Proceedings of the International Symposium on Weak Rock / Tokyo /21-24 September 1981

Significance of geological features

for the mechanical behaviour of rocks and rock masses

PIERRE M.DUFFAUTBRGM, Paris, France

ABSTRACTAs stress is tensor, the structure of materials is the first parameter controlling their behaviour. Natural earth's

crust materials have to be studied by engineers at three scales, the rock matrix, the rock mass, and the mountainscale. Each scale displays different chief features, classified after the departures from continuity, isotropy andhomogeneity. Geometrical defects of the joint surface increase the rock mass strength. On the contrary,geometrical defects of the schistosity or foliation surfaces increase the deformability and decrease the strength.The scattered anisotropy of the mylonite structure does not leave any direction for stability. At the rock matrixscale, void content or porosity is the chief parameter, then the continuity of the solid phase or the amount andstrength of the cement between the grains. Rock engineers have to feel naturalists and look at their materialsbefore any test or work.

Mr. Chairman, dear colleagues. It's for me a greathonour to address you by the end of this Symposium.Just before beginning, I would like to thank andcompliment the Japan Society of Civil Engineers forhaving prepared all of this synthetic work presentedjust before, about soft rock engineering in Japan.

I knew well through the International TunnellingAssociation meetings that the Japanese engineers usedto meet difficult conditions in soft rock tunnelling.And i am preparing myself to visit tomorrow theprestigious Seikan Tunnel. And I was very glad tohear about the other fields of ground engineering inJapan, such as slope engineering and foundationengineering, whatever for dams or other largestructures.

More than soft rocks, I would like to speak aboutweaknesses of the materials within our earth's crust.As an engineer involved since thirty-five years insurface and underground works, most of them civilbut some of them mining, I have been highly interest-ed in both rock mechanics and geological engineering.(Fig. I) I can remember having attended bothmeetings where LS.R.M. and LA.E.G. were to beborn, the first one in Salzburg, Austria, 1962, and tilesecond one in New Delhi, India, 1964. And sinceI attended or sent papers to more than half of thechief meetings of both societies, it's why I wouldn'tmiss this opportunity given to me by the OrganizingCommittee of the Symposium on Weak Rocks tospeak about the naturalistic side of our field.

I can not say that I read the whole pre-prints of theSymposium and no more here can I say I hear all thepresentations. Therefore, I understand that many

authors and speakers brought here substantialmaterial to the theme of my lecture, and I wouldapologize for not having introduced all of thismaterial into it - possibly some of this material willcontradict my thoughts. Anyway, I shall be glad totake advantage of them in the future.

Of course, I am not trying to cover all of thesubject, which would need hours of lecture, and delaythe farewell party tonight. But I would like to giveyou some examples in order for many of you tocomplete the task for the benefit of our society andof some neighbouring societies.

ABOUT CLASSIFICATIONS

As any of you know, rocks are different in manyways. I first wrote by many standards, but where arethe standards? Rocks are different in many ways,and for that reason, geologists use many systems ofclassification together (Table I). You may know howintricated and sophisticated these systems are, andhow much we need simplifications. Even inside theBasic Geological Description rules given by a formercommission of LS.R.M., there appear words that fewpeople know, and fewer use calcirudites for example.It's clear from that example that we need to list themost significant features, before choosing the bestcriteria for classification purpose.

Engineers have yet asked for some specific classifi-cations; for example, they have established classifi-cation for weathered rock. But it applies mostly togranitic rocks. We do not have any classification forcrushed rock along faults although it be a main

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concern in many underground works. In both cases,weathered rock and crushed rock are concepts ex-tending from sound rock to very soft, which may bestudied by soil mechanics methods.

No more have we a good classification for un-weathered and uncrushed soft rocks, that is thosevery fine grained detritic rocks covered by mud-stones, claystones, shales, c1ayshales, argillaceousshales, etc. I quote there words from the Sym-posium pre-prints, and I can't perceive exactly anygood reason to use one word instead of another one.In French and many other languages, some more,many more, are available, and of course, there is nopossibility for accurate translation.

My personal feeling from years working either onthe geological side or the mechanical one, in thelaboratory or on the job site, my personal feeling isthat the first basis for engineering classification isbehaviour along stress. And that behaviour dependsmainly on structure; structure being used here tocover any departure from the three classicalhypotheses, at the beginning of any rheological study,continuity, homogeneity, and isotropy.

SCALE

Let's first consider solid nature, like we consider abuilding (Fig. 2 and 3). At any scale of observation,in-situ earth looks like it were made of assembledparts. That is, at a lower scale, each part is made ofassembled sub-parts, and the same at a higher scale,all over the whole length range, from the angstrom tothe crystalline grain, to the mountain, and to theplates, along the modem theories of the earth's crust.

Two main scale ranges are classical in rockmechanics, the rock matrix, relevan t for any com-minution problems, as well as for any laboratory testson samples. Then, the rock mass, relevant for mostof foundation and tunnelling problems, and thosetwo main scales, appear as the common engineeringscales. But a higher level length scale would beneeded over some tens of meters. I call it now"mountain scale", expecting a better word from oneof you, which will be relevant for high slope problems- either natural mountain slopes, as you just sawbefore, or dams abutments, large open pit slopes, andalso large underground works, like mines, and bigsolu tion caverns.

Such classification in the three engineering scales isnot random, as structural features are different whengoing from one scale to another, as happens in con-struction. Alloy crystals are elements for steel. Steelis material for beams. And beams are elements forstructures.

Let me please use here the word "structure" inthe widest sense, covering the whole construction,the assembly of parts, the texture of the material ofthe parts, and the crystal lattice.

From this model, the first order structural featuresin rock mechanics would be listed on the Table 2,from which I am now to take some examples, inorder to go to second order features. [do not meanthat this bi-dimensional table exhausts the subject.Nature is not, bi-dimensional, and it's far more thantri-dimensional. But we have to begin with simple

schemes, and introduce sophistication only whereneeded.

Also, I limit myself from here to the three centralscales, the most significant for engineering mechanics.

DISCONTINUITY

Surfaces of discontinuity have long since beenrecognized as a major feature in rock mass mechanics,as they allow for easy opening under tensile forces,and easy shear displacement of bodies on each side.

Against the tensile forces, only solid bridges acrossthe surface arc efficient, until they perish fromfatigue. Against the shear force, on the contrary,a number of geometric features will provide strengthin excess of pure friction. (Fig. 4):, at the smallerscale, roughness, then, waviness or steps - all thosefeatures being generally oriented along a direction ofthe surface. And please remind that steps arc notsymmetrical. They prevent the movement only inone direction, like transistors.

And at a higher scale, a general curvature or bendor fold of the surface may occur. Of course, orienta-tion always is the major parameter of surfaces of dis-continuity.

Strike and dip of the plane, strike of the linearstructural on it, including folds if any. We all knowthat horizontal bedding joints are very favorable,and it's trivial to refer to conventional constructionwith bricks or stones. But the mason's work comesfrom very ancien t experience, which must not beunder-estimated today. (Fig. 5)

In natural sedimentary beds, the cross joints, thedia-clases (as Greek dia is for cross) are not so inter-rupted as in the brickwork, as a whole. And for thatreason, the stability of the natural assembly may beless than that of the brickwork. It happens that somerock mechanics researchers use models of brickworkfor limestone beds, maybe because geologists use justthis pattern as the standard sketch for such rocks.

More generally, join t sta tistics are now widely usedby engineers. Computers provide us easily with meanvalues and standard deviations. But we have to takecare. The mean value has no significance for aspecified problem. On the contrary, we shouldconsider the extreme value, which is on the dangerousside for the problem in cause.

Then, we have to go back to the site, and only ifthere is evidence of this dangerous joint not crossingthe rock mass, at the wrong place, we should take aless conservative design joint.

Within this chapter on discontinuity, I don't wantto refer to the rock matrix scale; the theories of crackaction are well developed; no more to the mountainscale; only for noticing that the bigger the scale youconsider a surface of discontinuity, the more thechance to discover departures from planeity , thenthe in fluence of join t strength decreases when thescale increases.

ANISOTROPY

Going to anisotropy, I would like to stay a fewseconds at the crystal scale, to mention that evencubic minerals are anisotropic as regards elastic defer-

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mation, contrary to their optical behaviour. The twodimensional model of a plain fabric may be referredto here (Fig. 6). Linear modulus is the same alongthe woof and the warp, but deformab ility is greaterby far, by an order of magnitude, or sometimes morein the diagonal direction, this direction called "bias"by dress-making people.

Here lies a simple explanation for those diagramsof modulus and strength of some anisotropicmaterials along the direction.

Of course, in a fabric, the space left between thethreads in both directions allows for large diagonaldeformation. The same may be true for crystals asthere is some space between atoms.

It doesn't apply as well to the blocks of an insiturock mass, except when their corners are softened.And this happens the more where the scale increases.

I just referred to the mason and to the tailor. BothUse different materials to build different structures.Should they have only one material at hand, theychange the pattern and the orientation to fit thebehaviour of the wall or dress. Should they havemore than one, they choose the right one at theright place, and in the right direction. I am sure wehave to learn from both ancient crafts.

May I refer to the pattern of retaining walls Ienjoyed to see in Japan along many roads (Fig. 7).

Another comment is about tunnelling throughrocks with a major schistosity. It's well known thatthe direction is relevan t. The easiest tunnelling isthen normal to a vertical schistosity. Myself, Iexperienced tunnelling parallel to a sub-verticalschistosity. And it was not too bad. But the worstPosition is a sub-horizontal schistosity, where thegravity field adds its action to the natural instability.

And the vertical schistosity makes me remind theproblem of the least cost tunnel between two pointswhen the shortest line is parallel to schistosity(Fig. 8). As I said previously, the tunnel cost in thenormal direction is lower: Should the cost -direction graph be elliptical, the least cost tunnelmight be designed as a zigzag, like the way of a sailboat against the wind, provided the ratio betweenmaximum and rniminum cost be over square rootof2.

This paradoxical result vanishes when you considera cost direction graph far from the ellipse with"horns".

In the stability of schistose rocks, the Oat surfaceparallel to the schistosity are critical. Due to theslenderness, they are likely to buckle one sheet afterthe other. Exactly as buckling is highly sensitive toany geometrical defect of beams or poles, thestability of schistose rocks is highly sensitive to anydefect, such as departure of planeity, cross join t, andthose frequent tectonic microstructures named"kink bands".

The more the schistosity is undulated and sheared,the easier the deformation. From this point of view,rock masses can be classified very roughly in onlythree structural models (Fig. 9). The classical box ofsugar pieces - (classical in France) with an ortho-tropic symmetry, or clinotropic, if you want todepart from the right angles, then, the stack of sheetsaxi-symmetrical and then, the mylonite structure,

with undulated crossbedded shear surfaces. Suchstructure has been described by Skernpton at theFirst I.S.R.M. Congress in Lisbon, 1966. And I thinkit has not since received all the attention it deserves.Actually, it's responsible of many within the mostdifficult tunnels in rock, such as the railway SimplonTunnel, bored around the tum of this centurybetween Switzerland and Italy, and the expressway,Tauern Tunnel, bored some years ago in Austria.

HETEROGENEITY

I kept heterogeneity as the last point, as it has notyet received so much attention as the two previousones.

Heterogeneity is the most important at the rockmatrix scale, and it widely appears, not only becausevarious minerals lie side by side in many rocks, buteven because voids are presen t in any rock and soilmaterials.

And void is by far the softest material.About minerals, as far as the strength of in-situ

rock mass is concerned, only two classes are needed _clay minerals, soft and highly water-sensitive; and theother ones (exception to be done for salt mineralswhere they exist).

It's only when studying comminution problems oraggregate problems, you may need to look further tominerals like calcite, quartz, and others; - thoseother ranging in hardness and strength betweenquartz and calcite, for the most of them.

Two days ago, I mentioned in my presentation,that two simple geometrical cases of heterogeneityare easy to handle. The isolated inclusion or void,and the sandwich. Simple and multiple geologicalsandwiches are widely distributed in the earth crust asfault zones in granite, hard dikes in soft rocks, andbanded sedimentary formations.

On the contrary, isolated holes or inclusions aresomewhat rare. The grains and voids in most ofdetritic rocks are too close to one another to benefitthis theory. However, the theories developed forconcrete can be a first approximation.

Many authors described here the behaviours of twoor more rocks and concluded that the differenceswere related to void content, that is, porosity. Andfor equal porosity, to the bond between grains. Agood example is the comparison between soft sand-stone, low cement, and a volcanic tuff with the sameporosity. The second rock has by far more strength,as the voids are dispersed into a solid continuousphase obtained from melt rock. The first rock, onthe contrary, exhibits a somewhat dispersed solidphase. The complete theory of the failure of thevolcanic rock, mono- and tri-axial, has been given bySantiago Uriel.

Such difference is found in weathering of compactigneous rocks. While some solid is removed bysolution, the skeleton remains continuous. And evenwith a porosity of 0.5 attained in tropical climates,the strength remains that of a rock. But the penetra-tion test gives a very wrong idea of the material, as itshigh void content, allows for easy penetration,whatever the strength.

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CONCLUSIONS

As concluding remarks, I would like to prevent youto rely too much on other men, whatever their skilland science, to perform what is a part of our job. Wedo have to make the rockmass a reliable part of ourengineered civilization. Then, we have to understandall of its elements ourselves.

I would refer to the image of a surgeon, whose taskis to cut across the live flesh to remove and replacesome parts, to implant new organs or structures.Surgeons are engineers working on the human body.And we are surgeons working on the earth crust.What could you think of a surgeon who would haveto rely before cutting on another man having beentaught anatomy? Fortunately, all surgeons havelearned anatomy themselves, and also physiology.And I would like to be sure that rock and soilengineers learn enough anatomy and physiology ofour earth.

Of course, specialists of anatomy and physiologyof the earth do exist: That is very good for us, andcan save a part of our time. But we have to considerand understand ourselves all the properties significantfor our engineering purposes. And we have tocooperate with engineering geologists to establishan efficient and reliable common language.

We have much to do ahead: The more, while ourfield has not yet been exhaustively ploughed. I justquote from the 21 st Rankine Lecture by ProfessorMorgnstern: "Geotechnical engineering is not yetmature, and far from being ready to standardization.Instead, the range of natural materials is so freat thatthe limits of our profession expand continually."No doubt that Japanese tuffs will add an interestingchapter to rock mechanics.

In fact, this Symposium brought some morematerial and some answers to questions I have raised.For example, a tentative classification of soft rockswas presented from South Africa yesterday. Butevery time a natural material exhibits an unforeseenbehaviour, that means we have something more tostudy. Then, we are sure to leave work ahead formany more symposia and congresses.

ARIGATO GOZAIMASU.

REFERENCES

DUFFAUT P. (1970) Essai de description structuraledes roches a l'usage de l'ingenieur, Proc. 1st Int.Congo IAEG, Paris

DUFFAUT P. (1981) Structural weaknesses in rocksand rock masses. Tentative classification andbehaviour, Proc. Int. Symp. on Weak Rock, Tokyo

MORGENSTERN (1981) 21st Rankine Lecture,Geotechnique, London

SKEMPTON (1966) Proc. 1st Int. Cong ISRM,Section 3 paper I 7, Lisbon

URIEL S. (1980) Le phenomene d'effondrement dansles roches volcaniques a forte porosite , Conf.French Com. on Rock Mech, to be published 1982in Revue Francaise de Geotechnique , Paris

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ENGINEERINGSCIENCES

EARTH SCIENCES

Fig. 1. Fields of Engineering and Earth Sciences

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BUILDING BEAM STEEL

DDDDOcoooo0000000000

~000000000000000coDe Cl

~ +----+--< ..........-.20m 0.2m 2mm

200m 0.2m 2mm.............-- +----+--< >---+-l

LV @MOUNTAIN BLOCK MATRIX

Fig. 2. Main Scales for Structures

10'EARTH PLATES

formations assembled

blocks or shce tsassembled

grains assembled

ROUGHNESS

WAVINESS

STEPS

BEND or FOLD

-

Fig. 4. Geometrical Defects of Surfaces of Discontinuity

BRICKWORK

II

I I

NATURE

Fig. 5. Patterns of Joints in Brick Work and in Nature

_1111_1111-'-1-1-1-1-1--1-1-1-1--1-1-1-1-1--1-1-1-1--1-1-1-1-1--1-1-1-1--,-1-1-1-1-,&IIIIIITI-,/

E

E

Fig. 6. Plain Fabric and Its Anisotropic Behaviour

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Fig. 7. Japanese Pattern for Retaining Walls

cost

schistosity

'''~~ .. --- --.\II

II

Fig. 8. The Least Cost Path in Anisotropic Medium

Sugarpieces

I~Sheet stack

Mylonitestructure

Fig. 9. The Three Major Rock Mass Structures

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horn

Table I Examples of Geological Classifications

Composition

{

QUATERNARYCENOZOIC

AGE MESOZOICPALEOZOICPROTEROZOIC

{

CRETACICJURASSICTRIASSIC

{

IGNEOUS

GENESIS METAMORPHIC

SEDIMENTARY

{EFFUSIVEINTRUSIVE

{ORTHOMETA

{SEA WATERCONTINENTAL

{ SILICA = QUARTZ

COMPOSITION SILICO ALUMINATES = CLAYS,FELDSPATHS, etc.

CALCIUM = CALCITE

{CLAY LUTiTESSILT

GRAIN SIZE SAND ARENITESGRAVELBOULDERS RUDITES

Table 2 Classification of Structural Features

Scale rock matrix rock mass mountain

dykes,bedding faults,

dis-continuity cracks joints contactscross joints between

formations

voidsfolds

hetero-geneity inclusionsaI terna ting laccoliths

cementslayers salt domes

etc.

an-isotropycleavage schistosi tyfaliation

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