The evolution of void spaces (permeability) in relation with rock joint shear behavior

To cite this paper: Int. J. Rock Mech. & Min. Sci. 34:3-4, paper No. 014. Copyright © 1997 Elsevier Science Ltd

Copyright © 1997 Elsevier Science Ltd

Int. J. Rock Mech. & Min. Sci. Vol. 34, No. 3-4, 1997 ISSN 0148-9062

To cite this paper: Int. J. RockMech. &Min. Sci. 34:3-4, Paper No. 014

THE E V O L U T I O N OF VOID SPACES ( P E R M E A B I L I T Y ) IN R E L A T I O N W I T H R O C K JOINT S H E A R B E H A V I O R

C~ A r c h a m b a u l t l; S. G e n t i e r 2 ; J. Riss3; R. F l a m a n d 1

1 Centre d'6tudes sur les ressources min6rales, Universit6 du Qu6bec/1 Chicoutimi, Chicoutimi (Qu6bec) G7H 2B1 Canada

2 Bureau de recherches g6ologiques et minibres, Direction de la recherche, BP 6009, 45060 Orl6ans, France

3 Centre de d6veloppement des g6osciences appliqu6es, Universit6 de Bordeaux I, Avenue des Facult6s, 33405 Talence, France

A B S T R A C T

A review is done on the geometrical description of the joint surface roughness morphology to define asperity angularities and spatial information on roughness structures. Characterization of damaged areas, in relation with shear displacement under three different constant normal stress magnitude is discussed within the more global contact area ratio. A synthetis of matched joint shear behavior offer the basis for a qualitative evaluation of the coupling complexity between the mechanical behavior and void space evolution between the joint walls during shear displacement on the basis of the morphological characteristics of the joint surface roughness and contact/damaged areas analyses sustained by joint shear test results from natural joint replicas.

Copyright © 1997 Elsevier Science Ltd

K E Y W O R D S

R o c k jo ints • stat ist ical ana lys i s • geostat is t ics • vo id m o r p h o l o g y • r o u g h n e s s m o r p h o l o g y • d i l a tancy • shear b e h a v i o r • fr ict ion • p e r m e a b i l i t y • h y d r o m e c h a n i c a l c o u p l i n g

1. I N T R O D U C T I O N

The hydraulic behavior of rock joints depends mostly on the geometry of the void spaces between their surfaces. Void space morphology is complex and vary over the joint surfaces with lateral changes in aperture, surface roughness, contact area, gouge material, applied normal and shear stresses and shear displacement. Lateral variations in the void space, with areas in contact surrounded by regions presenting openings (aperture) varying from point to point on the joint surfaces, result in highly irregular preferential flow paths with widely varying velocities. When fluid flows through such a fracture, it not only flows around the contact areas, but also has a tendency to flow through the channels with the largest apertures (Brown 1987) because hydraulic conductance is proportional to the cube of aperture (cubic law). This channel flow phenomenon is believed to be a major factor making flow in natural fractures deviate from the cubic law and cause a tortuosity in the flow (Tsang 1984; Tsang, Tsang 1987; Brown 1987; Pyrac Nolte et al. 1990; to cite a few).

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Joint deformation can take the form of normal closure, opening, shear and dilation. These deformation components result in changes of aperture and conductivity. So, modelling joint hydraulic behavior needs the coupling of normal stress, closure and conductivity and of shear stress, displacement, dilation and conductivity. In all cases, modelling of these couplings require a precise characterization of joint roughness morphology and its evolution with shear displacement in the case of shear loading conditions. Most of the models and experimental works done on hydromechanical coupling problems were performed mainly under normal loading conditions (Raven, Gale 1985; Barton et al. 1985; Gentier 1986; Billaux, Gentier 1990; Amadei, Illangasekare 1992; and authors cited previously). In the case of shear loading conditions the modelling of hydromechanical coupling is much more complex and experimental work quite difficult, so that fewer works were dedicated to this problem (Barton et al.

1985; Makurat et al. 1990; Olsson, Brown 1993; Esaki et al. 1995). The complexity of the problem in the latter case come from the evolution of the roughness morphology on the joint surfaces with applied normal stress and shear displacement. Also it comes from the various phases through which shear stress - displacement - dilatancy pass through during shear displacement.

This paper briefly review the geometrical description of the joint surface roughness morphology. Also, it deals with the characterization of damaged areas in relation with shear displacement under three constant normal stress magnitude. Finally, it discuss the coupling between the mechanical behavior and void space evolution between the joint walls during shear displacement on the basis of the joint shear behavior phases and damaged areas.

2. JOINT SURFACE R O U G H N E S S M O R P H O L O G Y

Joint walls characterization of roughness morphology is a prerequisite to estimate the parameters needed in their mechanical and hydrological analysis and modelling; particularly the 3D geometry of asperities shape on their surfaces affecting the stress distribution, the zones of degradation evolution and the related geometry of voids between the contact zones with shear displacement. This characterization means that a 3D quantitative description of roughness evolution on the joint surfaces was done during shear tests performed on a series of identical replicas from the walls of a natural fracture in a granite (Gu~ret, France) for which a detailed study of the morphology was done (Gentier 1986; Riss et al. 1995). The replicas were submitted to direct shear tests under three different normal stress (~N = 7, 14 and 21 MPa)

and each shear test was stopped at a defined shear displacement for five displacements (0.35, 0.55, 1.0, 2.0 and 5.0 ram), so that 15 shear tests were performed (Flamand et al. 1994). This procedure permits a control on the evolution 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 mm apart in four directions, digitized at a constant step (Ax = 0.5 mm) and kept constant for all shear tests as illustrated in Figure I(A) for the profiles in two direction (Flamand et al. 1994; Riss et al. 1995). For the first time, simultaneous mechanical shear test results and surface characteristics of the initial and evolving morphology for the various normal stress and shear displacements tested are available in an overall data set. It was already shown that joint roughness profiles provide an incomplete and biased characterization of joint surface morphology, not only because of the joint profiling system and procedure, but mainly because most of the profiles do not pass through asperities peaks, a situation that has low probability to occur in systematic profiling. So 2D colatitudes (02) or slope angle of

asperities are systematically underestimated (Riss et al. 1995; Archambault et al. 1996). This may be

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3

distribution of elementary plane facets giving true asperities slopes.

The analysis of the recorded data consists firstly, in the deduction of parameters characterizing the whole set of compiled points i.e. the joint wall surface expanding in a 3D space and, secondly, in detailed analysis of the (02) angles between a reference plane and a line segment linking two successive points on

the recorded profiles. This latter analysis is fundamental to restore the true 3D colatitudes (03)

distribution of the elementary plane facets composing the joint wall surfaces before and after testing and correcting by he same way the bias introduced by the profiles. The overall analysis of the joint wall surfaces gives a global view of the morphology and depends on whether we are interested 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 azimuth and 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 t o

b) the linear roughness coefficients or indices RL, z 2, z 3 and z 4 and the linear regression parameters

(0regressio n and the residual variance S2z,x) computed in Table 1.

The global or overall statistical analysis of the joint surfaces and the statistical detailed analysis of the 2D colatitudes (02) in shear direction, giving the classical linear roughness indices (RL, z2, z 3 and z4) as well

as the linear regression parameters (Table 1) were derived and computed in Riss et al. 1995 and their physical meaning for the joint roughness surfaces submitted to shear was discussed in Archambault et al.

1996. It was concluded that using one of the previous parameters or indices without taking into account informations from the others can introduce large errors and biased informations on the roughness morphology of the surfaces. Moreover, 2D roughness parameters are insufficient to describe adequately the joint wall surfaces morphology changing from point to point and correct the bias introduced by the profiles. A 3D representation of the roughness morphology can obviate to the previous problems. So, Gentier, Riss 1990 proposed a method to derive true 3D colatitudes (03) distribution by inference from

2D colatitudes (02) in vertical plane parallel to shear direction and fitting a 3D model on the basis of

generalized axial distributions (Fig. 2) and controlling the adequateness of both through the areal roughness parameter (Re). The characteristics deduced for Re, the mode and the mean (q3) are given in

Figure 2 and the position (directional mean) and dispersion (variance) can be computed from F(03). The

most important result was that the 3D elementary facets dip in any direction with a mean angle of 16 ° and this angle is greater than the 2D mean angle of individual segments (Riss et al. 1995).

Nevertheless, from these statistical analyses, physical conditions of the joint walls can be deduced regarding the impossibility to obtain a perfect matching of the joint wall surfaces with the differential variation between walls dipping and roughness as well as 2D and 3D colatitudes (02 and 03) distributions

on both walls. Also, the matching between profiles of both walls adjusted together show large void spaces and few contact area between them. 3D analysis of enclosed void spaces between the joint 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 show also that most of the

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contacts seems to be located on the slopes of asperities and it can happen that on particular profiles no contact can be seen. Of course, contact areas vary with applied normal and shear loads and also greatly with the degree of matching between the surfaces and with shear displacement (Archambault et al. 1996).

All these statistical analyses of the joint surface roughness gave no spatial information on the distribution of asperities and structures (shape, size, jogs, waviness, steps and others) on the joint surfaces for an adequate modelling of its shear deformation, strength and hydraulic behavior. Asperities spatial distribution and shape on joint wall surfaces are not necessarily at random and the presence of superposed structures cannot be detected on 3D angularities distribution (Fig. 2). Even if a major structure like a jog of high angularity will not change appreciably the distribution shown, it will control the shear strength, the dilatancy and the void spaces created with shear displacement. Various methods can do it and among them geostatistical analysis of asperities structures through variographic analysis of profiles to characterize size and shape of structures and kriging modelling and/or conditional simulation to restitute the topographic surfaces as illustrated in Figure I(B) (Gentier 1986; Gentier, Riss 1990; Rivard 1995). In the present case the shear direction chosen was parallel to the principal structure (-16 mm), so that only the roughness morphology represented by the 3D colatitudes (03) distributions (Fig. 2)

are controlling the joint shear behavior.

3. S E L E C T I O N P R O C E S S OF A S P E R I T Y A N G U L A R I T I E S IN OPENINCs F R I C T I O N AND F A I L U R E D U R I N G S H E A R P R O C E S S

The more important factors in the selection process of asperity angularities, submitted simultaneously to failure and friction mechanisms during shear process, are the operating normal stress magnitude (~N) and

the proportion of asperities for a given mobilized angularity. This latter factor depends mainly on the type of asperity angularities statistical distribution and particularly on the presence or not of structures characterizing the joint surfaces on which shearing and void spaces creation will be concentrated. As illustrated in Figure 2, under a given normal stress (~N = ~Nb) asperities within a restricted domain of

angularity, depending on their distribution, their structures and contact areas, are mobilized in friction and their angularity (or slope) control the rate or angle of dilatancy in the shear process. Asperities with higher angularity than the range of those mobilized in friction and in contact are deformed and failed for very small shear displacement and show frictional behavior thereafter. Asperities with lower angularity are developing voids by separation of the two surfaces in these areas and the increase in void spaces depends on dilatancy rate.

The selection process on angularities statistical distribution is done by the normal stress as illustrated by the superposition of the curve CYN/CYc, a logarithmic: log CYN/CY c or a negative exponential function:

exp[-K(CYN/CYc) ] in Figure 2 giving an hypothetical CYN/CY c variation in relation with the angularity density

of probabilities for the replicas and the shear direction under study, for which no structures intervene in the shear process. The function of CYN/CY c used, depends on the type of angularity statistical distribution

and on asperities structures at different scales. The 3D colatitudes (03) distribution show for 0 3 > 30 ° a

very low proportion of asperities having these angularities so that their mobilization in friction will occur at extremely low CYN/CY c value (CYN/CY c < 0.05). This is proven by the shear test results (Fig. 3) in which

normal stress as low as 7 MPa (CYN/CY c < 0.1) already show shear strength corresponding to i 0 values near

q3mean = 16° corroborated by dilatancy angle d°n ~ 14°-15 °. So, to mobilize 0 3 value (or i0) beyond the

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3 N c

low normal stress (0 < (yN/(y c < 0,05), i 0 values selected in friction will be 0 3 ~ 30 °, almost no asperities

failed, friction and dilation will be the only mechanisms intervening in the shear process. Increasing (YN/(Yc to higher values (0.05 < (YN/(Yc < 0,3) in which the values tested are, mobilize lower i 0 value near

q3mean ~ 16 ° with an increasing proportion of failed asperities at peak, proportional to (YN/CYc ratio with

increasing contact area. Further increments ((yN/(y c > 0,3) must lower i 0 values mobilized in friction

below mean (16 °) and mode (13 °) progressively with (yN/cy c increasing very rapidly towards transitional

normal stress.

This behavior is for asperity angularities distributed at random on the joint mean plane, corresponding to JRC = 0-10 (ISRM 1974), without the presence of major structures or overlapping of structures of different scales as observed for JRC = 10-20. Usually, these large structures, like wavinesses, show angularities (03) higher than 20 ° , but in the case of structures like jogs and steps, higher angularities will

be observed (03 > 30 °) not detected in the statistical distribution of angularities, except if angularities are

weighted by the dimension of the structures or detected as such by variographic analysis of the profiles. These structures introduce an anisoropy in the shear behavior between shear direction perpendicular and parallel to them. The whole shear process will be controlled by these structures and their angularity, even with 0 3 > 30 °, when the shear direction is perpendicular to them and the shear sense is against them.

Much higher (yN/(y c values will be needed to rip off these structures and lower their angularities. The

resulting dilatancy rate will be much higher, for moderate (YN/(Yc value, and the variation in the volume of

voids will increase greatly and will show particular morphologies. This anisotropy was observed in a recent experimental study on the flow of fluid through joint replicas under shear conditions (Gentier et

a l . , this symposium).

4. SHEAR BEHAVIOR PHASES OF MATCHED JOINT AND VOID SPACES E V O L U T I O N

The results of the 15 direct shear tests on joint replicas, whose conditions were described previously, are plotted in Fig. 3. Previous papers described the various conditions under which these tests were performed (Flamand et al. 1994) and discussed the various aspects of the results in relation with known models (Archambault et al. 1996). The shear process and mechanisms of a matched joint with irregular surfaces show the following phases (Fig. 3A):

• Pre-phase" This pre-phase of normal loading on the mean shear plane of the joint depends on the stress path used and only the constant normal load direct shear test shows this path. In this phase, if existing, normal stress concentrated on very few points (contact areas) with a normal closure depending on normal load and joint roughness morphology well studied in the literature (Goodman 1976; Bandis et al. 1981; Gentier 1986). Few of the works were dedicated to the evaluation of voids variation (Gentier 1986) with normal load and roughness morphology but a large number of them deal with fluid flow - conductivity - normal load coupling without considering joint roughness morphology.

• P h a s e I- Phase of elastic mobilization of shear stress by friction with the gradual application of the shear load from zero level. This phase of increasing shear load give rise to a transfer of the stresses on asperities positive slopes defined by their angularity. Friction is mobilized on their inclined planes (Fig. 3) and accompanied by their deformation. Application of normal load produced normal

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and shear force components on both positive and negative slopes of asperities. Adding the shear load produced another normal force component in the same direction than the previous one and a shear force component in opposite direction to the previous one, so that there is a progressive increasing in the resulting normal force component and a decrease of the resulting shear force component to zero, followed by reincreasing in opposite direction on the positive inclined plane while the force components on the negative slope of asperities are annulled (Fig. 3). This interaction of shear and normal loading causes a new closure (contractancy) on the joint surfaces. It results in an increase of the real contact area until gross slippage is imminent and it may reach three times of the initial static contact area without change in the normal load but changing normal stress locally. However for any two surfaces, the final area of contact is a numerical constant times the initial area of contact developed with the normal load only, so that the proportionality between both forces (shear and normal) at the point of slipping is maintained. This increase in contact area was called "junction growth" by Tabor 1959. This phenomenon was indirectly observed by a decreasing conductivity in fluid flow (Gentier et al . , this symposium) and an increase of induced interstitial fluid pressure (Poirier 1996) in this phase of shear displacement on joints indicating a reduction in permeability between the joint surfaces.

Phase II- A non-linear shear stress - shear displacement - dilatancy hardening mobilization phase to peak shear stress with increasing dilatancy rate, decreasing area of contact on the joint plane depending on (YN, increasing contact normal stress on it, in addition to friction on the facets slope of

asperities involved, till failure occurs. This phase show an increase of the volume of voids in relation with the progressive mobilization of dilatancy and this dilatancy opening phase was also observed indirectly in fluid flow by an increasing conductivity (Gentier et al . , this symposium) and a reduction of induced interstitial fluid pressure (Poirier 1996). This dilatancy hardening phase in joint shear behavior was not well studied and no known modelling was developed to take it into account.

Phase III- Peak shear stress (or shear strength) phase, following a behavior between constant peak displacement and constant stiffness models (Goodman 1976) in relation with (YN, usually at

maximum dilatancy rate with more or less asperities failure and the beginning of reincreasing contact areas with (YN. This phase was well studied and numerous models were formulated to predict

joint shear strength. Permeability in this phase is still increasing at a maximum rate but is far from the maximum opening possible. Volume of voids can be multiplied by 10 times in the following phases depending on (YN and amplitude of roughness asperities on the joint surfaces. So, peak shear

stress and dilatancy rate do not correspond to peak permeability on the joint, it is only the beginning of dilation between joint surfaces.

Phase IV: Progressive softening phase of unstable yielding with a progressive degradation of asperities on the joint surfaces by microfracturing, crushing, ploughing, indentation or asperities failure. This phase is characterized by a decreasing shear stress towards residual (or ultimate) strength under a given shear stress gradient (softening modulus) with increasing contact area, now composed essentially of the degraded zones on the joint surfaces. The increase in contact area released progressively the highly concentrated contact normal stresses towards the joint mean normal stress along the basic friction curve and this explains at the same time the progressive reduction in shear stress proportionally to the reduction in normal stress. Dilatancy increases highly during this phase at a decreasing rate with shear displacement to reach more than 50% of the total dilatancy (void openings) for low (YN values to 80% for moderate (YN values ((YN/CYc > 0.3).

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Permeability increases highly in this phase but not proportionally to the "cubic law" as expected because of expanding damaged contact zones between the joint surfaces and the production of gouge material filling parts of the volume of voids developed by dilatancy, so that fluid flow is channellized and the channels formed evolve with shear displacement and gouge obstruction of openings. Very little was done to model this phase except the empirical models proposed by Goodman 1976 and Rudnicki, Chen 1988 and the works done on degradation during cyclic shear (Hutson, Dowding 1990; Plesha 1987).

• P h a s e V: Residual or ultimate strength phase of stable sliding in which shear and normal stresses are relatively stable on a larger surface of contact indicating that Amonton's laws of friction are valid: the frictional force is independent of the size of the surfaces in contact and friction is proportional to the normal load. Dilatancy rate must be zero or at a very reduced rate for low to very low (YN values

and tend to disappear with shear displacement. Degradation on the joint surfaces is still operating with shear displacements by ploughing, wear, grinding and friction of asperities and particles on the joint shear plane to produce gouge material filling valleys and throughs so that the area of contact trends towards mean shear plane area. Even if total dilatancy is large giving greater openings, these openings are filled with gouge material so that permeability within the joint depends on the permeability of the filling material. This phase was well studied over the last two centuries, on the mechanical point of view, and various aspects were analyzed through numberless works.

In all these phases, the normal stress ((YN), roughness asperities morphology on the joint surfaces and

shear displacement play a major role on the joint shear behavior. In the case of mismatched joint surfaces the first phase will show a large contraction between the joint surfaces, phases II-III-IV will be replaced by a gradual raising of shear stress with a sustained contractancy towards stable sliding of phase V and no peak shear stress will be shown. Roughness damages and gouge material production will be much greater than in matched joints.

5. C O N T A C T AND D A M A G E D AREAS E V O L U T I O N ON J O I N T WALLS W I T H S H E A R BEHAVIOR: IMPLICATIONS ON C O N D U C T I V I T Y

Contact area between matched iregular joint plane surfaces, after normal loading, is quite small proportionnally to the joint mean plane area and depends on applied normal stress ((YN) on it (Gentier

1986). As stated previously, phase I of friction linear mobilization with raising shear load increases contact area by a factor as large as three times (Tabor 1959). Mobilization of dilatancy phase II reduces it progressively to peak shear strength (phase III) corresponding to around 0,55 mm of shear displacement. Most of the contact area, in these three first phases, is in friction; while the post-peak phases (IV & V) show a progressive degradation of the joint wall surfaces and contact area is essentially on the degraded zones after failure of asperities. The geometrical characteristics of the damaged zones (shape, size, position and/or orientation) are measured through an image analysis process (Piss et al. 1996 and this symposium) and permit to locate these areas on a topographical map of the initial joint wall surfaces, to correlate the damaged zones to asperities distribution. 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 the damaged areas are evaluated in relation with the normal stress for given shear displacement. Images of damaged areas present specific characteristics but maintained identical position in their distribution.

In general, there is an increasing degradation of surfaces asperities by extension of the damaged area

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with increasing roughness, normal stress and particularly with shear displacement. Analysis of results (Fig. 4) permit the following interpretation of the joint walls degradation. 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 degrading zones enlarged both depending on normal stress level and asperity heights. Increasing normal stress, gouge material is crushed more densely and sticked plastically on the surfaces with transfer of material from one wall to the other. After 5 mm of shear displacement, depending on 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. Whatever the applied normal stress and shear displacement, for the same surfaces roughness and shear direction, the damaged zones on upper and lower walls are generally symmetrical in location and size. With increasing shear displacement, the damaged zones extend from the initial ones and by linking them to become larger ones nearly perpendicular to shear direction. This analysis constitute a lower bound of the damaged zones because of gouge sticking from one wall to the other without change in color, by transfer of material without crushing or by tore out material from the walls without changing colors. Also, there is the particular case of damaging at peak shear strength (phase III) for which values computed were between 2 and 3%, for the three (YN values tested (Fig. 4). It

was proved that these values are irrelevant and for these low (YN values most of the contact areas in phase

III are under friction sliding not detected by the method based on degradation and change of color on the contact spots (Archambault et al. 1996).

The variations of contact area on the joint wall surfaces with normal stress and shear displacement (Fig. 4) show also the evolution of void spaces (permeability) between the degrading surfaces on which contact spots are in black, while void spaces are in white. The latter show large variations in aperture depending on joint roughness morphology and cumulated dilatancy with shear displacement on the joint plane for a given normal stress. Increasing the normal stress causes larger damaged zones (and contact spots), decreases cumulated dilatancy with fewer void spaces and decreases aperture for the same shear displacement on the joint plane. There could be equivalence in contact areas on joint surfaces for two different normal stress at two different shear displacement, the longer shear displacement corresponding to the lower normal stress and vice-versa for the same degraded surfaces in contact. But aperture in the void spaces will be larger for the low normal stress and longer shear displacement because of the greater cumulated dilatancy during shear. It is relatively easy to observe virtually that the fluid flow will follow the tortuous paths surrounding the areas in contact with widely varying velocities and preferential channels that will cause a varying anisotropy in the fluid flow with shear displacement as demonstrated by Gentier et al. (this symposium).

6. S H E A R - D I L A T A N C Y - C O N D U C T I V Y C O U P L I N G W I T H J O I N T S H E A R

B E H A V I O R

A synthesis of rock joint shear behavior is proposed in Figure 5 in which are related the various parameters and phenomena discussed previously and observations done on their behavior. Three arbitrary normal stress (low - moderate - high) are considered for which the shear stress - displacement - dilatancy curves and the corresponding Mohr diagram (x-~N) and r-~ N relationships were plotted on the basis of

the previous shear test results (Fig. 3) performed under low normal stress (0,1 < ~N/~c < 0,3). An

extrapolative attempt is done to define the behavior of contact area ratio and contact normal stress with joint mean normal stress and shear displacement on the basis of the previous discussions of the phases of joint shear behavior with shear displacement as well as the evolution analysis of contact and damaged

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areas with normal stress and shear displacement. The effect of the pre-phase normal loading on contact area ratio and contact normal stress is not illustrated in the figure. On the contrary of matched regular sawtooth joint roughness surfaces which show full contact area ratio at the beginning of shear loading, irregular joint roughness surfaces will show contact area ratio well under full contact except for very high normal stress. Shear loading increases contact area ratio while dilatancy mobilization decreases it progrogressively to peak shear strength and finally it reincreases during post-peak phases in the unstable sliding phase of softening and more slowly during the stable sliding friction phase. In these last phases, the contact areas are essentially composed of the damaged areas on which sliding occurs. The variation in void spaces is following the opposite behavior of the contact area ratio.

It must be noticed that most of the perturbations of closure-opening (conractancy-dilatancy) for a matched joint under shear loading, occur within 2,0 mm of shear displacement and even lesser than that. These perturbations influence largely the variation in void spaces on the joint surfaces, their conductivity and fluid flow behavior with very small shear displacement and this was sustained by experimental observations on fluid flow in fractures under shear loading (Gentier et al. , this symposium) and on the behavior of induced interstitial fluid pressure under similar conditions (Poirier 1996). This situation is quite important because very small shear displacement can cause large variations in conductivity on the joints in fractured rock masses in the vicinity of workings for which the hydromechanical coupling is important.

A C K N O W L E D G E M E N T

This is a BRGM contribution b94059; this work was financially supported by a BRGM research project, an NSERC of Canada research grant and a NSERC graduate student fellowship.

FIGURES

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To cite this paper: Int. J. Rock Mech. & Min. Sci. 34:3-4, paper No. 014. Copyr ight © 1997 Elsevier Science Ltd

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0 . 0 50 2 0 3 0 4 0 5 0 6 0 3D C e l a t i t u d e s

Figure 2 .3D reconstructed distributions and models (equation) for colatitudes 03 with characteristics of the fitted models for 3D distribution.

ISSN 0148-9062

To c i t e th i s paper : Int. J. R o c k M e c h . & Min . Sci . 34:3-4 , p a p e r No . 014. C o p y r i g h t © 1997 E l s e v i e r S c i e n c e L t d


N N N NL I s, ". i~I I N,

~ / ~ ~ " ' " ' " " "..., .

b b - -

~'0 ,

20.

lh •

=* 10--

III I[ I Iq ~."

~hl'.~ (2" ?~IP~I~ i I I II ~Iz~ ~ ..... I

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.:; • G I (:' 2E" . : '3

Normal s t ress (MPa)

Figure 3. Direct shear test results: A) shear stress-displacement-dilatancy relationships and B) calculated peak strength envelopes and dilatancy variation with normal stress according to models and test results with the loading modes on asperities (upper part of the figure).

I S S N 0 1 4 8 - 9 0 6 2

To cite this paper: Int. J. Rock Mech. & Mm. Sci. 34:3-4, paper No. 014. Copyright © 1997 Elsevier Science Ltd


14 M P.,&

, , . , , , . . , 2 1 , . , . , ,

• 2 : I . III I : : . :'.

4D 141 M p a . . . . . . . . . . . . , " _ . . . . ~ - ~ . . . . ~ . < . . . . . . . - _ _ - - - ~ p ~ ..

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(1 1 2 8 4 D i ~ p l a c e m e r ~ t ( n l m )

II" I::

Figure 4. Proportion of damaged area evaluated on upper and lower joint walls for ~n = 7, 14, 21 MPa and illustrated for three shear displacements (0.55 mm, 2 mm, 5 mm).

ISSN 0148-9062



f l :111 I',"

'~ 'E'~N hiBh

I t ) I- s h e a r dJ .3pI ~cf . ' / l lent

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.lib-'"

normal ~tr~J-%~

Figure 5. Schematic graphical synthesis of the various parameters characterizing joint shear behavior in relation with shear displacement and normal stress.

TABLES

ISSN 0148-9062


Paper 014, Table 1.

Table 1. Classical linear roughness coefficient measured on profiles parallel to the shear direction. Profile RL Z2 Z3 0regression S 2z,x mm2 z4

z=f¢x) . .

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

1 1.029 1.029 0.251 0.246 0.547 0.608 1059 1°62 0,108 0.092 0.089 0.190 2 1,030 1,031 0.249 0.254 0.528 0,551 0*56 0064 0.281 0.286 0.111 0,220 3 1.033 1,033 0.263 0.264 0.597 0.581 -(1°74 -0°76 0.327 0.321 -0,018 0,024 4 1.041 1,036 0.300 0.283 0.611 0.521 0*05 0002 0.585 0.552 0.057 0.101 5 1.022 1.030 0.212 0.212 0.254 0.448 -1°88 -1"68 0,702 0.637 -0.017 0,048 6 1.042 1.043 0.300 0.306 0.556 0 . 5 9 0 -0*34 -0*37 0.493 0.485 0.136 0.103

References

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ISSN 0148-9062

The evolution of void spaces (permeability) in relation with rock joint shear behavior

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