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 postclosure 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.

2.
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.
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, insitu 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 reactivated 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 systemspecific 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.
2.
3.
4.
NEA workshop on Excavation responses in geological repositories for radioactive
waste, Winnipeg, April 1988 [1]).
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].
Lessons learned from a NEA SEDE Topical Session which took place in 1998 on
Characterisation and Representation of the Excavation Disturbed Zone [3].
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 elasticviscoplastic 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
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
rock or averagely fractured rock lying between the repository and the nearest fracture zone.
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
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
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
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
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
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
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 designrelated 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.




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.
(ii)
(iii)




(iv)


(v)


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.
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.
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



(vi)



(vii)







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 selfhealing, 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.
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).
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
characterisation
Level of
understanding
Crystalline
Rock salt
Plastic clays
Indurated
clays
high
high
high
medium
high
high
high
medium
Difficulty
of
modelling
high
high
high
medium
Relative
importance of
EDZ
medium
low
low
high but should
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 selfhealing 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 preconstruction 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 waterbearing 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
by normal blasting
New part excavated
by careful blasting
D1
Drainage
material
D2
Bentonite blocks
and bentonite pellets
2.2 m
D3
D5
D4
A1
A2
D7
D6
A3
A4
A5
D10
D9
D8
A6
B2
B3
Bentonite
O-ring
Blocks 20/80
bentonite/crushed rock
D11
B4
B5
28 m
Concrete wall
30/70
bent./crushed rock
Crushed rock
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 smallscale 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
cross section,
m2
rock, 80
Virgin
no tunnel
Blasted tunnel
* Blast-EDZ
* Stress-EDZ
TB tunnel
* Stress-EDZ
* Virgin rock
20
20
60
80
18
62
Hydraulic
conductivity,
m/s
10-11
-8
10
10-9
10-10
10-11
Water
m3/s
flux,
8x10-10
2.6x10-7
2x10-7
6x10-8
2.6x10-9
2x10-9
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
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