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Influences of degree of saturation and stress cycle on gas permeability of unsaturated compacted Gaomiaozi bentonite
Engineering Geology 254 (2019) 54–62
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Influences of degree of saturation and stress cycle on gas permeability of unsaturated compacted Gaomiaozi bentonite
T
⁎
Tianyu Weia,b, Dawei Hua,b, , Hui Zhoua,b, Jingjing Lua,b, Tao Lüc a
State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China University of Chinese Academy of Sciences, Beijing 100049, China c China Nuclear Power Engineering Co., Ltd., Beijing 100840, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Gaomiaozi (GMZ) bentonite Gas permeability Gas migration mechanism Hydro-mechanical process Stress cycle
Compacted bentonite is usually used as the backfill material in underground storage of radioactive waste due to its swelling properties and low permeability. To understand the effects of the degree of saturation and stress cycle on the barrier properties of compacted bentonite, gas permeability tests were performed on highly compacted bentonite with different dry densities, water contents and stress cycles. The testing results show that the degree of saturation and stress cycle both have significant influences on the gas permeability of unsaturated compacted bentonite samples. As the saturation or confining stress increases, gas permeability of the samples is decreased by no more than 3 orders of magnitude. Moreover, under the condition of the same dry density, the greater the degree of saturation, the faster the decline of gas permeability of samples during loading; under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. A slight recovery in gas permeability was observed during unloading process of confining stress, and the decline of gas permeability caused by increasing confinements is mainly irreversible. Different from saturated/nearly-saturated samples, gas migration in the low/medially saturated compacted bentonite sample is considered to be dominated by single-phase advection in the connected pores of the unsaturated zone. The present work implicates that the effect of the degree of saturation and the stress path on the gas permeability of the backfill material should be taken into account during the construction period and early stage of operation of the radioactive waste.
1. Introduction In geological repository for disposing high- and low-level radioactive wastes, the waste container and buffer/backfill materials are initially in a state of long-term isolation from the atmosphere, for example oxygen, due to the presence of sufficient ground water. Subsequently, various types of gases (i.e., H2, CO2, CH4, H2S and vapor) will be produced on the contact surface of the metal container and buffer/backfill materials due to the corrosions of metal, microbial activities, vapor migration and radiolysis of water (Ortiz et al., 2002). If the generated gas cannot be removed or dissipated in a timely fashion, it may cause high gas pressures on the surface of buffer/backfill material and the waste containers, which in turn will damage the sealing performances of the buffer backfill materials (Liu et al., 2014). Therefore, it is of significance to investigate the gas migration mechanism of compacted buffer/backfill material during the design and operation
periods. The gas migration mechanism, laboratory equipment and experimental method have been extensively investigated. Halayko (1998) argued that the gas migration in compacted bentonite (sand-bentonite mixture) is basically controlled by pore structure, clay-water interaction, and gas gradient. Marschall et al. (2005) interpreted the gas migration in Opalinus clay in terms of 4 mechanisms: (a) advection and diffusion of dissolved gas; (b) visco-capillary flow of gas and water phase (“two-phase flow”); (c) dilatancy-controlled gas flow (“pathway dilation”); and (d) gas transport in tensile fractures (“hydro-/gasfrac”). For saturated or nearly-saturated samples, advection and diffusion of dissolved gas are subsistent all the time but too minimal to drive all the gases (Gallé, 2000). The gas pressure on the surface of waste containers will increase continuously until the critical capillary pressure occurs. Thus, gas will enter the capillary to introduce the gas/water co-migration of the viscous capillary two-phase flow (Graham et al., 2002; Ye
⁎ Corresponding author at: State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China E-mail address: [email protected] (D. Hu).
https://doi.org/10.1016/j.enggeo.2019.04.005 Received 20 May 2018; Received in revised form 3 April 2019; Accepted 3 April 2019 Available online 04 April 2019 0013-7952/ © 2019 Elsevier B.V. All rights reserved.
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the water content of the bentonite powder was measured in real time till it became stable after 48 h. Considering that the disadvantages of layering phenomenon (see Fig. 1(a)) in the traditional layered-bumping method, one double-end pressing method (see Fig. 1(b)) was proposed to produce the compacted bentonite samples in the following steps:
et al., 2014). However, for highly compacted and saturated bentonite, which usually possess a substantially high gas entry value, significant fluctuations of gas flow rates are still recorded although the injection pressure is lower than gas entry value during a gas injection test (Xu et al., 2015, 2017), and the degree of saturation of sample remains almost unchanged after the test completion (Pusch et al., 1985; Horseman et al., 1999; Harrington and Horseman, 2003; Ye et al., 2014). Both of above phenomena suggest the existence of the dilatancycontrolled gas flow, which is quite different from traditional visco-capillary flow. Recently, Guo and Fall (2018) numerically studied the dilatancy-controlled gas flow with double-porosity and double-effective stress concepts in saturated bentonite and the numerical results were validated by the results of laboratory tests conducted by Harrington and Horseman (2003). Xu et al. (2018) proposed that the capillary effect, mechanical effect, as well as the interfacial leakage can induce gas breakthrough but the mechanical gas breakthrough is perhaps the key for the low permeability specimens. In other words, the gas migration mechanism of saturated/nearlysaturated compacted bentonite shows great differences compared with that of generally unsaturated soil materials due to its significantly low permeability. Thanks to the favorable geological conditions, buffer/ backfill materials will be existing in a relatively dry environment during the construction period and in the early operation of the radioactive waste repository. Nevertheless, a more complex stress field variation might be caused by the evolutions of thermal field, geological excavation and other natural or artificial disturbances. The buffer/backfill materials at this stage are closer to the initial state, with a lower degree of saturation. According to Graham et al. (2002), when the degree of saturation is below 93% approximately in bentonite and sand-bentonite, there is only a small resistance to gas migration. Previous studies usually focus on the effect of stress variation on the permeability of saturated/nearly-saturated samples in unidirectional loading path (Cui and Tang, 2013; Cui, 2017; Ye et al., 2014; Xu et al., 2015). In this paper, several groups of samples with different dry densities (approximately 1.5 and 1.7 g/cm3) and water contents (approximately 13%, 15% and 18%) were prepared for the tests. The gas permeability of each sample was measured during the loading and unloading of confining stresses in order to investigate the influences of degree of saturation and stress cycle on the gas permeability of the bentonite samples.
(a) Fill bentonite powder into the mold for pre-compaction; (b) Slowly press stainless-steel block A into the mold with a static testing machine until the top surface of the mold is flushed on the top surface of the steel block to achieve the first compaction; (c) Reverse the mold and block A, and then press the stainless-steel block B into the mold completely from the other end; and (d) Due to dislocation and compaction of the bentonite particles, the thrust force Fp will gradually decrease. Once the thrust force Fp is almost constant, the sample can be removed from the mold. Compared with the traditional method that presses the samples only from one end, the samples prepared using double-end pressing method in the present study were more homogenous (Zheng et al., 2008). The prepared samples were cylindrical, and the height and diameter were 100 ± 1.0 mm and 50 ± 0.5 mm, respectively. The double-end pressing method described in this context can significantly shorten the sample preparation time period within 15 min, which can reduce the direct contact time between powder and air. Before the test, the water contents of the prepared cylindrical specimens were double-checked (see Table 2). The results suggest from the obtained data, that the water loss during sample preparation is negligible. To minimize the loss of moisture, the prepared samples were sealed and used to complete the following tests immediately. 2.2. Principle and procedures of quasi-stationary flow method Considering the characteristics of water-sensitivity and low permeability of the compacted bentonite, it is likely that conventional method for gas permeability tests (steady state method) is not suitable due to the long duration, and thus it was not employed in the present study. Instead, a gas permeability testing instrument for quasi-stationary flow method (Meziani and Skoczylas, 1999) was used in the following tests. The instrument consists of one hydrostatic pressure chamber, a highpressure gas tank, a buffer container, several precise barometers and other components (see Fig. 2(a)). It allows the triaxial deformation of the sample under confining stress and can automatically record and save the gas pressure readings in the pipeline with a computer at an accuracy of 0.001 MPa. The pressure range provided by the gas pressure tank is 0–20 MPa. Previous testing result (Mohammed and Al-Lami, 2016) indicated that the swelling pressure of compacted bentonite with dry density of approximately 1.75 kg/cm3 could be up to 4 MPa under the condition of constant volume constraint. In this scenario, considering the in-situ stress level in underground storage of radioactive waste, a 3-step loading process (confining stresses of 2, 4, and 6 MPa) and a 2-step unloading process (confining stresses of 4 and 2 MPa) were employed to study the influence of stress cycle on gas permeability. The stress path used in the tests is shown in Fig. 3. To minimize the influence of Klinkenberg effect (Klinkenberg, 1941), the initial injection pressure was set to be 0.5 MPa and remained unchanged during the tests. The quasi-stationary flow method (see Fig. 2(b)) was proposed to measure the permeability of tight materials under the hypothesis that the flow in the pores is one-dimensional, single-phase and laminar. Using this method, one can measure the gas permeability of tight materials in a short time period (Davy et al., 2007). Therefore, the quasistationary flow method rather than the conventional stationary method was adopted in the present work. Before the gas permeability test, the confining stress was first increased to a prescribed value. One manual pump was used to apply the confining stress, and a pressure transducer was utilized to record the value of confining stress. The deformation
2. Experimental programme 2.1. Sample preparation The material used in this study is sodium bentonite powder sampled from the Gaomiaozi (GMZ) mining area in Xinghe County, Inner Mongolia Autonomous Region in China. This region is characterized with the most potential materials for deep geological disposal of highlevel radioactive wastes in China (Ye et al., 2012). Its basic properties and mineral compositions are presented in Table 1. After ball milling and sieving, the bentonite used in present study was crushed into powder with particle size less than 0.075 mm, and placed in the oven at a constant temperature (105 ± 1°C) with continuous ventilation until the stability of weight (with an error of 0.1 g) was reached. Next, the bentonite powder was sprayed with water to obtain three different groups of water contents (approximately 13%, 15%, and 18%). For this, Table 1 Basic properties and mineral compositions of the tested soil. Natural water content (%)
Particle density ρs (g/cm3)
Main Minerals (by XRD) (%) Montmorillonite (bentonite)
Quartz
Albite
Illite
Calcite
13.80
2.68
66.58
12.35
10.01
6.79
4.27
55
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Fig. 1. (a) Photograph of samples made using the layered-bumping method (left and mid) and the double-end pressing method (right); (b) process of the double-end pressing method.
pressure inside the pores has also been stabilized and no pipeline leakage occurs to avoid possible test inaccuracy, it is also necessary that the decline of air pressure in the pipeline is less than 0.001 MPa within 10 min after closing the upstream valve (1) and downstream valve (3) (see Fig. 2(b)). During the test, the outlet was always connected to the atmosphere P0, and the valve (1) between the buffer reservoir and highpressure gas tank was closed. Thus, no additional gas was excited. The pressure of the buffer reservoir decreased with time during the tests, and the pressure was recorded by a computer in real time. In this study, it is assumed that the gas flow obeys Darcy's law, and it can be written as:
Table 2 Details of the samples during the preparation and the test. No. of Samples
A-1 A-2 A-3 A-4 A-5 A-6 B-1 B-2 B-3 B-4 B-5 B-6
Dry density ρd (g/cm3)
1.51 1.50 1.50 1.50 1.51 1.51 1.71 1.70 1.71 1.70 1.71 1.71
Water content during the preparation w (%) Before
After
13.08 13.08 15.20 15.20 17.92 17.92 13.08 13.08 15.20 15.20 17.92 17.92
13.02 13.05 15.17 15.14 17.89 17.89 13.05 13.03 15.15 15.15 17.84 17.82
Saturation S (%)
Moisture loss during the test (g)
45.24 44.56 51.78 51.78 61.98 61.98 61.80 60.81 71.81 70.66 84.66 84.66
0.2 0.1 0.1 0.2 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.4
→ K ⎯⎯⎯⎯⎯⎯→ V = −⎛⎜ ⎞⎟ grad P ⎝ μ⎠
(1)
→ where V (m/s) is the gas velocity, K(m2) is the gas permeability and μ is the fluid viscosity of argon and is equal to 2.20 × 10−5Pa ⋅ s herein. Assuming the ideal gas yields:
Note: “Before” means the water content of Gaomiaozi bentonite powder before the pressing; “After” means the water content of samples after the pressing.
ρ ρ (t ) = ⎛ 0 ⎞ P (t ) ⎝ P0 ⎠ ⎜
stabilization of samples is assumed once the value changes of confining stresses are less than 0.01 MPa. The argon gas was then injected into the samples from the inlet at an initial pressure of P1. To ensure that the
⎟
(2)
where ρ(t) and ρ0 are the specific mass of the gas at pressure P(t) and P0, respectively. 56
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Fig. 2. (a) Test devices and (b) schematic diagram for quasi-stationary flow method in the gas permeability tests.
sample can be written after integrating Eq. (3) with ∂P/∂t. Then we obtain:
Taking into account the mass balance equation and ignoring the effects of poromechanical coupling on the fluid pressure because of the high level of the latter's compressibility, the diffusivity equation is obtained as (Dana and Skoczylas, 1999):
⎯⎯⎯⎯⎯⎯→ K ∂P div(P grad P ) = ϕ (Sr ) μ ∂t
P (x ) =
x x P12 ⎛1 − ⎞ + P02 L⎠ L ⎝
(4)
After a time interval of Δt, the pressure of buffer reservoir was reduced to P2 = P1 − ΔP. This pressure difference ΔP was very small and two orders of magnitude less than P1. Considering that all the tests were conducted in a room temperature (25°C), the gas flow through the sample could be assumed constant at an average pressure Pmean = P1 − ΔP/2 on the upstream side, and the average upstream volume flow rate Qmean is derived from ΔP and Δt using (Davy et al., 2007):
(3)
where Sr is the saturation of the sample and ϕ(Sr) represents the fraction of porosity in the material which remains free for gas circulation at a given degree of saturation Sr. Another assumption is that the gas flow in the test is formed by a succession of quasi-stationary with following boundary conditions: a) P1 = P1(t) at x = 0; b) P0 = P0(t) at x = L.
Qmean =
V1 ΔP Pmean Δt 3
where L(m) is the length of the sample. For this, the pressure in the
(5)
where V1(m ) is the volume of the upstream buffer reservoir including 57
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Fig. 3. Stress path during the gas permeability tests.
pipeline. The gas permeability Kx(m2) in axial direction of the samples could be deduced according Eq. (5) and Darcy's law by considering Pmean = P1 as (Meziani and Skoczylas, 1999):
Kx =
μQmean 2LPmean 2 A (Pmean − P02 )
For the relationship between gas permeability and degree of saturation, the evolution of gas permeability with various degrees of saturation during loading and unloading is separately given in Figs. 5(a) and (b). The dotted arrows indicate the changes in gas permeability of the samples with similar degree of saturation with respect to the stress cycle, in which the downward arrow indicates the decrease of gas permeability due to applied loading; and the upward arrow indicates the increase due to unloading. In the case of the same stress state and dry density, the gas permeability of sample is reduced by 2–3 orders of magnitude with the increase of degree of saturation. For the initial confining stress of 2 MPa and dry density of approximately 1.7 g/cm3, the gas permeability was decreased by 94.5% when increasing the degree of saturation from 60.81% (B-2) to 84.66% (B-6). The results match that of Gallé (1998) for the gas permeability test of the Fo-Ca clay in France (see Fig. 6). Moreover, under the condition of almost the same dry density, the greater the degree of saturation, the faster the gas permeability decline of the sample during loading (see A-1 & A-6 and B1 & B-6 in Fig. 5(a)). Under the condition of almost the same degree of saturation, the larger the initial dry density, the gentler the gas permeability decrease (see A-6 & B-1). Compared with the decrease in loading process, the recovery of gas permeability during unloading could be negligible (see Fig. 5). Similar to the saturated/nearly-saturated samples, Table 2 shows that the moisture losses during the test are also negligible for the samples with a lower degree of saturation. Unfortunately, we did not get any specific regulations of the moisture loss between the groups.
(6)
2
where A(m ) is the cross-sectional area of sample. 3. Testing results Two samples were tested in each testing group, and the averaged value of three measurements was used for each case to ensure the validity and the representativeness of the test results. The samples have been weighed before and after each test to determine the moisture loss during permeation. The details of the samples before and after the test are summarized in Table 2, and the testing results observed in 12 gas permeability measurements versus confining stress and degree of saturation are plotted in Figs. 4 and 5. Under the condition of almost the same dry density, the gas permeability of samples decreased with the increase of confining stress (see Fig. 4). Compared with the measurement by the initial confining stress Pc = 2MPa, the gas permeability values of sample A-1 (ρd = 1.51g/cm3, Sr = 45.24%) were decreased by 21.4% (at confining stress of 4 MPa) and 49.0% (at confining stress of 6 MPa), respectively. The gas permeability values of sample A-6 with higher water content (ρd = 1.51g/ cm3, Sr = 61.98%) were decreased by 90.6% and 99.8% at confining stresses of 4 and 6 MPa, respectively, during the same process (see Fig. 4(a)). A similar phenomenon was also observed in another group of dry density as shown in Fig. 4(b). The gas permeability values of sample B-1 (ρd = 1.71g/cm3, Sr = 61.80%) were decreased by 14.5% and 46.8% at the confining stresses of 4 and 6 MPa, respectively. Those of sample B-6 (ρd = 1.71g/cm3, Sr = 84.66%) are 49.0% and 92.3% during the same process at confining stresses of 4 and 6 MPa, respectively. Next, the gas permeability of the samples recovered to a relatively slight extent during the decrease of confining stress. The value of gas permeability of sample A-1 recovered to 53.3% (at confining stress of 4 MPa) and 58.5% (at confining stress of 2 MPa) of the initial gas permeability under confining stress of 2 MPa. A similar recovery scenario was also noticed on samples A-6, B-1 and B-6. Note that the gas permeability of all the samples could not recover to their initial value at the end of unloading of the confining stress.
4. Discussion Based on the above testing results, in view of the macroscopic and microscopic gas migration mechanism, the evolutions of gas permeability of unsaturated compacted bentonite with different degrees of saturation and dry densities with respect to the confining stress cycle process are analyzed in this section. 4.1. Influences of degree of saturation on gas permeability The testing results of the compacted bentonite samples of two dry densities illustrated in Fig. 5 show that the degree of saturation has a significant effect on the gas permeability of the compacted bentonite. In case of the same stress state and dry density, the gas permeability of sample is markedly reduced by 2–3 orders of magnitude with increase of degree of saturation. It was evidenced (Ye et al., 2009) that there are 58
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Fig. 4. Variations of the gas permeability during loading and unloading of confining stress: (a) ρd ≈ 1.5g/cm3 and (b) ρd ≈ 1.7g/cm3.
pass. Therefore, it is indicative that gas flow in the low/medially saturated samples (Sr < 93%, according to Graham et al., 2002) is dominated by the single-phase advection in unsaturated zone (see Fig. 7). It is also reasonable to interpret the decreasing gas permeability with increasing degree of saturation as the decline of the connected pores in the unsaturated zone which was caused by pore water filling. On the other hand, due to the swelling potential of the clay mineral during hydration, the initial connected pores in the unsaturated zone will be reduced when the sample is prepared under confined conditions (Ye et al., 2009). This reduction of the initially connected pores caused by the swelling properties of bentonite during hydration is also considered to be an important indicator accounting for of the above phenomena. Comparing the testing results of the two groups of samples with different dry densities, it is obvious that the gas permeability of samples
still a large number of macro- and micro-connected pores between the particles of the unsaturated compacted bentonite. In the process of sample compaction, the pores are filled with water unevenly, and different proportions of saturated and unsaturated pores will be formed, depending on the degree of saturation of the samples. As the degree of saturation of the sample before and after the test does not change significantly, it is likely that the gas flow in the sample may be merely a single-phase advection in the non-saturated region or possible preferential pathway flow (Cuss et al., 2014) caused by local gas accumulation. Similar behavior is also reported in the saturated/nearly-saturated samples (Horseman et al., 1999; Harrington and Horseman, 2003; Ye et al., 2014; Graham et al., 2016). According to Xu et al. (2015), the condition of “gas induced-dilatancy” effect is in fact that the gas injection pressure P1 must be greater than the threshold Pd, and the observed “gas induced-dilatancy” effect allows very limited airflow to 59
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Fig. 5. Variations of gas permeability with saturation degree during (a) confining stress loading and (b) confining stress unloading.
triaxial deformation of the sample, the following can be inferred: the samples will become more compacted under the rising confining stress during the loading process, and the connected pores in the unsaturated zone are also greatly reduced by the compressive deformation of samples. A similar phenomenon has also been reported in the investigation of compacted bentonite-sand mixture (Liu et al., 2015). Fig. 5 shows that under the condition of almost the same dry density, the greater the degree of saturation, the faster the gas permeability decline of the sample during loading. Under the condition of almost the same degree of saturation, the larger the initial dry density, the gentler the gas permeability decrease. The variations may be caused by the saturation and dry density, which affects the deformation capability of the samples. However, due to the limited experimental data available, the specific mechanism with respect to the deformation is not fully understood in this work.
with higher dry densities (B-1 and B-2) is much smaller than that of samples with lower dry densities (A-5 and A-6) under the initial confining stress of 2 MPa. A higher dry density corresponds to a smaller initial volume fraction of connected pores in the unsaturated zone, and thus a lower gas permeability.
4.2. Influences of stress cycle on gas permeability 4.2.1. Influences of stress increment The stress path used in the tests is illustrated in Fig. 3. Figs. 4(a) and (b) show that the gas permeability of compacted bentonite blocks is closely related to the stress state and the stress path. The gas permeability of the sample with a degree of saturation of 61.98% (A-6) is decreased by a maximum of 99.8% with the increment of confining stress. Considering that the equipment adopted in this paper allows for 60
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Fig. 6. Variations of gas permeability with dry density and the degree of saturation for Fo-Ca clay (Gallé, 1998) and bentonite samples in this paper (CV: Constant Volume, CP-2 MPa: Confining stress of 2 MPa).
5. Conclusions The gas permeability of highly compacted bentonite with different dry densities and degrees of saturation was studied using the quasistationary method during a confining stress cycle. The gas migration mechanism in compacted bentonite blocks was analyzed when the degree of saturation of buffer/backfill material was relatively low during the construction period and in the early stage of operation of the repository. The main conclusions are drawn as follows: (1) The gas permeability of the sample under confining stress of 2 MPa was decreased by 2–3 orders of magnitude with the increase in degree of saturation. The gas migration in the low/medially saturated compacted bentonite sample (Sr < 93%) was considered to be dominated by single-phase advection in the unsaturated zone. Due to pore water filling and the swelling potential of bentonite during hydration, an increase in the degree of saturation led to a decline in connected pores in the unsaturated zone, which caused a decrease in the gas permeability subsequently. (2) The gas permeability of sample decreases significantly with the increase of confining stress. Under the condition of almost the same dry density, the greater the degree of saturation, the steeper the decline of the gas permeability of samples. Under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. Under the condition of allowing free deformation of sample, the connected pores in the unsaturated zone are significantly reduced via the compressive deformation of the sample, which in turn increases the resistance to gas flow. (3) Unlike the significant reduction in the gas permeability of sample caused by stress increase, the recovery caused by stress decrease was relatively smaller. The effect of stress increase on the gas permeability of sample was basically irreversible. However, there was no specific relationship between this recovery and the degree of saturation of the sample. (4) For the engineering stability purpose, it is argued that the effect of the degree of saturation and the stress path on the gas permeability of the buffered/backfill material cannot be ignored during the construction period and in the early stage of operation of the repository.
Fig. 7. Schematic diagram of gas permeation in the low/medially saturated samples.
4.2.2. Influences of stress drop As mentioned above, the low/medially saturated samples still contain a large proportion of unsaturated zones after compaction, and the connected pores serve as natural preferential pathway allowing gas to flow in a single-phase advection. The increases in both degree of saturation and confining stress cause a significant decrease in the gas permeability. However, it is indicative from Figs. 4 and 5 that a slight permeability recovery was achieved when the confining stress decreased. Therefore, it can be inferred that the decrease in gas permeability caused by confining stress increase is basically irreversible. Unfortunately, the mechanism of this recovery and the relationship associated with the degree of saturation are still unclear, which requires further investigations.
Unfortunately, due to the limited capacities of the experimental facilities used in the context, no quantitative analyses of volumetric 61
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deformation or other variables were conducted. This, of course, needs further investigations. Acknowledgements We acknowledge the support by the National Key R&D Program of China (Grant Nos. 2018YFC0809600 and 2018YFC0809601), the National Natural Science Foundation of China (Grant Nos, 51779252 and 51479193) and the Chinese Academy of Science Pioneer Hundred Talents Program. The authors wish to thank the Editor as well as the anonymous reviewers for their valuable and constructive suggestions, which significantly improve the quality of this manuscript. References Cui, Y.J., 2017. On the hydro-mechanical behaviour of MX80 bentonite-based materials. J. Rock Mech. Geotech. Eng. 9 (3), 565–574. Cui, Y.J., Tang, Anh Minh, 2013. On the chemo-thermo-hydro-mechanical behaviour of geological and engineered barriers. J. Rock Mech. Geotech. Eng. 5 (3), 169–178. Cuss, R.J., Harrington, J.F., Noy, D.J., Graham, C.C., Sellin, P., 2014. Evidence of localised gas propagation pathways in a field-scale bentonite engineered barrier system; results from three gas injection tests in the large scale gas injection test (Lasgit). Appl. Clay Sci. 102, 81–92. Dana, E., Skoczylas, F., 1999. Gas relative permeability and pore structure of sandstones. Int. J. Rock Mech. Min. Sci. 36 (5), 613–625. Davy, C.A., Skoczylas, F., Barnichon, J.D., Lebon, P., 2007. Permeability of macro-cracked argillite under confinement: Gas and water testing. Phys. Chem. Earth 32 (8), 667–680. Mohammed, Y. Fattah, Al-Lami, Aysar H.S., 2016. Behavior and characteristics of compacted expansive unsaturated bentonite-sand mixture. J. Rock Mech. Geotech. Eng. 8 (5), 629–639. Gallé, C., 1998. Evaluation of gas transport properties of backfill materials for waste disposal: H-2 migration experiments in compacted Fo-Ca clay. Clay Clay Miner. 46 (5), 498–508. Gallé, C., 2000. Gas breakthrough pressure in compacted Fo-Ca clay and interfacial gas overpressure in waste disposal context. Appl. Clay Sci. 17 (1), 85–97. Graham, J., Halayko, K.G., Hume, H., Kirkham, T., Gray, M., Oscarson, D., 2002. A capillarity-advective model for gas break-through in clays. Eng. Geol. 64 (2), 273–286. Graham, C.C., Harrington, J.F., Sellin, P., 2016. Gas migration in pre-compacted bentonite under elevated pore-water pressure conditions. Appl. Clay Sci. 132 (SI), 353–365.
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Influences of degree of saturation and stress cycle on gas permeability of unsaturated compacted Gaomiaozi bentonite
T
⁎
Tianyu Weia,b, Dawei Hua,b, , Hui Zhoua,b, Jingjing Lua,b, Tao Lüc a
State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China University of Chinese Academy of Sciences, Beijing 100049, China c China Nuclear Power Engineering Co., Ltd., Beijing 100840, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Gaomiaozi (GMZ) bentonite Gas permeability Gas migration mechanism Hydro-mechanical process Stress cycle
Compacted bentonite is usually used as the backfill material in underground storage of radioactive waste due to its swelling properties and low permeability. To understand the effects of the degree of saturation and stress cycle on the barrier properties of compacted bentonite, gas permeability tests were performed on highly compacted bentonite with different dry densities, water contents and stress cycles. The testing results show that the degree of saturation and stress cycle both have significant influences on the gas permeability of unsaturated compacted bentonite samples. As the saturation or confining stress increases, gas permeability of the samples is decreased by no more than 3 orders of magnitude. Moreover, under the condition of the same dry density, the greater the degree of saturation, the faster the decline of gas permeability of samples during loading; under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. A slight recovery in gas permeability was observed during unloading process of confining stress, and the decline of gas permeability caused by increasing confinements is mainly irreversible. Different from saturated/nearly-saturated samples, gas migration in the low/medially saturated compacted bentonite sample is considered to be dominated by single-phase advection in the connected pores of the unsaturated zone. The present work implicates that the effect of the degree of saturation and the stress path on the gas permeability of the backfill material should be taken into account during the construction period and early stage of operation of the radioactive waste.
1. Introduction In geological repository for disposing high- and low-level radioactive wastes, the waste container and buffer/backfill materials are initially in a state of long-term isolation from the atmosphere, for example oxygen, due to the presence of sufficient ground water. Subsequently, various types of gases (i.e., H2, CO2, CH4, H2S and vapor) will be produced on the contact surface of the metal container and buffer/backfill materials due to the corrosions of metal, microbial activities, vapor migration and radiolysis of water (Ortiz et al., 2002). If the generated gas cannot be removed or dissipated in a timely fashion, it may cause high gas pressures on the surface of buffer/backfill material and the waste containers, which in turn will damage the sealing performances of the buffer backfill materials (Liu et al., 2014). Therefore, it is of significance to investigate the gas migration mechanism of compacted buffer/backfill material during the design and operation
periods. The gas migration mechanism, laboratory equipment and experimental method have been extensively investigated. Halayko (1998) argued that the gas migration in compacted bentonite (sand-bentonite mixture) is basically controlled by pore structure, clay-water interaction, and gas gradient. Marschall et al. (2005) interpreted the gas migration in Opalinus clay in terms of 4 mechanisms: (a) advection and diffusion of dissolved gas; (b) visco-capillary flow of gas and water phase (“two-phase flow”); (c) dilatancy-controlled gas flow (“pathway dilation”); and (d) gas transport in tensile fractures (“hydro-/gasfrac”). For saturated or nearly-saturated samples, advection and diffusion of dissolved gas are subsistent all the time but too minimal to drive all the gases (Gallé, 2000). The gas pressure on the surface of waste containers will increase continuously until the critical capillary pressure occurs. Thus, gas will enter the capillary to introduce the gas/water co-migration of the viscous capillary two-phase flow (Graham et al., 2002; Ye
⁎ Corresponding author at: State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China E-mail address: [email protected] (D. Hu).
https://doi.org/10.1016/j.enggeo.2019.04.005 Received 20 May 2018; Received in revised form 3 April 2019; Accepted 3 April 2019 Available online 04 April 2019 0013-7952/ © 2019 Elsevier B.V. All rights reserved.
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the water content of the bentonite powder was measured in real time till it became stable after 48 h. Considering that the disadvantages of layering phenomenon (see Fig. 1(a)) in the traditional layered-bumping method, one double-end pressing method (see Fig. 1(b)) was proposed to produce the compacted bentonite samples in the following steps:
et al., 2014). However, for highly compacted and saturated bentonite, which usually possess a substantially high gas entry value, significant fluctuations of gas flow rates are still recorded although the injection pressure is lower than gas entry value during a gas injection test (Xu et al., 2015, 2017), and the degree of saturation of sample remains almost unchanged after the test completion (Pusch et al., 1985; Horseman et al., 1999; Harrington and Horseman, 2003; Ye et al., 2014). Both of above phenomena suggest the existence of the dilatancycontrolled gas flow, which is quite different from traditional visco-capillary flow. Recently, Guo and Fall (2018) numerically studied the dilatancy-controlled gas flow with double-porosity and double-effective stress concepts in saturated bentonite and the numerical results were validated by the results of laboratory tests conducted by Harrington and Horseman (2003). Xu et al. (2018) proposed that the capillary effect, mechanical effect, as well as the interfacial leakage can induce gas breakthrough but the mechanical gas breakthrough is perhaps the key for the low permeability specimens. In other words, the gas migration mechanism of saturated/nearlysaturated compacted bentonite shows great differences compared with that of generally unsaturated soil materials due to its significantly low permeability. Thanks to the favorable geological conditions, buffer/ backfill materials will be existing in a relatively dry environment during the construction period and in the early operation of the radioactive waste repository. Nevertheless, a more complex stress field variation might be caused by the evolutions of thermal field, geological excavation and other natural or artificial disturbances. The buffer/backfill materials at this stage are closer to the initial state, with a lower degree of saturation. According to Graham et al. (2002), when the degree of saturation is below 93% approximately in bentonite and sand-bentonite, there is only a small resistance to gas migration. Previous studies usually focus on the effect of stress variation on the permeability of saturated/nearly-saturated samples in unidirectional loading path (Cui and Tang, 2013; Cui, 2017; Ye et al., 2014; Xu et al., 2015). In this paper, several groups of samples with different dry densities (approximately 1.5 and 1.7 g/cm3) and water contents (approximately 13%, 15% and 18%) were prepared for the tests. The gas permeability of each sample was measured during the loading and unloading of confining stresses in order to investigate the influences of degree of saturation and stress cycle on the gas permeability of the bentonite samples.
(a) Fill bentonite powder into the mold for pre-compaction; (b) Slowly press stainless-steel block A into the mold with a static testing machine until the top surface of the mold is flushed on the top surface of the steel block to achieve the first compaction; (c) Reverse the mold and block A, and then press the stainless-steel block B into the mold completely from the other end; and (d) Due to dislocation and compaction of the bentonite particles, the thrust force Fp will gradually decrease. Once the thrust force Fp is almost constant, the sample can be removed from the mold. Compared with the traditional method that presses the samples only from one end, the samples prepared using double-end pressing method in the present study were more homogenous (Zheng et al., 2008). The prepared samples were cylindrical, and the height and diameter were 100 ± 1.0 mm and 50 ± 0.5 mm, respectively. The double-end pressing method described in this context can significantly shorten the sample preparation time period within 15 min, which can reduce the direct contact time between powder and air. Before the test, the water contents of the prepared cylindrical specimens were double-checked (see Table 2). The results suggest from the obtained data, that the water loss during sample preparation is negligible. To minimize the loss of moisture, the prepared samples were sealed and used to complete the following tests immediately. 2.2. Principle and procedures of quasi-stationary flow method Considering the characteristics of water-sensitivity and low permeability of the compacted bentonite, it is likely that conventional method for gas permeability tests (steady state method) is not suitable due to the long duration, and thus it was not employed in the present study. Instead, a gas permeability testing instrument for quasi-stationary flow method (Meziani and Skoczylas, 1999) was used in the following tests. The instrument consists of one hydrostatic pressure chamber, a highpressure gas tank, a buffer container, several precise barometers and other components (see Fig. 2(a)). It allows the triaxial deformation of the sample under confining stress and can automatically record and save the gas pressure readings in the pipeline with a computer at an accuracy of 0.001 MPa. The pressure range provided by the gas pressure tank is 0–20 MPa. Previous testing result (Mohammed and Al-Lami, 2016) indicated that the swelling pressure of compacted bentonite with dry density of approximately 1.75 kg/cm3 could be up to 4 MPa under the condition of constant volume constraint. In this scenario, considering the in-situ stress level in underground storage of radioactive waste, a 3-step loading process (confining stresses of 2, 4, and 6 MPa) and a 2-step unloading process (confining stresses of 4 and 2 MPa) were employed to study the influence of stress cycle on gas permeability. The stress path used in the tests is shown in Fig. 3. To minimize the influence of Klinkenberg effect (Klinkenberg, 1941), the initial injection pressure was set to be 0.5 MPa and remained unchanged during the tests. The quasi-stationary flow method (see Fig. 2(b)) was proposed to measure the permeability of tight materials under the hypothesis that the flow in the pores is one-dimensional, single-phase and laminar. Using this method, one can measure the gas permeability of tight materials in a short time period (Davy et al., 2007). Therefore, the quasistationary flow method rather than the conventional stationary method was adopted in the present work. Before the gas permeability test, the confining stress was first increased to a prescribed value. One manual pump was used to apply the confining stress, and a pressure transducer was utilized to record the value of confining stress. The deformation
2. Experimental programme 2.1. Sample preparation The material used in this study is sodium bentonite powder sampled from the Gaomiaozi (GMZ) mining area in Xinghe County, Inner Mongolia Autonomous Region in China. This region is characterized with the most potential materials for deep geological disposal of highlevel radioactive wastes in China (Ye et al., 2012). Its basic properties and mineral compositions are presented in Table 1. After ball milling and sieving, the bentonite used in present study was crushed into powder with particle size less than 0.075 mm, and placed in the oven at a constant temperature (105 ± 1°C) with continuous ventilation until the stability of weight (with an error of 0.1 g) was reached. Next, the bentonite powder was sprayed with water to obtain three different groups of water contents (approximately 13%, 15%, and 18%). For this, Table 1 Basic properties and mineral compositions of the tested soil. Natural water content (%)
Particle density ρs (g/cm3)
Main Minerals (by XRD) (%) Montmorillonite (bentonite)
Quartz
Albite
Illite
Calcite
13.80
2.68
66.58
12.35
10.01
6.79
4.27
55
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Fig. 1. (a) Photograph of samples made using the layered-bumping method (left and mid) and the double-end pressing method (right); (b) process of the double-end pressing method.
pressure inside the pores has also been stabilized and no pipeline leakage occurs to avoid possible test inaccuracy, it is also necessary that the decline of air pressure in the pipeline is less than 0.001 MPa within 10 min after closing the upstream valve (1) and downstream valve (3) (see Fig. 2(b)). During the test, the outlet was always connected to the atmosphere P0, and the valve (1) between the buffer reservoir and highpressure gas tank was closed. Thus, no additional gas was excited. The pressure of the buffer reservoir decreased with time during the tests, and the pressure was recorded by a computer in real time. In this study, it is assumed that the gas flow obeys Darcy's law, and it can be written as:
Table 2 Details of the samples during the preparation and the test. No. of Samples
A-1 A-2 A-3 A-4 A-5 A-6 B-1 B-2 B-3 B-4 B-5 B-6
Dry density ρd (g/cm3)
1.51 1.50 1.50 1.50 1.51 1.51 1.71 1.70 1.71 1.70 1.71 1.71
Water content during the preparation w (%) Before
After
13.08 13.08 15.20 15.20 17.92 17.92 13.08 13.08 15.20 15.20 17.92 17.92
13.02 13.05 15.17 15.14 17.89 17.89 13.05 13.03 15.15 15.15 17.84 17.82
Saturation S (%)
Moisture loss during the test (g)
45.24 44.56 51.78 51.78 61.98 61.98 61.80 60.81 71.81 70.66 84.66 84.66
0.2 0.1 0.1 0.2 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.4
→ K ⎯⎯⎯⎯⎯⎯→ V = −⎛⎜ ⎞⎟ grad P ⎝ μ⎠
(1)
→ where V (m/s) is the gas velocity, K(m2) is the gas permeability and μ is the fluid viscosity of argon and is equal to 2.20 × 10−5Pa ⋅ s herein. Assuming the ideal gas yields:
Note: “Before” means the water content of Gaomiaozi bentonite powder before the pressing; “After” means the water content of samples after the pressing.
ρ ρ (t ) = ⎛ 0 ⎞ P (t ) ⎝ P0 ⎠ ⎜
stabilization of samples is assumed once the value changes of confining stresses are less than 0.01 MPa. The argon gas was then injected into the samples from the inlet at an initial pressure of P1. To ensure that the
⎟
(2)
where ρ(t) and ρ0 are the specific mass of the gas at pressure P(t) and P0, respectively. 56
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Fig. 2. (a) Test devices and (b) schematic diagram for quasi-stationary flow method in the gas permeability tests.
sample can be written after integrating Eq. (3) with ∂P/∂t. Then we obtain:
Taking into account the mass balance equation and ignoring the effects of poromechanical coupling on the fluid pressure because of the high level of the latter's compressibility, the diffusivity equation is obtained as (Dana and Skoczylas, 1999):
⎯⎯⎯⎯⎯⎯→ K ∂P div(P grad P ) = ϕ (Sr ) μ ∂t
P (x ) =
x x P12 ⎛1 − ⎞ + P02 L⎠ L ⎝
(4)
After a time interval of Δt, the pressure of buffer reservoir was reduced to P2 = P1 − ΔP. This pressure difference ΔP was very small and two orders of magnitude less than P1. Considering that all the tests were conducted in a room temperature (25°C), the gas flow through the sample could be assumed constant at an average pressure Pmean = P1 − ΔP/2 on the upstream side, and the average upstream volume flow rate Qmean is derived from ΔP and Δt using (Davy et al., 2007):
(3)
where Sr is the saturation of the sample and ϕ(Sr) represents the fraction of porosity in the material which remains free for gas circulation at a given degree of saturation Sr. Another assumption is that the gas flow in the test is formed by a succession of quasi-stationary with following boundary conditions: a) P1 = P1(t) at x = 0; b) P0 = P0(t) at x = L.
Qmean =
V1 ΔP Pmean Δt 3
where L(m) is the length of the sample. For this, the pressure in the
(5)
where V1(m ) is the volume of the upstream buffer reservoir including 57
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Fig. 3. Stress path during the gas permeability tests.
pipeline. The gas permeability Kx(m2) in axial direction of the samples could be deduced according Eq. (5) and Darcy's law by considering Pmean = P1 as (Meziani and Skoczylas, 1999):
Kx =
μQmean 2LPmean 2 A (Pmean − P02 )
For the relationship between gas permeability and degree of saturation, the evolution of gas permeability with various degrees of saturation during loading and unloading is separately given in Figs. 5(a) and (b). The dotted arrows indicate the changes in gas permeability of the samples with similar degree of saturation with respect to the stress cycle, in which the downward arrow indicates the decrease of gas permeability due to applied loading; and the upward arrow indicates the increase due to unloading. In the case of the same stress state and dry density, the gas permeability of sample is reduced by 2–3 orders of magnitude with the increase of degree of saturation. For the initial confining stress of 2 MPa and dry density of approximately 1.7 g/cm3, the gas permeability was decreased by 94.5% when increasing the degree of saturation from 60.81% (B-2) to 84.66% (B-6). The results match that of Gallé (1998) for the gas permeability test of the Fo-Ca clay in France (see Fig. 6). Moreover, under the condition of almost the same dry density, the greater the degree of saturation, the faster the gas permeability decline of the sample during loading (see A-1 & A-6 and B1 & B-6 in Fig. 5(a)). Under the condition of almost the same degree of saturation, the larger the initial dry density, the gentler the gas permeability decrease (see A-6 & B-1). Compared with the decrease in loading process, the recovery of gas permeability during unloading could be negligible (see Fig. 5). Similar to the saturated/nearly-saturated samples, Table 2 shows that the moisture losses during the test are also negligible for the samples with a lower degree of saturation. Unfortunately, we did not get any specific regulations of the moisture loss between the groups.
(6)
2
where A(m ) is the cross-sectional area of sample. 3. Testing results Two samples were tested in each testing group, and the averaged value of three measurements was used for each case to ensure the validity and the representativeness of the test results. The samples have been weighed before and after each test to determine the moisture loss during permeation. The details of the samples before and after the test are summarized in Table 2, and the testing results observed in 12 gas permeability measurements versus confining stress and degree of saturation are plotted in Figs. 4 and 5. Under the condition of almost the same dry density, the gas permeability of samples decreased with the increase of confining stress (see Fig. 4). Compared with the measurement by the initial confining stress Pc = 2MPa, the gas permeability values of sample A-1 (ρd = 1.51g/cm3, Sr = 45.24%) were decreased by 21.4% (at confining stress of 4 MPa) and 49.0% (at confining stress of 6 MPa), respectively. The gas permeability values of sample A-6 with higher water content (ρd = 1.51g/ cm3, Sr = 61.98%) were decreased by 90.6% and 99.8% at confining stresses of 4 and 6 MPa, respectively, during the same process (see Fig. 4(a)). A similar phenomenon was also observed in another group of dry density as shown in Fig. 4(b). The gas permeability values of sample B-1 (ρd = 1.71g/cm3, Sr = 61.80%) were decreased by 14.5% and 46.8% at the confining stresses of 4 and 6 MPa, respectively. Those of sample B-6 (ρd = 1.71g/cm3, Sr = 84.66%) are 49.0% and 92.3% during the same process at confining stresses of 4 and 6 MPa, respectively. Next, the gas permeability of the samples recovered to a relatively slight extent during the decrease of confining stress. The value of gas permeability of sample A-1 recovered to 53.3% (at confining stress of 4 MPa) and 58.5% (at confining stress of 2 MPa) of the initial gas permeability under confining stress of 2 MPa. A similar recovery scenario was also noticed on samples A-6, B-1 and B-6. Note that the gas permeability of all the samples could not recover to their initial value at the end of unloading of the confining stress.
4. Discussion Based on the above testing results, in view of the macroscopic and microscopic gas migration mechanism, the evolutions of gas permeability of unsaturated compacted bentonite with different degrees of saturation and dry densities with respect to the confining stress cycle process are analyzed in this section. 4.1. Influences of degree of saturation on gas permeability The testing results of the compacted bentonite samples of two dry densities illustrated in Fig. 5 show that the degree of saturation has a significant effect on the gas permeability of the compacted bentonite. In case of the same stress state and dry density, the gas permeability of sample is markedly reduced by 2–3 orders of magnitude with increase of degree of saturation. It was evidenced (Ye et al., 2009) that there are 58
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Fig. 4. Variations of the gas permeability during loading and unloading of confining stress: (a) ρd ≈ 1.5g/cm3 and (b) ρd ≈ 1.7g/cm3.
pass. Therefore, it is indicative that gas flow in the low/medially saturated samples (Sr < 93%, according to Graham et al., 2002) is dominated by the single-phase advection in unsaturated zone (see Fig. 7). It is also reasonable to interpret the decreasing gas permeability with increasing degree of saturation as the decline of the connected pores in the unsaturated zone which was caused by pore water filling. On the other hand, due to the swelling potential of the clay mineral during hydration, the initial connected pores in the unsaturated zone will be reduced when the sample is prepared under confined conditions (Ye et al., 2009). This reduction of the initially connected pores caused by the swelling properties of bentonite during hydration is also considered to be an important indicator accounting for of the above phenomena. Comparing the testing results of the two groups of samples with different dry densities, it is obvious that the gas permeability of samples
still a large number of macro- and micro-connected pores between the particles of the unsaturated compacted bentonite. In the process of sample compaction, the pores are filled with water unevenly, and different proportions of saturated and unsaturated pores will be formed, depending on the degree of saturation of the samples. As the degree of saturation of the sample before and after the test does not change significantly, it is likely that the gas flow in the sample may be merely a single-phase advection in the non-saturated region or possible preferential pathway flow (Cuss et al., 2014) caused by local gas accumulation. Similar behavior is also reported in the saturated/nearly-saturated samples (Horseman et al., 1999; Harrington and Horseman, 2003; Ye et al., 2014; Graham et al., 2016). According to Xu et al. (2015), the condition of “gas induced-dilatancy” effect is in fact that the gas injection pressure P1 must be greater than the threshold Pd, and the observed “gas induced-dilatancy” effect allows very limited airflow to 59
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Fig. 5. Variations of gas permeability with saturation degree during (a) confining stress loading and (b) confining stress unloading.
triaxial deformation of the sample, the following can be inferred: the samples will become more compacted under the rising confining stress during the loading process, and the connected pores in the unsaturated zone are also greatly reduced by the compressive deformation of samples. A similar phenomenon has also been reported in the investigation of compacted bentonite-sand mixture (Liu et al., 2015). Fig. 5 shows that under the condition of almost the same dry density, the greater the degree of saturation, the faster the gas permeability decline of the sample during loading. Under the condition of almost the same degree of saturation, the larger the initial dry density, the gentler the gas permeability decrease. The variations may be caused by the saturation and dry density, which affects the deformation capability of the samples. However, due to the limited experimental data available, the specific mechanism with respect to the deformation is not fully understood in this work.
with higher dry densities (B-1 and B-2) is much smaller than that of samples with lower dry densities (A-5 and A-6) under the initial confining stress of 2 MPa. A higher dry density corresponds to a smaller initial volume fraction of connected pores in the unsaturated zone, and thus a lower gas permeability.
4.2. Influences of stress cycle on gas permeability 4.2.1. Influences of stress increment The stress path used in the tests is illustrated in Fig. 3. Figs. 4(a) and (b) show that the gas permeability of compacted bentonite blocks is closely related to the stress state and the stress path. The gas permeability of the sample with a degree of saturation of 61.98% (A-6) is decreased by a maximum of 99.8% with the increment of confining stress. Considering that the equipment adopted in this paper allows for 60
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Fig. 6. Variations of gas permeability with dry density and the degree of saturation for Fo-Ca clay (Gallé, 1998) and bentonite samples in this paper (CV: Constant Volume, CP-2 MPa: Confining stress of 2 MPa).
5. Conclusions The gas permeability of highly compacted bentonite with different dry densities and degrees of saturation was studied using the quasistationary method during a confining stress cycle. The gas migration mechanism in compacted bentonite blocks was analyzed when the degree of saturation of buffer/backfill material was relatively low during the construction period and in the early stage of operation of the repository. The main conclusions are drawn as follows: (1) The gas permeability of the sample under confining stress of 2 MPa was decreased by 2–3 orders of magnitude with the increase in degree of saturation. The gas migration in the low/medially saturated compacted bentonite sample (Sr < 93%) was considered to be dominated by single-phase advection in the unsaturated zone. Due to pore water filling and the swelling potential of bentonite during hydration, an increase in the degree of saturation led to a decline in connected pores in the unsaturated zone, which caused a decrease in the gas permeability subsequently. (2) The gas permeability of sample decreases significantly with the increase of confining stress. Under the condition of almost the same dry density, the greater the degree of saturation, the steeper the decline of the gas permeability of samples. Under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. Under the condition of allowing free deformation of sample, the connected pores in the unsaturated zone are significantly reduced via the compressive deformation of the sample, which in turn increases the resistance to gas flow. (3) Unlike the significant reduction in the gas permeability of sample caused by stress increase, the recovery caused by stress decrease was relatively smaller. The effect of stress increase on the gas permeability of sample was basically irreversible. However, there was no specific relationship between this recovery and the degree of saturation of the sample. (4) For the engineering stability purpose, it is argued that the effect of the degree of saturation and the stress path on the gas permeability of the buffered/backfill material cannot be ignored during the construction period and in the early stage of operation of the repository.
Fig. 7. Schematic diagram of gas permeation in the low/medially saturated samples.
4.2.2. Influences of stress drop As mentioned above, the low/medially saturated samples still contain a large proportion of unsaturated zones after compaction, and the connected pores serve as natural preferential pathway allowing gas to flow in a single-phase advection. The increases in both degree of saturation and confining stress cause a significant decrease in the gas permeability. However, it is indicative from Figs. 4 and 5 that a slight permeability recovery was achieved when the confining stress decreased. Therefore, it can be inferred that the decrease in gas permeability caused by confining stress increase is basically irreversible. Unfortunately, the mechanism of this recovery and the relationship associated with the degree of saturation are still unclear, which requires further investigations.
Unfortunately, due to the limited capacities of the experimental facilities used in the context, no quantitative analyses of volumetric 61
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