Introduction
In Switzerland, Opalinus Clay, a Mesozoic shale formation of about 180 My in age, has been selected as the host rock for the disposal of high-level nuclear waste (BFE, 2011). Opalinus Clay features several
beneficial properties such as its low permeability, the high radionuclide retention,
and the potential for self-sealing. However, the favourable characteristics
of the rock mass may change during tunnel excavation. Excavation is
accompanied by stress redistribution and the development of an excavation
damage zone (EDZ). The evolution of the EDZ is an important factor for the
long-term safety of a nuclear repository as it may significantly influence
the permeability of the confining host rock and offer pathways for
radionuclide transport. Unloading and/or exposure to atmospheric conditions
with a low relative humidity (RH) may lead to suction and, if the
air-entry value is exceeded, to desaturation of the rock mass close to the
tunnel. These processes can lead to shrinkage and the formation of
desiccation cracks (Tsang et al., 2012). During the open-drift stage of a
nuclear repository, seasonal atmospheric changes, especially RH variations,
may alter the rock mass and influence the long-term crack evolution.
Möri et al. (2010) measured crack apertures of Opalinus Clay in the
framework of the cyclic deformation (CD) experiment at Mont Terri
Underground Rock Laboratory (URL) located in the Jura Mountains,
Switzerland. They found that the cracks close during summer (i.e. when the
RH is high) and open during winter (i.e. when the RH is low). Crack closure
and opening are associated with swelling and shrinkage of Opalinus Clay.
Möri et al. (2010) also observed a net closure of the cracks over
several seasonal cycles, which indicates an irreversible deformation
component that is likely associated with time-dependent processes such as
consolidation, creep, or slaking. These irreversible deformation components
can contribute to both tunnel convergence and self-sealing of the EDZ. The
self-sealing effect is the ability of clay shales to close previously
developed cracks and therefore reduce their permeability through hydro-mechanical, hydro-chemical, and/or hydro-biochemical processes
(Bernier et al., 2007). Among others, the adsorption of water on clay
minerals and related volumetric expansion can be associated with this
effect.
Numerous studies have been conducted to show the influence of drying–wetting
cycles on clay or clay shale specimens (e.g. Chu and Mou, 1973; Popescu,
1980; Chen and Ma, 1987; Osipov et al., 1987; Dif and Bluemel, 1991; Day,
1994; Al-Homoud et al., 1995; Basma et al., 1996; Pejon and Zuquette, 2002).
In those studies, however, swelling was performed by allowing the
specimens to fully soak in water. Few studies exist in which the influence of
cycles in RH on the drying–swelling characteristics of clay shales has been
investigated (e.g. Grice, 1968; Van Eeckhout, 1976; Olivier, 1979; Pham et
al., 2007; Farulla et al., 2010; Cardoso et al., 2011; Yang et al., 2012;
Pineda et al., 2014). Grice (1968) noted that specimens of Utica shale that
were immersed in water disintegrated completely after oven drying. Specimens
that were exposed to RH fluctuations between 60 and 90 % for a period of
9 months, however, showed only minor cracking. Van Eeckhout (1976)
equilibrated specimens of Beatrice coalmine shale to various levels of RH to
study the mechanisms of reduction in rock strength resulting from variations
in RH. During moisture absorption he measured a volumetric expansion in the
order of 0.2–1 %. The strains were larger in direction normal to bedding
and occurred mostly between 48 and 100 % RH. Subsequent drying of the
specimens to the initial level of RH showed that about 0.25 % of the
strains were not recoverable. Van Eeckhout (1976) identified these
expansion–contraction characteristics and the associated lengthening in
internal cracking as a possible cause for the lowering in strength he observed
due to humidity fluctuations. Similar observations have been made by
Olivier (1979) for specimens of a Lower Triassic mudrock. With the help of
water retention curves for several wetting–drying cycles between 10 and
99 % RH, Cardoso et al. (2011) showed that the air-entry value of an
Upper Jurassic marl decreases with an increasing number of cycles. The decrease
is accompanied by an increase in void ratio indicating a degradation of the
material. Pham et al. (2007) subjected specimens of mudstones from Bure to
one cycle of RH from 98 to 32 % and back to 98 %. The measurement of
strains as well as the ultrasonic velocity showed a hysteresis between drying
and wetting curves. Additionally, a non-linearity has been observed as more
strain was induced by a change in RH at high levels of RH (i.e. between 76
and 98 %) than for the same change at lower RH. Yang et al. (2012) used
digital image correlation techniques to study the deformation behaviour of
Callovo–Oxfordian argillaceous rock specimens subjected to axial load
(between 0.3 and 8.5 MPa) and RH cycles (between 39 and 85 %). A linear
relationship between RH and strain has been observed for RH smaller than
75 %. Furthermore, the strains were reversible during cycles of hydration
and dehydration at low axial stress (0.3 and 2 MPa), whereas irreversible
strains (i.e. a net shrinkage) have been measured for RH cycles at 8.5 MPa
axial load. Pineda et al. (2014) experimentally investigated the influence of
RH cycles on the degradation of Lilla claystone in a long-term RH cycling
experiment using ultrasonic wave velocity measurements and Brazilian tensile
strength tests. The applied RH cycles caused an irreversible increase in the
specimens' volumes as swelling always exceeded the amount of shrinkage.
Pineda et al. (2014) found that higher peak-to-peak amplitudes in RH cycles
(cycles between 20 and 99 % were compared to cycles between 50 and
99 %) led to larger volumetric swelling. This effect was less pronounced
for specimens that were tested under higher confinements. With the help of
microstructural analyses, cracking has been identified as the main cause for
irreversible swelling for Lilla claystone. Furthermore, the degradation of
the material was manifested in a decrease in tensile strength from 2.9 to
0.2 MPa after four cycles and a decrease in dynamic Young's modulus by more
than 50 %. For both quantities, the reduction was largest for the first
cycle; afterwards a decreasing degradation rate was observed.
All studies mentioned above showed that cyclic variations in RH can have a
significant influence on rock mechanical parameters such as tensile strength
and can lead to irreversible volume changes which might contribute to the
destabilization of underground excavations but also favour processes that
are considered to control self-sealing.
This study aims at contributing to the understanding of the influence of RH
variations on the mechanical and hydro-mechanical behaviour of Opalinus
Clay. A series of specimens were exposed to RH variations under unstressed
conditions and tested for their Brazilian tensile strength. The study
focuses on answering the question of whether Opalinus Clay shows a damage evolution
when exposed to RH cycles that affects the tensile strength and causes
irreversible volumetric expansion that might be relevant for long-term
deformations and/or self-sealing.
Geological map of the Mont Terri URL (modified from Nussbaum et al.,
2011). The specimens used in this study were obtained from the borehole BHM-1
that was drilled parallel to bedding in Gallery 08. The location of the
borehole is indicated approximately.
![]()
Tested material and experimental procedure
Material description
For this study, samples from the shaly facies of Opalinus Clay from the Mont
Terri URL, Switzerland, were used. The formation is part of the Mont Terri
anticline that formed during the folding of the Jura Mountains. The present
overburden at the URL lies between 230 and 330 m but is estimated to have
reached up to 1350 m in the late Tertiary (Thury and Bossart, 1999; Mazurek
et al., 2006). The shaly facies of Opalinus Clay contains a clay content of
50–80 % (Mazurek, 1998; Klinkenberg et al., 2009; Nagra, 2002; Bossart,
2005). The clay minerals can be subdivided into illite (15–25 %),
illite–smectite mixed-layer phases (10–15 %), kaolinite (20–30 %), and
chlorite (5–15 %). Beside the clay minerals, Opalinus Clay consists of
quartz (10–20 %), feldspar (0–5 %), carbonates (5–25 %), pyrite
(0–3 %), and organic material (0–1 %). The clay particles are aligned
sub-parallel to each other leading to a distinct macroscopic bedding. As a
result, the physical properties of Opalinus Clay show a strong transversely
isotropic behaviour. The hydraulic conductivity, for example, varies between
10-12 m s-1 parallel to bedding and 10-14 m s-1 normal to bedding
(Marschall et al., 2004). The water loss porosity of the shaly facies is in
the order of 15–19 % (Bossart, 2005; Amann et al., 2011,
2012; Wild et al., 2015; Wild, 2016). The pore water of the Opalinus Clay at
the Mont Terri URL can be classified as Na-Cl water and, with regard
to its composition, is still close to that of the seawater where the material
was deposited (Thury and Bossart, 1999; Pearson et al., 2003).
Sampling and specimen preparation
In total, 31 specimens were taken from two 67.5 mm diameter bore cores obtained from a
25 m long borehole (BHM-1) that was drilled in Gallery 08 in the shaly
facies of Opalinus Clay at the Mont Terri URL (Fig. 1). A triple-tube core
barrel with compressed air cooling was used. The bore axis of BHM-1 was
oriented parallel to the bedding. The samples were immediately sealed in
vacuum-evacuated aluminium foil after core extraction. Core samples were cut
under dry conditions to a diameter-to-length ratio of approximately 2 : 1.
Sample sections from 2.5–3.3 m depth (N specimens) and 8.5–9.4 m depth
(P specimens) were selected. Both sections were located outside of the EDZ
as no EDZ-related fractures were detected at these depths. Additionally, a
cubic specimen (S4) was cut in such a way as to allow for the measurement of
the principal strains (i.e. the strains perpendicular, ε1,
and parallel, ε2, to the bedding plane orientation and
the strain in the plane of isotropy, ε3). The layout of
the electric resistive strain gauges (HBM type: K-LY46) that were directly
glued onto the specimen's faces is shown in Fig. 2a. Furthermore, one
cylindrical specimen (E19) was used to measure the strain perpendicular and
parallel to bedding (Fig. 2b). The environmental exposure time of the
specimens during the installation of the strain gauges was minimized to
about 30 min.
Illustration of strain gauge arrangement for measurements of
principal strains: (a) on a cubic Opalinus Clay specimen (S4)
measuring all principal strains individually; (b) on a cylindrical
Brazilian test specimen (E19) measuring only two principal strains.
![]()
Experimental layout
The specimens were exposed in desiccators to an alternating sequence of low
and high RH levels under unstressed conditions. The RH was controlled by
using supersaturated salt solutions. Sodium nitrite (NaNO2, RH = 66 % at 20 ∘C) and ammonium di-hydrogen phosphate
(NH4H2PO4, RH = 93 % at 20 ∘C) were used based
on the seasonal variations at the Mont Terri URL between 1997 and 2011
reported by Swisstopo (2014). It was found that the RH follows a cyclic
annual variation following a sine curve with maximum humidity values of
93.8 % during summer and minimum humidity values of 62.6 % during
winter. At each level of RH, the specimens were allowed to equilibrate with
the environment. Equilibration was achieved when the weight of the specimens
(periodically measured every 1–7 days, accuracy 0.01 g) remained constant.
Homogeneity of the ambient conditions within the desiccator boxes was ensured by installing computer fans at the back of the boxes. Brazilian
tensile strength tests were conducted after each cycle when the specimens
were equilibrated at 66 % RH. In total 4.5 cycles were applied, starting
with an equilibration phase at 93 % RH to establish the same initial
conditions for all specimens. The experimental setup is schematically shown
in Fig. 3. For the monitoring of the RH, a Honeywell HIH-4000-001 sensor
(sampling rate of about 12 h, accuracy ±3.5 %) was used.
Temperature was monitored by a resistance thermometer (Pt100) (sampling rate
of about 12 h, accuracy 0.015 ∘C). The temperature in the
laboratory was kept between 19 and 23 ∘C throughout the
experiment.
Suction and strain calculations
The water content was determined according to the International Society for Rock Mechanics (ISRM)-suggested methods
(ISRM, 1979). From the RH and the temperature that were monitored during the
experiment, the total suction can be calculated according to Kelvin's
relationship:
Ψ=-RTVw0ωwlnpp0,
where Ψ is the suction in pascal, R is the ideal gas constant (i.e. 8.314 J mol-1 K-1), T the absolute temperature in kelvin, Vw0 the specific volume
of water (i.e. about 0.001 m3 kg-1), ωw the molecular mass
of water vapour (i.e. 0.018 kg mol-1), p the vapour pressure of water in the
system in megapascal, and p0 the vapour pressure of pure water in megapascal. The
ratio p/p0 equals the RH.
Schematic illustration of the experimental setup of the stepwise
cyclic RH laboratory experiment.
![]()
On the cubic specimen (S4), strains were measured in all three principal
directions. Thus, the volumetric strain (εv) can be
calculated by adding all three principal strains (i.e. εv=ε1+ε2+ε3).
On the cylindrical specimen (E19), the strain parallel (ε2) and perpendicular (ε1) to the bedding was
recorded. Assuming a transversely isotropic material, the strain parallel to
bedding (ε2) equals the strain in the plane of isotropy
(ε3). Hence, the volumetric strain can be calculated from
the sum of the strain perpendicular to bedding and twice the strain parallel
to bedding (i.e. εv=ε1+2ε2). For all strains, expansion is taken as positive.
Properties and test configurations of specimens. Water content, dry
density, porosity, and initial saturation were determined according to
ISRM (1979). Furthermore, the direction of the applied load with respect to
bedding during the Brazilian tensile strength tests is indicated.
Specimen
Diameter
Thickness
Initial water
Bulk dry
Porosity
Initial
Direction of
no.
content
density
saturation
applied stress
(–)
(mm)
(mm)
(%)
(g cm-3)
(%)
(%)
(–)
A1
67.32
35.35
6.95
2.30
15.85
100.8
parallel
A2
67.20
34.78
7.13
2.28
16.47
98.7
parallel
A3
67.66
34.70
7.33
2.26
17.10
97.0
parallel
A4
67.61
32.33
7.23
2.24
17.96
90.1
parallel
A5
67.64
35.76
7.02
2.27
16.69
95.7
parallel
A6
67.37
35.94
6.97
2.28
16.47
96.5
parallel
A7
67.59
34.49
7.04
2.28
16.49
97.3
parallel
A8
67.44
35.01
7.01
2.30
15.74
102.4
parallel
A9
67.39
35.27
7.34
2.26
17.09
97.2
parallel
A10
67.58
35.50
7.21
2.27
16.80
97.4
parallel
A11
67.52
34.77
7.20
2.28
16.33
100.7
parallel
A12
67.63
35.06
7.16
2.27
16.96
95.7
parallel
A14
67.54
35.16
7.26
2.26
17.29
94.8
parallel
A15
67.51
34.89
7.27
2.27
17.02
96.8
parallel
E1
67.06
36.47
7.23
2.30
15.59
106.9
normal
E2
67.12
34.51
7.24
2.28
16.35
101.1
normal
E3
67.08
35.17
7.25
2.30
15.74
105.9
normal
E4
67.1
35.18
7.14
2.30
15.82
103.7
normal
E6
67.14
34.79
7.21
2.30
15.85
104.5
normal
E7
67.02
35.74
7.13
2.29
15.97
102.4
normal
E8
67.15
35.06
7.07
2.29
16.15
100.2
normal
E9
67.21
35.64
7.12
2.28
16.42
98.9
normal
E10
67.24
35.17
7.10
2.28
16.62
97.3
normal
E11
67.47
35.07
7.23
2.26
17.29
94.4
normal
E12
67.43
34.98
7.15
2.27
16.79
96.8
normal
E13
67.56
36.08
7.19
2.24
18.08
88.9
normal
E14
67.50
36.06
7.16
2.27
16.73
97.2
normal
E15
67.20
35.78
7.15
2.28
16.65
97.7
normal
E16
67.08
34.23
7.06
2.28
16.41
98.1
normal
E17
67.28
35.40
7.04
2.28
16.57
96.7
normal
E18
67.25
35.26
7.05
2.27
16.73
95.8
normal
Mechanical testing procedure
Brazilian tensile strength tests were conducted at ETH Zurich utilizing a
modified 2000 kN servo-hydraulic rock-testing machine (Walter and Bai,
Switzerland). The tests were conducted according to the ISRM-suggested
methods (ISRM, 1978) immediately after removal from the desiccator. Load was
applied parallel (P specimens) or normal to bedding (N specimens) (Fig. 4)
using a constant loading rate of 0.08 kN s-1 (except for specimens N1–N6, for
which a loading rate of 0.05 kN s-1 was used). The accuracy is < 1 %
of the actual reading.
Loading configuration for the Brazilian tests with respect to
bedding (bedding orientation is indicated by the light grey pattern). In
panel (a) the load is applied normal to bedding, allowing the
measurement of the Brazilian tensile strength parallel to bedding
(σt,p). In panel (b) the load is applied parallel
to bedding, allowing the measurement of the Brazilian tensile strength normal
to bedding (σt,n).
![]()
Results
The specimens' dimensions and initial properties are given in Table 1. The
initial water contents of the specimens range between 6.95 and 7.34 %,
which is comparable with the water content measured on cores right after core
extraction (Pearson et al., 2003; Amann et al., 2011; Wild, 2016). The
initial saturation was estimated from the initial water content, the bulk dry
density, and the porosity of the specimens according to the ISRM-suggested
methods (ISRM, 1979). A grain density of 2.73 g cm-3 was used to
calculate the porosity (Pearson et al., 2003; Bossart, 2005; own data).
Values of saturation that exceed 100 % can be related to the uncertainty
in the grain density (±0.03 g cm-3) and the specimen's volume.
Figure 5 shows the results of the RH, temperature, and strain measurements for
the N specimens. The specimens were first equilibrated to a RH of 93 % and
then subjected to 4.5 cycles with peak–peak amplitudes of between 30 and 36 %
(i.e. RH variation between 63 and 94 %; Fig. 5a). The resulting
suction applied to the specimens was calculated according to Eq. (1) and is
plotted together with the corresponding response of the water content in
Fig. 5b. Similar trends with respect to the water content changes were
observed for all specimens. A constant water content and a small change in
volumetric strain were observed during the first equilibration phase, indicating a high initial saturation degree of the specimens. During
cycling, the water content changed by ±2.2–2.4 %. Except for the
first drying period, in which 0.6–0.8 % water content was lost, the water
content was reversible. A comparable response was observed for the series of
P specimens (Fig. 6) which were subjected to the same testing procedure,
although RH or strains were not measured explicitly. Strain measurements on
the specimens E19 and S4 showed an immediate response to changes in RH (Fig. 5c, d). Swelling occurred during wetting, shrinkage during drying phases.
The magnitude of swelling exceeded the shrinkage, which accumulated to an
irreversible volumetric strain of 0.55–0.75 % at the end of the experiment
(Fig. 5d). The individual strain measurements showed that mainly
deformations normal to bedding contributed to the overall expansion of the
rock specimen. The strain measured parallel to bedding was significantly
smaller and approximately reversible.
Results of the stepwise cyclic RH experiment for the N specimens,
including (a) the changes in relative humidity (RH) and temperature,
(b) the water content and apparent suction calculated from the
measured RH, (c) the principal strains normal to bedding
(ε1) and parallel to bedding (ε2 and
ε3) of specimens S4 and E19, and (d) the volumetric
strains of specimens S4 and E19.
![]()
Results of the stepwise cyclic RH experiment for the P specimens,
including (a) the theoretically applied levels of relative humidity
(RH) and the measured temperature (in the laboratory) and (b) the water
content of the specimens.
![]()
Although irreversible strain was measured, no significant change in
Brazilian tensile strength was observed (Fig. 7). The Brazilian tensile
strength parallel to bedding remained constant over three to five cycles, while corresponding values for the direction normal to bedding only indicate
insignificant decreasing trends that lie within the strength values reported
by Wild et al. (2015) for specimens that were equilibrated to 56–87 MPa
suction. Thus, a change in Brazilian tensile strength as a response to the
RH variations was not measurable or insignificant.
The median value and the associated ranges of the Brazilian tensile
strength of specimens from the stepwise cyclic RH experiment with tension
parallel (σt,p) and normal (σt,n) to
bedding. Furthermore, the range of Brazilian tensile strength values reported
by Wild et al. (2015) for specimens that were equilibrated to 56–87 MPa
suction are indicated.
![]()
Discussion
Water retention
Figure 8 shows the relationship between suction and water content for
specimen E7 during the stepwise cyclic RH experiment. The system is in
equilibrium at the highest and lowest suction values (turning points between
wetting and drying paths) but not in between. Also shown are the main drying
and wetting paths reported by Wild et al. (2015).
The first drying path for the specimen follows the main drying path as it
represents the drying of the intact rock starting from initial conditions
which were comparable to the study of Wild et al. (2015). Since the specimens
were not dried to their residual water content, the following scanning curves
lie between the main drying and main wetting paths. Hysteresis can be
observed between drying and wetting path caused by non-homogeneous pore size
distribution, different contact angles between wetting and drying, or
entrapped air bubbles during wetting (Birle et al., 2008). Therefore, the
initial water content cannot be re-established anymore and a water loss of
0.6–0.8 % occurred. However, the scanning curves of the specimen
subjected to the stepwise RH cycles approximately lie within the main drying
and wetting paths, indicating that the water retention characteristics are
not significantly affected by the variations in RH. This is consistent with
findings by Pineda et al. (2014).
Strain and damage
Strain results of the dynamic and stepwise cyclic RH experiments (Fig. 5c)
indicate that the Opalinus Clay follows a cyclic expansion and contraction
associated with water absorption and desorption processes. Thereby, the
Opalinus Clay shows a strongly transversely isotropic deformation behaviour
where the strain in the direction normal to bedding (ε1)
dominates the bulk deformation. These results are consistent with findings by
Minardi et al. (2016), who also found an anisotropic response of strain for
an Opalinus Clay specimen subjected to one cycle of wetting and drying. This
observation can be related to the absorption of water into parallel-orientated clay interlayers (i.e. parallel to the bedding planes) leading to
swelling in normal direction. Moreover, according to Houben et al. (2013),
the pores of Opalinus Clay are elongated along the bedding.
Water retention curve for specimen N7 displaying suction as a
function of the water content. Also shown are the water retention curves
(main drying and main wetting) reported by Wild et al. (2015) for specimens
which were equilibrated to the applied level of RH at any point.
![]()
Irreversible volumetric expansion took place during the stepwise cyclic RH
exposure. Many studies on single mineral types (e.g. Na-montmorillonite)
have demonstrated that clay minerals show distinct hydration states when
exposed to different levels of RH (e.g. Mering, 1946; Mooney et al., 1952;
Gillery, 1959; Emerson, 1962; Van Olphen, 1965; Glaeser and Mering, 1968;
Chipera et al., 1997; Ferrage et al., 2005; Likos and Lu, 2006). These
hydration stages reflect the intercalation of one to four discrete layers of
water molecules between the mineral interfaces and are driven by the
hydration of the cations of the clay minerals (Norrish, 1972). During
transition between these stages, the interlayer spacing can increase by up to a
factor of 2 (Norrish, 1954). For the hydration of Na-montmorillonite, for
example, the interlayer spacing increases from 10 to 12.5 Å between 0
and 20 % RH, from 12.5 to 15.5 Å between 50 and 70 % RH, and further
to about 19 Å for RH > 98.5 % (Mooney et al., 1952; Gillery,
1959; Emerson, 1962; Glaeser and Mering, 1968). Between 70 and 95 % a
two-layer hydration state is present for both Ca- and Na-montmorillonite
(Seedsman, 1985). Similar stages for other clay minerals are given by
Gillery (1959). They all indicate a relatively stable state between 80 and
90 % RH. Furthermore, sorption and adsorption paths for clay minerals show
hysteresis, indicating that crystalline swelling is an irreversible
thermodynamic process (Laird et al., 1995).
This might explain the accumulated irreversible volumetric expansion as most
of the clay minerals transition between the one- and two-layer hydration
state in the RH range covered within the experiments in this study. This is
also supported by macroscopically detectable cracking that was observed
during the experiment.
Although slight cracking of the specimens was detected and irreversible
volumetric strain was observed, no significant influence on the Brazilian
tensile strength was observed after three to five cycles for both
experiments. It is therefore concluded that the observed degradation caused
by the cyclic variations of RH in this study is not sufficient to cause
severe damage that influences the strength of the material. The lower
degradation potential for Opalinus Clay compared to other clay shales when
subjected to RH cycling is in agreement with findings reported by Pineda et
al. (2011). Compared to Lilla Claystone (Pineda et al., 2008,
2014), the (tensile) strength and (dynamic) stiffness of Opalinus Clay is
significantly less affected by cyclic RH variations.
Conclusions
This study demonstrates that cyclic RH variations have the potential to
internally damage the Opalinus Clay leading to irreversible volumetric
expansion. Internal damage mainly takes place along the bedding,
supported by the fact that irreversible strain was almost exclusively
observed in the direction normal to the bedding.
The Brazilian tensile strength of Opalinus Clay seems to be unaffected by
cyclic RH variations (i.e. a change was not measurable or insignificant).
The Brazilian tensile strength parallel to bedding remained constant over
three to five cycles while corresponding values for the direction normal to
bedding only indicate insignificant decreasing trends. Water retention
characteristics of Opalinus Clay were not significantly altered by the
observed environmental degradation.
The experimental study demonstrates that RH variations can lead to
irreversible volumetric strains and therefore supports the hypothesis that
long-term environmental variations might contribute to long-term
deformations of underground excavations and favour processes that are
considered to control self-sealing in Opalinus Clay.