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Simplified evaluation on hydraulic conductivities of sand–bentonite mixture backfill
Applied Clay Science 26 (2004) 13 – 19 www.elsevier.com/locate/clay
Simplified evaluation on hydraulic conductivities of sand–bentonite mixture backfill Hideo Komine * Department of Urban and Civil Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi, Ibaraki, 316-8511, Japan Received 7 March 2003; received in revised form 29 August 2003; accepted 2 September 2003 Available online 28 February 2004
Abstract Sand – bentonite mixtures are planned for use as backfill materials for high-level nuclear waste disposal in Japanese project. Sand – bentonite mixtures are attracting greater attention as backfill materials because they offer properties of very low permeability and high swelling. We must investigate the hydraulic properties by experiments and evaluate quantitatively the hydraulic-conductivities of sand – bentonite mixtures to design specifications, such as dry density and bentonite content, of backfill materials. For that purpose, this study investigated hydraulic conductivities at different bentonite contents and dry densities by experimentation. In addition, we discussed the relationship between hydraulic conductivity and bentonite content from the viewpoint of bentonite swelling in backfill voids. Furthermore, this study proposed simplified evaluation for hydraulic conductivity using a parameter proposed by the author: swelling volumetric strain of montmorillonite. This evaluation method can obtain hydraulic conductivity of backfill materials at various dry densities and bentonite contents. Therefore, this evaluation method can be used for designing bentonite content and compaction density to achieve very low permeability. D 2003 Elsevier B.V. All rights reserved. Keywords: Bentonite; Hydraulic conductivity; Simplified evaluation; Montmorillonite; Radioactive waste disposal; Barriers
1. Introduction Design and development of buffer and backfill materials to fill disposal facilities are important for developing technology for high-level nuclear wastes disposal (see Fig. 1). Compacted bentonite and sand –bentonite mixtures are attracting greater attention as buffer and backfill materials because they
* Fax: +81-294-84-2941. E-mail address: [email protected] (H. Komine). 0169-1317/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2003.09.006
offer impermeability and swelling properties. We must investigate hydraulic properties by experimentation and quantitatively evaluate hydraulic-conductivities of compacted bentonite and sand – bentonite mixtures to design specifications such as dry density and bentonite content of buffer and backfill materials. Toward that goal, this study investigated hydraulic conductivities of different bentonite contents and dry densities through experimentation. Furthermore, this study proposes a simplified evaluation method for hydraulic conductivity using a parameter pro-
14
H. Komine / Applied Clay Science 26 (2004) 13–19
Fig. 1. An example of disposal facility and pit for high-level radioactive wastes in Japan (Vertical emplacement type, after Ogata et al., 1999).
posed by the author: swelling volumetric strain of montmorillonite.
2. Test procedure 2.1. Samples This study used commercial bentonite, KunigelV1, produced at the Tsukinuno Mine in Japan. Table 1 Typical profile of the Japanese bentonite, Kunigel-V1 Type
Sodium bentonite
Particle density (Mg/m3) Liquid limit (%) Plastic limit (%) Plasticity index Activity Plastic ratio Clay ( < 2 Am) content (%) Montmorillonite content (%) Cation exchange capacity (meq./g) Exchange capacity of Na+ (meq./g) Exchange capacity of Ca2 + (meq./g) Exchange capacity of K+ (meq./g) Exchange capacity of Mg2 + (meq./g)
2.79 473.9 26.61 447.3 6.93 16.81 64.5 48 0.732 0.405 0.287 0.009 0.030
The montmorillonite content is calculated at the ratio of the methylene blue absorption values of bentonite used and montmorillonite (White and Michael, 1979). Exchange capacities of Na+, Ca2 +, K+, and Mg2 + are measured values of eduction by a onenormal CH3COONH4 solution. The cation exchange capacity is the sum of the above values.
Table 1 shows fundamental properties of that bentonite (Komine and Ogata, 1994, 1999; Komine, 2001). This is a sodium-type bentonite containing nearly 48% montmorillonite; it is frequently used in studies of materials for artificial barriers against nuclear waste in Japan. Bentonite materials were kept at a constant temperature (22 F 1 jC) and almost constant humidity (70 – 80%). Water content of this material was in the range of 6.5– 10.0%. In addition, this study used Mikawa silicate sand no. 6. Sand particle density was 2.66 Mg/m3 and particle diameter was 0.053 – 0.590 mm. For backfill materials, this study used mixtures with bentonite contents of 5%, 10%, 20%, 30% and 50%. 2.2. Measurement of hydraulic conductivity of backfill specimens Backfill specimens were produced by tamping mixtures with bentonite contents of 5%, 10%, 20%, 30% and 50%. The mixtures’ water contents were adjusted to the optimum water content of respective samples that are shown in Table 2. Results shown in Table 2 suggest that a bentonite content of 30% provides the highest maximum dry density and lowest optimum water content. These results indicate that the backfill material for bentonite content of 5 – 30% offers compaction characteristics of sandy soil. On the other hand, backfill material for bentonite content of 50% shows compaction—characteristics of clayey soil because of its relatively high bentonite content.
H. Komine / Applied Clay Science 26 (2004) 13–19 Table 2 Optimum water content and maximum dry density Bentonite content (%)
Average soilparticle density (Mg/m3)
Maximum dry density (Mg/m3)
Optimum water content (%)
5 10 20 30 50
2.67 2.68 2.69 2.70 2.73
1.61 1.64 1.68 1.72 1.66
19.4 17.6 17.0 14.6 17.5
measure specimens’ vertical swelling pressures, as shown in Fig. 2. We measured the hydraulic-conductivity of backfill-specimen after vertical swelling pressure reached a constant value. In the hydraulic conductivity test, we measured outflow quantities on the condition that the pressure of burette for measuring inflow was 245 kPa and the pressure of burette for outflow was 49 kPa. Hydraulic conductivity was calculated by Eq. (1) with the measured value of outflow: k¼
Fig. 2 shows the test apparatus for measuring hydraulic conductivity of backfill specimens. The backfill specimens were set up on a pedestal as shown in Fig. 2. Specimens were covered with a rubber membrane. Silicone grease was added to the outside of rubber-membrane to reduce degradation of that membrane. All backfill specimens were tamped by free fall of a rammer from 20 mm above the specimens. The rammer mass was 329.9 g; its bottomdiameter was 32.0 mm. The set point of the backfill specimen diameter was 60 mm and that of height was 10 mm. Dry densities of backfill specimens were within the range of 1.43– 1.79 Mg/m3. After setting up the test apparatus, distilled water was supplied to backfill specimens at 9.8 kPa from the bottom of specimen. A load transducer was used to
15
q H0 q g ; ðm=sÞ A w Pin Pout
ð1Þ
where q represents the average outflow quantity at unit time (m3/s), A is the cross-sectional area, Pin is the water-pressure of bottom of specimen, Pout is the waterpressure at the top of the specimen, H0 is the specimen height, qw is the water density (kN s2/m4), and g represents gravitational acceleration ( = 9.80665 m/s2) The water content of each specimen was 100 – 119% after experimentation.
3. Hydraulic conductivity of sand –bentonite mixture backfill Fig. 3 shows relationships between hydraulic conductivity and bentonite content of backfill mate-
Fig. 2. Test apparatus for measuring hydraulic conductivity of backfill materials.
16
H. Komine / Applied Clay Science 26 (2004) 13–19
Fig. 3. Relationship between hydraulic conductivity and bentonite content for backfill materials.
rials. Experimental results show that hydraulic conductivity of the sand – bentonite mixture backfill was in the range of 2.66 10 10 to 4.85 10 12 m/s for 5– 20% bentonite content; it was within 6.87 10 12 to 1.21 10 12 m/s for 30 –50% bentonite content. Experimental results indicate that hydraulic conductivities of sand – bentonite mixture backfill decrease as dry density and bentonite content increase. Reduction of hydraulic conductivity is particularly
marked for backfill with bentonite content of 5– 20%. Komine and Ogata (1999) observed swelling behavior of bentonite, as shown in Fig. 4, using a scanning electron microscope. It can control the temperature and vapor pressure around samples. Observation showed that filling of voids in the backfill material was inadequate in the case of mixtures with bentonite contents of 5% and 10%. For mixtures with 20% bentonite contents or more, the mixture voids were filled up completely by swelling deformations of bentonite that had absorbed water. Observations described above suggest that reduction of hydraulic conductivity is greatest for 5– 20% mixtures because void filling conditions change. Reduction of hydraulic conductivity is less in materials containing more than 20% bentonite because voids are almost entirely filled by swollen bentonite. De Magistris et al. (1998) and Sivapullaiah et al. (2000) reported similar results. Their experimental results are attributable to the mechanism described above.
4. Simplified evaluation using ‘‘swelling volumetric strain of montmorillonite’’ Komine and Ogata (1999) also discussed the swelling processes of backfill material as shown in Fig. 5. The author proposed a new parameter, swelling volumetric strain of montmorillonite, esv* (%), in
Fig. 4. Swelling behavior of bentonite in the mixture (bentonite content 50%).
H. Komine / Applied Clay Science 26 (2004) 13–19
17
Fig. 5. Concept of processes of the swelling behavior of bentonite-backfill material.
Komine and Ogata (1999) to evaluate the relationship between swelling characteristics of sand – bentonite mixtures and swelling behavior of montmorillonite in bentonite. Fig. 5 shows the composition of bentonite-backfill material: voids, montmorillonite minerals, component minerals excluding montmorillonite, and sand. The parameter esv* is the percentage volume increase of swelling deformation of montmorillonite when dry. Eqs. (2) – (4) of the parameter esv* are obtained from the above discussions. Komine and
Ogata (1999) related details of swelling processes and derivations of Eqs. (2) – (4). n o e 100 qm * ¼ e0 þ s max ðe0 þ 1Þ 1 þ 1 esv Cm 100 qnm 100 100 qm 1 þ 100 ð2Þ a Cm qsand e0 ¼
qsolid 1 qd0
ð3Þ
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H. Komine / Applied Clay Science 26 (2004) 13–19
100 100 Cm a qm qsolid ¼
100 100 1 þ Cm 1 qqm þ 100 a 1 Cm
qm qsand
nm
ð4Þ
In those equations: esmax is the maximum swelling strain (%) ( = 0% in this study); e0 is the void ratio; Cm is the montmorillonite content of bentonite (%); qd0 represents dry density (Mg/m3); a indicates the bentonite content (%); qm shows the particle density (Mg/ m3) of montmorillonite; qnm is the particle density (Mg/ m3) of minerals excluding montmorillonite; and qsand is the particle density (Mg/m3) of sand. This study proposes simplified evaluation for hydraulic conductivity using the parameter described above: swelling volumetric strain of montmorillonite, e*sv (%). Fig. 6 shows the relationship between hydraulic conductivity, k, and e*sv for bentonite backfill materials. This figure indicates that hydraulic conductivity is highly correlated with e*sv. Eq. (5) can elucidate the relationship between hydraulic conductivity, k (m/s) and e*sv (%). *Þ1:6245 k ¼ 2:2307 106 ðesv
ð5Þ
This evaluation method, a combination of Eqs. (2) – (5), can indicate the hydraulic conductivity of backfill materials at various dry densities and bentonite contents. Therefore, this evaluation method can be used for designing the bentonite content and compaction density to achieve very low permeability. However, Eq. (5) is an empirical formula based on experimental results obtained under conditions of sodium-type bentonite
and distilled water. This equation is also based on test results with bentonite content a of 5 –50%. Therefore, the evaluation method proposed in this study is applicable to conditions of sodium-type bentonite backfill with bentonite content a of 5 –50% and fresh water.
5. Conclusions This study investigated hydraulic conductivities of different bentonite contents (5 –50%) and dry densities (1.43 –1.79 Mg/m3) by experimentation. Experimental results showed that hydraulic conductivity of backfill was in the range of 2.66 10 10 to 4.85 10 12 m/s for 5 – 20% bentonite contents and was within 6.87 10 12 to 1.21 10 12 m/s for 30 – 50% bentonite contents. Moreover, this study addressed the relationship between hydraulic conductivity and bentonite content from the viewpoint of bentonite swelling in backfill voids. Electron micrographs show that hydraulic conductivity of backfills depends strongly on swelling of montmorillonite in bentonite. On the basis of the considerations suggested by the experimental results, this study proposed the simplified evaluation for hydraulic conductivity using a parameter proposed by the author: swelling volumetric strain of montmorillonite. This evaluation method can yield hydraulic conductivity of backfill materials at various dry densities and bentonite contents. Therefore, this evaluation method is useful for designing bentonite contents and compaction densities to achieve very low permeability. Acknowledgements Experimental results were obtained by the apparatus of the Central Research Institute of Electric Power Industry. The author wishes to thank Dr. Nishi, Dr. Tanaka and Dr. Komada of the Central Research Institute of Electric Power Industry. References
*. Fig. 6. Relationship between k and esv
De Magistris, F.S., Silvestri, F., Vinale, F., 1998. Physical and mechanical properties of compacted silty sand with low bentonite fraction. Canadian Geotechnical Journal 35, 909 – 925.
H. Komine / Applied Clay Science 26 (2004) 13–19 Komine, H., 2001. Evaluation of swelling characteristics of buffer and backfill materials considering the exchangeable-cations compositions of bentonite and its applicability. Proceedings of the 15th International Conference on Soil Mechanics and Geotechnical Engineering, vol. 3, pp. 1981 – 1984. Komine, H., Ogata, N., 1994. Experimental study on swelling characteristics of compacted bentonite. Canadian Geotechnical Journal 31 (4), 478 – 490. Komine, H., Ogata, N., 1999. Experimental study on swelling characteristics of sand – bentonite mixture for nuclear waste disposal. Soils and Foundations 39 (2), 83 – 97. Ogata, N., Kosaki, A., Ueda, H., Asano, H., Takao, H., 1999.
19
Execution techniques for high level radioactive waste disposal: IV. Design and manufacturing procedure of engineered barriers. Journal of Nuclear Fuel Cycle and Environment 5 (2), 103 – 121 (in Japanese with English abstract). Sivapullaiah, P.V., Sridharan, A., Stalin, V.K., 2000. Hydraulic conductivity of bentonite – sand mixtures. Canadian Geotechnical Journal 37, 406 – 413. White, D., Michael, G.P., 1979. A proposed method for the determination of small amounts of smectites in clay mineral mixtures. Proceedings of British Ceramics Society, vol. 28, pp. 137 – 145.
Simplified evaluation on hydraulic conductivities of sand–bentonite mixture backfill Hideo Komine * Department of Urban and Civil Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi, Ibaraki, 316-8511, Japan Received 7 March 2003; received in revised form 29 August 2003; accepted 2 September 2003 Available online 28 February 2004
Abstract Sand – bentonite mixtures are planned for use as backfill materials for high-level nuclear waste disposal in Japanese project. Sand – bentonite mixtures are attracting greater attention as backfill materials because they offer properties of very low permeability and high swelling. We must investigate the hydraulic properties by experiments and evaluate quantitatively the hydraulic-conductivities of sand – bentonite mixtures to design specifications, such as dry density and bentonite content, of backfill materials. For that purpose, this study investigated hydraulic conductivities at different bentonite contents and dry densities by experimentation. In addition, we discussed the relationship between hydraulic conductivity and bentonite content from the viewpoint of bentonite swelling in backfill voids. Furthermore, this study proposed simplified evaluation for hydraulic conductivity using a parameter proposed by the author: swelling volumetric strain of montmorillonite. This evaluation method can obtain hydraulic conductivity of backfill materials at various dry densities and bentonite contents. Therefore, this evaluation method can be used for designing bentonite content and compaction density to achieve very low permeability. D 2003 Elsevier B.V. All rights reserved. Keywords: Bentonite; Hydraulic conductivity; Simplified evaluation; Montmorillonite; Radioactive waste disposal; Barriers
1. Introduction Design and development of buffer and backfill materials to fill disposal facilities are important for developing technology for high-level nuclear wastes disposal (see Fig. 1). Compacted bentonite and sand –bentonite mixtures are attracting greater attention as buffer and backfill materials because they
* Fax: +81-294-84-2941. E-mail address: [email protected] (H. Komine). 0169-1317/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2003.09.006
offer impermeability and swelling properties. We must investigate hydraulic properties by experimentation and quantitatively evaluate hydraulic-conductivities of compacted bentonite and sand – bentonite mixtures to design specifications such as dry density and bentonite content of buffer and backfill materials. Toward that goal, this study investigated hydraulic conductivities of different bentonite contents and dry densities through experimentation. Furthermore, this study proposes a simplified evaluation method for hydraulic conductivity using a parameter pro-
14
H. Komine / Applied Clay Science 26 (2004) 13–19
Fig. 1. An example of disposal facility and pit for high-level radioactive wastes in Japan (Vertical emplacement type, after Ogata et al., 1999).
posed by the author: swelling volumetric strain of montmorillonite.
2. Test procedure 2.1. Samples This study used commercial bentonite, KunigelV1, produced at the Tsukinuno Mine in Japan. Table 1 Typical profile of the Japanese bentonite, Kunigel-V1 Type
Sodium bentonite
Particle density (Mg/m3) Liquid limit (%) Plastic limit (%) Plasticity index Activity Plastic ratio Clay ( < 2 Am) content (%) Montmorillonite content (%) Cation exchange capacity (meq./g) Exchange capacity of Na+ (meq./g) Exchange capacity of Ca2 + (meq./g) Exchange capacity of K+ (meq./g) Exchange capacity of Mg2 + (meq./g)
2.79 473.9 26.61 447.3 6.93 16.81 64.5 48 0.732 0.405 0.287 0.009 0.030
The montmorillonite content is calculated at the ratio of the methylene blue absorption values of bentonite used and montmorillonite (White and Michael, 1979). Exchange capacities of Na+, Ca2 +, K+, and Mg2 + are measured values of eduction by a onenormal CH3COONH4 solution. The cation exchange capacity is the sum of the above values.
Table 1 shows fundamental properties of that bentonite (Komine and Ogata, 1994, 1999; Komine, 2001). This is a sodium-type bentonite containing nearly 48% montmorillonite; it is frequently used in studies of materials for artificial barriers against nuclear waste in Japan. Bentonite materials were kept at a constant temperature (22 F 1 jC) and almost constant humidity (70 – 80%). Water content of this material was in the range of 6.5– 10.0%. In addition, this study used Mikawa silicate sand no. 6. Sand particle density was 2.66 Mg/m3 and particle diameter was 0.053 – 0.590 mm. For backfill materials, this study used mixtures with bentonite contents of 5%, 10%, 20%, 30% and 50%. 2.2. Measurement of hydraulic conductivity of backfill specimens Backfill specimens were produced by tamping mixtures with bentonite contents of 5%, 10%, 20%, 30% and 50%. The mixtures’ water contents were adjusted to the optimum water content of respective samples that are shown in Table 2. Results shown in Table 2 suggest that a bentonite content of 30% provides the highest maximum dry density and lowest optimum water content. These results indicate that the backfill material for bentonite content of 5 – 30% offers compaction characteristics of sandy soil. On the other hand, backfill material for bentonite content of 50% shows compaction—characteristics of clayey soil because of its relatively high bentonite content.
H. Komine / Applied Clay Science 26 (2004) 13–19 Table 2 Optimum water content and maximum dry density Bentonite content (%)
Average soilparticle density (Mg/m3)
Maximum dry density (Mg/m3)
Optimum water content (%)
5 10 20 30 50
2.67 2.68 2.69 2.70 2.73
1.61 1.64 1.68 1.72 1.66
19.4 17.6 17.0 14.6 17.5
measure specimens’ vertical swelling pressures, as shown in Fig. 2. We measured the hydraulic-conductivity of backfill-specimen after vertical swelling pressure reached a constant value. In the hydraulic conductivity test, we measured outflow quantities on the condition that the pressure of burette for measuring inflow was 245 kPa and the pressure of burette for outflow was 49 kPa. Hydraulic conductivity was calculated by Eq. (1) with the measured value of outflow: k¼
Fig. 2 shows the test apparatus for measuring hydraulic conductivity of backfill specimens. The backfill specimens were set up on a pedestal as shown in Fig. 2. Specimens were covered with a rubber membrane. Silicone grease was added to the outside of rubber-membrane to reduce degradation of that membrane. All backfill specimens were tamped by free fall of a rammer from 20 mm above the specimens. The rammer mass was 329.9 g; its bottomdiameter was 32.0 mm. The set point of the backfill specimen diameter was 60 mm and that of height was 10 mm. Dry densities of backfill specimens were within the range of 1.43– 1.79 Mg/m3. After setting up the test apparatus, distilled water was supplied to backfill specimens at 9.8 kPa from the bottom of specimen. A load transducer was used to
15
q H0 q g ; ðm=sÞ A w Pin Pout
ð1Þ
where q represents the average outflow quantity at unit time (m3/s), A is the cross-sectional area, Pin is the water-pressure of bottom of specimen, Pout is the waterpressure at the top of the specimen, H0 is the specimen height, qw is the water density (kN s2/m4), and g represents gravitational acceleration ( = 9.80665 m/s2) The water content of each specimen was 100 – 119% after experimentation.
3. Hydraulic conductivity of sand –bentonite mixture backfill Fig. 3 shows relationships between hydraulic conductivity and bentonite content of backfill mate-
Fig. 2. Test apparatus for measuring hydraulic conductivity of backfill materials.
16
H. Komine / Applied Clay Science 26 (2004) 13–19
Fig. 3. Relationship between hydraulic conductivity and bentonite content for backfill materials.
rials. Experimental results show that hydraulic conductivity of the sand – bentonite mixture backfill was in the range of 2.66 10 10 to 4.85 10 12 m/s for 5– 20% bentonite content; it was within 6.87 10 12 to 1.21 10 12 m/s for 30 –50% bentonite content. Experimental results indicate that hydraulic conductivities of sand – bentonite mixture backfill decrease as dry density and bentonite content increase. Reduction of hydraulic conductivity is particularly
marked for backfill with bentonite content of 5– 20%. Komine and Ogata (1999) observed swelling behavior of bentonite, as shown in Fig. 4, using a scanning electron microscope. It can control the temperature and vapor pressure around samples. Observation showed that filling of voids in the backfill material was inadequate in the case of mixtures with bentonite contents of 5% and 10%. For mixtures with 20% bentonite contents or more, the mixture voids were filled up completely by swelling deformations of bentonite that had absorbed water. Observations described above suggest that reduction of hydraulic conductivity is greatest for 5– 20% mixtures because void filling conditions change. Reduction of hydraulic conductivity is less in materials containing more than 20% bentonite because voids are almost entirely filled by swollen bentonite. De Magistris et al. (1998) and Sivapullaiah et al. (2000) reported similar results. Their experimental results are attributable to the mechanism described above.
4. Simplified evaluation using ‘‘swelling volumetric strain of montmorillonite’’ Komine and Ogata (1999) also discussed the swelling processes of backfill material as shown in Fig. 5. The author proposed a new parameter, swelling volumetric strain of montmorillonite, esv* (%), in
Fig. 4. Swelling behavior of bentonite in the mixture (bentonite content 50%).
H. Komine / Applied Clay Science 26 (2004) 13–19
17
Fig. 5. Concept of processes of the swelling behavior of bentonite-backfill material.
Komine and Ogata (1999) to evaluate the relationship between swelling characteristics of sand – bentonite mixtures and swelling behavior of montmorillonite in bentonite. Fig. 5 shows the composition of bentonite-backfill material: voids, montmorillonite minerals, component minerals excluding montmorillonite, and sand. The parameter esv* is the percentage volume increase of swelling deformation of montmorillonite when dry. Eqs. (2) – (4) of the parameter esv* are obtained from the above discussions. Komine and
Ogata (1999) related details of swelling processes and derivations of Eqs. (2) – (4). n o e 100 qm * ¼ e0 þ s max ðe0 þ 1Þ 1 þ 1 esv Cm 100 qnm 100 100 qm 1 þ 100 ð2Þ a Cm qsand e0 ¼
qsolid 1 qd0
ð3Þ
18
H. Komine / Applied Clay Science 26 (2004) 13–19
100 100 Cm a qm qsolid ¼
100 100 1 þ Cm 1 qqm þ 100 a 1 Cm
qm qsand
nm
ð4Þ
In those equations: esmax is the maximum swelling strain (%) ( = 0% in this study); e0 is the void ratio; Cm is the montmorillonite content of bentonite (%); qd0 represents dry density (Mg/m3); a indicates the bentonite content (%); qm shows the particle density (Mg/ m3) of montmorillonite; qnm is the particle density (Mg/ m3) of minerals excluding montmorillonite; and qsand is the particle density (Mg/m3) of sand. This study proposes simplified evaluation for hydraulic conductivity using the parameter described above: swelling volumetric strain of montmorillonite, e*sv (%). Fig. 6 shows the relationship between hydraulic conductivity, k, and e*sv for bentonite backfill materials. This figure indicates that hydraulic conductivity is highly correlated with e*sv. Eq. (5) can elucidate the relationship between hydraulic conductivity, k (m/s) and e*sv (%). *Þ1:6245 k ¼ 2:2307 106 ðesv
ð5Þ
This evaluation method, a combination of Eqs. (2) – (5), can indicate the hydraulic conductivity of backfill materials at various dry densities and bentonite contents. Therefore, this evaluation method can be used for designing the bentonite content and compaction density to achieve very low permeability. However, Eq. (5) is an empirical formula based on experimental results obtained under conditions of sodium-type bentonite
and distilled water. This equation is also based on test results with bentonite content a of 5 –50%. Therefore, the evaluation method proposed in this study is applicable to conditions of sodium-type bentonite backfill with bentonite content a of 5 –50% and fresh water.
5. Conclusions This study investigated hydraulic conductivities of different bentonite contents (5 –50%) and dry densities (1.43 –1.79 Mg/m3) by experimentation. Experimental results showed that hydraulic conductivity of backfill was in the range of 2.66 10 10 to 4.85 10 12 m/s for 5 – 20% bentonite contents and was within 6.87 10 12 to 1.21 10 12 m/s for 30 – 50% bentonite contents. Moreover, this study addressed the relationship between hydraulic conductivity and bentonite content from the viewpoint of bentonite swelling in backfill voids. Electron micrographs show that hydraulic conductivity of backfills depends strongly on swelling of montmorillonite in bentonite. On the basis of the considerations suggested by the experimental results, this study proposed the simplified evaluation for hydraulic conductivity using a parameter proposed by the author: swelling volumetric strain of montmorillonite. This evaluation method can yield hydraulic conductivity of backfill materials at various dry densities and bentonite contents. Therefore, this evaluation method is useful for designing bentonite contents and compaction densities to achieve very low permeability. Acknowledgements Experimental results were obtained by the apparatus of the Central Research Institute of Electric Power Industry. The author wishes to thank Dr. Nishi, Dr. Tanaka and Dr. Komada of the Central Research Institute of Electric Power Industry. References
*. Fig. 6. Relationship between k and esv
De Magistris, F.S., Silvestri, F., Vinale, F., 1998. Physical and mechanical properties of compacted silty sand with low bentonite fraction. Canadian Geotechnical Journal 35, 909 – 925.
H. Komine / Applied Clay Science 26 (2004) 13–19 Komine, H., 2001. Evaluation of swelling characteristics of buffer and backfill materials considering the exchangeable-cations compositions of bentonite and its applicability. Proceedings of the 15th International Conference on Soil Mechanics and Geotechnical Engineering, vol. 3, pp. 1981 – 1984. Komine, H., Ogata, N., 1994. Experimental study on swelling characteristics of compacted bentonite. Canadian Geotechnical Journal 31 (4), 478 – 490. Komine, H., Ogata, N., 1999. Experimental study on swelling characteristics of sand – bentonite mixture for nuclear waste disposal. Soils and Foundations 39 (2), 83 – 97. Ogata, N., Kosaki, A., Ueda, H., Asano, H., Takao, H., 1999.
19
Execution techniques for high level radioactive waste disposal: IV. Design and manufacturing procedure of engineered barriers. Journal of Nuclear Fuel Cycle and Environment 5 (2), 103 – 121 (in Japanese with English abstract). Sivapullaiah, P.V., Sridharan, A., Stalin, V.K., 2000. Hydraulic conductivity of bentonite – sand mixtures. Canadian Geotechnical Journal 37, 406 – 413. White, D., Michael, G.P., 1979. A proposed method for the determination of small amounts of smectites in clay mineral mixtures. Proceedings of British Ceramics Society, vol. 28, pp. 137 – 145.