ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and...

download ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation During Shear Displacement

of 8

Transcript of ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and...

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    1/8

    Eurock '96, Barfa (ed.)!$;) 1996 Ba fkema, Ro tterdam. ISBN 90 5 41 08 43 6

    Rock joint shear mechanical behavior with 3D surfaces morphology

    and degradation during shear displacement

    Comportement mecanique avec modelisation 3D d 'un joint en cisaillement

    Das mechanische Verhalten und die 3D-Modellierung der Morphologie der

    Oberflachen und ihrer Beschadigung in Direcktschnitt-Proben

    Guy Archambault &Rock Flarnand - Centre d' Etudes sur les Ressources Minerales, Universite du Quebecd Chicoutimi. Que .. Canada

    Sylvie Gentier - BRGM. Direction de /0 Recherche. Or/eons, France

    Joelle Riss - Centre de Developpement des Geosciences Appliquees, Universite de Bordeaux I, Talence, France

    Colette Sirieix -ANTEA, Direction de /0 Geotechnique, Orleans, France

    ABSTRACT: Joint shear behavior is analyzed in relation with profiles 20 statistical description, 3D statisticalmodelling of asperities angularity, geostatistical analysis and krigeage modelling to detect superposed structuresand restitute surfaces topography of a joint roughness morphology on replicas of a natural fracture submitted to

    direct shear tests performed under various normal stresses and stopped at defined shear displacements. Theshear processes and progressive degradation on the replicas joint surfaces as well as their evolution are evaluatedthrough measurements of the damaged areas using image analysis. The evolution of the size and location of thedamaged areas arc analyzed in relation with normal stress for given shear displacement.

    RESUME: Le comportement d'un joint en cisaillement est analyse en fonction de la description stanstique 20des profils, de la rnodelisation statistique 3D de l'angularite des asperites, de l'analyse geostatistique et dukrigeage permenant de detecter la superposition de structures et de restituer la topographie des surfaces de lamorphologie de la rugosite du joint, sur des repliques d'une fracture naturelle sollicitees en cisaillement directsous diverses contraintes normales et pour des deplacernents en cisaillement definis. Les mecanismes de cisail-Iernent et la degradation progressive des surfaces du joint sur les repliques, ainsi que leur evolution sont cvaluesen rnesurant les aires endornmagees a I'aide de la technique d'analyse images. L'evolution de la dimension et dela localisation des aires endornrnagees sont analysecs en fonction de la contrainte norrnale appliquee pour des

    deplacements en cisaillemcnt definis.

    ZUSAMMENFASSUNG: Das Verhalten von Kluftscherung wird analysiert in Benzug auf 2D-statitischeBeschreibung del' Profile, 3D-statitische Modellisierung del' Winkel von Asperiten, geostatitische Analyzen undKrigeagemodellierung, urn aufeinandcrgereihte Strukturen einer rauhen Kluftmorphologie auf Bausteinmodelleelnes naturlichen Bruches, festzustellen. Scheversuche an diesen Modellisierungsversuch wurden unterverscheidene norma Ie Spannungen ausgefuhrt und bei hestimmten Scherdeplacierungen angehalten. Del'Scherungsprozess und progressive Degradierung del' Kluftflachen des Bausteinrnodels wurden in den gestortcnZonen durch lmageanalysierung ausgewertet. Die Entwicklung del' Grosse und genaue Stellung diesel'Storlingszonen wurden analysiert in Bezung auf Norrnalspannung fur einen bestimmten Scherungswert.

    fNTRODUCTION

    The prolific literature on characterization and behavior

    of single, irregular rock joints submitted to various

    normal and direct shear loading conditions, to eva-

    luate the needed mechanical and hydraulic parameters

    In hydromechanical stability analysis of workings in

    fractured rock masses (Stephansson 1985; Barton &

    Stephansson 1990; Myel' et al. 1995; to cite only the

    three organized symposium of the .ISRM commission

    On rock joints) show their inextricable complex

    behavior and characteristics. These numberless

    research works on various rock joints problems

    Confirm Scholtz (1990) statement that there is noconstitutive law for friction quantitatively built upon

    micromechanical framework because of the com-

    plexity of shear contacts, the topography of con-

    tacting surfaces and the evolving surfaces topography

    during sliding. A recent review of the literature onrock joints testing and modelling (Stephansson &

    ling 1995) pointed out that there is still a large

    number of problems to solve before having an overall

    understanding of the phenomenon, particularly the

    roughness morphology and the difficulty of its

    characterization and modelling.

    In this paper, an approach is reviewed for geo-

    metrical description of joint surfaces morphology on

    the basis of a 3D statistical description and modelling.

    A contribution to the characterization of damaged

    areas in relation with shear displacement under

    constant normal stress is also presented, in which theevolution and geometrical characteristics of the

    damaged zones are evaluated in relation with the

    normal stress magnitude, for given shear displace-

    247

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    2/8

    Table I. Classical linear roughness coefficients measured on profiles parallel to the shear direction.

    Profile RL Z2 Z3 8rcgrcssion S /x mm2 Z4

    z=f(x)

    A B A B A B A B A B A B

    I 1.029 1.029 0.251 0.246 0.547 0.608 1059 1063 0.108 0.092 0.089 0.190

    2 1.030 1.031 0.249 0.254 0.528 0.551 00

    5 6 00

    6 4 0.281 0.286 0.111 0.220

    3 1.033 1.033 0.263 0.264 0.597 0.581 - 007 4 - 007 6 0.327 0.321 -0.018 0.0244 1.041 \ .036 0.300 0.283 0.611 0.521 0

    005 0002 0.585 0.552 0.057 0.101

    5 1.022 1.030 0.212 0.254 0.448 0.56\ -1088 -1068 0.702 0.637 -0.017 0.048

    6 1.042 1.043 0.300 0.306 0.556 0.590 - 00

    3 4 - 00

    3 7 0.493 0.485 0.136 0.103

    248

    damaged zones are evaluated in relation with the

    normal stress magnitude, for given shear displace-

    ment.

    2 JOINT ROUGHNESS CHARACTERIZATION

    Joint walls morphology characterization means here

    that a 3D quantitative description of roughness evolu-

    tion on the joint surfaces was done during laboratory

    shear tests performed on a series of identical replicas

    from the walls of a natural fracture in a granite

    (Gueret, France) for which a detailed study of the

    morphology was done (Gentier 1986; Riss & Gentier

    1990, 1995). The replicas were submitted to direct

    shear tests under three different normal stress and

    each shear test was stopped at a defined shear dis-

    placement for five displacements (Flamand et a!'

    1994). This procedure permits a control on theevolution of the joint wall surfaces morphology with

    shear displacement.

    The shear stress, shear displacement, normal

    stress and normal displacement being available at the

    end of each test; a morphological analysis was

    performed using five profiles, z=f(x,y), recorded on

    the joint wall surfaces 15.26 rnm apart in four direc-

    tions, digitized at a constant step (L'l.x=0.5 rnrn) and

    kept constant for all shear tests (Flamand et a!' 1994;

    Riss et a!' 1995). The analysis of these data consists

    first in the deduction of parameters characterizing the

    whole set of recorded points, i.e. the joint wallsurface expanding in a 3D space and, secondly, in

    detailed distribution analysis of the 82 angles between

    a reference plane and a line segment linking two

    successive points on the recorded profiles. This

    analysis is fundamental to restore the true 3D colati-

    tude (83) distribution of elementary plane facets

    composing the joint wall surfaces before and after

    testing and correcting by the same way the bias intro-

    duced by the profiles.

    2. I Statistical description and analysis of the joint

    surfaces

    The overall analysis of the joint wall surfaces gives a

    global view of the morphology and depends on

    wheter we are interrested in the 3D spatial reality of

    each joint wall surface or to each joint wall surface

    recording directions related respectively to:

    a) the total variance of all profiles set of points,

    the residual variance and correlation coefficient after

    linear regression of the altitudes z in function of the

    coordinates (x,y) on the reference plane, the azimuthand colatitude in a given reference system of the

    regression plane and of the principal plane resulting

    from the diagonalization of variance and covariance

    matrices of the whole set of points; or

    b) the linear roughness coefficient RL, the z2, z3

    and z4 coefficients and the linear regression parame-

    ters (8rcgrcssion and residual variance ~.~).As the total variance is an invariant for a given

    set of points (x, y, z referred to given reference

    systems: VI01al = V x+Vy+V z) the spatial dispersion of

    the set of recorded points and its evolution during

    shear displacement can be measured and comparedproviding profiles are recorded at the same positions

    on each wall surfaces of the fracture replica and kept

    constant in each shear test. Taking into account both

    variances and covariances, it is also possible to

    compute estimations of the local mean plane of the

    fracture that could be slightly different from the

    regional one. They are based firstly on a linear

    regression of z on x and y and secondly on deriving

    the first principal plane from the diagonalization of the

    variance-covariance matrix. The following analysis

    performed on the joint wall surfaces under study

    permits to establish that the local mean plane dipsslightly (5) in a direction perpendicular to the shear

    direction, that the upper wall dips slightly more than

    the lower one and that the lower wall is rougher than

    the upper one. The detail of these analyses and

    computations are given in Riss et a!' (1995).

    From the computed linear roughness coefficient

    (RL) (Table I), dependent on 82 distributions, and on

    the basis of experimental 20 colatitudes distributions

    characteristics, it is observed that probabilities of 82

    is slightly higher than for the negative 82, but the

    mean values of the latter are smaller than the other

    one. In

    average, there are more asperities in thepositive direction than in the opposite one but these

    asperities are smoother than in the negative direction.

    On the whole, repartitions of positive and negative

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    3/8

    angles can be considered as identical for wall A and

    slightly more dispersed for wall B. Experimental 20

    colatitudes (82) for both walls are quite similar and

    the mean fW > is 10.310 for wall A and 10.080 for wallB while 8~-) is -11.910 and -12.490 for wall A and8respectively. Also, from Table I, the parameters

    8rcgrcssion and residual variance S;.x indicate the trend

    of the profiles to dip and the latter measures the part

    of the elevations z that are not explained by the global

    dip of the fracture replica. The dipping trend of

    profiles can indicate probable zones of contact where

    asperities may be damaged, particularly for the

    profiles dipping towards the shear direction and

    against the sense of shear displacement while, for

    oppositc dipping, there will be a trend of the surfaces

    to separate in creating voids between them. But these

    situations depend also on asperities heights (CLA or

    RS), profile roughness (RL), roughness dispersion

    (S;.x) and other linear coefficients, So, looking at

    one of them without taking into account informationsfrom the others can introduce large errors and biased

    informations on the morphology of the surfaces.

    Moreover, 20 roughness parameters are insufficient

    to describe adequately the joint wall surfaces

    IllOllJhology changing from to point to point.

    2.2 Statistical modelling in 3D of roughness

    angularities

    Digitized profiles look like polygonal lines and a

    polygonal surface results from a joint wall inter-

    section wih a set of contiguous hexagonal prismsorthogonal to the mean regional plane. Then, the

    surface is subdivided into small facets, small enough

    to be considered as planar (Fig. IA). Angles between

    the normal to facets and z axis are the real colatitudes

    (83). As shear direction is parallel to the local mean

    plane horizontal direction, it is assumed that 20

    colatitudes (82), measured in vertical planes in this

    direction, can represent any colatitude measured in a

    plane perpendicular to the local mean plane. Using

    classical method of 3D colatitude reconstruction

    (Gentier 1986), inference of the 3D colatitudes distri-

    butions is done (Fig. IB). The reconstructed distri-butions are not strictly similar for each wall, like the

    20 distributions. Then areal roughness (RA) is

    computed from these distributions in order to

    evaluate, by comparison, the reconstructed distribu-

    uons F(83) using RA values deri ved from a stereo-

    logical method. The F(83) distributions being

    acceptable, then theoretical models must be fi t to them

    In order to have an expression useful for further

    developments such as a simulation of the fracture wall

    Surfaces and for estimating RA. Inference is done

    either by fitting the 3D empirical distribution derived

    frOIll the 20 (82) distribution to a model or by fillingthe 20 experimental (82) to a 20 distribution from a

    3D model (Riss & Gentier 1989, 1990). The 3D

    mOdels used are generalized axial distributions:

    A)

    Section

    (prOfil~eFacet

    Line of "'-reference T 8] I

    rue ang e8,

    Apparenl angle

    B)3.5

    3.0

    (IN l(Jc

    1.0

    '" ~ 25.c

    o0 .20

    '0'

    ~.~ 1.5 .o

    1.0

    0.75

    0.5

    0.25

    0.5

    o

    6010 20 30 403D Cotaliludes

    50

    K ~ R

    A I M o d e l fhWaliA 16.58 1.10 1.0634 13

    I

    1611

    WallB 15.08 1.25 1.0625 [ 13 15'96

    A+B 12.06 1.65 1.0632 13 1588

    I

    Figur-e I. AlDcfinitions of 02 and 03 and stercogrnphic projection showing

    the dependence of 02 on 03 the direction of the vertical section plane; ll) 3D

    r ec onstruc te d distr ibutions and models for colatitudcs 83 with characteristics

    of the filled models for 3D distribution.

    Exp (Kcos~83)F(e3) =f l sin 03

    o Exp(Kt~)dtModels derived from the fitting process are shown in

    Figure IB with the parameters used and the charac-

    teristics deduced for RA, the mode and the mean (8 3).The most important result is that the 3D elementary

    facets dip in any direction with a mean angle of 16

    and this angle is obviously greater than the 20 mean

    angle of individual segments (Riss et al. 1995).

    These statistical analyses and deduced physical

    conditions of the joint walls permit to establish that a

    perfect matching between the surfaces is highly

    improbable with the differential variation between

    walls dipping and roughness as well as between 20and 3D colatitudes (82 and 83) or asperities slopes

    distributions on both walls. Moreover, Gentier

    (1986), in its evaluation of voids between the joint

    249

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    4/8

    walls, has illustrated a series of profiles of both walls

    adjusted together in which large void spaces and few

    contact areas could be seen between them. 3D analy-

    sis of enclosed void spaces between the fracture

    surfaces to evaluate voids morphology, either by

    statistical simulation or casting of voids, show also

    few contact areas between joint surfaces (Gentier &Riss 1990). These studies also show that most of the

    contacts seem to be located on the slopes of asperities

    and it even may happen that on particular profiles no

    contact can be seen. Equally, an anisotropic joint

    shear behavior with shear direction and sense may be

    deduced from the previous analyses and be attributed

    to the slight dip (5) of the tested joint local mean

    plane perpendicular to the shear direction used, the

    dip variations of the profiles with the recorded direc-

    tion, the variability in roughness of each joint wall

    and the dissymmetry between the positive and nega-

    tive colatitudes (82) on the profiles.

    2.3 Geostatistical analysis a/joint sur/aces

    Even with all these quantitative statistical analyses and

    the more sophisticated 3D statistical modelling and

    characterization of joint roughness angularities giving

    a more realistic figure than the 2D statistical evalua-

    tion, no spatial structural information is given

    regarding asperities structures (shape, size, jogs,

    waviness and others) for an adequate modelling of

    joint shear deformation and strength. Asperities spa-

    tial distribution and shape on joint wall surfaces arenot necessarily at random and the presence of super-

    posed structures cannot be detected on 3D angularities

    distribution (Fig. IB) and the presence of a major

    structure like a jog showing high angularity (>45),

    this will not change appreciably the distribution

    shown. But the mechanical shear behavior will be

    greatly affected by this structure which will control

    the joint shear strength and the related dilatancy

    behavior. Various methods are available to do it and

    among them geostatistics with which variograms and

    variographic analysis of profiles permit to characterize

    the size of asperities structures (range) related to theheights, curvature radii and angularity distributions

    while krigeage modelling and simulation can restitute

    the topographic surfaces (Fig. 2) (Gentier 1986;

    Gentier &Riss 1990).As an example, the variograms analyses applied

    to heights, curvature and angularity statistical

    distributions on various samples of Gueret granite

    joint surfaces show at least two overlapping asperity

    structures: 4 to 6 rnrn and 18 to 20 mm, and also a

    large one (40 mm) causing a 5 dipping of the joint

    plane sample. The chosen shear direction was

    parallel to the principal structure, so that only theroughness morphology represented by the 3D

    colatitudes (83) distributions (Fig. IB) are controlling

    the joint shear behavior.

    Fi~urc 2. Example of krigcage of a j o in t s u r fa ce based 011 an isotropic

    spherical variogram model. ({lJler Gcnticr, /986)

    3 JOINT SHEAR BEHAVIOR PHASES WITH

    SHEAR DISPLACEMENT

    3.1 Direct shear testing and results

    The direct shear test results come from a shear test

    program (Flarnand et al. 1994) on joint replicas

    submitted to three different constant normal stress (7,14 and 21 MPa). The 15 shear tests were performed

    and stopped at various shear displacements (0.35,

    0.55, 1.0, 2.0 and 5.0 rnrn) and the results are

    summarized in Figure 3. In Figure 3A the shear

    stress-shear displacement-dilatancy relationships arc

    recorded for the shear tests done. These results arc

    plotted, for the main characteristics, in a Mohr

    diagram (upper part, Fig. 3B) while the lower

    diagram shows the dilatancy rate (or angle) in relation

    with normal stress (ON)' These results are compared

    with LA DAR model (Ladanyi &Archambault 1969)

    for io values of 15 and 30 and with Barton's model(Barton 1973) for JRC values of 10 and 14, values

    estimated with the Z2 coefficient in Table I (Tse &

    Cruden 1979). A good agreement between experi-

    mental results and io values of LADAR model

    between 15 and 17 very near the mean ih value of16 (Fig. IB) evaluated statistically for the 3D

    angularity of asperities on the joint surfaces. The

    same observation regarding dilatancy rate (or angle

    dnr) behavior with the normal stress where dl~r took

    values of 14.1,11.7 and 10.4 for ON= 7,14 and21 MPa respectively for very low ON/Oc (values

    between 0.1 and 0.3). This also is in better

    agreement with the reconstructed 3D distribution of

    asperity angularity discussed in detail in Archambault

    et al. (1995). A compilation of peak dilation angles

    250

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    5/8

    and 1/(JN values from direct shear test results

    performed on various rock joints by different workers

    (Barton 1973) including Barton's own results as well

    as those from Ladanyi & Archambault (1980) on

    irregular tensile fractures and more in the last decade

    confirm a certain trend for io values to be between IS O

    and 35 limits and for dnf, variation between 0 and

    25 at peak, was observed.

    3.2 Shear behavior phases

    The shear process and mechanisms of a joint with

    irregular surfaces, on the basis of the previous

    results, may be summarized in the following phases

    (Fig.3A):

    Pre-phase: Normal loading on the mean shear

    plane of the joint concentrates the normal stress on

    very few points (contact areas) with a normal closure

    depending on normal load magnitude and joint

    surfaces asperities morphology well studied in theliterature (Goodman 1976; Gentier 1986; Bandis et al.

    1981 ) .

    A

    35

    IV V30

    " '~ 25lJl

    lJl 20eV i

    :;; 15

    ' ".c:(J)

    10

    5

    (JNl (7 MPa)

    (JN3(21 MPa)

    (JN2 (14 MPa)

    oo 1 234

    Shear displacement (mm)

    5

    60 III(JNI (7 MPa)

    III IV VN". . ,~ (JN2 (14 MPa)> < 40E.s: . .cc:

    '" 20

    ]1Q

    0-

    0 1 2 3 4 5

    Shear displacement (mm)

    Phase I: Elastic mobilization of shear stress by

    friction, with the shear load gradual application from

    zero level causes a new closure (negative dilatancy).

    It results in an increase of the real contact area until

    gross slippage is imminent and it may reach three

    times the initial static area without change in the

    normal load. However for any two surfaces, the final

    area is a numerical constant times the initial area

    developed with the normal load only, so that the

    proportionality between both forces (shear and

    normal) at the point of slipping is maintained, and this

    increase in contact area was called "junction growth"

    by Tabor (1959). This phenomenon was indirectly

    observed by an increase of induced interstitial

    pressure (Poirier 1996) in this phase of shear

    displacement on joints. This phase of increasing

    shear load give rise to a transfer of the stresses on

    asperities positive slopes defined by their angularity

    and friction is mobilized on their inclined planes and

    accompanied by their deformation. Phase 1 1 : A non-linear shear stress-shear dis-

    placement-dilatancy hardening mobilization phase to

    B

    35 Fairhurst

    30 Ladar(i = 30)

    l.adar (i= 15)

    ~,,/' Barton (JRC=14)

    / . .,< Barton (JRC = 10),," .....;/ Basic friction

    f r ; ; ~ ~ i > < ! :i ' , ' : m?{

    /) - ....I. ....

    25

    .3.5mm

    + 5.0mm

    -10I I

    0 10 20 30

    Normal stress (MPa)

    30

    25

    dn20

    15

    10

    I I - .

    5 . . . . dnr al5 mm0

    -v , --. ___ __ ~I~placemenl

    0 10 20 30

    Normal stress (MPa)

    Figure 3. Direct shear lest results: A) shear srress-displnccrncnt-dilntancy relationships and U) calculated peak strength envelopes ami dilatancy

    variation with nor mal stre ss ac cor ding 10 m o de ls a n d test results.

    251

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    6/8

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    7/8

    4.2 Contact and damaged areas and their variation

    with normal stress and shear displacement

    Contact area between irregular joint plane surfaces,

    after normal loading, is quite small in regards with the

    total joint area (Gentier 1986). Phase I of friction

    linear mobilization with the raising shear loadincreases the contact area (Tabor 1959) by a factor as

    large as three times, as stated previously, while

    mobilization of dilatancy in phase II reduces it

    progressively to peak shear strength corresponding to

    around 0.55 rnm of shear displacement. Following

    this scenario the post-peak phases will show a

    progressive degradation of the joint wall surfaces. To

    quantify asperities degradation on the joint wall

    surfaces, after each shear test performed on the fifteen

    samples, an image analysis was undertaken on the

    thirty images (15 replicas and 2 images per replica)

    and the geometrical characteristics of the damaged

    zones are measured: shape, size, position and/or

    orientation. The whole process is detailed in Riss et

    al. (1996). The damaged areas being defined, it was

    Possible, with the sequence of five shear tests (for a

    given normal stress) stopped at different shear

    displacement, to analyze the evolution of the damaged

    areas with shear displacement under constant normal

    stress (Fig. 4). Also the evolution of damaged areas

    7 MPA

    50

    40

    te lQl

    : ; : 30

    Ql

    Cl

    te l

    c:te l 20c" g0

    10

    0

    0

    are evaluated in relation with the normal stress, for

    given shear displacement.

    In general, there is an increasing degradation of

    surfaces asperities by damage area extension with

    increasing roughness, normal stress and particularly

    with shear displacement. The analysis can produce a

    slight underestimation of the damaged zones becauseof gouge sticking from one wall to the other without

    change in color or by transfer of material without

    crushing. From the analysis of results (Fig. 4), the

    sequence of joint wall degradation may be summa-

    rized as follow: first, for a given normal stress,

    material from superficial parts of one wall is broken

    away and crushed with shear displacement during

    which the number of these deteriorating parts

    increased and the size of the degradation zones

    enlarged both depending on stress level. As normal

    stress increased gouged material is crushed more

    densely and sticked plastically on the surfaces with a

    transfer of material from one wall to the other. The

    anisotropy of the joint surfaces morphology show

    degradation zones location depending on shear direc-

    tion. After 5 mm of shear displacement and in rela-

    tion with the applied normal stress (7, 14 and 21

    MPa) the total damaged area evaluated, combining the

    degraded zones of both walls (upper and lower) is at

    most 23%, 33% and 58% respectively. With

    21 MPA

    7 M~a.

    Walls% Area sheared

    Upper: --Lower: -----

    2 3 4 5

    Displacement (mill)

    Figure 4. Proportion of damaged area evaluated 011 Lipper and lower joint walls for an ::: 7, 14, 21 MPa and illustrated for three shear

    displacements (0.55 111111,2 mill, 5 nun).

    253

  • 8/12/2019 ISRM-EUROCK-1996-031_Rock Joint Shear Mechanical Behavior With 3D Surfaces Morphology and Degradation Dur

    8/8

    increasing shear displacement, the damaged zones are

    extending from the initial ones and by linking them to

    become larger zones nearly perpendicular to shear

    direction. More details on areas of damaged zones

    and on position, orientation and spatial correlation

    between upper and lower damaged areas are

    discussed in Riss et al. (1996).

    Observed sheared area proportions at peak shear

    strength (phase III) are extremely low, between 2 and

    3%, for the three aN values tested (7, 14 and 21 MPa)

    (Fig. 4) and there was little variation with aNlaC in

    this range varying from 0.1 to 0 .3 on them as i f aN

    has no influence on sheared area. It is shown (Fig.

    4) that most of asperity degradation resulting in

    damaged (sheared) area occurs between peak and 3

    mm of shear displacement corresponding to the

    progressive softening phase IV of joint shear

    behavior. Thus, at peak shear strength the normal

    stress on c ontact areas is much higher than the

    average joint applied normal stress, but despite this

    fact for the three relatively low o-, values used, veryfew asperities were sheared off. If equating contact

    and sheared areas, it means for aN =7, 14 a nd 21

    MPa and contact area of I to 3 %, contact normal

    stress of around 700 MPa (or 8 ac) an irrelevant

    value. More appropriate values related to aN::; 2ac

    means contact areas between 5 and 15%, so t he

    difference must be in friction giving no damaged area

    at failure.

    ACKNOWLEDGEMENT

    This is a BRGM contribution n 94059; this work

    was financially supported by a BRGM research pro-

    ject, an NSERC of Canada research grant and an

    NSERC graduate student followship.

    REFERENCES

    Archambault G., Gentier S., R iss J., Flamand R. &Sirieix C 1995. A reevaluation of irregular joint

    shear behavior on t he basis of 3D modelling of

    their morphology. Part II: Joint shear behavior

    mechanical modelling. Proc.Mcchauics of

    Jointed and Faulted Rock (MJFR-2): 163-168.Rotterdam: Balkema.

    Bandis S.C., Lumsden A.C & Barton N.R. 1981.Fundamentals of rock joint deformation. lilt. 1.

    Ruck Mech. Sci. &Geomech. 13: 255-279.Barton N. 1973. Review of a new shear-strength

    criterion for rock joints. Eng. Geol. 7: 287-332.Barton N. & Stephansson o. (Eds.) 1990. Rock

    Joints. Proc. International Symposium on RockJoints, Loen, Norway, June 4-6.

    Flamand R., Archambault G., Gentier S., Riss J. &Rouleau A. 1994. An experimental study of the

    shear behavior of irregular joints based on

    angularies and progressive degradation of thesurfaces. Proc. 47th Canadian GeotechnicalConference of the c.G.s., Halifax, Nova Scotia,253-262.

    Gentier S. 1986. Morphologie et comportement

    hydrornecanique d'une fracture naturelle dans Ie

    granite sous contrainte normale; etude experimen-

    tale e t theorique. Documents du B RGM n 134,

    BRGM, Orleans, France, 597 p.

    Gentier S. & Riss J. 1990. Quantitative description

    and modelling of joints morphology. Rock

    Joints: 375-382. Barton & Stephansson (eds).Rotterdam: Balkema.

    Goodman R.E. 1976. Methods of GeologicalEngineering. West Publishing Co. 472 p.

    Ladanyi B. & Archambault G. 1969. Simulation of

    shear behavior of a jointed rock mass. Rock

    mechanics - Theory and practice, Proc. l l tli

    Sytnp. on Roch Mech., California: 105-125.Ladanyi B. & Archambault G. 1980. Direct and

    indirect determination of shear strength of rock

    mass. Proc. SME o/AIME Annual Meeting, LasVegas, Nevada, Preprint No. 80-25, 16 p.

    Myer L.R., Tsang CF., Cook N.G.W. & Goodman

    R.E. (editors) 1995. Fractured and Jointed RockMasses. B alkema, 772 p.

    Patton F.D. 1966. MUltiple modes of shear failure in

    rock. Proc. l st Congress ISRM, Lisbon: 583-590.

    Poirier S. 1996. Etude experirnentale du comporte-

    ment de la pression interestitielle et de s on

    influence sur Ie comportement physico-rnecani-

    que d'un materiau poreux intact ou fracture par

    essais triaxiaux non draines. Unpublished M.Sc.

    Thesis, UQAC.

    Riss J. &Gentier S. 1989. Linear and areal rough-

    ness of non planar rock surfaces of fracture.

    Acta Steorol 8: 677-682.

    Riss J. &Gentier S. 1990. Angularity of a natural

    fracture. Proc. lilt. Con]. on Mech. of Jointedand Faulted Rock, Rossmanith P. ( Ed.),Balkema, pp. 399-406.

    Riss J., Gentier S., Archambault G., Flamand R. &Sirieix C 1995. A reevaluation of irregular joint

    shear behavior on the basis of 3D modelling of

    their morphology. Part I: Morphology descrip-

    tion and 3D modelling. Mechanics of Jointed

    and Faulted Rock (MJFR-2) Proc., Balkerna, pp.157-163.

    Riss J., Gentier S., S irieix C, Archambault G. &Flamand R. 1996. Degradation characterization

    of sheared joint wall surface morphology. Proc.2nd HARMS Symposium, Montreal.

    Scholtz CH. 1990. The mechanics of eartquakesand faulting. Cambridge Univ. Press, 439 p.

    Stephansson O. (Ed.) 1985. Fundamentals of Rock

    Joints. Proc. of the lnternational Symposium allFundamentals of Rock Joints. Bjorkl iden,Sweden, September 15-20.

    Stephansson O. & Jing L. 1995. Testing and

    modeling of rock joints. Proc. Mechanics of

    Jointed and Faulted, Rossmanith (Ed.),Balkema, pp. 37-47.

    Tabor D. 1959. Junction growth in metallic friction.

    Proc. Roy. Soc. A. 251, 378.Tse R. & Cruden D.M. 1979. Estimating joint

    roughness coefficients. Int. 1. Rock Mech. Min.Sci. & Geomech. Abstr. 16: 303-307.

    254