Swelling-Pressure and Hydraulic Conductivity of Compacted Clays Focusing on the Clay-Mineral Type

Abstract

It is very important to notice clay minerals, for resource development, stability evaluation of underground utilization, and others. However, few quantitative data on the effects of clay-mineral type on the swelling characteristics and permeability of clays and clay minerals have been reported to date. This study was conducted to investigate the effects of clay-mineral type on the swelling characteristics and permeability of compacted clays using one-dimensional swelling-pressure and constant-pressure permeability tests. Comparative tests revealed that the difference of clay-mineral type in the clays influences the swelling-pressure and hydraulic conductivity. The swelling-pressure and hydraulic conductivity were closely associated with the specific surface area of clays. Furthermore, hydraulic conductivity was almost consistent that measured with the Kozeny-Carman equation. This result suggests that hydraulic conductivity can be estimated based on a specific surface area and void ratio of compacted clays.

 

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 70 (2021) 272–278. The caption of Fig. 2, and Figs. 2 and 4 are slightly changed.

1. Introduction

Clay materials composed mainly of clay minerals are used in various industrial applications owing to their unique properties (plasticity, expansion, swelling, ion exchange, dispersion, flocculation, and adsorption).¹⁾ Geomaterials, including clay minerals, are weak materials, which undergo large amounts of mechanical and chemical degradation in texture because of clay mineral content. Therefore, clay minerals (especially smectite) are considered a factor that causes landslides due to heaving and collapse during construction work. Engineered barriers and backfill for the geological disposal of radioactive waste require ultra-long-term stabilization, and the use of compacted sodium-type bentonite for this purpose has attracted considerable attention. When it comes to the geotechnical field, many researchers/engineers have studied the mechanical properties of montmorillonite-bearing materials^2–6) and swelling characteristics and permeability of montmorillonite.^7–14) Montmorillonite is a type of smectite called swelling clay mineral.

Alteration by volcanic hydrothermal systems and the formation of crushing and weathering zones over geologic time have occurred widely throughout the Japanese archipelago. As a result, a wide variety of clay minerals can be found in the Earth’s crust. In general, the clay layer is a weak layer with low permeability. In rock masses with macro-fractures filled with clay minerals, the clay layer significantly affects the mechanical and hydraulic behavior of the rock mass. Thus, in natural resources (geothermal energy, petroleum gas, shale gas, methane hydrate) mining, further to discontinuity, we need to be careful of the presence of clay minerals. Furthermore, it is generally considered that the physical and mechanical properties, including the permeability of rock materials intercalating clay minerals, vary significantly based on clay minerals, as evidenced by their structures and chemical compositions. Therefore, knowing the clay mineral type is crucial to evaluate the mechanical properties of clay-mineral-bearing geomaterials. As mentioned above, in natural resource mining, the necessity of understanding the permeability of rock mass, including macro-fractures filled with clay minerals, is increasing. Many studies have been conducted on the permeability of macro-fractured rocks filled with clay minerals.^15–18) However, in these studies, little is known about the type of filled clay minerals. Although there is an example that examined the swelling characteristics and permeability of kaolinite and other clay minerals,^19,20) few studies have investigated the swelling characteristics and permeability of various clay minerals under the same conditions.

This study clarified the swelling characteristics and permeability of compacted clays based on the clay mineral type, using a constant-pressure permeability test. Further, the hydraulic conductivity of compacted clays was estimated using the Kozeny-Carman equation.^21,22) In this equation, the specific surface area and void ratio of compacted clays were employed. It is considered particularly important for evaluating the swelling characteristics and water permeability of clay samples mainly composed of various clay minerals, considering various rock engineering projects for underground utilization, such as geological disposal, LPG stockpiling, and CCS.

2. Experimental Procedure

2.1 Clay samples

One-dimensional swelling pressure and constant-pressure permeability tests with compacted clays containing one clay mineral were carried out to clarify the influence of clay mineral type on swelling pressure and hydraulic conductivity. The constant-pressure test is widely used in the evaluation of compacted bentonite-based materials, and it also has the advantage that the experiment time is not long. The schematic crystal structure of the layered silicates is shown in Fig. 1. Layer silicates, such as clay minerals, mostly consist of tetrahedral silicate sheets and octahedral hydroxide sheets. By stacking these sheets continuously, clay minerals can be classified as a 1:1 or 2:1 layer structure. Furthermore, these are subdivided by the differences in composition caused by cation isomorphic substitution. Eight types of clays were used as clay samples for this study, and the type/structure of clay minerals shown in Fig. 1 could be covered, except for serpentine. The clay samples were composed mainly of kaolinite, 10 Å halloysite, pyrophyllite, talc, mica, chlorite, smectite, and vermiculite. These samples were commercially available clay powder samples except for 10 Å halloysite (collected primarily from the surface of the Earth in a field at Tottori, Japan). The particle density and specific surface area (Shimadzu Micromeritics Gemini 2375, BET method, N₂ adsorption, 150–200°C/h) of clay mineral powder samples are shown in Table 1, and X-ray diffraction patterns (Rigaku Ultima IV diffractometer, CuKα, 40 kV, 20 mA, 0.15 mm receiving slit, 0.5° divergence slit, 2° scattering slit) of the clay mineral powder samples are shown in Fig. 2. The specific surface area was highest for the smectite (swelling clay minerals) powder sample, followed by the 10 Å halloysite (swelling clay minerals) and kaolinite powder samples. The specific surface areas of the other clay samples were one digit.

Fig. 1

Schematic crystal structure of layer silicates (based on Shirozu²³⁾).

Table 1 Particle density (ρ_s) and specific surface area (S_m) of clay mineral powder samples.

Fig. 2

Unoriented X-ray powder diffraction (XRD) patterns of kaolinite (A), 10 Å halloysite (B), pyrophyllite (C), talc (D), mica (E), chlorite (F), smectite (G), and vermiculite (H) powders.

2.2 One-dimensional swelling pressure and constant-pressure permeability tests

The specimens used for the one-dimensional swelling pressure and constant-pressure permeability tests had a diameter of 50 mm and a height of 10 mm. Each dried clay mineral was statically compacted to a dry density of 1.5 g/cm³. The compacted specimens were produced using an apparatus composed of a cylindrical mold, pistons, and an oil pressure jack. The tests were performed at 22 ± 1°C in a temperature-controlled room using equipment with sufficient rigidity (Fig. 3).

Fig. 3

Schematic diagram of the experimental configuration for the one-dimensional swelling pressure and the constant-pressure permeability tests.

In the one-dimensional swelling pressure test (Fig. 3, left), the clay specimen was immersed in distilled water with the volume change constrained. Further, the load F generated in the vertical direction at the time was measured at 1-second intervals. The axial displacement of the specimen was measured using a displacement transducer. The maximum values of the axial displacement of the specimen were 0.01 mm (smectite sample), 0.002 mm (kaolinite and 10 Å halloysite samples), and below the detection limit (other samples). Therefore, it is considered that stress and volume change of the order that cannot be ignored when evaluating the swelling pressure did not occur. The swelling pressure P_s was calculated using eq. (1).   

\begin{equation} P_{\text{s}} = \frac{F}{A} \end{equation} (1)

where A is the cross-sectional area of the specimen. The test was conducted for at least 7 days. After the test was completed, the stainless steel upper base plate was set on the specimen. The test specimen was immersed for 2 days using a water-filled decompression container and further immersed in atmospheric pressure for 2 days. Then, a constant-pressure permeability test was conducted.

The constant-pressure permeability test (Fig. 3, right) was carried out by passing water (distilled water) through the specimen at a constant water pressure using a compressor and pressure water tank. The hydraulic conductivity k of the specimen can be calculated using Darcy’s law:   

\begin{equation} k = \frac{QL}{hAt} \end{equation} (2)

where Q is the runoff volume, L is the height of the specimen, h is the difference in water level, A is the cross-sectional area of the specimen, and t is the measurement time. The runoff volume was measured at intervals of 60 s, and the data were transmitted from an analytical balance (Shimadzu AUX120, with an accuracy of 0.1 mg) to a personal computer. Measurements were taken ≥24 h after the first confirmation of water runoff. h was calculated based on the pressure in the pressure water tank (at water pressure 0.1 MPa, h is 1020.4 cm). The water pressure was kept lower in the constant-pressure permeability test than in the swelling pressure test (Table 2) to avoid deformation and form of the water path of the specimen, which greatly affects the hydraulic conductivity. Further, it was confirmed that there was no enormous difference in the hydraulic conductivity owing to the difference in the water pressure. The runoff volume measured by the analytical balance decreases momentarily due to evaporation; however, this decrease was added to the runoff volume at the stage of data organization, and the evaporation volume was also taken into consideration. The constant-pressure permeability device was the same as that previously reported,²⁰⁾ and it has been confirmed that the hydraulic conductivity (5.3 × 10⁻¹³ m/s)²⁰⁾ of bentonite obtained using this device is the same as the hydraulic conductivity (4.8 × 10⁻¹³ m/s)²⁵⁾ of the same sample and under the same conditions.

Table 2 Hydraulic conductivities (k, k_KC) of clay mineral powder samples.

3. Results and Discussions

3.1 Swelling pressure of compacted clays

The results of the one-dimensional swelling pressure tests of compacted clays are shown in Fig. 4. Except for the smectite sample, the equilibrium swelling pressure was reached approximately 2 days after the start of the test for all clay samples. The swelling pressure was highest for the smectite sample (P_s = 1.10 MPa), followed by the 10 Å halloysite (P_s = 0.63 MPa). The swelling pressure of bentonite, mainly composed of smectite at an effective clay density of 1.5 g/cm³, is in the range of approximately 0.4 to 1.2 MPa,²⁶⁾ and the smectite sample treated in this study is also of this category. The swelling pressure of mica, pyrophyllite, talc, and chlorite samples is exceedingly small at 0.03 MPa or less. Therefore, it was found that the magnitude of the swelling pressure of the clay sample differed depending on the clay mineral type. Compared with the swelling pressure of clay samples (10 Å halloysite: P_s = 0.24 MPa, kaolinite: P_s = 0.17 MPa, mica: P_s = 0.02 MPa, talc: P_s = 0.01 MPa, and chlorite: P_s < 0.002 MPa)²⁰⁾ at a compaction dry density of 1.4 g/cm³, the swelling pressure of all samples is large except for chlorite samples. The swelling pressure tends to increase as the compaction dry density increases. As seen from the schematic crystal structure (Fig. 1) of the clay minerals, smectite and 10 Å halloysite samples (swelling clay minerals) absorb a large amount of water between interlayers when immersed in water, which increased the distance between the unit layers and the swelling pressure of both samples. This phenomenon is also observed in several previous studies. However, the vermiculite sample, which is a swelling clay mineral, does not swell at room temperature,¹⁾ it is considered that it did not show a large swelling pressure. Other clay samples, which are non-swelling clay minerals, also showed little swelling pressure, which does not mean that water is absorbed between the unit layers and swells. It can be considered the apparent swelling pressure, which can be attributed to the generation of pore water pressure due to water immersion in the particle gaps. However, the large swelling pressure of the kaolinite sample, a non-swelling clay mineral, can be attributed to the specific surface area (Table 1). In other words, the specific surface area of the kaolinite sample was larger than that of the clay samples, which are other non-swelling clay minerals. Therefore, it is considered that as the adsorbed water due to surface tension increases, the pore water pressure acting on the particle gap increases accordingly, and as a result, the swelling pressure becomes larger than that of other clay samples. The difference in the swelling pressure of clay samples, which are non-swelling clay minerals, needs to be clarified quantitatively in the future, considering the measurement results of pore water pressure.

Fig. 4

Swelling pressure of clay mineral powder sample (dry density = 1.5 g/cm³).

3.2 Hydraulic conductivity of compacted clays

The results of the constant-pressure permeability tests are shown in the second row of Table 2. The hydraulic conductivity shown here is the average of the experimental values of three to nine specimens. Further, the maximum and minimum values are also shown. It is considered that there is no large variation in any of the experimental values.

The average hydraulic conductivity of the clay samples ranged from 10⁻⁸ to 10⁻¹³ m/s. Hydraulic conductivity was lowest for the smectite sample (k = 1.9 × 10⁻¹³ m/s), followed by 10 Å halloysite and kaolinite (k = 10⁻¹¹ m/s). The hydraulic conductivity of the other samples was between 10⁻⁸ and 10⁻⁹ m/s. Therefore, like the results of the swelling pressure test, the hydraulic conductivity of compacted clays differed depending on the clay mineral type. Smectite is a swelling clay mineral containing water and cations between layers. However, a significantly larger electric double layer¹⁾ is formed compared with that of other clay minerals. It is believed that the formation of an immobile water film by the electric double layer and physicochemical action^27,28) by the swelling clay mineral contributed strongly to the hydraulic conductivity of the smectite sample. Similar to the swelling pressure test results, the low hydraulic conductivities of 10 Å halloysite and kaolinite samples are attributed to the size of the specific surface area. As mentioned above, when the specific surface area is large, the swelling pressure increases. However, it is considered that the permeability decreases because the gaps are involved in the permeability decrease. Furthermore, when the specific surface area is large, the amount of adsorbed water due to surface tension increases. However, this adsorbed water is highly viscous,¹⁾ which is a factor that reduces the hydraulic conductivity. In the next section, an attempt was made to estimate the hydraulic conductivity using the specific surface area. Like the swelling pressure test results, the hydraulic conductivities of clay samples of this study are about one order smaller than hydraulic conductivities of clay samples (compacted dry density = 1.4 g/cm³; 10 Å halloysite: k = 10⁻⁹ m/s, kaolinite: k = 10⁻¹⁰ m/s, mica: k = 10⁻⁸ m/s, talc: k = 10⁻⁸ m/s, chlorite: k = 10⁻⁸ m/s).²⁰⁾ The effect of compaction density was seen.

The relationship between the swelling pressure and hydraulic conductivity of clay mineral samples is shown in Fig. 5. The hydraulic conductivity of the clay mineral materials decreased with increasing swelling pressure. Further, the analysis revealed a strong correlation (correlation coefficient: 0.99).

Fig. 5

Relationship between swelling pressure and hydraulic conductivity of clay mineral powder sample (dry density = 1.5 g/cm³).

From this result, it can be said that the clay sample with a larger swelling pressure tends to exhibit the ability to fill the voids, and the hydraulic conductivity tends to be smaller. Furthermore, it is suggested that the hydraulic conductivity can be estimated only from the swelling pressure without performing the permeability test, or the swelling pressure can be estimated from the hydraulic conductivity without performing the swelling pressure test. The above experimental results indicate that when evaluating the permeability of rock mass, including clay minerals, the permeability may differ depending on the type of clay minerals included. In other words, it is important to focus on the types of clay minerals. Further, when using an ultra-low-permeability material such as smectite, for example, when smectite in the material changes to other clay minerals due to alterations, it causes an increase in the hydraulic conductivity and changes the physical properties of the material.

3.3 Estimate of hydraulic conductivity by Kozeny–Carman equation

As seen in Table 1, the specific surface area and void ratio differ due to differences in clay mineral type. Among the models representing the hydraulic conductivity of porous media, the Kozeny-Carman equation,^21,22) shown in eq. (3), is used as it uses the specific surface area. As this equation incorporates macroparameters (void ratio) and microparameters (specific surface area), it is often used to evaluate the hydraulic conductivity of low-permeability materials such as bentonite (Ren et al.;²⁹⁾ Kobayashi et al.³⁰⁾).   

\begin{equation} k_{\text{KC}} = C\frac{\rho_{\text{w}}g}{\eta}\cdot \frac{1}{S_{\text{m}}{}^{2}\rho_{\text{s}}{}^{2}} \cdot \frac{e^{3}}{1 + e} \end{equation} (3)

where k_KC (m/s) is the hydraulic conductivity, C is a dimensionless shape constant with an approximate value of 0.2,²²⁾ ρ_w (= 997.770 kg/m³) is the density of water at 22°C, g is the gravitational acceleration (= 9.8 m/s²), η (= 0.000958 Pa·s) is the coefficient of viscosity at 22°C, S_m (m²/kg) is the specific surface area per unit mass of particles, ρ_s (kg/m³) is the particle density, and e (ρ_s/ρ_d − 1, ρ_d = 1,500 kg/m³) is the void ratio. In this study, the hydraulic conductivity of compacted clays was estimated using the Kozeny-Carman equation.^21,22) The equation uses the specific surface area and void ratio of compacted clays. The obtained results were compared with the experimental values.

The hydraulic conductivity k_KC by the Kozeny-Carman equation is shown in the fourth row of Table 2. Figure 6 shows the relationship between the hydraulic conductivity k and k_KC. For all clay samples except for the smectite sample, the hydraulic conductivity k obtained from the experiment was larger than the hydraulic conductivity k_KC by the Kozeny-Carman equation. The specific surface areas used in eq. (3) may vary depending on the pretreatment time and temperature of the sample during measurement.¹⁾ Therefore, there may be a difference between k and k_KC. However, the difference between k and k_KC was approximately one order of magnitude for all the clay samples. The Kozeny-Carman equation may be an effective method for evaluating the permeability of compacted clay samples. However, when estimating the hydraulic conductivity using the specific surface area and pore ratio, there are slight differences in the obtained values.

Fig. 6

Comparison between hydraulic conductivities (k, k_KC).

4. Conclusions

To clarify the swelling characteristics and permeability of compacted clay samples, mainly composed of various clay minerals, were subjected to one-dimensional swelling pressure and constant-pressure permeability tests. These experiments produced the following results.

1. (1)    It was found that the swelling pressure of the clay sample differed depending on the clay mineral type. In particular, smectite and 10 Å halloysite samples (swelling clay minerals) absorb a large amount of water between the interlayers when immersed in water, which increased the distance between the unit layers and the swelling pressure of both samples. The low swelling pressure of non-swelling clay minerals does not mean that water is absorbed between the unit layers and swells; however, it can be considered that it is the apparent swelling pressure mainly due to the generation of pore water pressure due to water immersion in the particle gaps.
2. (2)    The hydraulic conductivity of compacted clays differed depending on the clay mineral type. The hydraulic conductivity of compacted clays is closely related to the specific surface area of the clay samples. Furthermore, the hydraulic conductivity of the clay mineral materials decreased with increasing swelling pressure. The analysis revealed a very strong correlation.
3. (3)    The hydraulic conductivity, k_KC, was calculated according to the Kozeny-Carman equation. There were no significant differences between the experimental hydraulic conductivity and k_KC. The Kozeny-Carman equation provides an efficient means to evaluate the permeability of compacted clay samples.

A further study of the relationship between specific surface area and adsorbed water and the relationship between pore pressure associated with adsorbed water and swelling pressure should be conducted. We will reveal an entire picture of this study in the future. Based on these results, it is considered that clay mineral type is one of the factors that greatly affect the properties of clay materials, and this study contributes to the evaluation of the properties of rock materials and others, including clay minerals.

Acknowledgments

This work was partly supported by Grants-in-Aid for Scientific Research ‘KAKENHI’ (grant number 19K15489) of the Japanese Society for the Promotion of Sciences (JSPS). The support is gratefully acknowledged. The authors thank Dr. Toshiyuki Tanaka from the Tottori Institute of Industrial Technology and Mr. Masaki Asano and Mr. Ryohei Hano from the Tottori University for their assistance in the analysis of the specific surface areas of clay mineral powder samples.

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