Lessons Learned in Salt T. Rothfuchs, K. Wieczorek, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, Germany S. Olivella, A. Gens Universidad Politècnica de Catalunya, Spain Summary Detailed research on EDZ evolution in salt formations has been started about a decade ago in order to enable the assessment of its importance for the long-term safety analysis of radioactive waste repositories. Following the laboratory investigation of dilatancy and the accompanying change in porosity and permeability, orientating field investigations were done to determine extension and evolution of the EDZ. Dilatancy and permeability/porosity functions have been determined by several investigators. The data available today enable their adequate consideration in numerical simulations. The field investigations resulted that the EDZ commonly reaches by about 0.5 m into the drift wall and up to 1.5 m into the drift floor. The EDZ develops immediately after room excavation. Its further evolution with time is limited. After the installation of supporting plugs, the EDZ starts to heal in consequence of the continuing creep deformation of the viscoplastic rock. Investigations around an old plug installed in the Asse mine resulted a significant reduction of permeability within 90 years. A complete healing, however, is not achieved within this period of time. Constitutive models to predict damage, dilatancy and permeability distribution around excavated drifts have been developed recently. The results of first numerical simulations of underground experiments are very promising and thus it seems possible to assess the reversibility of EDZ evolution which cannot be observed in situ because of its long duration. Anyway, the constitutive models used need to be confirmed by further selected in-situ measurements, preferably around excavations of different age and, whenever possible, in the surroundings of plugs installed longer times ago. 1. Introduction In all types of rock formations, excavation of openings has an influence on the nearby rock zone. The so-called excavation disturbed zone (EDZ) has hydraulic or mechanical properties different from the undisturbed rock. The type and degree of disturbance and the extent of the EDZ depend on different factors, such as type of rock, hydraulic and mechanical state, dimensions of the opening, and excavation procedure. The primary cause for the EDZ formation is the change in stress state imposed by excavation. Next to the opening, the stress state will be highly inhomogeneous, which can lead to microand macrofracturing affecting the mechanical integrity and the hydraulic properties. A second effect of excavation is ventilation of the nearby rock, which can lead to drying and thus affects the hydraulic behaviour. 101 In contrast to crystalline rocks like granite, rock salt has a pronounced viscoplastic behaviour. It reacts to stress changes by creep deformation, reducing the stress. Thus, the risk of macrofracturing is widely reduced. On the other hand, a microfractured EDZ with typical extents of several decimetres up to one or two metres develops around openings. While the mechanical integrity of the rock is hardly affected, the hydraulic properties change significantly. The permeability close to the rock surface may be increased by four or five orders of magnitude. It can, however, be reduced again when the stresses return to a more or less homogeneous state, e. g., after creep of the rock onto engineered barrier systems. Whether EDZ formation is fully reversible depends on various parameters and is still under investigation. 2. Current understanding of EDZ Nature and Properties 2.1 Process understanding As explained above, the development of EDZ in salt formations is mainly dependent on the state of stress or the evolution of dilatancy around the openings. Dilatancy in salt is defined as the inelastic volume increase in consequence of opening of grain boundaries under deviatoric loading. Fig. 1: Dilatancy boundary for rock salt. Differential stress versus confining pressure  The principle behaviour can be seen in the stress space (Fig. 1). During deformation, two boundaries are visible: first the dilatancy limit above which the opening of grain boundaries occurs and second, the failure boundary or short term failure strength above which fracturing takes place. Below the dilatancy boundary only compression can occur and any increase of porosity and permeability even with time is impossible . Exceeding of the dilatancy boundary on the contrary results in an dramatic increase of permeability by 3 - 5 orders of magnitude. In  it is reported that the dilatancy limit of rock salt is more a band than a distinct line. Of special importance for safety analyses is the relation between permeability and porosity which was investigated in detail in the laboratory. 2.2 Experiences from laboratory testing It has been shown by laboratory testing that the permeability is dependent on the effective minimum principle stress (in the laboratory the confining pressure) and the effective porosity, respectively. In deformation experiments on natural rock salt  and synthetic salt  a drastic increase of permeability (up to 5 orders of magnitude) was found at onset of dilatancy (d< 1%). In  it is demonstrated that progressive failure is accompanied by an increase in pore space and 102 permeability, and also of acoustic emissions, a change of electrical resistivity and damping of seismic waves as well as by a decrease of seismic wave velocities. The latter effect was investigated by combined measurements  of compressive (Vp) and shear (Vs) wave velocities and of permeability. Figure 2 demonstrates in an impressive way that this method is a strong tool to determine onset of dilatancy. Fig. 2: Example for the indication of dilatancy development by increasing permeability and decreasing ultrasonic wave velocity measured on a rock salt sample at 2 MPa confining pressure at 30 °C; Dilatancy boundary is at about 10 MPa. (after Popp et al., 2001) Fig. 3: Permeability versus porosity of rock salt  In  it is reported that several investigators have shown that stress induced micro-crack opening occurs parallel to the axis of maximum compression. This implies that the permeability is anisotropic under deviatoric loading which is normally the case close to underground openings. Several investigations have shown that the permeability evolves in different stages corresponding to stress, strain and time. Data published in  are shown in Fig. 3. In a first approximation the data can be fitted to a simple relation of the type k = n . Figure 3 indicates, however, that an approximation with a two-regime model is to be preferred. At a pressure of 2 MPa the initial opening of the pore space (regime 1) corresponds to an exponent of 4. In regime II the increase of permeability is only weak (n 0.2). This behaviour is interpreted as a change from crack opening to crack widening after a connected flow network is established. In  it is also shown that the permeability of dilated samples can be restored at a confining pressure of 10 MPa to a value of 10-20 m2 which is more or less the permeability of undisturbed rock salt . A more comprehensive modelling of the different stages of permeability generation in the a priori tight rock salt can be described by the percolation model of Alkan . The base model of percolation is quite simple : k C n ( p pc ) t 103 In this equation, C is the constant that relates the permeation to the geometrical parameters of the material such as crack length, opening and density of a defined continuum model. The Penny-Shaped crack model of Gueguen and von Dienen  was used to define C in this study. Based on the Penny-Shaped cracks model the permeability can be formulated as follows: k l6 30 c 2 4 3 In this study, for the definition of the percolation probability, p and critical percolation probability, pc the number of the detected acoustic emissions are used with the assumption that the acoustic events are the indicators for the openings of the cracks. On the other hand, the exponent t is related to the backbone probability of the observed fractal dimension of the ASSE salt cores. Introducing the definition of the measurable dilatancy () instead of the porosity and the characteristic values of the ASSE rock salt, the formulation results in: k 0,00337 l 2 ( ) 4 ( N NC / D t ) Np Np The above given equation is used to calculate the permeability development in some of the triaxial experiments (Fig.4). 1,E-15 AT-015 s 3=6 MPa, Pg= 2,5 MPa Gemessen Berechnet 1,E-16 2.3 Experiences from experimental observation and characterization 1,E-17 k, m2 Various measuring methods have been recently applied with promising success to characterize and determine dilatant zones or the extent of EDZ around underground openings. 1,E-18 1,E-19 1,E-20 In-situ permeability is usually determined by packer testing. A test interval in a borehole is Fig. 4: Measured and with percolation sealed by packers and gas is injected. The model calculated relation between axial flow rate and the gas pressure in the test stress and permeability interval and possibly in adjacent guard intervals are recorded during the injection phase and a subsequent shut-in phase. From the recorded data, the permeability is derived by analytic solution of the diffusion equation or by inverse modelling. Computer codes for the evaluation are available. Such tests were performed at the Asse salt mine in the frame of the projects ALOHA  and BAMBUS II (see Extended Abstract #36). Due to the necessary sealing length, conventional packer tests are not suitable for measurements very close to the opening. Therefore, a special system involving a plastic sheet for sealing the drift surface has been developed in the frame of the BAMBUS II (Extended Abstract #36). Numerous measurements performed in the above mentioned projects showed an extent of the EDZ of 1 to 1.5 m below the floor of drifts and less than 0.5 m into the walls. The permeability increased to maximum values in the range of 10-15 m2. As expected, the permeability in the EDZ is highly anisotropic, with the highest component parallel to the drift surface. 0 10 20 30 1, MPa 40 50 60 104 A special issue of the EDZ is its potential healing. Prerequisite for healing is a stress state which leads to compaction instead of dilatancy. Other conditions influencing healing are time, temperature and moisture content. At the 700-m level of the Asse mine, it was possible to investigate a site where a cast steel bulkhead had been emplaced in a three-year-old drift as early as 1914. 89 years later in summer 2000, boreholes for permeability testing were drilled through the bulkhead into the salt. It was found that the highest permeability values were below 10-18 m2, while around the adjacent open drift a normal EDZ was detected. So, a significant reduction in EDZ permeability due to the compressive stress state was confirmed. A complete healing, however, did not take place yet. A further method to determine the actual state of the rock is represented by active and passive seismics. As already mentioned in section 2.2 it is well known that seismic velocities of rock change significantly with progressive failure. Therefore, velocity distributions around underground openings have been measured in different rocks, and as expected low velocity zones turned out to give a good estimation of the EDZ . As an example Fig. 5 shows the seismograms of a downhole measurement in a borehole in the Asse mine. The single traces are plotted vertically in black while the amplitudes are colour coded. This allows to judge the complete contained information in a qualitative but very effective way. It can clearly be seen that the travel times are considerably longer up to a depth of about 0.4 m. At greater depths the signal characteristics are consistent and no major changes can be detected. In addition to seismic methods also the application of geoelectric monitoring is suited to detect zones of higher porosity in salt. Within the ALOHA-Project  liquid injection tests were performed to identify the extent of the EDZ below the drift floor in the Asse mine (Fig. 6). The injection tests were accompanied by geoelectric measurements which can be used to determine the electric resistivity distribution in the rock. Here an electric current is injected via a pair of electrodes and the resulting potential differences between other pairs of electrodes are measured. The resistivity is correlated to the water content of the rock salt. The correlation function is known from numerous laboratory tests. It can be seen that the resistivity around 1 m depth is significantly lower than at greater depth indicating penetration of brine into the dilatant zone below the floor.. The value of 200 to 1000 m represents a water content of 0.3 to 0.5 % by volume which is a factor of 3 – 5 higher than that of natural rock salt. Fig. 5: Colour coded seismic section obtained Fig. 6: Distribution of electric resistivity in from interval measurements in a borehole in the planes between the electrode boreholes salt  after liquid injection at 1 m depth 105 2.4 Modelling capabilities Some new modelling approaches have been developed recently. The general constitutive model of Olivella and Gens  for rock salt incorporates both creep and viscoplasticity. It permits to predict dilatancy in the EDZ induced by shear stress development. The model is implemented in the code CODE_BRIGHT. Modelling of the EDZ behaviour in field conditions has been carried out within the BAMBUS project for the TSDE test. A two-dimensional model has been considered, built using triangular elements (1033 elements and 2108 nodes) (Fig. 7). The limitation of the 2D representation were accepted in order to be able to use an optimum mesh refinement near the drift wall. Three materials have been considered: rock salt, backfill and steel (canister). The main properties of the materials used in this analysis have been included in BAMBUS project reports. A constant stress of 12 MPa and a temperature of 36.4ºC are assigned to the upper Fig. 7: View of the finite element mesh used in the and right boundaries, analyses and detail of the drift area. whereas at the bottom and left boundary zero displacements have been assumed. A heat source of 19.61 W has been considered in the canister. When the analysis begins, porosity increases due to dilatancy induced by shear stresses, especially for short distances around the cavity. The initial porosity of the rock was considered to be very low. In fact a residual porosity of 0.001 is normally present as unconnected voids. The highest porosity values were obtained in the walls of the drift, in the area close to the roof (see Fig. 7, right side). The maximum porosity, slightly higher than 0.025, was computed in the upper part of the pillar wall between cavities, for about 600 day’s simulation. Later, due to progressive drift closure and backfill pressure increase, porosity values decreased at all points as shown by the slopes of curves in Fig. 8, left side. After 3500 days, the values reached fall in the interval between 0.005 and 0.015. The rock permeability was determined on basis of a permeability-porosity relationship determined in the UPC laboratory. The mathematical expression is as follows: k k 0 ( n / 0 ) 10 21( 3.5 / 0.0013.5 ) 3.2 x10 11 3.5 where k is intrinsic permeability (m2), is porosity and n is the power of porosity. This law neglects anisotropy effects which could be incorporated later. Permeability evolution for some points in the EDZ is plotted in Figure 8, right side. Permeability evolution is found to be influenced by the heating and the presence of the backfill in the TSDE test drift. Heating increases shear stresses but the backfill provides confinement for the rock in the EDZ. 106 Porosity development in the EDZ (Po=15, M=0.95, A=0.005) Permeability development in the EDZ (Po=15, M=0.95, A=0.005) 1E-12 0.05 element 287 element 284 element 305 element 308 element 312 element 278 0.03 element 287 element 284 element 305 element 308 element 312 element 278 1E-14 Permeability porosity 0.04 0.02 1E-16 1E-18 0.01 1E-20 0 0 1000 2000 time (d) 3000 0 4000 1000 2000 time (d) 3000 4000 Fig. 8. Time evolution of porosity (above left) and permeability (above right) in the rock around the opening for elements near drift wall. Location of elements for evolution plots (below left) and porosity distribution near the drift wall after heating (below right). EDZ-Modelling studies Evolution of permeability for a line perpendicular to the cavity In order to compare, at least in a qualitative way, 1E-15 the results of the model with measurements, it is 1E-16 better to represent the variation of permeability 1E-17 with distance to the drift. line situation Permeability as a function 1E-18 of distance is presented in Fig. 9 for different times: 1E-19 0, 250, 1000, 1500, 2000, 2500 and 3000 days of 1E-20 This plot 0 0.5 1 1.5 2 2.5 3 simulation. distance [m] corresponds to a line which starts in the upper Fig. 9. Permeability as a function of distance from drift part of the drift wall and wall and for several simulation times. goes through the rock in the direction where the most important dilation is expected. The highest permeability was reached after 500 days of heating approximately. Later on, permeability decrease and the values were (near to the drift wall) of about 10-16-10-17 m2. The computed permeability distributions are of the same order of magnitude as the measured ones. It should be noted that the initial situation was not recovered after 10 years of simulation and that the dilation effects on permeability are not significant for distances greater than one meter. 1E-14 Permeability [m2] 1000 - 3000 days 1000 1500 2000 2500 3000 250 0 days of heating 250 days of heating 1000 days of heating 1500 days of heating 2000 days of heating 2500 days of heating 3000 days of heating DRIFT 0 -1000 days The Hou/Lux constitutive model  considers elasto-viscoplastic behaviour, too. It permits prediction of stress induced damage, damage healing, dilatancy and contractancy as well as coupling with hydraulic models such as Darcy's law and permeability-porosity relation (e g., that shown in Fig. 4). The model is implemented in the code MISES 3 and it has been used recently to simulate development and healing of EDZ around an underground sealing system . Figure 10 shows the FEM model with rotation symmetry. The dam is situated in a horizontal drift of 4 m radius at a depth of 1000 m below ground. The following history is considered: The plug is installed 50 years after drift opening. 10 years later a brine inrush leads to loading of the left plug head face. Air is prevailing on the on the right side. Induced by the secondary stress field influenced by the supporting plug and the hydraulic processes creep deformation 107 continues because of the viscoplastic rock behaviour. The almost complete healing of the EDZ after 70 years can be seen in Fig. 11. This result is in good agreement with the in-situ observation of EDZ permeability reduction described in section 2.3 (see also Extended Abstract #36). Fig. 10: FE model of drift plug  Fig. 11: Evolution of permeability around an drift plug in a salt formation 1000 m below ground 2.5 Discussion of scale effects Salt formations selected for radioactive waste disposal are generally very homogeneous and due to its visco-plasticity not as fractured as crystalline rocks. Thus, they can be considered as continuum. Small samples investigated in the laboratory can be considered very homogeneous, too and the laboratory results can be directly transferred to in-situ conditions. The mechanical treatment during drilling leads to micro-fracturing and relaxation of the core samples leads to widening of the micro-fractures and thus to an increase of porosity and permeability. However, the samples can be restored into the original condition if being loaded by a confining stress representing the in-situ conditions. Figure 12 shows as an example the permeability measured on salt cores as a function of confining pressure . The lower curve represents initial data of re-consolidated samples and the upper curve represents data after dilatant deformation. As can be seen, the permeability decreases again to original values below 10-21 m2 at confining pressures higher than 10 MPa. The results obtained from small samples in the laboratory can thus be considered transferable to in-situ conditions. This is even more valid for URLs if they are transferred into a repository at a later stage or if the type of formation is similar Fig. 12: Permeability of core samples as a to that expected at the repository site. function of confining pressure  Caution is to be taken only if the type of formation and its properties differ significantly from those expected at the repository. Also the depth of the URL should be comparable to repository conditions, but EDZ evolution can be studied in principle if the local stress state is 108 known sufficiently. This holds also for URLs located in old abandoned mines where the state of stress is commonly disturbed due to the existence of large old excavations. 3. Practical Consequences of EDZ for Repository Evolution Stages Fig. 13: Horizontal convergence of a 10 m wide and 7 m high drift at the Asse mine The importance of the EDZ in a salt repository is mostly connected to the combined sealing effectiveness of host rock and geotechnical seals or backfill placed in drifts or disposal rooms. As explained above, EDZ develops if dilatancy occurs under deviatoric stress conditions. If an underground room is excavated the stress state in the immediate vicinity of the room changes significantly, e.g., the radial stress component increases spontaneously leading to rapid room convergence. A typical example is given in Fig. 13. The maximum convergence rates occur in the first three months and decrease to a constant value during the following years. From this it is clear that dilatancy and EDZ evolution occur in the very first months and a further increase of EDZ extension with time is limited. This has the practical consequence that EDZ evolution can only be avoided if the host rock is supported either by installation of a stiff seal immediately after or better accompanying to room excavation. Anyway, healing of EDZ takes place in rock salt also in case of longer room opening. After supporting structures like plugs are installed in drifts, the rock creeps onto the surface of the stiff inclusion and leads to compaction and reduction of porosity and permeability. As shown in section 2.2, permeability decreases significantly within times less than ten decades. Hence, special measures to reduce or cut off EDZ around disposal rooms or access drifts may not be needed in a salt repository. 4. EDZ Relevance for PA In all generic PA for repositories in saliferous rocks carried out so far, the increased permeability of the EDZ has never been explicitly considered from the following reasons: it can in principle be included in PA with an assumption of an integral permeability for a seal system, a considerable effect is only expected for the first decades of the post-closure period and thus of minor importance for the long-term-safety, with the construction of the repository, there is always an option to reduce an EDZ by technical measures. Anyhow, if an explicit treatment of the EDZ permeability is required, e.g. in a licensing procedure or in studies of gas pressure build-up, the PA codes can be correspondingly amended. For this, first a relation describing the initial spatial extension of the EDZ and the associated permeability is required and second, a relation describing the temporal variation of the EDZ permeability under the petrostatic pressure of the converging rock. A first attempt to 109 derive such relations from the analysis of experimental results was performed in . Here it was stated that the experimental database is still too small for a sound modelling of the permeability decrease due to the crack healing in the post-operational phase of the repository. From the available experimental results it can be expected that the water content and temperatures will have a strong influence to the re-establishment of the EDZ permeability. 5. Conclusions and Recommendations for Further Research Needs The mechanisms leading to the development of EDZ in rock salt are known to a satisfactory degree. Laboratory investigations performed by several investigators resulted adequate information about the dilatancy limit in salt rocks. The relation between permeability and porosity, the hydraulic properties being of primary interest for long-term safety analyses, have also been determined to a satisfactory degree and are thus available for PA studies. Anisotropy of permeability is obvious, but is rarely considered in models and needs to be quantified by further experimental investigations. So far, evolution of EDZ including its healing has been addressed in only one field investigation programme at the Asse mine (extended abstract #36). It was shown that the EDZ permeability reduces significantly within several decades, but a complete healing is not achieved. Models to predict damage and dilatancy as well as permeability distributions in rock salt have been developed recently and first predictions done in comparison to in-situ measurements show a reasonable agreement between modelling and measuring results. These models enable an assessment of reversibility of EDZ evolution which cannot be observed in situ because of its long duration (several centuries?). These important issue can only be solved by numerical simulations the constitutive models in which, however, being still to be confirmed by further selected in-situ measurements, whenever possible at plugs installed longer times ago. 6. Acknowledgements Most of the work cited above has been co-funded by German and Spanish governmental organizations and by the Commission of the European Communities (CEC). The authors would like to thank for this support. 7. References  Schulze, O., Popp, T., Kern, H., 2001: “Development of damage and permeability in deforming rock salt”, Engineering Geology 61, 163 – 180, Elsevier Science B.V.  Hou, Z., Lux, K.-H.,: Mechanische Modellierung und mechanisch-hydraulische Tragwerksanalyse von Auflockerungszonen des konturnahen Gebirges von Damm- und Verschlussbauwerken im Salinar, BMBF- Abschlussbericht 02 C 0588, 30.06.2002.  Cristescu, N., Hunsche, U., 1998: "Time Effects in Rock Mechanics, John Wiley, New York. 110  Stormont, J.C., Deamen, J.J.K., 1992: “Laboratory study of gas permeability changes in rock salt during deformation”, Int. J. Rock Mech. Min. Sci.Geomech. Abstr. 29, 325 – 342.  Peach, C.J., Spiers, C.J., 1996: "Influence of crystal plastic deformation on dilatancy and permeability development in synthetic salt rock, Tectonophysics, 256, 101-128.  Popp, T., Kern, H., Schulze, O., 2001: „Evolution of dilatancy and permeability in rock salt during hydrostatic compaction and triaxial deformation”, J. of Geophys. Res., Vol. 106, No. B3, 4061 – 4078.  Miehe, R., Harborth, B., Klarr, K., Ostrowski, L., 1993: “Permeabilitätsbestimmungen im Staßfurth Steinsalz in Abhängigkeit von einer Streckenauffahrung“, Kali+Steinsalz, 11, 175 – 184.  Alkan, H., Cinar, Y., Pusch, G., : Hydraulische Modellbildung von Auflockerungszonen des konturnahen Gebirges von Damm- und Verschlussbauwerken im Salinar, BMBF- Abschlussbericht 02 C 0598, 30.06.2002.  Guegen Y., Dienes, J., 1989: "Transport Properties of Rocks from Statistics and Percolation Porous Media, Mathematical Geology, Vol. 21,1, 1-13.  Wieczorek, K., Zimmer, U., 1998: “Untersuchungen zur Auflockerungszone um Hohlraeume im Steinsalzgebirge”, Final Report, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, GRS-A-2651, Braunschweig.  Alheid, H. J., 2002: “Geophysical characterization of excavation disturbed zones”, Proc. NARMS02, Toronto Workshop on “Excavation Damage Zone”.  Olivella S. and A. Gens, 2002, “A constitutive model for crushed salt”, Int. J. Numer. Anal. Meth. Geomech., 26:719-746.  Hou, Z., 2003: “Mechanical behaviour of salt in the excavation disturbed zone around underground facilities”, Int. J. of Rock Mechanics and Mining Sciences, Vol. 40/Issue 5, 725-738.  Müller-Lyda, I., 1999: “Permeabilität von aufgelockertem Steinsalz. Ableitung einer Permeabilitäts- Druck-Relation für Langzeitsicherheitsanalysen“, GRS-151, Braunschweig. 111 Lessons Learned in Indurated Clays H.-J. Alheid Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany Summary Detailed research on the evolution of excavation disturbed zones (EDZ) around tunnels in indurated clay formations has been performed during the last decade. The important mechanisms governing the development of the EDZ in indurated clays are generally well understood and very good documented by laboratory and in-situ tests as well as model calculations. This knowledge allows analysing how design parameters influence the development of the EDZ around a tunnel and thus help to optimise nuclear waste disposal concepts. In addition long term development can be described qualitatively. The presented results were mainly obtained from the Underground Rock Laboratories (URL) in Tournemire and Mont Terri. In-situ measurement techniques to characterise the extent and properties of the EDZ are well established but some of them allow only qualitative descriptions. Laboratory experiments suffer from sample disturbances during sampling and transportation. Accordingly, proper sampling and sample preparation is difficult and expensive. Even though the EDZ seems to have no impact on long term safety considerations, the knowledge of the site specific development of the EDZ in time and space and its reliable modelling are essential. All necessary tools (experimental and numerical) have to be further developed to guarantee a continuing optimum state of the art analysis of the EDZ. 1. Introduction During and after excavation of underground openings an excavation disturbed zone (EDZ) develops around the opening. The hydraulic and mechanical properties of the EDZ differ from those of the undisturbed rock. The type and degree of disturbance and the extent of the EDZ depend upon many factors, such as hydraulical and mechanical properties of the rock, state of in-situ stresses, dimensions and geometries of the opening, and the excavation procedure. In the vicinity of the opening, excavation induced deviatoric stresses induce micro- and macrofracturing affecting the mechanical integrity and increasing the permeability. Especially the possible increase of the permeability in the range of several orders of magnitude after construction of the opening has to be analysed and considered carefully in view of a safe containment of radioactive waste over very long time. With respect to clay formations, research has been performed to measure the extent and the properties of the EDZ and, based on these experimental results, to develop numerical tools to model the development of the EDZ in time. Knowledge of the development of the EDZ is necessary to optimise the design of underground tunnels for nuclear waste disposal with regard to construction, operation and long term safety. The results presented in this paper are mainly based on research performed in the Underground Rock Laboratories (URL) at Tournemire (France) and Mont Terri (Switzerland). During the next years additional important results are expected from the URL Meuse/Haute Marne in France that is under construction. 2. Current understanding of EDZ Nature and Properties 2.1 Process understanding As in all rocks the development of EDZ in indurated clays is driven by the initiation of failure due to excessive stresses which can not be accommodated by the material around underground openings during construction and operation. This leads to the evolution of dilatancy around the openings. Dilatancy is defined as an inelastic volume increase in consequence of the initiation and opening of micro or macro fractures under deviatoric stresses. In isotropic rocks generally tensile fractures oriented parallel to the free surface occur. Most indurated clays show a significant anisotropy of strength due to the existence of Bedding plane failure Extensional fractures Fig. 1: Development of extensional fractures and bedding plane fractures in the case of horizontal bedding planes. (adopted from ) bedding planes. In this case two different failure modes can be observed: Extensional fractures parallel to the tunnel wall and combined tensile and shear failure of the bedding planes. The relative occurrences of these two mechanisms depend mainly on the state of stresses around the opening, the orientation of the bedding planes in regard to the axis of the tunnel and the degree of anisotropy. From laboratory experiments, in-situ observations in tunnels and boreholes as well as from results of modelling calculations bedding plane failure is most pronounced if the tunnel axis is oriented parallel to the bedding planes. These observations are summarised in Figure 1. The opening of fractures is always accompanied by an increase of permeability parallel to the fractures and therefore permeability in the EDZ is generally anisotropic. The amount of permeability increase depends strongly on the interconnectivity of the fractures. Partial desaturation of the EDZ during construction and operation as well as gradual resaturation after closure plays an important role in the development of the EDZ in time. 2.2 Experiences from laboratory testing The mechanical behaviour of indurated clays has been studied in many laboratory tests. Thus deformation, strength and dilation parameters are well known for specific clays. The degree of saturation has a large influence on the mechanical behaviour. Desaturation in the vicinity of underground openings strengthens the rock and results in increased selfsupporting capability of the clay around the opening. Weakening of clay with increased water content leads to creep and borehole closure in laboratory experiments (Fig.: 2). The damage around boreholes in samples is most pronounced if the boreholes are oriented parallel to bedding [2, 3]. Laboratory experiments that allow to correlate the degree of saturation and the electrical conductivity or resistivity  help to analyse the distribution of saturation within the EDZ by in-situ geo-electrical measurements. Fig. 2: Weakening of clay with increased water content. Left: Water circulated in the inner borehole. Right: Dry inner borehole (adopted from ) Permeability measurements of intact Opalinus clay in isostatic cell tests resulted in hydraulic conductivities from 6*10-14 m/s perpendicular to the bedding planes to 2*10-13 m/s at 45° to the bedding plane. For permeability changes induced by deviatoric stresses there are only few results from laboratory measurements available up to now, but the development of suitable testing devices is in progress . Results from laboratory experiments on indurated clays depend strongly on the degree of sample disturbance achieved during sampling, transportation and preparation. Several techniques have been developed to guarantee disturbance free samples for laboratory testing. These techniques include special drilling methods (using Nitrogen or oil based mud as drilling fluid and special core catchers) and special treatments after core recovery to contain part of the initial confinement during transportation (concrete confinement, pressure cells). Sample disturbance especially complicates laboratory research on dilatancy and progressive failure. 2.3 Experiences from in-situ experimental observation and characterization Various measuring methods have been applied with promising success to characterise and determine dilatant zones or the extent of EDZ around underground openings. Descriptions of the development of the EDZ in indurated clays are available from observations during construction and operation of tunnels at depths up to 800m . Damage to the tunnels (especially floor heaving) was mainly induced by swelling and strength reduction due to water access during construction or operation. Detailed qualitative analyses of the observed features of the EDZ were performed in the Tournemire and Mont Terri Rock Fig. 3: Structure developed during complete closure of a borehole at the Mont Terri URL. Black circle indicates the initial diameter of the borehole. Black lines enhance failure patterns. Laboratories [7, 8]. During most excavations at the Mont Terri Rock Laboratory water access was carefully avoided. As a result no significant damage is observed up to now. Most of the tunnels are oriented oblique to the bedding planes. In order to get insight into the structure of the EDZ if the axis of the opening is oriented parallel to the tunnel the deformation behaviour of boreholes was analysed. Figure 3 shows the structure developed during complete closure of a borehole parallel to bedding. The structure was fixed by injection of resin before overcoring the initial borehole. The observed features correspond very well to the process understanding shown in figure. 1. The most important parameter to characterise the EDZ is the permeability. Therefore a large number of water and gas permeability measurements have been performed in situ. Figure 4 shows results from the Tournemire Rock Laboratory obtained in the 100 years old railway tunnel. The permeability (closed symbols) is increased several orders of magnitude close to the tunnel wall. It exceeds the range of the applied measuring tool especially designed to measure low permeabilities (closed triangles). Seismic velocities measured in the same hole showed good qualitative correlation to the measured permeabilities. First results from constant head injection tests close to the wall of the EB tunnel during the “Engineered Barrier Experiment” at Mont Terri show the highest transmissivities not at the wall but at some distance to the wall (Fig.5) this corresponds to other observations at Mont Terri. In addition a reduction of the transmissivities can be observed with time. This can result from local resaturation around the probes due to the applied technique or indicate healing of the damaged rock due to resaturation and reloading of the EDZ because of the saturation of the applied bentonite backfill. Unfortunately permeability measurements are expensive and time consuming. In addition the applied technique may influence the results if repeated measurements are used to analyse the development in time. Therefore other Fig. 4: Apparent permeabilities around the old part Fig. 5: Transmissivities close to the tunnel wall of of the Tournamire Rock Laboratory the EB tunnel at Mont Terri parameters are often favoured to characterise the extent and condition of the EDZ. The most promising techniques are seismic and geo-electrical methods. Seismic velocity distributions around underground openings have been measured in different rocks, and low velocity zones turned out to give a good estimation of the EDZ . Complete seismogram sections show a very good indication of the extent of the EDZ characterised by reduced amplitudes due to high material damping and low velocities in the mechanically disturbed zone (Fig.6). Geo-electric measurements reflect the degree of saturation of the rock. As desaturation is enhanced in the EDZ, the resistivity distribution around the tunnel can be used to characterise the EDZ (Fig. 7, ). Radar measurements gave no reliable results up to now . Acoustic emissions during the development of the EDZ are only sparsely observed in Opalinus clay . Fig. 6: Colour coded seismic section obtained from Fig. 7: Distribution of electric resistivity around interval measurements in a borehole in indurated clay the New Gallery at Mont Terri  The permeability in the EDZ is highly anisotropic, with the highest component parallel to the drift surface. Up to now, this anisotropy is difficult to determine by permeability measurements in low permeable rocks. Seismic measurements offer a possibility to analyse this anisotropy at least qualitatively. In-situ observations and measurements in indurated clays demonstrate that the EDZ is limited to the close vicinity of the tunnels and extents up to 1m or 2m from the tunnel wall. The development of the EDZ with time is subject of several in-situ experiments in the Mont Terri Rock Laboratory. The “Engineered Barrier Experiment” analyses the effect of resaturation and backfill swelling, the “Heater Experiment” concentrates on the effect of temperature, the “Ventilation Experiment” deals with the influence of desaturationresaturation cycles and the “Self Healing Experiment” directly looks to the self healing capabilities of the clay. All these experiments are ongoing experiments and final analysis of the results has not been performed up to now. Available data indicate a decrease of permeability with time if resaturation occurs or internal pressure is applied to the opening. Up to now it is not clear whether this decrease in permeability is just an effect of the closure of fractures due to compressional stresses and swelling (self sealing) or reflects redistribution to the structure of the intact rock. Existing documents on laboratory and in-situ tests as well as on EDZ-prototype experiments at Mont Terri have been reviewed and summarised into a comprehensive data base of the rock mechanical parameters of the Opalinus Clay at Mont Terri . 2.4 Modelling capabilities Preoperational modelling has been performed for several experiments at Mont Terri. These calculations consider in most cases the hydromechanical (Engineered Barrier Experiment) or the thermo-hydromechanical (e.g. Heater Experiment ) coupling calculating the desaturation and resaturation of tunnels or boreholes during experiments. Such calculations have been performed using elastic or isotropic elastoplastic constitutive laws implemented in different codes (e.g. CODE_BRIGHT or Flac 3D). Other approaches use an isothermal, fully coupled three-phase code for liquid and gas flow in deforming porous media (MHERLIN). These calculations consider the EDZ as desaurated and/or plastified zone around the opening but do not model the processes of EDZ development in detail. Great efforts have been made to model the initiation and development of the EDZ in indurated clays based on a conceptual model and micro-mechanical modelling developed in the frame of the “Rock Mechanics Analysis Experiment” at Mont Terri. The conceptual model is based on the assumption that the deformation of clay like the Opalinus Clay is mainly controlled by its internal structure and the interaction of the clay particles, the bound and the free water. Micro mechanical modelling using the code PFC was performed to get an understanding of the material behaviour on the microscopic scale . Based on this insight model a constitutive law was developed that considers fully hydromechanical coupling, anisotropic hydraulic conductivity, a bilinear ubiquitous joint model with strain softening and creep. Comparison of calculations based on parameters obtained from laboratory experiments with in-situ results showed that this model overestimates the time dependent deformations unless a threshold value for the onset of creep was introduced. With this threshold included the results of modelling the development of the EDZ  agree excellent with observations made in-situ. Figure 8 shows on the left hand side the results from calculations modelling a tunnel oriented 45° to the bedding plane. This orientation results in tensile fracturing at the walls (almost vertical grey lines) and very limited bedding plane failure in roof and floor as observed in the new gallery at Mont Terri. The asymmetry in damage distribution is induced by the orientation of the bedding planes. Results obtained for a tunnel oriented parallel to horizontal bedding is shown on the right hand side. The EDZ is characterised by some tensile fractures close to the walls and additional significant bedding plane failure in roof and floor. This corresponds to observations in boreholes and laboratory experiments as described above. Comparison of calculated and measured pore pressures and deformations in the EDZ as well as tunnel convergences also show good agreement. The results demonstrate that the developed numerical tools can model the important features of the development of the EDZ during construction and operation of underground tunnels in indurated clays. Fig. 8.: Evolution of the EDZ with time around a tunnel in indurated clay with anisotropic HM-behaviour due to bedding planes. Left: Tunnel axis oriented about 45° to bedding. Right: Tunnel axis oriented parallel to bedding 2.4 Discussion of scale effects Most laboratory experiments are performed on small samples and at relatively high deformation rates. Thus scale effects may be important in space and time. The validity of parameters obtained from small scale tests for the description of the behaviour of a large rock masses can only be proven by comparison to in-situ results and model calculations. In case of the Opalinus clay, laboratory results for elastic parameters obtained on samples from Mont Terri show good agreement with in-situ measurements. Also, model calculations of tunnel deformations proof the validity of elastic parameters determined in laboratory experiments. The situation is different for strength parameters. Due to sample disturbances, as discussed in 2.2, the measured parameters often underestimate the in-situ values. Thus laboratory results seem to be conservative in view of strength. In addition strength parameters seem to depend on the applied deformation rates . The dependency increases with increasing saturation. This may be explained by the development of non uniform pore pressure distribution in the sample during the test due to the low permeability of the clay. Pore pressure measurements during laboratory tests on small samples are difficult and have been performed successfully in a few cases only. 3. Practical Consequences of EDZ for Repository Evolution Stages Stress changes during construction lead to formation of an excavation disturbed zone around the excavated openings. The extent of this zone is very restricted in terms of space and time. The damaged rock will not completely disintegrate but will generally keep a residual strength. The choice of tunnel geometry, orientation and support can reduce the extent of the EDZ even more. During the construction and operation the hydraulic conductivity of the excavation disturbed zone is several orders of magnitude higher than that of the undisturbed host rock. Geochemical alterations in the vicinity of the tunnel during construction and operation of a repository seem to be negligible, due to the short period of time during which emplacement tunnels remain open. After closure of the repository the excavation disturbed zone and the backfill begin to saturate and the stresses in the excavation disturbed zone will become compressive. This will lead to a compaction and to a sealing of existing fractures. The question whether the conditions after closure will finally lead to self healing of the damaged rock can not finally be answered today. Nevertheless, it is expected that the properties of the excavation disturbed zone will gradually approach the properties of the intact rock. Increasing temperature probably accelerates the disintegration of the rock and the homogenisation of the pore space in the excavation disturbed zone. For sealing off access tunnels and the shaft different models exist to treat the EDZ. The damaged material can either be completely removed for the total length of the EBS prior to construction or the EDZ is cut by narrow radial cuts filled with bentonite to stop transport parallel to the tunnel axis. 4. EDZ Relevance for PA If the EDZ will gradually seal after closure of the repository and in addition pathways along the EDZ of the access drifts and the shaft are cut off the effect of the EDZ for long term safety considerations may be neglectable even if the EDZ remains with a slightly increased porosity and an effective axial hydraulic conductivity around one order of magnitude greater than that of the intact rock. Hydrogeological model calculations in the frame of the “Entsorgungsnachweis” have shown that water flow through a deep repository will not be significantly increased even in the case of a significantly higher permeability of the excavation disturbed zone, as water flow depends in the first instance on the permeability of the intact rock . Nevertheless for every selected site the validity of the above assumption of a limited extent of the EDZ, the long term reduction of permeability and the self sealing capacity of the EDZ has to be proven by site specific data, observations and calculations. It seems that the experimental database is still too small for sound modelling of the permeability decrease due to the healing in the post-operational phase of the repository. 5. Conclusions and Recommendations for Further Research Needs The important mechanisms leading to the development of the EDZ in indurated clays generally are well understood. The main factors influencing the development of the EDZ are very well documented by laboratory and in-situ tests and model calculations. This knowledge allows to reduce the EDZ during construction to a certain degree and to have a qualitative understanding of the long term development. However, it should be kept in mind that the present knowledge is mainly based on research performed in two underground laboratories at Tournemire (France) and Mont Terri (Switzerland) and that the rocks at these sites are rather similar to each other. In addition the depths of these laboratories are only moderate compared to the depth of a final repository. Experiences from tunnels at greater depth are only sparse and qualitative. Therefore the transferability of the up to know knowledge about the EDZ around tunnels to other sites with indurated clays with differing properties has to be checked carefully. But the research performed so far has developed important process understanding as well as experimental and modelling techniques. Some aspects, like the biological effects in the EDZ, have just been identified and research is in its initial stage. Other problems are generally solved but are not known quantitatively, as e.g. the anisotropy of permeability induced by the excavation that superimposes the anisotropy of the intact rock. This combined anisotropy is obvious, but is rarely considered in models and needs to be quantified by further experimental investigations. The long term behaviour has to be extrapolated from short term observations and experiments using reliable numerical models. Models to predict damage and dilatancy as well as permeability distributions have been developed. Predictions, done in comparison to in-situ measurements, show a reasonable agreement between modelling and measurement results. These models enable an assessment of the EDZ evolution over long times. These numerical simulations are based on constitutive models that still need to be further confirmed by selected in-situ measurements. This holds especially true for the influence of repeated desaturation –saturation cycles addressed in the “Ventilation Test”, for the influence of increased temperature addressed in the “Heater Experiment” and the interaction between backfill, EDZ and intact rock addressed in the “Engineered Barrier Experiment”. Although these experiments will lead to large additional information and knowledge during the next months or years it can be expected that not all questions will be answered by the achieved results. One important lesson learned from these experiments will be that the design of large scale demonstration tests can be improved considerably. An in depth understanding of the long term development of damaged clay under confining stress conditions is urgently needed to show whether the permeability decrease in the EDZ is due to fracture closure or real self-healing processes. Even though the EDZ seems to have no impact on long term safety considerations, the knowledge of the site specific development of the EDZ in time and space and its reliable modelling are essential. Therefore all necessary tools (experimental and numerical) have to be further developed to guarantee also in future the optimum state of the art analysis of the EDZ. 6. Acknowledgements Most of the work cited above has been co-funded by members of the Mont Terri Consortium and by the Commission of the European Communities (CEC). The author would like to thank them for this support. 7. References  Nagra: Projekt Opalinuston - Synthese der geowissenschaftlichen Untersuchungsergebnisse. Entsorgungsnachweis für abgebrannte Brennelemente, verglaste hochaktive sowie langlebige mittelaktive Abfälle. Nagra Technical Report NTB 02-03, Nagra, Wettingen, Switzerland, 2002.  Økland, D. & Cook, J.M. Bedding-related borehole instability in high-angle wells. 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