual particles submitted to contactforces and, as a consequence, the mechanical behaviour ofthe whole granular mass.

Size effect for individual grainsExperimental results in the literature (e.g. Marsal, 1972;

Lee, 1992; Nakata et al., 1999; Miura et al., 2003) outlinehow, statistically, the intrinsic strength of an individualparticle decreases as the particle size increases (Figs 2 and3). The strength of the particle is commonly determined byapplying a compressive load on it, placed between two stiffplatens. It is calculated from the maximum load reachedbefore global failure of the particle (Fig. 2). This specificfeature of grain behaviour has to be attributed to initialmicrocracks randomly distributed in size and orientationwithin the particles. They are, statistically, larger in largegrains (Fig. 4). Physically, during loading, the initial micro-cracks grow and interconnect to form a macrocrack atfailure. Griffiths theory for brittle materials (Griffith, 1921)established that the strength is inversely proportional to thesquare root of the initial microcrack length. McDowell &Bolton (1998) suggested later that the gradation of crushablesoils tends towards a fractal distribution because of particlecrushing.

Size effect in granular materialsExperimental size effects were clearly apparent in a series

of triaxial tests carried out by Marsal (1967) on specimensof different sizes, as well as in a series of crushing tests onparticles of different sizes. Similarly, Marachi et al. (1969)performed an outstanding series of triaxial tests on threekinds of rockfill specimen of different sizes. For each mater-ial, three specimen groups of different sizes were reconsti-tuted with the same compactness and closely parallel grainsize distributions (Fig. 5), adjusted so that the mean andmaximum grain sizes were in proportion to the specimensizes (fine-, medium- or coarse-grained; Table 2).At the end of the shearing phase, evolution of the grada-

Table 1. Key features of large triaxial apparatus of Marsal and Marachi et al.

Sample features Maximum loading range

References Diameter: mm Height: mm Max. grain size: mm Confining pressure: MPa Axial load: kN

Marsal (1967, 1972) 1130 2500 180 2.5 15 000Marachi et al. (1969, 1972) 915 2286 152 5 18 000

Table 2. Key features of multi-scale series of triaxial testsperformed by Marachi et al. (1969)

Type of material MaterialDmax: mm (inches)

Specimen for testing

Diameter: mm(inches)

Height:mm

Coarse grained 152 (6) 915 (36) 2286Medium grained 51 (2) 305 (12) 762Fine grained 12 71 (2.8) 178

2 FROSSARD, HU, DANO AND HICHER

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovDroite

atzenkovDroite

atzenkovDroite

atzenkovDroite

atzenkovRectangle

tions was measured, by determination of a global grainbreakage factor determined by the difference between theinitial and final gradations (according to the definition andprocedure detailed in Marsal, 1972). Marachi et al. (1969)found that the reduction observed in the value of the internalfriction angle can be closely related to the grain breakageintensity, particularly developed in large specimens. Theirresults showed clearly that the maximum shear strength ofthe specimen decreases and the amount of particle breakageincreases when the particle size increases. Similar relation-ships were obtained for the three different granular mater-ials, the only difference being in the intensity of particlecrushing. One other key experimental result of these investi-gations (Fig. 6) was that, within each of the three familiesof similar granular materials, the correspondence betweenthe internal friction and the amount of grain breakage was

Platen displacement

0

1

2

3

4

5

6

7

8

Bearing failureat contact point

f

Fixed platenFo

rce,

: kN

f

Fig. 2. Method for crushing test on particles (after Lee, 1992)

Oolitic limestone

Carboniferous limestoneLeighton Buzzard sand

1 2 3 5 10 20 30 50 100Average particle size: mm

50

20

10

5

2

1

05

02

Cru

shin

g st

ress

: MP

a

Fig. 3. Experimental evidence of size effect in strength ofindividual grains (Lee, 1992).

Large specimen F

Largecracks

F

(a) (b)

Small specimen

Small cracks

Fig. 4. Schematisation of origin of size effect: (a) close-up of freshly cut rock surface, showing flaws, cracks and porecavities; (b) within a given mineral matrix, the large specimen will tend statistically to include larger flaws or cracksaffecting its crushing resistance than the smaller specimen

ROCKFILL SHEAR STRENGTH EVALUATION 3

atzenkovDroite

atzenkovDroite

atzenkovDroite

atzenkovDroite

atzenkovDroite

atzenkovRectangle

atzenkovRectangle

found to be unique, regardless of the characteristic size(coarse-, medium- or fine-grained), if all the specimenswithin one family were given the same initial compactness.Hardin (1985), Lee (1992), Coop & Lee (1993), Biarez &

Hicher (1997), Daouadji et al. (2001), Deluzarche & Cam-bou (2006) and Lade & Bopp (2005) also studied the break-age of particles in granular media, which was found to be asource of size effects in crushable granular materials.

1

2in1

US standard sieve sizes

200 100 50 30 16 8 4 3 in 6 in100

80

60

40

20

0

Per

cent

fine

r

0005 001 005 01 05 10 5

Particle size: in(c)

28

dia.

spe

cim

ens

12di

a. s

peci

men

s

Fiel

dan

d36

dia.

spe

cim

ens

US standard sieve sizes

200 100 50 30 16 8 4 3 in 6 in 12 in 24 in100

80

60

40

20

0

Per

cent

fine

r

0005 001 005 01 05 10 5 10Particle size: in

(a)

28

dia.

spe

cim

ens

12di

a. s

peci

men

s

36di

a. s

peci

men

s

Fiel

dgr

adat

ion

US standard sieve sizes

200 100 50 30 16 8 4 3 in 6 in 12 in 24 in100

80

60

40

20

0

Per

cent

fine

r

0005 001 005 01 05 10 5 10Particle size: in

(b)

28

dia.

spe

cim

ens

12di

a. s

peci

men

s

36di

a. s

peci

men

s

in34

in38

1

2in1in

3

4in3

8

1

2in1in

3

4in3

8

Fig. 5. Rockfill grain size distributions for different specimen diameters of three materialstested by Marachi et al. (1969): (a) Pyramid Dam material; (b) crushed basalt; (c) OrovilleDam material (1 in 25.4 mm)

4 FROSSARD, HU, DANO AND HICHER

atzenkovRectangle

atzenkovRectangle

PROPOSED SIZE EFFECT RELATIONS BASED ONGRAIN BREAKAGEAssumed key features of grain breakageOn the basis of experimental data, Marsal (1967, 1972)

proposed representing the average experimental crushingstrength f by a power function of the average diameter d ofthe particles (Fig. 7)

f d (1)

where and are experimental constant material values.

Grain-crushing statistics can also be described by a Weibulldistribution, considering the probability of survival within apopulation of brittle objects exposed to loading conditions(Weibull, 1951). In this approach, the probability of survivalPs(V , ) within a set of brittle particles defined by theirvolume V and submitted to an applied stress is given by

Ps V , exp VV0

0

m" #(2)

where V0 is a given reference volume, and 0 corresponds tothe applied stress for which Ps(V0) 1=e 0:37, meaningthat 37% of the specimens (with volume V0) survived the

54

50

46

42

38

3410 200

(c)

54

50

46

42

38

34

Ang

le o

fin

tern

al fr

ictio

n: d

egre

es

Specimen dia.: in

36

1228

0 25 50

(a)

e0 045

e0 074

54

50

46

42

38

3425 50

Particle breakage factor, : %(b)

B

0

Fig. 6. Internal friction angle against grain breakage factor for different specimen sizes of three materials tested by Marachi etal. (1969): (a) Pyramid Dam material; (b) crushed basalt; (c) Oroville Dam material (1 in 25.4 mm)

1 10

18

10

1

Average grain size, : mmd

Ave

rage

cru

shin

g fo

rce,

: kN

f

Fig. 7. Experimental relation between average grain size andparticle-crushing load for San Angel basalt (redrawn from datapresented by Marsal, 1967)

10

08

06

04

02

0

PV

s0

()

10

20

m 5

04 06 08 10 12 140

Stress

Fig. 8. Influence of Weibull modulus m on shape of Weibulldistribution

ROCKFILL SHEAR STRENGTH EVALUATION 5

loading without breaking. This equation is represented inFig. 8 for V V0 and for different values of the materialparameter m. The smaller m is, the more disperse is thestrength distribution. The two representations, Marsal (equa-tion (1)) and Weibull (equation (2)), can be connected inorder to relate the parameters and m. Indeed, as the grainvolume V is statistically proportional to the cube of itsdiameter d, if failure is statistically defined by a givenprobability of survival, then the corresponding averagecrushing strength will be proportional to a power function ofthe diameter d (McDowell & Bolton, 1998)

Ps V , fixed) / d3=m (3)

Given that the mean stress inside the grain is propor-tional to the applied force f during loading, divided by themean section of the grain, which itself is statistically propor-tional to the square of its diameter, then merging the twoapproaches gives a simple means to set, for a given material,a representative Weibull distribution. From equations (1), (2)and (3) the following relation is derived

2 3m

or m 32 (4)

In the range of materials investigated by Marsal (1967,1972), the exponent was found to be between 1.2 and 1.8,corresponding to a Weibull modulus m between 4 and 15,with a central value of of about 1.5, corresponding to aWeibull modulus m central value equal to 6. These centralvalues are not a simple random result; they correspondprecisely to a distribution of the sizes of flaws or micro-cracks inside particles, for which the significant flaw ormicrocrack size governing their failure is found to bestatistically proportional to their diameter.In equations (2) and (3), corresponds to the mean

stress applied on one grain. The core assumption of theapproach developed herein is that the significant stressapplied to one grain within a granular assembly may beconsidered as being proportional to the average macroscopicstress applied to the granular material. Then, in the abovesituation of a central distribution of flaws, the limit ofaverage macroscopic stress that the granular medium canwithstand before crushing will be inversely proportional tothe square root of a characteristic grain diameter.

Basic size effect relation for simple particle setsComing back to the more general situation, consider

particles from two sets of particles, A and B, issued fromthe same homogeneous mineral stock, with similar grainshapes, and characteristic diameters dA and dB, described byequations (1)(4).

(a) Submit the particles in these two sets to crushing forcesfA and fB:

(b) In order to obtain the same probability of ruptures ofparticles within the two sets, fA and fB must be adjusted tothe same proportion of the average resistance of particlesin sets A and B, resulting in the following basic sizeeffect relation regarding grain breakage for simpleparticle sets (Frossard, 2009d).

fB fA dBdA

or fB fA dB

dA

23=m(5)

Size effect relation for granular materials (Table 2)In a granular material, the macroscopic stresses may be

related to the set of local interparticle contact forces,

integrated in a representative volume. Following the LoveWeber approach of stress averaging, the macroscopic stresstensor ij can be obtained by the integration of contactforces and branch vectors for all contacts (Love, 1927;Weber, 1966; Christofferson et al., 1981; Rothenburg &Selvadurai, 1981; Chang & Hicher, 2005)

1V

Xn, p

dB p dA p DB

DA

lB n= p lA n= p DBDA

VB VA DBDA

3(7)

Regarding the forces and internal equilibrium in the twosimilar materials, some basic assumptions will be made. Asthe particles of both granular media are issued from thesame mineral stock, the statistics of their crushing strengthsare assumed to be described by the same Weibull distribu-tion; and the friction coefficients at homologous contacts areassumed to be identical.Consider now these similar granular media submitted to

macroscopic stresses, close to limit equilibrium, inducingslow motion inside both media: the resulting contact forcesinduce some grain breakage. Consider the probability offailure of homologous particles p in the two media under theeffect of contact forces exerted by homologous particles n.Equation (5) shows that, in order to secure the sameprobability of grain breakage, the homologous contact forcesshould be in proportion to the crushing resistances of homo-logous particles p

f B n= p f A n= p DB

DA

23=m(8a)

As the scaling ratio is the same for all contact forces, thisrelation, valid locally for each contact force f ( n= p), alsoholds globally for the complete sets of contact forces {f}.

f Bf g f Af gDB

DA

23=m(8b)

Macroscopic stresses in the two granular materials derivefrom equation (6).

B 1VB

Xn, p

granular material A, then the corresponding stress state {B}on the shear strength envelope of granular material B can beobtained by applying a homothety with ratio (DB=DA)

3=m

on the vector OfAg. So from the simple geometric con-structions of Fig. 11, the shear strength envelope of granularmaterial B can be obtained from that of granular material A,regardless of any analytical formulation of the shear strengthenvelope.If the shear strength envelope of granular material A can

expressed analytically as

A fA nA (11)then by reversing equation (10), the following relationsresult between shear stresses and normal stresses in granularmaterials A and B.

A B DBDA

3=m

nA nB DBDA

3=m (12)

Then equations (11) and (12) lead to

B DBDA

3=mfA nB

DB

DA

3=m" #(13)

Equation (13) gives the expression for the shear strengthenvelope for granular material B, on the basis of the expres-sion for the shear strength envelope for granular material A.The mathematical expression of the shear strength envel-

ope for material A can be any of those usually used forgranular materials; with De Mellos criterion (De Mello,1977), the shear strength envelope is given by

f A 9n b (14)

where f is the maximum shear stress, 9n is the effectivenormal stress, and A and b are material constants. Assumingnow that the expression for the shear strength envelope forgranular material A is given by equation (14) and charac-terised by the parameters AA and bA, the expression for theshear strength envelope of granular material B can bederived from equation (13) as

fB DBDA

3=mfA

AA DBDA

3 1bA =mnB bA

(15)

Observe that in equation (15) the coefficient AA is modifiedby a size effect factor, but the power exponent bA remainsunchanged.

VALIDATION WITH EXPERIMENTAL DATAA wide set of experimental results published indepen-

dently by various authors has been analysed for this valida-tion, including results by Lee (1992) and Marsal (1967),analysed in Hu (2009), and results from Marachi et al.(1969) and Charles & Watts (1980), analysed in Frossard(2005a, 2006, 2009a, 2009c).

Analysis of results by Lee (1992)Two limestone granular materials, subsequently termed

MOLS (as material A) and POLS (as material B), withparallel gradations (Fig. 12(a)) and different particle sizes,

were chosen by Lee (1992), with the ratio between theirrespective mean grain diameters D50 about 10.Crushing tests (Fig. 1) were performed on different parti-

cle sizes. The crushing strength function adjusted for thislimestone (equation (1)) is

f 4:51d1:65 (16)

Sets of triaxial tests were performed by Lee (1992), definingthe shear strength envelopes for both materials.From triaxial experiment data, the shear strength envelope

for material MOLS can be fitted according to

A n 0:71 0:89n (17)

Choosing the characteristic diameter D50 to represent thesize ratio, D50(A) 0.22 mm and D50(B) 1.73 mm formaterials MOLS and POLS respectively, the theoreticalshear strength envelope for POLS can be predicted fromequations (15) and (17) as

B n 0:66 0:89n (18)

400

300

200

100

00 250 500 750 1000 1250

: kPa(b)

: k

Pa

1

3

2

1 Experimental shear strength envelope for MOLS material2 Experimental shear strength envelope for POLS material3 Shear strength for POLS material extrapolated from MOLS

material by using size effect rule

100

80

60

40

20

0

Per

cent

age

pas

sing

: %

MOLS

POLS

01 1Particle size: mm

(a)

Fig. 12. (a) Parallel gradations for materials MOLS and POLS;(b) comparison of experimental and predicted shear strengthenvelopes (redrawn from data presented by Lee, 1992)

8 FROSSARD, HU, DANO AND HICHER

Shear strength envelopes obtained by this method andthose from triaxial experiments have been compared (Fig.12(b)). The theoretical prediction fits the experimental re-sults very well, although the size effect is not very muchmarked in this example.

Analysis of results from Marsal (1967)Marsal (1967) performed a series of triaxial experiments

on silicified conglomerate rockfill used at El Infiernillo dam,with the specimen diameters and corresponding gradationsshown in Fig. 13(a). Two different granular sizes are selectedas materials A and B, corresponding respectively to speci-mens 20 cm and 113 cm in diameter. The coefficients ofuniformity for the two granular materials are nearly thesame, at about 10.From particle-crushing tests data in Marsal (1967),

1.2, so

m 32 3

:75 (19)

For material A triaxial tests, the fitted equation of the shearstrength envelope is (Fig. 13(b))

A n 1:30 0:91n (20)

Characteristic grain sizes are D50(A) 6.8 mm andD50(B) 68.9 mm for materials A and B. Applying the sizeeffect relation to the shear strength envelope of material A,the shear strength envelope for material B can be predictedas

B 68:9

6:8

3=3:75A n

1:30 68:9

6:8

3 10:91 =3:75 0

:91n

1:10 0:91n

(21)

The comparison between the shear strength envelopeobtained from the experimental results and the computedone (equation (21)) is shown in Fig. 13(b), and displaysquite a good fit.

Analysis of results from Marachi et al. (1969)As described earlier, three different sets of similar granu-

lar materials were investigated, first prepared with parallelgradations, as displayed in Fig. 5, and subsequently tested intriaxial apparatuses with the same initial compactness withineach set.The three source materials were

(a) a quarried sedimentary argillite used in Pyramid Dam,with angular, medium-resistance particles

(b) a quarried crushed basalt, with angular, quite strongparticles

(c) a borrowed granular alluvium used in Oroville Dam, withstrong, rounded cobbles, gravels and particles of fine-grained amphibolite.

No statistical data on crushing tests of particles are avail-able, so no direct determination of the Weibull parameter mis possible. However, measurements of the global breakagefactor B% by systematic comparison of gradations measuredbefore and after testing allows the Weibull parameters to beevaluated for each set of materials. Indeed, the size effectrelation first sets a correspondence between the stress statesassociated with the same grain breakage amount withinsimilar materials. Thus, for similar materials A and B, thedata for breakage factor B% against confining stress shouldcorrespond with the same similarity as the failure envelopes.So the parameters m can be fitted by adjusting the similarityfactors on the breakage factor data, and then applying themto the correspondence between the shear strength envelopes.The results of this fit on breakage factor data are dis-

played in Fig. 14(a), with m values of 8.3, 9 and 7 for thethree materials. The application of these adjusted similarityratios to the internal friction data (Fig. 14(b)) is particularlygood: the predicted shear strength envelopes for rockfill(Dmax 150 mm), reconstituted from shear strength envel-opes measured on gravel (Dmax 12 mm), correspond wellwith the experimental data.

Analysis of results from Charles & Watts (1980)The data compilation published by Charles & Watts

(1980) gathers experimental shear strength data from numer-ous previous tests on granular fills and rockfills, shown asthe hatched areas in Fig. 15(a), together with new experi-mental results, shown as curves A, B1, B2 and C.This original diagram displays some apparent distortion:

the new experimental results of Charles & Watts (outlinedby the dotted ellipse) appear curiously shifted towards theupper right corner of the diagram, as compared with the

5

4

3

2

1

0

She

ar s

tres

s,: M

Pa

0 1 2 3 4 5Normal stress, : MPa

(b)n

Gradation 2 (test results)

Gradation 1 (test results)

Gradation 1 (model results)

100

80

60

40

20

0

Per

cent

age

pas

sing

: %

Silicified conglomeratefrom EI Infiernillo Dam

Specimen diameterGradation 1: 113 cmGradation 2: 20 cm

Gradation 2

Gradation 1

01 1 10 100Particle size: mm

(a)

Fig. 13. (a) Gradation for El Infiernillo Dam material;(b) comparison of experimental and predicted shear strengthenvelopes for silicified conglomerate material (redrawn from datapresented by Marsal, 1967)

ROCKFILL SHEAR STRENGTH EVALUATION 9

older results of Marsal and Marachi. This holds also forslate material B2, deliberately chosen as weak by theseauthors, in order to complete the range of tested materials:so curve B2 should lie rather below the usual range ofrockfill.This apparent distortion can also be attributed to size

effects, and corrected by the size effect relation. Indeed, thenew measurements have been performed on granular mater-ials limited to Dmax 38 mm, whereas the older results fromMarsal and Marachi were drawn from coarser materials(Dmax 150180 mm). Here, the absence of data on particlebreakage compels reliance on the central value m 6 forthe Weibull parameter (see above). The corrected diagram(Fig. 15(b)) has been produced by this method, with ahorizontal similarity factor on confining stresses of(38 mm/150 mm) (this transformation is facilitated by thelogarithmic scale used here for confining stresses: the simi-larity operation reduces to a simple shift of results towardsthe left).This correction for size effects resolves the apparent

distortion of the original diagram between the new resultsand the older ones.

DISCUSSION ON SIZE EFFECT RELATION ANDPRACTICAL APPLICATIONSKey assumptions involved in size effect relationThree key assumptions have been made to reach the size

effect relation (equation (10)) and its consequences, asfollows.

(a) The mineral particles constituting the granular materialsunder consideration do crush when the materials arestrained, because of the development of pre-existing flawsand microcracks within the mineral matter, causingfailures ruled primarily by the laws of fracture mechanicslaws, with a predominant mode of particle failure bytensile-induced fracture, known as Mode I.

(b) The way these pre-existing flaws and microcracks aredistributed in the material affect the variability of theparticle crushing strength, whose statistics follow aWeibull distribution.

(c) Complete physical similarity is materially achievable (i.e.with reasonable accuracy) between granular materialsconstituted from a common homogeneous mineralsource, compacted to the same density (i.e. the samevoid ratio) through similar compaction procedures.

Attention should be paid to the third of these key assump-tions, as it may not be always satisfied with some granularmaterials, such as heterogeneous alluvium made of mixedsedimentary or metamorphic rocks, in which hard rockspredominate in the coarser fractions of granulometry, andweaker minerals accumulate in the finer fractions of granu-lometry.A fourth key assumption is related to the implicit use of

the size effect relation as a necessary and sufficient condi-tion.Starting from local contact forces, the size effect relation

has been shown to be a necessary condition for obtainingthe same amount of grain breakage. However it cannot be

60

40

20

40

Pyramid Dam material

Specimen dia.: in

36012028

Crushed basalt40

20

0

Par

ticle

bre

akag

e fa

ctor

,: %

B

Oroville Dam material20

00 200 400 600 800

Confining pressure: lbf/in2

(a)

55

45

3555

Pyramid Dam material

Specimen dia.: in36012028

Crushed basalt

45

35Ang

le o

fin

tern

al fr

ictio

n: d

egre

es

Oroville Dam material

45

00 200 400 600 800

Confining pressure: lbf/in2

(b)

55

Fig. 14. For the three materials tested by Marachi et al. (1969): (a) adjusting size effect factors on grain breakage data againstconfining pressure; (b) application of size effect factors on internal friction data against confining pressure (1 in 25.4 mm;1 lbf/in2 6.89 kPa)

10 FROSSARD, HU, DANO AND HICHER

proved to be also a sufficient condition, because for a givenmacroscopic stress state, the corresponding distribution oflocal contact forces is not unique. Nevertheless, the valida-tion on experimental data shows that, with a good level ofapproximation, it is also a sufficient condition for a widerange of granular materials.

Rationale for a new practical method for rockfill shearstrength determinationThe above results on size effects lead to a new rational

method to assess, for a given material, the shear resistanceenvelope for rockfill, in a four-step procedure involvingreasonable laboratory tests.

(a) From the rockfill under consideration, prepare a reducedmaterial, geometrically similar to the complete rockfill.

(b) Perform a large set of normalised rock mechanicscrushing tests (Protodiakonov or Franklin tests), as afunction of grain diameter, in order to adjust a Weibulldistribution on the crushing strength distribution.

(c) Perform a set of shear tests (triaxial or others) on thereduced, gravel-sized material from the same stock(provided that the grain size distributions are geome-trically similar, and compacted to the same porosities), inorder to measure the shear strength envelope for thegravel-sized material.

(d ) Use the size effect relation, with the measured Weibullparameter m, to assess the rockfill shear strength envelopefrom the measured gravel one.

Prudently limiting the scaling factor (DB/DA) to 15, this

method makes it possible to assess the shear strengthenvelope of rockfill with Dmax 600 mm, representative ofmost of practical situations, from triaxial tests on gravel-sized material with Dmax 40 mm, acceptable in a 250 mmdiameter probe apparatus. This procedure, based on our newphysical similarity rule, resulting from the laws of fracturemechanics governing particle breakage, makes it possible toreduce the material mass of a representative sample by afactor of (1/15)3 1/3400.

Other practical applications of size effect relationAn important consequence of the size effect relation is its

relevance to safety margins and safety factors of compactedrockfill slopes. This material size effect has already beenintegrated in the limit equilibrium stability analysis method(Frossard, 2006, 2008, 2009a, 2009b, 2009c) by developingthe previous work of Charles & Soares (1984). The resultingmethod makes it possible to assess the relevance for safetyfactors of the recent trend towards the use of coarser mater-ials in higher works with steeper slopes (Fig. 16). Theresults obtained have shown that direct extrapolation of mid-sized rockfill slopes to very high ones with coarser materialsis likely to reduce the safety factors quite significantly.To close the discussion on consequences of the size effect

equation (10), consider stressstrain curves measured withintwo materials under complete physical similarity, shearedwith confining stresses adjusted according to equation (10).For a given strain, the global amount of grain breakage willbe the same in the two materials, and the principal stressratio will also be the same. As a consequence, equation (10)should also set direct correspondences between the stressstrain curves in similar materials, which should also extendto critical state conditions.Indeed, a material size effect has been detected in statis-

tics for measured apparent rigidity modulus on a wide set ofrockfill dam bodies (Hunter & Fell, 2003), and the trend inthe measured results has been found to correspond to thesize effect relation (Frossard, 2009a). However, before thiscorrespondence between stressstrain curves and (even-tually) critical state conditions can be investigated further, itstill needs to be thoroughly compared with detailed experi-mental data.

Effect of change in void ratioThe size effect relation links sets of similar materials

made of the same mineral matter, with similar gradations(parallel on a log scale of grain size) and the same initialvoid ratios, implemented through similar compaction proce-dures. In such a situation, the correspondence between theinternal friction and the amount of grain breakage has beenfound to be unique, regardless of the characteristic size(coarse-, medium- or fine-grained), if all the specimens ofone set are given the same initial compactness (see Fig. 6,and other results from Marachi et al., 1969). So experimentalcurves such as those in Fig. 6 could be measured for variousvalues of initial void ratio on fine-grained model material, asdiscussed above. Such experimental curves will also hold forthe coarse rockfill corresponding to the model material.

CONCLUSIONSA rational approach of the physics of size effects due to

grain breakage in granular materials has been presented,which sets a new method for evaluating the mechanicalbehaviour of coarse granular materials widely used in civilworks, through reasonable laboratory testing efforts. Thewide set of experimental proofs presented has already shown

20

15

109876

5

4

3

210 30 100 300 1000 3000 10000

Confining pressure, : kN/m(a)

32

AC

B1

B2

Marsal (1972)Marachi

. (1972)et al

A SandstoneB1 SlateB2 SlateC Basalt

) fM

axim

um p

rinci

pal s

tres

s ra

tio, (

/ 1 3 Charles & Watts

(1980)

20

15

109876

5

4

3

210 30 100 300 1000 3000 10000

Confining pressure, : kN/m(b)

32

A C

B1

B2

) fM

axim

um p

rinci

pal s

tres

s ra

tio, (

/ 1 3

C

A

B2

150 mm 38 mm

Fig. 15. (a) Data compilation on shear strength against confiningstress by Charles & Watts (1980); (b) Evidence of size effects inshear strength compilation

ROCKFILL SHEAR STRENGTH EVALUATION 11

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

atzenkovRectangle

the global validity of the approach. A significant programmeis in progress in France to develop further confirmations ofthis validity, and explore its limitations through a wideexperimental programme, including 1 m diameter triaxialtests. The key assumptions underlying the approach aresimple, and are likely to be satisfied by many of the granularmaterials encountered in civil engineering practice, as con-firmed by the experimental proofs presented. However, asoutlined in the paper, some exceptions may be found withparticular granular materials.Significant practical applications have been developed, in

the area of material testing methods, and in the area ofstability analysis methods.Finally, the present paper has been focused specifically on

the effects of grain breakage on shear strength in granularmaterials. For a comprehensive insight into the physicsunderlying this shear strength in granular materials, thepresent results should be considered together with the prop-erties resulting from the effects of friction, and otherfeatures such as gradation and compactness, grain shape andmineralogy (e.g. Frossard, 1979, 1983, 1986, 2001, 2005a,2005b; Duncan, 2004).

ACKNOWLEDGEMENTSThis paper is part of a collective work under develop-

ment since June 2008 in the French research ProjectECHO, sponsored by the French Agence Nationale pour laRecherche (ANR), involving a wide partnership, including:EIFFAGE Travaux Publics; Electricite de France-CIH;Laboratoire de Mecanique des Solides from Ecole Poly-technique; Institut de Recherche en Genie Civil et Mecani-que from Ecole Centrale Nantes; Centre dEtudesTechniques de lEquipement- Laboratoire Regional desPonts et Chaussees Lyon; Coyne et Bellier Ingenieurs-Conseils (now incorporated in Tractebel Engineering), who

originated the project and coordinated its implementation,as coordinating partner. The authors gratefully thank: DrN. D. Marachi for permission to reproduce the originaldata shown in Figs 5, 6, 14(a) and 14(b) and Geotechni-que journal for permission to reproduce the original datashown in Figs 15(a) and 15(b). The second author alsowants to acknowledge the financial support provided bythe Chinese Scholarship Council.

NOTATIONA, b material parameters in De Mello failure criterion

DA, DB characteristic lengths or sizes for granular materials A andB respectively (e.g. maximum grain size Dmax, mean grainsize D50, or other given length)

d average diameter of a set of particlesf average crushing resistance (force) measured on a set of

particlesf ( n= p) contact force (vector) exerted by particle n on particle pfA, dA fixed crushing force exerted on a set of particles of given

diameterf fAg set of contact forces (vectors) within granular material Al( n= p) branch vector joining centres of particles in contact, from

particle n towards particle pm material parameter in Weibull statistical distributionPs probability of survivalV volume

, experimental material parameters in Marsal crushingstrength formula

stress, ij macroscopic stress tensor and its components

A, nA shear stress and normal stress at failure in granularmaterial A

REFERENCESBarton, N. & Kjaernsli, B. (1981). Shear strength of rockfills.

J. Geotech. Engng ASCE 107, No. 7, 873891.

A bn

n(b)

H0

0

G D( )0

Reference prototype

G D( )

H

Extrapolated project(c)

G D( )0 G D( )

D0

D(a)

Fig. 16. Combined impacts of size effects and other key parameters on rockfill embankment safety factors: (a) granulometry; (b) shearstrength; (c) size H and slope of the works

12 FROSSARD, HU, DANO AND HICHER

Bazant, Z. P. & Planas, J. (1998). Fracture and size effect inconcrete and other quasibrittle materials. London: CRC Press.

Becker, E., Chan, C. K. & Seed, H. B. (1972). Strength and deforma-tion characteristics of rockfill materials in plane strain and triaxialcompression tests, Report No. TE-72.3. Department of Civil andEnvironmental Engineering, University of California, Berkeley.

Biarez, J. & Hicher, P. Y. (1997). Influence de la granulometrie etde son evolution par ruptures de grains sur le comportementmecanique de materiaux granulaires. Rev. Fr. Genie Civ. 1, No.4, 607631.

Bolton, M. D. (1986). The strength and dilatancy of sands. Geo-technique 36, No. 1, 6578, http://dx.doi.org/10.1680/geot.1986.36.1.65.

CBDB-ICOLD (2007). Proc. 3rd Symp. on Concrete Face RockfillDams, Florianopolis.

Chang, C. S. & Hicher, P. Y. (2005). An elasto-plastic model forgranular materials with microstructural consideration. Int. J.Solids Struct. 42, No. 14, 42584277.

Charles, J. A. & Soares, M. M. (1984). Stability of compactedrockfill slopes. Geotechnique 34, No. 1, 6167, http://dx.doi.org/10.1680/geot.1984.34.1.61.

Charles, J. A. & Watts, K. S. (1980). The influence of confiningpressure on the shear strength of compacted rockfills. Geotech-nique 30, No. 4, 353367, http://dx.doi.org/10.1680/geot.1980.30.4.353.

Chavez, C. & Alonso, E. E. (2003). A constitutive model forcrushed granular aggregates which includes suction effects. SoilsFound. 43, No. 4, 215227.

Christofferson, J., Mehrabadi, M. M. & Nemat-Nasser, S. (1981).A micromechanical description on granular material behavior.J. Appl. Mech. ASME 48, No. 2, 339344.

Coop, M. R. & Lee, I. K. (1993). The behavior of granular soils atelevated stresses. In Predictive soil proceedings of the Wrothmemorial symposium (eds G. T. Houlsby and A. N. Schofield),pp. 186198. London: Thomas Telford.

Daouadji, A., Hicher, P.-Y. & Rahma, A. (2001). An elastoplasticmodel for granular materials taking into account grain breakage.Eur. J. Mech. A Solids 20, No. 1, 113137.

De Mello, V. F. B. (1977). Reflections on design decisions ofpractical significance to embankment dams. Geotechnique 27,No. 3, 281355, http://dx.doi.org/10.1680/geot.1977.27.3.281.

Deluzarche, R. & Cambou, B. (2006). Discrete numerical modellingof rockfill dams. Int. J. Numer. Anal. Methods Geomech. 30, No.11, 10751096.

Duncan, J. M. (2004). Friction angles for sand, gravel, and rockfill.Kenneth Lee Memorial Seminar, Long Beach, CA, 13 pp.

Frossard, E. (1979). Effect of sand grain shape on interparticlefriction: indirect measurements by Rowes stressdilatancy theo-ry. Geotechnique 29, No. 3, 341350, http://dx.doi.org/10.1680/geot.1979.29.3.341; Discussion by W. B. Whalley (1982), 32,No. 2, 161165, http://dx.doi.org/10.1680/geot.1982.32.2.161.

Frossard, E. (1983). Une re`gle decoulement simple pour lesmilieux granulaires. Geotechnique 33, No. 1, 2129, http://dx.doi.org/10.1680/geot.1983.33.1.21 (in French).

Frossard, E. (1986). Dilatance, dissipation denergie et crite`re derupture tridimensionnel dans les materiaux granulaires. Rev. Fr.Geotech., No. 34, 1731 (in French).

Frossard, E. (2001). Lapproche energetique en mecanique desmateriaux granulaires. Poudres & Grains, No. NS-2, 156 (inFrench).

Frossard, E. (2005a). Comportement macroscopique des materiauxgranulaires mis en oeuvre dans les barrages, Special Report inFrench Research Project Micromechanics of Rockfill Dams, spon-sored by the French Ministry of Research and Technology, Paris(in French).

Frossard, E. (2005b). A micromechanical constitutive law resultingfrom energy dissipation by friction. Proc. 5th Int. Conf. onPowders and Grains, Stuttgart 1, Part 1, 141145.

Frossard, E. (2006). A new energy approach in granular mediamechanics: applications to rockfill dams. Proc. 22nd Int. Con-gress on Large Dams, Barcelona 5, 191208.

Frossard, E. (2008). Seguridad de grandes presas de escollera:efectos de escala en la resistencia al corte, y en los factores deseguridad. Senior Lecture. Actas Jornada Tecnica Geotecnia dePresas. Madrid: Sociedad Espanola de Presas y Embalses (inSpanish).

Frossard, E. (2009a). On the structural safety of large rockfill dams.Proc. 23rd Int. Congress on Large Dams, Brasilia, Q.91R.39(CD-ROM).

Frossard, E. (2009b). Scale effects in large rockfill dams behav-iour and safety margins. Keynote lecture. Proc. 10th ICOLDBenchmark Workshop on Numerical Analysis of Dams, Paris(CD-ROM).

Frossard, E. (2009c). Effets dechelle dans la resistance au cisaille-ment des remblais granulaires et dans la stabilite de grandsouvrages en enrochements. Proc. 17th Int. Conf. Soil Mech.Geotech. Engng, Alexandria 1, 97101 (in French).

Frossard, E. (2009d). La relation deffet dechelle sur les courbesintrinse`ques dans les materiaux granulaires du genie civil:Bases physiques, ECHO Research Project internal report. Paris:Coyne et Bellier (in French).

Griffith, A. A. (1921). The phenomena of rupture and flow insolids. Phil. Trans. R. Soc. London A 221, No. 587,163198.

Hardin, B. O. (1985). Crushing of soil particles. J. Geotech. EngngASCE 111, No. 10, 11771192.

Hu, W. (2009). Contribution a` letude de leffet dechelle dans lesmateriaux granulaires. PhD thesis, Ecole Centrale de Nantes (inFrench).

Hunter, G. & Fell, R. (2003). Rockfill modulus and settlement ofconcrete faced rockfill dams. J. Geotech. Geoenviron. EngngASCE 129, No 10, 909917.

ICOLD (2002). Concrete face rockfill dams: Concepts for designand construction. Paris: International Commission on LargeDams, Committee on Materials for Fill Dams.

Indraratna, B., Ionescu, D. & Christie, H. D. (1998). Shearbehaviors of railway ballast based on large-scale triaxial tests.J. Geotech. Geoenviron. Engng ASCE 124 No. 5, 439449.

Lackenby, J., Indraratna, B., McDowell, G. & Christie, D. (2007).Effect of confining pressure on ballast degradation and deforma-tion under cyclic triaxial loading. Geotechnique 57, No. 6, 527536, http://dx.doi.org/10.1680/geot.2007.57.6.527.

Lade, P. & Bopp, P. A. (2005). Relative density effects on drainedsand behavior at high pressures. Soils Found. 45, No. 1, 113.

Lee, D. M. (1992). The angles of friction of granular fills. PhDthesis, Cambridge University.

Love, A. E. H. (1927). A treatise on the mathematical theory ofelasticity. Cambridge: Cambridge University Press.

McDowell, G. R. & Bolton, M. D. (1998). On the micromechanicsof crushable aggregates. Geotechnique 48, No. 5, 667679,http://dx.doi.org/10.1680/geot.1998.48.5.667.

Marachi, N. D., Chan, C. K., Seed, H. B. & Duncan, J. M. (1969).Strength and deformation characteristics of rockfills materials,Report No. TE-69-5. Department of Civil Engineering/Geo-technical Engineering, University of California, Berkeley.

Marachi, N. D., Chan, C. K. & Seed, H. B. (1972). Evaluation ofproperties of rockfill materials. J. Soil Mech. Found. Div. 98,No. SM1, 95114.

Marsal, R. J. (1967). Large-scale testing of rockfills materials.J. Soil Mech. Found. Engng Div. ASCE 93, No. 2, 2744.

Marsal, R. J. (1972). Mechanical properties of rockfill. In Embank-ment dam engineering: Casagrande volume (eds R. C. Hirsch-feld and S. J. Poulos), pp. 109200. New York: Wiley.

Miura, S., Yagi, K. & Asonuma, T. (2003). Deformation-strengthevaluation of crushable volcanic soils by laboratory and in-situtesting. Soils Found. 43, No. 4, 4757.

Nakata, Y., Hyde, A. F. L., Hyodo, M. & Murata, H. (1999). Aprobabilistic approach to sand crushing in the triaxial test.Geotechnique 49, No. 5, 567583, http://dx.doi.org/10.1680/geot.1999.49.5.567.

Rothenburg, L. & Selvadurai, A. P. S. (1981). Micromechanicaldefinitions of the Cauchy stress tensor for particular media. InMechanics of structured media (ed. A. P. S. Selvadurai), pp.469486. Amsterdam: Elsevier Science.

Weber, J. (1966). Recherches concernant les contraintes intergranu-laires dans les milieux pulverulents. Bull. Liaison des Ponts etChaussees Paris, No. 20, 120 (in French).

Weber, J. (1971). Les applications de la similitude physique auxproble`mes de la mecanique des sols. Paris: Eyrolles et GauthierVillars (in French).

Weibull, W. (1951). Statistical distribution function of wide applic-ability. J. Appl. Mech. ASCE 18, No. 3, 293297.

ROCKFILL SHEAR STRENGTH EVALUATION 13

BACKGROUNDFigure 1

SIZE EFFECT IN GRANULAR MATERIALS USED IN CIVIL WORKS: PREVIOUS WORK&mkb1;Size effect for individual grainsSize effect for individual grains&mkb2;Size effect in granular materialsSize effect in granular materialsTable 1Table 2Figure 2Figure 3Figure 4Figure 5

PROPOSED SIZE EFFECT RELATIONS BASED ON GRAIN BREAKAGE&mkb3;Assumed key features of grain breakageAssumed key features of grain breakageEquation 1Equation 2Figure 6Figure 7Figure 8Equation 3Equation 4&mkb4;Basic size effect relation for simple particle setsBasic size effect relation for simple particle setsEquation 5&mkb5;Size effect relation for granular materials (&kay;Size effect relation for granular materials (Table 2)Equation 6aEquation 6bFigure 9Figure 10Equation 7Equation 8aEquation 8bEquation 9aEquation 9bEquation 10

SIZE EFFECT RELATION AND SHEAR STRENGTH ENVELOPES&mkb6;The link with shear strength envelopesThe link with shear strength envelopes&mkb7;Geometric and analytical correspondences set by the size effect relationGeometric and analytical correspondences set by the size effect relationFigure 11Equation 11Equation 12Equation 13Equation 14Equation 15

VALIDATION WITH EXPERIMENTAL DATA&mkb8;Analysis of results by &kaydollar;Analysis of results by Lee (1992)Equation 16Equation 17Equation 18Figure 12&mkb9;Analysis of results from &kaydollar;Analysis of results from Marsal (1967)Equation 19Equation 20Equation 21&mkb10;Analysis of results from &kaydollar;Analysis of results from Marachi et al. (1969)&mkb11;Analysis of results from &kaydollar;Analysis of results from Charles & Watts (1980)Figure 13

DISCUSSION ON SIZE EFFECT RELATION AND PRACTICAL APPLICATIONS&mkb12;Key assumptions involved in size effect relationKey assumptions involved in size effect relationFigure 14&mkb13;Rationale for a new practical method for rockfill shear strength determinationRationale for a new practical method for rockfill shear strength determination&mkb14;Other practical applications of size effect relationOther practical applications of size effect relation&mkb15;Effect of change in void ratioEffect of change in void ratio

CONCLUSIONSFigure 15

ACKNOWLEDGEMENTSNOTATIONREFERENCESBarton & Kjaernsli 1981Figure 16Bazant & Planas 1998Becker et al. 1972Biarez & Hicher 1997Bolton 1986CBDB-ICOLD 2007Chang & Hicher 2005Charles & Soares 1984Charles & Watts 1980Chvez & Alonso 2003Christofferson et al. 1981Coop & Lee 1993Daouadji et al. 2001De Mello 1977Deluzarche & Cambou 2006Duncan 2004Frossard 1979Frossard 1983Frossard 1986Frossard 2001Frossard 2005aFrossard 2005bFrossard 2006Frossard 2008Frossard 2009aFrossard 2009bFrossard 2009cFrossard 2009dGriffith 1921Hardin 1985Hu 2009Hunter & Fell 2003ICOLD 2002Indraratna et al. 1998Lackenby et al. 2007Lade & Bopp 2005Lee 1992Love 1927McDowell & Bolton 1998Marachi et al. 1969Marachi et al. 1972Marsal 1967Marsal 1972Miura et al. 2003Nakata et al. 1999Rothenburg & Selvadurai 1981Weber 1966Weber 1971Weibull 1951