The Importance of the Excavation Damaged Zone for Post-Closure Safety of Deep Geological Repositories: Some Introductory Remarks

Piet Zuidema Nagra, Switzerland

Summary

This presentation aims at listing observations and critical issues concerning the importance of the excavation damaged zone around underground excavations for post- closure safety of deep geological repositories. The observations are reported in the format of a list of bullet points with some explanatory text. It is not the aim to give a scientific overview or discuss specific scientific issues; this will be done by the scientific papers later in the conference.

1. Introduction

The excavation damaged or disturbed zone (EDZ) is an important issue in the assessment of post-closure safety of deep geological repositories. Therefore, much work has been done both within the different national disposal programmes and also within the framework of international projects and a considerable amount of information is available. Despite the effort spent, no general conclusions can be drawn and system-specific evaluations are necessary. In some cases the EDZ may be negligible, in other cases the EDZ may be an important feature that strongly affects the performance of a repository. Nevertheless, an attempt is made in this presentation to compile a set of general remarks about the role of the EDZ in the performance of deep geological repositories in the post-closure phase.

The first introductory remarks below refer to observations concerning communication:  The EDZ is an issue that has been discussed for many years and has explicitly been considered in a number of performance assessment studies. There are many different issues related to the EDZ that involve several disciplines. Because terminology is not always the same for all disciplines, there has been some confusion on EDZ related issues. Thus, it is considered important to always clearly define what is meant by the EDZ, which phenomena are considered and in what context the effects of the EDZ are assessed.

 In this paper, the EDZ is defined as that zone around underground openings that may have altered properties relevant to the post-closure performance of the overall repository system due to excavation of these openings.

2. Some broad observations

Some broad observations can be made on the current understanding of the EDZ in relation to its long term effects on repository performance:  The EDZ cannot be avoided but measures can be taken to minimise its effects. Its extent and properties depend upon the system-specific circumstances (rock type, in- situ conditions, geometry / orientation of excavated rooms, excavation techniques, 73 liner design & emplacement, etc.). An appropriate layout of the underground openings and use of adequate mining techniques can certainly mitigate the effects.

 The phenomena occurring due to excavation (e.g. re-activation of existing fractures, generation of new fractures; geochemical alterations; etc.) depend upon the specific properties of the system under investigation. The importance of these phenomena with respect to repository behaviour in the post-closure phase needs to be assessed in the context of the overall repository system behaviour (e.g. kinetics and time available for geochemical alteration reactions; role of EDZ in hydrodynamic situation of overall system).

 The properties of the EDZ and the nature of phenomena occurring within the EDZ may have a strong transient behaviour. This may e.g. include self-sealing of re- activated or newly generated fractures. Thus, it is important to "translate" the results from early (and short-term) observations on EDZ properties into information relevant to the system for times and conditions that are relevant for the post-closure performance of the system. Furthermore, it is important to note that often the experimental observations made are under different conditions than those occurring in the post-closure phase (observations in open tunnels instead of closed tunnels with swelling pressure in the post-closure phase of the repository).

 It is clear that the EDZ may have an effect on the post closure performance of the repository system and, therefore, the EDZ needs to be addressed in performance assessment.

3. Role of the EDZ in performance assessment

The potential role of the EDZ in system performance and its assessment:  The EDZ exists when emplacing the wastes, the backfill material and the seals and is thus an integral part of the system that will evolve with time. There may be a need to consider the EDZ when assessing the temporal evolution of the overall repository system (engineered barriers, access routes, host rock) to define the relevant system conditions at the time when eventually some radionuclides may be released. The EDZ may e.g. affect the hydrogeological, geochemical, geomechanical and thermal conditions but also phenomena and processes related e.g. to the release of gas generated within the repository in ways that may be both favourable and/or detrimental to system performance.

 The EDZ may form a potential pathway for migration of radionuclides from the waste emplacement rooms to the surface environment. However, this requires an evaluation of the behaviour of the EDZ for the time and conditions when radionuclide release actually might occur (after canister breaching and breakthrough of radionuclides through the buffer which may in the case of a repository for SF/HLW be tens of thousands of years or even more).

 The behaviour of the EDZ and the changes with time need to be assessed on a system- specific basis. As mentioned above, the behaviour and the processes occurring and their magnitude may be such that in some systems the EDZ may be only of very limited importance to post-closure performance whereas in other systems it may be a critical factor.

74 4. The importance of an interdisciplinary approach

The close interaction between experimentalists, process and system modellers, design specialists and performance assessment experts is a necessity to arrive at an optimum design and a credible post-closure safety case:  The focus of experiments should be maintained as much as possible on the behaviour of the EDZ, thus EDZ characterisation studies should be performed with a view to how this information may contribute to the assessment of EDZ behaviour and evolution in the overall system context at relevant times. The same principle also applies to process and system models.

 Design approaches that might limit the impact of the EDZ on radionuclide transport and on detrimental effects on the different repository components may have to be developed and tested for some repository concepts.

 Performance assessment needs to have a good understanding of the properties and the behaviour of the EDZ and its temporal evolution. This may for some systems require including the different nuances of the EDZ (and the corresponding favourable and/or detrimental effects on overall system performance) into the phenomenological analysis that will lead to the definition of the broad quantitative radionuclide release calculations.

75

Review of the Conclusions of Previous EDZ Workshops

Tim McEwen Safety Assessment Management, UK

Summary

A review has been carried out of four previous workshops on the Excavation Damage(d) or Disturbed Zone (EDZ) that took place between 1988 and 2001. It considers developments in the study of the EDZ generally only up to the time of the Clay Club meeting in 2001 [4] and, although reference is made occasionally to more recent work, this fact needs to be borne in mind when reading this paper. Two of these workshops considered only one rock type. A distinction is made in this review between the Excavation Disturbed Zone and the Excavation Damage(d) Zone, although in some rock types it may not be possible to distinguish between the two. This review examines the change in understanding that has developed over this period regarding the significance that should be attached to the EDZ from the point of view of both operational and long-term safety. It also examines the level of understanding of the processes that are involved in determining the extent and magnitude of the EDZ, from both a practical and a phenomenological standpoint. The questions regarding the EDZ that remain to be studied are presented with reference to the different rock types of interest.

1. Introduction

This review considers four previous workshops on the EDZ which have taken place over a period from 1988 to 2001. These workshops are:

1. NEA workshop on Excavation responses in geological repositories for radioactive waste, Winnipeg, April 1988 [1]). 2. EDZ workshop: Designing the Excavation Disturbed Zone for a nuclear repository in hard rock as part of the International Conference on Deep Geological Disposal of Radioactive Waste [2]. 3. Lessons learned from a NEA SEDE Topical Session which took place in 1998 on Characterisation and Representation of the Excavation Disturbed Zone [3]. 4. Self-healing topical Session Proceedings of the 11th Clay Club Meeting, Nancy, France, May 2001 [4].

Whilst two of these workshops (2 and 4) considered only one rock type, the other two considered all types, however, up to about 1996 there was considerably greater interest in investigation the EDZ in crystalline rocks and salt than in argillaceous rocks. It also appears that, at that time, it was thought that the EDZ was probably of more significance in crystalline rocks than in salt, as there was believed to be a continuous, high permeability pathway around underground openings, which would provide a preferential pathway for radionuclide transport. In contrast, the emphasis in salt at that time seemed to be concerned more with the stability of the underground openings rather than with the impact that any EDZ would have

77 on the long-term performance of a repository, although work at the WIPP was being carried out in this area. In fact, it has only been in the last few years that much interest has been shown in studying the EDZ in indurated clays, which is interesting because it is this rock type that probably displays the most complex and extensive EDZ. This is, of course, linked to the increase in interest over the same period in the use of this rock type to host repositories for long-lived waste.

This review considers developments in the study of the EDZ generally only up to the time of the Clay Club meeting in 2001 [4] and, although reference is made occasionally to more recent work, this fact needs to be borne in mind when reading this paper. Recent developments in the understanding of the EDZ in indurated clays are, therefore, not included, and the most recent work to be included on crystalline rocks and salt is that that took place up to approximately 1998.

2. Terminology and understanding of the EDZ

The acronym EDZ has a variety of meanings which are, however, relative similar in their description of what has taken place in the rock mass due to the construction of an underground opening. These different meanings have developed because EDZs have been studied in different types of rock, where effects can be markedly different, and in different geological environments but within the same types of rock. The latter is the situation for crystalline rocks which have been studied in very different structural terrains, for example, in the Canadian Shield (at the URL site at Pinawa), in the Alpine basement rocks of southern Switzerland (at the Grimsel Test Site) and in the Baltic Shield (at the Äspö HRL). The granitic basement at Pinawa at depths of more then a few hundred metres has an extremely low fracture density and unusually high in situ stresses. These factors have had a considerable influence on the extent and form of the EDZ in the Canadian URL compared with those developed either at Grimsel or Äspö. The essential elements of the EDZs in all these crystalline rocks are, however, similar.

There is no universally agreed definition of the EDZ, nor is there agreement as to what precisely the acronym EDZ means, however these differences are probably not important, as they are related to the slightly different use of terms such as disturbed and damaged. This current variance in the use of this terminology does not seem to have caused any problems to date, neither in investigating EDZs nor in the way in which they have been treated in performance assessments.

The various ways in which the term EDZ is or has been used are listed below:

1. In Canada the terms Excavation Disturbed Zone or Excavation Damage (or Damaged) Zone (both EDZ) are synonymous, and in the USA and Canada the term Disturbed Rock Zone (DRZ) has also been applied. Fairhurst and Damjanac (in [2]), for example, make no distinction in their use of these terms and state that they are all synonymous.

2. In Sweden and Switzerland the Excavation Damaged Zone is distinguished from the Excavation Disturbed Zone. The Damaged Zone is limited to the part of the rock mass closest to the underground opening which has suffered irreversible deformation and where fracture propagation and/or the development of new fractures has occurred. This is distinguished from the Disturbed Zone, which is further into the rock mass, and in which only reversible (recoverable) elastic deformation has occurred.

78 3. 1 and 2 above are both concerned with the development of EDZs in crystalline rock. The development of an EDZ in a plastic rock, such as a soft (plastic) clay or rock salt is somewhat different from that in an essentially brittle, crystalline rock. In both the Boom Clay and the Zechstein salt at Asse the term Excavation Disturbed Zone is used. In both these rocks the macro response of the rock is essentially plastic, however, this disguises the fact that the large changes in stress accompanying the development of an underground opening produce dilatancy and microfraturing in the salt, which is modelled as a elastic- viscoplastic material. The deformation in the Boom Clay around underground openings, in contrast, can be adequately modelled as a perfectly plastic material, although recent work has demonstrated the presence of extensive fracturing around excavated tunnels [5].

4. The EDZ developed in indurated clays, such as the Jurassic clays (or shales) at Tournemire and the Opalinus Clay in Switzerland, is more like the EDZ developed in crystalline rocks. In the short term the two are likely to be similar in extent and possible also in character. In the long term, however, an EDZ in an indurated clay may heal or partially heal, whereas the same seems less likely in the more brittle and stronger crystalline rock. The main difference between the EDZs developed in these two types of rock may be related to the chemical changes that takes place within them. This subject is discussed later in the report.

The terminology used in this review is in line with that used in crystalline rocks, in that a distinction is made between an Excavation Disturbed Zone and an Excavation Damaged Zone. In rocks such as the Boom Clay or the Zechstein salt at Asse it is assumed that it not possible to distinguish between these two zones, and the term Excavation Disturbed Zone is used, in line with its current usage at both sites1. It is also suggested that this subdivision of an EDZ is retained for use in the future in any rock types where it is warranted.

3. Excavation Damage Zones in different rock types

EDZs have been studied most frequently in crystalline rocks. This is partly due to the fact that the majority of early work in URLs was carried out in this rock type and partly because an EDZ is more obviously visible in this type of rock than it is in less competent rocks, such as clays and rock salt. In particular, there was a considerable amount of early work in the Canadian URL as the lack of fractures at depth and the high in situ stresses suggested that the EDZ would provide a significant and more transmissive pathway through the rock mass.

EDZs have been most frequently studied in crystalline rock in Canada and Sweden, at the Canadian URL at Pinawa and at both Stripa and Äspö in Sweden. They have also been investigated at the Grimsel Test Site in Switzerland and, more recently, at Kamaishi, in Japan. Work started on EDZs quite early in URL programmes, as it was considered that an EDZ could have a considerable influence on the performance of a repository in crystalline rock. This is because its formation could result in the production of a more permeable pathway parallel to a disposal tunnel, which could result in the more rapid transport of radionuclides in the rock mass immediately adjacent to the repository. This part of the rock mass is sometimes referred to as the near-geosphere, i.e. what is sometimes termed the intact rock or averagely fractured rock lying between the repository and the nearest fracture zone. 1 It may be that these two zones do not exist in these types of rock, in particular in plastic clay. Conversely, it is also possible that, although the Boom Clay can be modelled adequately as a perfectly plastic material, in phenomenological terms it does possess these two zones. Even if this were to be the case, it is may be difficult to distinguish between these two zones in such material and the possibility of their existence may have no impact on the role of the EDZ in relation to the long-term performance of a repository. 79 This part of the rock mass is significant, as it is often provides an important component of the geosphere barrier in performance assessments.

Such more permeable pathways could also provide connections between the faster natural pathways in the rock mass, which are normally associated with fracture zones. Such increased connectivity could result in a hydraulic cage effect, in which a component of the groundwater flow by-passes the waste disposal tunnels or vaults, which could be potentially advantageous, as it could reduce the flux of groundwater around the waste canisters. However, the potential for increased flow immediately outside the Engineered Barrier System (EBS) results in the possibility of lowered solute concentrations in the EDZ, thereby increasing the potential flux of radionuclides through the EBS, because the concentration gradient through the EDZ is increased2.

In other rock types, notably clays and rock salt, the problems caused by the formation of an EDZ have always been considered to be less acute. There has commonly been an assumption of self-healing within an EDZ in both plastic clays and rock salt, although it is only recently that it has been possible to demonstrate the actual extent of this process over many years, for example at Mol. The possible extent self-healing process in argillaceous rocks is discussed further in Section 4.4.

More indurated clays pose possibly a greater problem. There was initially less interest in the potential of such rocks for the disposal of long-lived wastes and it has only been in last few years that there has been an increased level of investigation of these rock types, notably at Tournemire, France and Mt Terri, Switzerland. Complex couplings are believed to exist, in which the deformation of these clays is linked to geochemical and other processes, and these types of rocks display short and long-term creep properties and aspects of brittle behaviour that are difficult to understand and investigate in situ. In addition to the lower level of understanding of the phenomenological factors associated with the deformation of such clays, compared with crystalline rocks, it is currently believed that an EDZ in such rocks may be more significant than one in a plastic clay or in rock salt. It would appear that the emphasis of any research into EDZs needs to be placed in this area.

4. Summary of previous EDZ workshops

At the time of the NEA Workshop in 1988 it was considered that, at least in hard rocks, the EDZ was likely to be of great significance for performance assessment. At that time there had been only limited research into the development of EDZs in clays whereas, in rock salt, there had been extensive experimentation, both underground and in the laboratory, on the effects associated with underground excavations.

4.1 The NEA Workshop in 1988

In 1988 a workshop on excavation response was held in Winnipeg organised by the NEA ISAG (The In Situ Advisory Group), a forerunner of the SEDE [1]. A review had been conducted by the ISAG of current requirements for in situ research and a primary topic identified concerned the excavation response; that is the effects that could be induced in the rock mass by the construction of a repository. Specifically, it was noted that residual stresses, creep or subsidence and induced fracturing, possibly leading to an increased potential for

2 This is most applicable to the situation where spent fuel or HLW is surrounded by a low permeability barrier, which is normally compacted bentonite. It is, however, also applicable to other waste disposal applications in saturated rock. 80 groundwater flow, were phenomena that required analysis to determine their influence on the safety of a repository and on the design of its engineered barriers.

It was considered at the time that the boundary between the engineered tunnel or vault and the host geological environment was conceptually one of the least well understood components of the disposal system. There were many questions relating to the implications of excavation effects and damage in this zone for the performance and design of repositories, and this was believed to apply to all rock types. The aspects of the excavation response that were considered to require explanation were thought to include:

1. the importance of excavation response to the design and safety of a repository, 2. the type of in situ tests and measurements conducted and 3. the conceptual and mathematical models that had been developed.

The validation of the latter was thought to be of particular importance if they were to be used in support of performance assessment.

The objectives of the workshop were to:

1. review the influence of excavation effects on the engineering design and performance of repositories, 2. develop conclusions and recommendations on ways of accounting for excavation effects in clay, rock salt and crystalline rocks and 3. provide a forum for discussions between scientists conducting in situ studies and measurements and those developing predictive models for use in safety assessments of repositories.

Five key considerations emerged from the presentations and discussions at the workshop:

1. the relevance or importance of excavation response and damage to performance assessment, 2. the extent to which fundamental processes or the physics of excavation response were reliably understood, 3. the future needs for the development of instrumentation and measurement methods, 4. appropriate strategies for model development and validation and 5. implications for repository design and engineering.

The relevance or importance of excavation response and damage to performance assessment

Differences in priorities emerged from the studies of the different rock types being studied with respect to the relevance of excavation effects to performance assessment. For repositories in clays it was concluded that excavation effects, if properly managed, would have no adverse influences on long-term safety, due to the self-healing characteristics of the clay3. For repositories in salt, whilst it was felt that some short-term safety assessment concerns could exist, excavation effects were not considered to be an issue for the repository’s long-term performance. However, it was noted that a better understanding of the

3 At the time of the workshop the dominant interest in the use of clays to host a repository was centred on the plastic clays at Mol and those in Italy. A paper presented by Roehl (in [1]) on the considerably more indurated and deeper Jurassic ironstone formation at Konrad alluded to the presence of an EDZ, but did not discuss it specifically and the presence of both plastic and brittle deformation zones around the existing underground openings was not considered as being significant, either in terms of operational or long-term safety (e.g. [6], [7]) . 81 combined effects of excavation, pressure, heat and radiation was required, as they influenced long-term creep phenomena. For repositories in crystalline rocks, excavation effects were not considered important for their short-term performance, however, it was felt that the implications for long-term safety had yet to be fully assessed.

The extent to which fundamental processes or the physics of excavation response were reliably understood

It was concluded that further work was needed to identify precisely the relevant phenomena and, in particular, it was recommended that the focus of future excavation response testing and analysis should be on identifying governing parameters that will potentially affect waste isolation. It was also concluded that excavation effects in salt were understood well enough for the purposes of repository construction, however, the combined effects of various excavation-induced processes as they affect long-term creep phenomena needed to be assessed further. For clay, since the effects were only expected to be short-term, it was believed that a detailed understanding would not be important or necessary. For crystalline rock, it was considered that more information was required to determine the need for remedial measures (e.g. the design of seals) and to assess the long-term safety implications. In this respect, it was concluded that the phenomena of most potential significance were likely to occur in the vicinity of repository access shafts, tunnels and emplaced waste containers.

Future needs for development of instrumentation and measurement methods

It was concluded that the current uncertainties in processes and phenomena related to excavation damage and effects arose primarily from a lack of reliable measurements. Although much recent progress had been made, reliable instrumentation or measurement techniques did not exist for measurements of excavation effects near the host rock/excavation interface. Work was needed to evaluate the accuracy and applicability of existing instrumentation for measurement of rock-mass response, and to develop instrumentation of increased accuracy. Particularly for crystalline rock, suitable methods needed to be developed for measuring very low permeabilities in the excavation damage zone. Finally, there was also thought to be a need to develop instruments of sufficient longevity under in situ conditions so as to allow testing and monitoring for periods extending to several years

Appropriate strategies for model development and validation

Some differences in opinion were expressed as to whether there was a need to improve testing and data collection or to improve model development. However, it was generally agreed that there was a need to encourage more interaction between experimenters and modellers. Rock-mass response to excavation was considered to be a very useful tool in validating models for long-term performance assessment. A recommendation was made that an international workshop should be organised to address the use of experimental data in the validation of relevant geomechanical, thermal-mechanical and fracture flow models. It was also recommended that an international benchmark exercise be established to review models designed to analyse the geomechanical and thermal-mechanical response of fractured rock or EDZs. It was felt that such an exercise could be held as part of the validation workshop outlined above, or as a separate activity.

82 Implications for repository design and engineering

The EDZ was considered to have a major effect on the design and effectiveness of seals for repository shafts and drifts, particularly in crystalline rocks and clay. It was suggested that the shaft and drift sealing tests carried out as part of the Stripa Project had demonstrated that some techniques could prove suitable for isolating and sealing excavation-induced, high permeability zones around shafts and tunnels in granitic rock. It was also suggested that further work was required to demonstrate that sealing methods worked under representative in situ conditions and that more consideration should be given to trying to limit the EDZ by evaluating the use of controlled blasting or boring techniques.

4.2 The CNS Workshop

The 1996 CNS Conference Workshop on the EDZ was held in Winnipeg, Canada in 1996, with its main objective of providing a forum for informal discussions of international work performed since 1987 on characterising and modelling the EDZ in hard rock environments, i.e. crystalline rock [2]. The focus of the 1996 EDZ workshop was related to the design- related aspects of the EDZ, such as methods of minimising the EDZ during repository development and methods that could be used to seal the EDZ to eliminate it as a pathway for the release of radionuclides from a repository.

A keynote introductory presentation was given by Fairhurst [2] and numerous case studies and examples were drawn from in situ studies of EDZs in hard rocks at AECL’s URL, PNC’s Kamaishi Mine, Nagra’s Grimsel Facility, SKB’s Äspö HRL and Posiva’s research tunnel at Olkiluoto. The presentations covered the current state-of-the-art in:

 methods for characterising the location and geometry of the EDZ in hard rocks,  methods for modelling rock mass response to predict the location and extent of the EDZ in hard rocks,  methods for designing excavations in hard rocks to minimise the EDZ and  methods for sealing the EDZ.

The workshop concluded that significant progress had been made since the 1988 NEA Workshop [1] in methods that could be used to characterise and measure the extent of the mechanical damage surrounding excavations in hard rocks. In particular, the acoustic emission-microseismic monitoring technique (AE/MS) was shown from examples in AECL’s URL and SKB’s Äspö facility to be extremely useful in revealing the extent of damage that develops around excavations during construction. The workshop also revealed that good progress had been made in advancing some rock mechanics models to the point of being able to predict the location and extent of the EDZ around openings in hard rocks. Examples were shown to illustrate how this knowledge and the models could be used to design opening with better construction methods (such as improved blast designs), geometries and orientations that could significantly reduce and almost completely eliminate the EDZ. Examples were also given to illustrate how this knowledge could be used to design seals that could effectively cut off the EDZ pathways around underground openings in hard rock.

The CNS workshop did reveal however, that there was still a general lack of understanding regarding the significance of the EDZ as a potential pathway for the release of radionuclides from hard rock repositories. It also showed that little work had been done to represent the EDZ as a transport path in models being used to assess the long-term performance of repositories. Furthermore there was a general lack of information about the radionuclide

83 transport properties of the EDZ pathway, particularly with respect to values of its effective transport porosity, in particular its flow-wetted surface area, its dispersion and diffusion characteristics and its chemical sorption properties. The workshop concluded that more work was needed in these areas before the importance of the EDZ as a radionuclide release pathway could be properly understood and sealing methods could be developed to eliminate it effectively. It was also concluded that work was needed to determine if such seals would be required to ensure or significantly enhance long-term repository performance.

4.3 Synthesis of the main conclusions from the 1998 SEDE Topical Session

The main conclusions of the SEDE Topical Session on the EDZ [3] were published under eight headings, as follows:

(i) Experimental work in different rock types:  the majority of the work has been carried out in crystalline rocks and  little work has been carried out in indurated clays.

(ii) Characterisation of the EDZ:  suitable characterisation methods exist and have been tested in various URLs,  the geometry of the EDZ in all rock types in sufficiently well known,  it is difficult to determine the hydraulic characteristics of EDZs and  it is even more difficult to determine radionuclide transport characteristics of the EDZ.

(iii) Level of understanding of EDZs:  the mechanisms of EDZ formation are understood in crystalline rocks,  the mechanisms are also relatively well understood in plastic clays and in rock salt, however the extent of self-healing and the time over which it may take place is less clear (chemical changes in clays are also likely to more significant than those in crystalline rocks and rock salt),  the mechanisms of EDZ formation are less well understood in indurated clays, due to the uncertain levels of self-healing processes, the period over which they may take place and the effects due to chemical changes in the clays and  the effects on the repository system due to the formation of the EDZ may not all be negative and may not, in any case, be very significant.

(iv) Solute transport:  little is known about radionuclide transport in the EDZ, but it is anticipated that an EDZ should have similar properties to those of the host rock (in terms of its sorption capacities, extent of matrix diffusion, etc.) and  in order to demonstrate the validity of this premise, it is considered important to carry out experiments on the transport characteristics of fractures within an EDZ and to compare them with existing fractures in the host rock.

(v) Modelling:  EDZs are frequently represented and modelled overconservatively as active hydraulic units,  if an EDZ is treated in such a manner in the performance assessment of a spent fuel/HLW repository, its role is significant as it maximises the rate of diffusion in the bentonite backfill (e.g. in SKB’s KBS-3 concept); it is important to represent the EDZ

84 in a more representative and, therefore, less conservative manner in order to avoid this artefact,  an EDZ should not be modelled in isolation, as its significance depends upon its relationship and interactions with the other components of the repository (e.g. the types of seals and backfill, the presence and form of any tunnel lining, etc.),  the presence of a lining or other forms of support, such as shotcreting, may be very interesting from an EDZ point of view (in, for example, effecting its rate of self- healing, etc.) but difficult to accommodate within a PA (as new potential interactions are introduced) and  the effects of time-dependency in the growth and possible recovery of an EDZ have not normally been considered.

(vi) Elimination and/or reduction of an EDZ:  methods exist to limit the extent of an EDZ (e.g. the use of a TBM in crystalline rocks and possibly in other rock types, the importance of lining design and seals with swelling capacities in soft clays, etc.),  lining backfill designs can be tailored to minimise the post-excavation growth of an EDZ (e.g. by maximising the support provided to the rock and by preventing or minimising time-dependent dilatancy),  minimising the development of an EDZ should also be viewed in relation to the operational programme for the repository (e.g. in a crystalline rock the ease of bentonite block emplacement in relation to the excavation method applied).

(vii) The ways forward and priorities (future research targets):  the extent of self-healing in indurated clays needs to be examined,  the relationship between the effective permeability (i.e. along a tunnel, taking into account the interconnectivity of EDZ fractures) vs. the local permeability (i.e. the permeability measured normal to the tunnel in radial boreholes),  radionuclide transport within the EDZ (e.g. sorption in EDZ fracture vs. fractures in the host rock, access to matrix diffusion, fracture connectivity within the EDZ, colloid filtration, etc.),  coupling of processes within the EDZ (e.g. effects of heat, clogging of fractures),  the extent of long-term evolution of the EDZ (e.g. self-healing in indurated clays, maintenance of the hydraulic cage effect, etc.),  at Yucca Mountain, fracturing of the host rock is so intense that the EDZ does not appear to represent an important issue,  planned experiments of relevance to understanding the EDZ are: tracer testing in the Canadian URL; testing of seals at Äspö and Mont Terri.

(viii) Overall conclusions:  a distinction is now made between disturbed and damaged zones within an EDZ,  large differences exist between rocks that self-heal (if only partially) and those that do not,  the conclusions of the 1988 workshop [1], in which the EDZ was considered as providing a highly permeable conduit parallel to galleries are no longer considered generally valid,  since the CNS workshop [2], considerable work has been carried out in rocks other than crystalline rocks.

85 A summary of the anticipated level of significance of the EDZ in different rock types and the difficulty in characterising and modelling them is provided in Table 4.1. The overall conclusion of the 1998 workshop was that the significance of the EDZ in terms of repository safety has declined over the last two decades, as our level of understanding of EDZs has increased. There is now sufficient confidence in our understanding of the EDZ in crystalline rocks, plastic clays and rock salt not to require additional large-scale R&D programmes to study its effects. It is still likely to be necessary to carry out site specific investigations of EDZs at potential repository sites, so as to provide site-specific data on their properties and to aid in the design of sealing systems. Residual problems and uncertainties associated with EDZs in indurated argillaceous rocks are likely to be resolved over the next few years, as more in situ work is carried out in these types of rocks.

Table 4.1 A summary of the general level of significance and understanding of different aspects of the EDZ in various rock types (based on the conclusions of the 1998 EDZ workshop, [3]).

Rock type Difficulty of Level of Difficulty Relative characterisation understanding of importance of modelling EDZ Crystalline high high high medium Rock salt high high high low Plastic clays high high high low Indurated medium medium medium high but should clays tend towards medium*

*Note: the actual impact of an EDZ in indurated clays may, in many cases, be rather limited due to the potential for self-sealing (due probably to the overconsolidated nature of the clay and its long-term creep properties). There is currently, however, insufficient understanding of the processes active in the EDZ in such rocks to be confident about the extent of such self-healing and the time over which it might be expected to occur (see Section 4.4).

4.4 Clay Club Topical Session on self-healing

The self-healing property of argillaceous rocks, which is often quoted as one of the advantages of such formations for the disposal of long-lived wastes, is an important subject of study as it could mitigate the effect of an EDZ around underground openings [4]. This workshop, which was multidisciplinary in nature, investigated a process which had been identified as of interest in controlling the flow of fluids through fractures in clays and provided an overview of the subject of self-healing. The aims of the workshop were to exchange information on:

 the general understanding of self-healing from both a geomechanical and a geochemical standpoint and  the approaches that needed to be followed in dealing with the subject of self-healing.

Several different argillaceous rocks were considered, varying from those that were highly indurated, such as the Palfris Formation at Wellenberg, to those that had retained their plasticity, such as the Boom Clay at Mol. The range of geomechanical properties that is demonstrated by these rocks is, therefore, considerably greater than that shown by crystalline rocks and by the majority of evaporitic formations. The overall conclusion of the workshop 86 was that the multidisciplinary overview produced as a result of the workshop was useful, as one had not been previously available, as it had demonstrated the necessity of including the potential for self-healing in all the types of argillaceous rocks considered as potential host formations and had illustrated the need to reconcile the geomechanical and the geochemical approaches vis-à-vis the process of self-healing.

There was agreement on the clarification of the terms of self-healing and self-sealing, and it was concluded that self-sealing should be considered to be a largely hydromechanical process, in which the transmissivity of a fracture is reduced, possibly to zero, whereas self- healing was a more general term that should be described as the capability of a host formation to heal newly-formed discontinuities, so as to restore the rock mass to its original state regarding its hydraulic and transport properties. Self-sealing should, therefore, be viewed as a specific contributory mechanism within the overall context of self-healing. It was also possible for the more indurated argillaceous rocks only to affect a partial self-healing.

Examples of self-healing or partial self-healing were presented for a range of clays from the Boom Clay at Mol where, although fractures formed due to construction activities, there was initial evidence that self-healing had taken place by measuring a return to the pre- construction conditions regarding the flow of water and gas through the clay. It was explained that further work in this area would be carried out as part of the EC-funded SELFRAC project, in which the Opalinus Clay at Mt Terri would be compared with the Boom Clay at Mol.

In the more indurated clays considered, e.g. the Opalinus Clay and Palfris Marl in Switzerland and the Boda Claystone Formation in Hungary, evidence for self-healing was adduced from a variety of sources, such as:

 the lack of hydrochemical anomalies and mineral veining and the absence of flow into underground openings, even in highly tectonised zones of the Opalinus Clay, indicating that there is current little or no flow and that there had been no palaeoflow;  the closed-system behaviour of the Palfris Marl derived from chemical and isotopic characteristics of the deep groundwater and the existence of hydraulic underpressures, which suggested a continuous very low permeability of the marl, even during extensive deformation and temperatures up to 200-250C;  the measurable reduction in transmissivity in in situ experiments in the Opalinus Clay to determine the extent of seal-healing in a fracture or fracture network within the EDZ and in self-healing following gas-fracing; and in laboratory experiments where swelling was initiated in induced fractures by changing the fluid chemistry;  the precipitation of non-argillaceous infilling minerals, the rehydration of clay minerals in situ and degradation of non-swelling clay minerals in the URL in the Boda Claystone Formation at 1000 m depth, and a considerable change in in situ stresses around underground openings with a resulting effect on the hydraulic connectivity within the EDZ.

5. Developments over the period of the four workshops

There are certain trends in the development of understanding of the EDZ that can be seen to have taken place over the period of these four workshops. These trends are, in part, due to a change in emphasis in R&D from that mainly on crystalline rocks to a greater interest in argillaceous rocks, in particular in indurated clays and marls. There has also been a greater

87 appreciation of the presence and potential significance of EDZs in more plastic rocks, such as less indurated clay and salt.

The belief that the EDZ necessarily provides a continuous, high permeability pathway parallel with all underground openings, especially in crystalline rock, has been replaced over the years by the realisation that such an assumption is not necessarily valid and that it may be possible to limit any effects of the EDZ by the appropriate use of seals or plugs. The potential significance of self-sealing processes in argillaceous rocks and salt is also now appreciated, although there are still questions as to the extent of such self-healing and the time over which it might take place.

6. Remaining questions to be studied

The questions regarding the EDZ that remain to be studied are listed here with reference to the different rock types of interest.

Crystalline rocks:  EDZs in crystalline rocks in relatively low stress environments, e.g. in Scandinavia at expected repository depths, should not be extensive if appropriate excavation techniques are used, and should not, therefore, have a significant effect on groundwater ingress or rock mass stability. It is unclear, however, to what extent the EDZ is significant in providing a more transmissive pathway through the rock and how easy it is to demonstrate convincingly that low permeability seals or plugs in the rock can prevent such an effect.  In high stress environments, e.g. at the Canadian URL, the EDZ is more extensive and more significant with respect to providing continuous extension fractures parallel to tunnels, which result in a more transmissive pathway. Again, it needs to be demonstrated convincingly that low permeability seals or plugs in the rock can intercept the EDZ and limit its significance.

Argillaceous rocks:  There is a need to reconcile the geomechanical and geochemical approaches vis-à-vis self-healing.  The potential significance of the EDZ which, even in plastic clays, is associated with extensive fracturing (such as at Mol), has still to be resolved. Any such resolution is dependent on further work on the extent of such fracturing, its effect on fluid and gas flow around tunnels and the extent to which self-sealing process will return the rock to its pre-construction state. Any advance in understanding the processes that are operative is likely to require the synthesis of information from diverse sources in order to provide a sound scientific basis for future safety assessments and for developing a better understanding of a disposal site.

Salt:  More permeability data and more reliable stress measurements are required around drifts to allow the use of a recently-developed permeability stress relationship in safety assessments.  Although it is known that the permeability within the EDZ is anisotropic, this fact is rarely considered in models and needs to be quantified by further experimental investigations.

88  The extent of reversibility or healing of an EDZ cannot be observed in situ, as the rate of the process involved is too slow (it takes possibly several hundred years), so that this issue can only be considered by developing numerical simulations, which in turn require further in situ measurements.

7. Acknowledgements

Thanks to Tilmann Rothfuchs, Frédéric Bernier, Wim Bastiaens, Steve Horseman, Peter Blümling and Derek Martin for supplying some of the information used in this paper.

8. References

[1] NEA 1989. Excavation response in geological repositories for radioactive waste, Proceedings of an NEA Workshop, Winnipeg, Canada, 26-28 April 1988, OECD/NEA, Paris.

[2] CNS 1996. Designing the Excavation Disturbed Zone for a nuclear repository in hard rock, Proceedings of the EDZ Workshop, International Conference on Deep Geological Disposal of Radioactive Waste, Winnipeg, Canada, 20 September 1996, Canadian Nuclear Society.

[3] NEA 2002. Characterisation and representation of the Excavation Disturbed or Damaged Zone (EDZ). Lessons learnt from a SEDE Topical Session, September 1998, Paris. NEA Report NEA/RWM(2002)1.

[4] NEA 2001. IGSC working group on measurement and physical understanding of groundwater flow through argillaceous media (The Clay Club). Proceedings of Topical Session on Self-healing, May 2002, Nancy, France. NEA Report, NEA/RWM/CLAYCLUB(2001)5.

[5] Bastiaens, W & Demarche, M 2003. The extension of the URF HADES: realisation and observations. Proceedings of WM’03 Conference, Tucson, February 2003.

[6] Berg, H-P, Lange, F, Müller, W, Thomauske, B, Wurtinger, W 1986. Safety analysis as a basis for the design and construction of the Konrad repository. In: Proceedings of Symposium on Siting, design and construction of underground repositories for radioactive waste, Hanover, 1986. IAEA, pp 653-668.

[7] Diekmann, N, Koniecczny, R, Meisyet, D and Schnier, H 1986. Geotechnical and rock mechanical investigations for the design of the Konrad repository. In: Proceedings of Symposium on Siting, Design and Construction of Underground Repositories for Radioactive Waste, Hanover, 1986. IAEA, pp 385-400.

89

Lessons Learned with Respect to EDZ in Crystalline Rock

Roland Pusch, SKB/Geodevelopment AB, Aespoe/Lund, Sweden

Introduction

The issue of excavation disturbance was raised decades ago when transport of hazardous ion species from waste repositories was identified as a possible mechanism in contamination of groundwater. More than twenty years ago calculation of elastic excavation-induced movement of rock in shafts and related increase in porosity for isotropic stress fields indicated that there is a practically important stress-induced EDZ. Assuming statistically uniform fracture aperture and spacing and applying the cube law, it was demonstrated that the longitudinal hydraulic conductivity of “onionskin” fractures, Kze, can be increased by 10 times at the rock wall and by about 10 % at a distance of 1.5 times the radius, while the radial conductivities will be slightly decreased (Figure 1). These calculations were highly hypothetical because of the assumption of infinitely long idealized fractures and poorly known relations between aperture and conductivity. Various field experiments have therefore been planned and performed to find out whether EDZ is really of importance to transport of water and contaminants in repositories. The lessons learned by conducting such tests are described and commented below.

Figure 1. Predicted change in hydraulic conductivity of the disturbed zone around a circular excavation at isotropic initial stress conditions. Kzr=Conductivity of radial fractures in axial direction, Kze=Conductivity of onion-skin fractures in axial direction, Kr=Conductivity in radial direction, Ko=Conductivity of undisturbed rock [Kelsall et al, 1982]. Blasted tunnels and drifts

General considerations

SKB’s studies of the hydraulical and mechanical behaviour of rock have consistently included the rock structure and this made detailed examination of the walls of blasted tunnels natural for getting a basis for developing conceptual models of the EDZ. An early finding was that the degree of mechanical degradation varies along the length of a blasted tunnel because of the distribution of charge, a typical schematic illustration being that in Figure 2. It means that spot-wise determination of the hydraulic conductivity in short boreholes drilled normal to the rock walls is expected to give strongly varying data and that it is impossible to get definite information on the water transport capacity of the most shallow, disturbed rock over longer axial distances by performing such tests.

Figure 2. Major types of damage by tunnel blasting. 1a-zones are characterized by regular sets of plane fractures extending radially from the central parts of the blast-holes. 1b-zones represent strongly fractures parts at the tips of the blast-holes [Pusch, 1994].

Stripa experiments

The first experimental evidence that the EDZ of blasted tunnels has a significant water- bearing capacity was offered by the Buffer Mass Test in the first phase of the international Stripa Project. It was conducted at 360 m depth in granite and represented a half-scale version of the Swedish KBS-3 concept, consisting of a blasted drift with 25 m2 cross section and a length of 35 m, the inner 12 m part being isolated from the outer part by a concrete bulwark (Figure 3). Two 760 mm diameter holes with 3 m depth and 6 m spacing, simulating canister deposition holes, were core-drilled in the inner part that was backfilled with clayey soil, and four ones in the outer part. The proof of the existence of a significantly water-bearing EDZ was the strong inflow into the temporarily empty hole (No 3) located just outside the bulkhead when the backfill behind it started to be saturated. This indicated that water flowing from the rock to the backfilled drift was redirected by the EDZ and discharged into this hole. Figure 3. Main features of the Stripa Buffer Mass Test [Pusch, 1989].

Experiments performed in the Canadian Shield at about the same time suggested that the EDZs of blasted tunnels are not very conductive. Thus, large-scale EDZ experiments made in AECL’s underground rock laboratory by constructing dams in blasted tunnels for measuring axial flow along them indicated that the hydraulic conductivity of the near-field rock below the floor was not significantly higher than that of the virgin rock. The contrast to the Stripa data were explained by the very high rock stresses in the AECL rock, yielding a tight skin zone, and by difficulties in defining the pressure and flow distributions. Still, uncertainty remained with respect to the actual hydraulic performance of the EDZ in blasted tunnels in crystalline rock.

The contradictory results from the field experiments called for large-scale field tests tailored for determination of the hydraulic performance of the EDZ of tunnels that had been excavated by use of common drill-and-blasting, and such an experiment was made in the second phase of the Stripa project (1986-1992). The blast-holes had 3 m length and 0.5 m spacing, the charge being 0.5 kg Gurite per meter of the contour holes and 0.5 kg Dynamex as bottom charge. The test arrangement in the so-called BMT drift is shown in Figure 4. It comprised drilling of 76 radially oriented 2” boreholes with 7 m length at the inner end of the drift and a corresponding set at the bulkhead. They overlapped to about 0.75 m depth thus forming a slot of this depth, which was separated from the holes and the drift by packers. At their outer ends the boreholes had a spacing of 0.9 m. The holes extending from the slot were equipped with packers at 3 m depth and mutually connected in 4 separated sets to represent the roof, the two walls, and the floor so that in- and outflow of the nearest 0.75 m rock annulus, the 0.75-3 m annulus and the outer 3-7 m annulus could be measured sector-wise. Each sector at the inner end of the drift was equally and simultaneously pressurized while measuring the inflow sector-wise into the outer slot and holes with respect to the amount and distribution. For eliminating inflow of water into and along the open part of the drift it was coated with epoxy and rubber liners and filled with a 100 m3 rubber bladder embedded by Na bentonite slurry. The bladder was pressurized by water to a level corresponding to the highest rock water pressure along the tested part. The rock in the tested drift was equipped with a large number of piezometers extending to different depths and recording of the pressures in the course of the test gave the hydraulic gradient along it.

A finite element flow model was worked out and applied for evaluating the hydraulic conductivity using the continuously recorded pressure and flow data. The prerequisite for this evaluation was a linear drop in pressure from the inner to the outer galleries, which was verified by the piezometer readings at steady state.

Assuming straight horizontal flow paths from the inner to the outer borehole galleries, and applying Darcy flow, it was concluded that the shallow 0.75 m zone, where the number of fractures with water-bearing capacity in drillcores was 3 to 7, had an average hydraulic conductivity of 1.2x10-8 m/s, i.e. 2-3 orders of magnitude higher than that of the virgin rock. The average conductivity of the virgin rock was evaluated as 3x10-11 to 10-10 m/s. The evaluation showed that the rock from 0.75 to 3 m depth, which was taken to represent the stress-induced EDZ, had an axial average conductivity that was 10 times than that of the virgin rock, and a radial average conductivity of about one fifth of that of the virgin rock, hence indicating a “skin” zone. Today, the Stripa BMT flow test is still the only experiment that has been performed on a sufficiently large scale to verify that tunnel excavation by blasting has a very significant effect on the conductivity of the near-field rock, and the fact that it included a length corresponding to four consecutive blast series (3 m each) clearly demonstrates that the EDZ-effect is not local.

The hydraulic conductivity of the rock from 0.15 to 0.9 meters depth was measured by packer tests in 80 boreholes and this study gave largely varying data with an average of 10 -9 m/s, i.e. somewhat lower than the values derived from the full-scale test. The outer packers needed to be located at 15 cm depth thereby excluding the most shallow, fractured EDZ. Figure 4. Test set-up for determination of the arrangement for evaluating the hydraulic conductivity of the EDZ of the BMT test drift [Börgesson et al, 1992]. A is the water-filled bladder and B the bentonite slurry that prevented water in the rock to flow into the drift. Aespoe experiments - ZEDEX

Further information of the hydraulic properties of the EDZ over a longer distance of blasted drifts at 300-400 m depth can be drawn from ongoing tests in the so-called Zedex drift in the Aespoe underground laboratory. Figure 5 is a schematic illustration of the field test with the prime purpose to investigate the conductivity and rate of saturation of tunnel backfills. The innermost part of the Zedex drift is filled with very permeable crushed rock that can be pressurized to several hundred kPa, while the rest of the drift contains inclined compacted layers of mixed bentonite/crushed rock (30/70). Both parts are about 14 m long. The outermost part consists of crushed rock over which blocks of highly compacted bentonite and pellets are placed for compensating the expected settlement of the underlying crushed rock fill that would otherwise lead to an open gap at the roof.

The Zedex drift was excavated even more carefully than the Stripa test drift, using smooth blasting technique designed by ANDRA [Bauer et al, 1996a] and the EDZ would hence be less conductive but measurements indicate a hydraulic behaviour of the EDZ that is similar to that at Stripa. Both rock masses behave similarly as indicated by the figure 6x10-11 m/s for the bulk conductivity of undisturbed Aespoe rock used in ongoing flow modelling. The Zedex experiment hence supports the conclusion from the Stripa test that the EDZ in blasted tunnels in crystalline rock is a very important water conductor over longer distances.

Spot-wise determination of the conductivity of shallow rock in the Zedex drift has been made in a comprehensive and careful study performed by ANDRA through Ecole de Géologie de Nancy, using the Seppi Tool [Bauer et al, 1996b, Bauer et al 1996c]. The evaluated conductivity has a maximum close to the rock – around E-10 to E-9 m/s – but the most shallow 10 cm part, which has the highest conductivity, could not be investigated as in the Stripa experiments. Since the rock around the Zedex drift is estimated to have an average hydraulic conductivity of at least E-8 m/s to a depth of a few decimetres, ANDRA’s study verifies that spot-wise determination cannot give a reliable value of the average conductivity over a long distance.

Old part excavated Bentonite O-ring by normal blasting Blocks 20/80 New part excavated 2.2 m Bentonite blocks bentonite/crushed rock by careful blasting and bentonite pellets

D9 D10 D1 D2 D3 D4 D5 D6 D7 D8 D11 Drainage material A1 A2 A3 A4 A5 A6 B2 B3 B4 B5

28 m 30/70 bent./crushed rock Crushed rock Concrete wall Concrete Plug Drainage and deairing layers, permeable mats

Figure 5. Longitudinal section of the blasted Zedex drift [Gunnarsson et al, 2001]. Experience from German projects

BGR’s work for determining EDZ properties in blasted tunnels and drifts in crystalline rock using spot-wise testing has given very similar results as the Stripa and Aespoe experiments [Liedtke, 2003]. Thus, the EDZ of conventionally blasted tunnels in the central Aare granite area, which is characterized by a very tight somewhat gneissic structure and fractures healed by biotite, chlorite or epidote and quartz, was formed by induced macro- and micro-fractures to about 0.3 m depth, within which depth associated stress relaxation of cores was also found. Where the original discontinuities were more or less parallel to the tunnel the EDZ had a depth of more than one meter. The hydraulic characteristics of the EDZ rock was measured by use of a surface packer system and a short interval packer system developed and used for in situ tests. The conductivity within 0.3 m depth was found to vary between 10-10 and 10-3 m/s.

TBM-excavated drifts and tunnels

While full-scale testing of the entire near-field is required for adequate interpretation of the extension and nature of the EDZ in blasted crystalline rock, spot-wise determination of the hydraulic conductivity can give good information on the conductivity of the TBM-induced EDZ because of its relative homogeneity. Techniques for determining the hydraulic conductivity by spot measurements have been developed by BGR in Germany, and SKB, Sweden. BGR’s field method will be described in papers presented at this conference and compared with the outcome of SKB’s study, which comprised hydraulic testing of drill cores in triaxial cells in the laboratory.

The field experiments were made at about 450 m depth in crystalline rock in SKB’s underground research laboratory at Äspö, Sweden. In situ Surface Packer Tests were carried out with an equipment consisting of a hollow metal cylinder fixed by a metal ring to the tunnel wall. The horizontal holes were oriented perpendicularly to the tunnel wall. At each test spot one experiment with pressurizing the packer with water and one with gas were made. The recorded pressure drop over time in the control space was interpreted in terms of conductivity by applying “Two Phase Flow Theory” using a finite element method. This gave values in the interval 10-10 to 2.5x10-10 m/s for the most shallow 10 mm part of the EDZ. No significant differences were found between water and gas. For rock extending from the rock wall to 100 mm depth the average hydraulic conductivity is estimated at 10-11 to 10-10 m/s.

The rock samples used for determining the hydraulic conductivity of the shallow rock in the same tunnel where the field tests took place were prepared from 100 mm cores with a length of 250 to 500 mm taken perpendicularly to the rock wall. Several series of 10 mm diameter cores were extracted by diamond drilling perpendicular to the large cores, i.e. parallel to the tunnel at different distances from the tunnel wall, and several series of 3 mm discs were sawed from the large cores to allow determination of hydraulic conductivity perpendicular to the tunnel wall. Hydraulic testing of the samples used a triaxial apparatus and a hydraulic gradient of about 100. The measurements showed that the isotropic hydraulic conductivity was in the range of 2x10-9 to 5x10-12 m/s from the tunnel wall to 4.5 mm depth, and 5x10-12 to 5x10-13 m/s from 4.5 to 10 mm depth. The undisturbed crystal matrix has a hydraulic conductivity of about 5x10-13 m/s. Fluorescence microscopy of epoxy-impregnated samples gave the porosity and showed the pattern of fissuring caused by the bits. The majority of the fissures formed an angle of +/- 25 - 45o to the rock wall and were responsible for the increase in porosity from less than 0.5 % of the undisturbed crystal matrix to 2 - 5.6 % within 10 mm distance from the wall. A number of macroscopic fractures were also identified, their depth and spacing being 10 - 50 mm, and they are assumed to cause an additional increase in hydraulic conductivity of larger volumes of the matrix than represented by the small samples and hence yield values on the same order of magnitude as the ones obtained from the small- scale packer tests.

Comparison of the field measurements and the lab testing of small samples shows that the lab tests at depths of less than 4.5 mm from the tunnel wall have a higher conductivity than in the field tests but from a depth of 4.5 mm onwards the relationship is inversed. This verifies the belief that stress-generated fissures in the larger rock volume played a major role in the in situ tests. The high uniform hydraulic conductivity of the shallowest part of the EDZ serves to distribute water flowing in from discrete water-bearing fractures over the entire periphery of deposition holes and tunnels in the early period of water saturation of buffer clay embedding canisters with HLW.

Experience from attempts to seal the EDZ of blasted tunnels

The high axial hydraulic conductivity of the blast-induced EDZ means that rather much water may flow into the tunnels before, during and after backfilling despite the “skin” effect in the stress-induced EDZ because inflow to the EDZ can take place through discrete water-bearing fractures and fracture zones that intersect the tunnels more or less perpendicularly. Where this takes place the backfilling operation may be difficult or even impossible and attempts have therefore been made to investigate if the blast-induced zone can be sealed by grouting. The work, which was conducted in the Stripa BMT drift after completion of the large-scale conductivity test, comprised drilling of 345 radially oriented holes with about 1 m length and 0.7-0.9 m spacing and grouting using an earlier developed “dynamic injection” technique [Börgesson & Pusch, 1989]. Very fine-grained cement slurry with plasticizer was used for the purpose. The sealing effect was poor, which was primarily explained by the clogging effect of debris that prevented cement penetration to more than a few centimetres depth and by too large spacing of the holes as well as by the fineness of the fractures. A possible additional reason is that the lack of counter-forces at the injection resulted in loss in injection energy, which is the reason for avoiding after-injection in practical rock construction and instead injecting grout prior to excavation.

Conclusive remarks – major lessons learned

The typical property of undisturbed crystalline rock to transport water through relatively few discrete fractures with a considerable spacing, often several meters, is changed in the vicinity of drifts and tunnels. Here, the dynamic impact and very high gas pressure caused by blasting produces rich fracturing around the blast-holes and can activate discrete fractures and make them propagate and become effective flow paths at several meters distance from the excavated room. In the near-field the frequency of fractures with a potential to transport water hence increases and the blast-induced change in hydraulic performance can be very obvious, especially where one fracture set is parallel or close to parallel to the axis of the tunnel. Within a distance of 1-1.5 m from a tunnel wall blasting even by using careful technique can increase the net bulk hydraulic conductivity by orders of magnitude.

Stress-induced excavation disturbance has much less influence on the hydraulic conductivity but it affects the rock to a larger distance. It is estimated that the axial conductivity of this zone can increase by 10 times while the radial conductivity may drop to one fifth of the conductivity of the virgin rock. The effect most probably depends on the rock structure. The overall hydraulic effect of the EDZ is illustrated by the table below showing the estimated change in axial flux across the entire nearfield, 80 m2, around a blasted tunnel with 25 m2 cross section area. It is composed of the blast-affected EDZ equalling 20 m2 and of the surrounding stress-induced EDZ of 60 m2. For TBMs with the same tunnel size, the EDZ extending to 0.1 m depth represents 18 m2. For comparison of the two cases the net flux across the same section area, 80 m2, is given in the table, implying that the larger part of the section is made up of virgin rock for the TBM case.

Assuming the conductivity of the virgin rock to be 10-11 m/s and that of the blasted zone to be 10-8 m/s and taking the axial conductivity of the stress-affected EDZ around the blasted tunnel to be 10-9 m/s, one finds the total flux across the 80 m2 near-field section area to be about 100 times higher than for a TBM tunnel, for which the conductivity of the EDZ is taken as10-10 m/s.

Table showing the approximate water flux across the assumed 80 m2 near-field for the hydraulic gradient i=unity distributed over the various EDZ components. The backfilled tunnel is assumed to be impermeable.

Case Permeated Hydraulic Water flux, cross section, conductivity, m3/s m2 m/s Virgin rock, 80 10-11 8x10-10 no tunnel Blasted tunnel 20 10-8 2.6x10-7 * Blast-EDZ 20 10-9 2x10-7 * Stress-EDZ 60 6x10-8 TB tunnel 80 2.6x10-9 * Stress-EDZ 18 10-10 2x10-9 * Virgin rock 62 10-11 6.2x10-10

Comparing these data one finds the expected water transport capacity of a cross section corresponding to the total near-field of TBM tunnels to be no more than about 1 % of that of blasted tunnels. The importance from the point of safety analysis of this fact is obvious.

The following important additional conclusions can be drawn from the various attempts to characterize the EDZ from a hydraulic point of view:

* Careful blasting of the type applied to the Stripa BMT experiment causes a blast-induced EDZ that extends to about 1 m from the periphery and is at least 100 times more conductive than the virgin rock. The surrounding stress-induced EDZ extends to about 3 m from the periphery and has an axial conductivity that is about 10 times higher than that of the virgin rock, while the radial conductivity is about 5 times lower than this conductivity. The consequence of this is that a plug intended to totally cut off the EDZ of a blasted tunnel must extend to about 3 m from the periphery. For cutting off the blast-induced EDZ it must extend to 1.5 m depth in the floor and 1 m in the walls while 0.5 m may be sufficient in the roof. * The blasting techniques employed in the discussed cases were careful but improved methods and even greater care may reduce the disturbance caused by this excavation method.

* Unlike the EDZ in salt and certain clay layers no self-sealing is expected in the EDZ of crystalline rock. However, the hydrothermal conditions that will prevail for a couple of thousand years in certain HLW repository concepts may lead to formation of clay minerals in fractures by converting feldspars and certain heavy minerals. This matter is worth studying.

* Sealing of the EDZ in blasted tunnels by grouting of short holes has a poor effect. For minimizing water inflow into tunnels during backfilling grouting must be made prior to the excavation focusing on significantly water-bearing fracture zones that can be identified before the excavation starts.

References

Bauer C, Homand F, Ben Slimane K, 1996b: Disturbed zone assessment with permeability measurements in the ZEDEX tunnel.

Bauer C, Homand F, Ben Slimane K, 1996a. Proceeding of the EDZ Workshop “ Designing the Excavation Disturbed Zone for Nuclear Repository in Hard Rock ". Winnipeg September 1996. Canada. CNS pp 87-96.

Bauer C, Homand F, Ben Slimane K, Hinzen K.G, Reamer S.K , 1996c. Damage zone in the near field in the Swedish ZEDEX tunnel using in situ and laboratory measurements. EUROCK Congress in Turin, September 1996.

Börgesson L, Pusch R, 1989. Rock sealing by dynamic injection; Stripa Project Phase III. Proc. NEA/CEC Workshop on Sealing of Radioactive Waste Repositories at Braunschweig, OCDE/OEDC, Paris.

Börgesson L, Pusch R, Fredriksson A, Hökmark H, Karnland O, Sandén T, 1992. Final Report of the Rock Sealing Project – sealing of Zones Disturbed by Blasting and Stress Release. Stripa Project Technical Report 92-21. SKB, Stockholm.

Gunnarsson D, Börgesson L, Hökmark H, Johannesson L-E, Sandén T, 2001. Äspö Hard Rock Laboratory, Report on the installation of the Backfill and Plug Test. SKB IPR-01-17, SKB Stockholm.

Kelsall C, Case B, Chabannes R. Preliminary evaluation of the rock mass disturbance resulting from shaft, tunnel and borehole excavation, D. Appolonia, Project No NM79-137, Battelle Memorial Institute, Office of Nuclear Waste Isolation, Columbus, Ohio.

Liedtke L, 2003. Personal communication.

Pusch R, 1989. Alteration of the hydraulic conductivity of rock by tunnel excavation. Rock Mech. & Mining Sciences, Vol.26, No.1 (pp.71-83). Pusch R, 1994. Waste Disposal in Rock, Developments in Geotechnical Engineering,76. Elsevier Publ. Co. ISBN:0-444-89449-7.