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