Clay shale specimens were exposed to cyclic relative humidity (RH) variations to investigate the response of the material to natural environmental changes. Opalinus Clay, a clay shale chosen as host rock for nuclear waste disposal in Switzerland, was utilized. The specimens were exposed to stepwise relative humidity cycles in which they were alternately allowed to equilibrate at 66 and 93 % relative humidity. Principal strains were monitored throughout the experiments using strain gauges. After each relative humidity cycle, Brazilian tensile strength tests were performed to identify possible changes in tensile strength due to environmental degradation.
Results showed that Opalinus Clay follows a cyclic swelling–shrinkage behaviour with irreversible expansion limited to the direction normal to bedding, suggesting that internal damage is restricted along the bedding planes. The Brazilian tensile strength in direction parallel and normal to bedding as well as the water retention characteristic remained unaffected by the RH variations.
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.
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
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,
Illustration of strain gauge arrangement for measurements of
principal strains:
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 (NaNO
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:
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 (
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.
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
Loading configuration for the Brazilian tests with respect to
bedding (bedding orientation is indicated by the light grey pattern). In
panel
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
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
Results of the stepwise cyclic RH experiment for the N specimens,
including
Results of the stepwise cyclic RH experiment for the P specimens,
including
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 (
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 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 (
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.
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.
The data of this study can be provided by the authors upon request.
The experiment was carried out by Patric Walter under the supervision of Florian Amann and Katrin M. Wild. Katrin M. Wild prepared the paper with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This study was funded by the Swiss Federal Nuclear Safety Inspectorate ENSI. The authors would like to thank Reto Seifert and Stewart Bishop (ETH Zurich) for their support with the mechanical and electrical challenges during the setup of the experiment. We are also grateful to Matthew Perras and Wilfried Winkler (ETH Zurich) for fruitful discussions. Furthermore, we would like to thank Claudio Madonna (ETH Zurich) for help provided during the laboratory work and for feedback on this paper. Edited by: M. Oliva Reviewed by: two anonymous referees