- Email: [email protected]
Experimental Evaluation of Geomechanical Behaviour of Bentonite-Sand Mixture for Nuclear Waste Disposal
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 191 (2017) 386 – 393
Symposium of the International Society for Rock Mechanics
Experimental Evaluation of Geomechanical Behaviour of Bentonite-Sand Mixture for Nuclear Waste Disposal L.K. Sharmaa*, Rajesh Singha,b, M. Ahmada, R.K. Umraoa, T.N. Singha a
Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai- 400076, India b Department of Geology, University of Lucknow, Lucknow- 226007, India
Abstract Nuclear waste disposal and spent nuclear fuel are a major concern in most countries. International Atomic Energy Agency (IAEA), the world's central intergovernmental forum for scientific and technical co-operation in the nuclear field, recommended the deep geological disposal for high level radioactive spent fuel. For such disposal, selection of the host rock and the barrier system may be different, but almost all the programs are considering an engineered barrier. A mixture of sand and bentonite is primarily selected as a possible artificial buffer for thermal disintegration that surrounds and protects the individual nuclear waste canisters. In this study, the behavior of geotechnical properties was observed for various bentonite-sand mixing ratios (10-50 %). The geomechanical experiments such as specific gravity, compaction, unconfined compression, direct shear and falling head permeability were executed to define an optimum amount of bentonite and sand proportion for designing and constructing a better nuclear waste disposal facility. The addition of bentonite increases the geotechnical parameters such as maximum dry density, specific gravity, unconfined compressive strength, Young’s modulus and cohesion. However, the addition leads to a decrease in the optimum moisture content, angle of internal friction and the hydraulic conductivity of sand mixture. Optimum performance of the bentonite-sand mixture was observed when the mixture contained equal amounts of sand and bentonite. © Published by Elsevier Ltd. This ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017. Peer-review under responsibility of the organizing committee of EUROCK 2017 Keywords: Bentonite; sand; nuclear waste; geomechanical behaviour; permeability
1. Introduction On the basis of waste’s origin, radioactive nuclear wastes can be roughly classified into two groups as low-level radioactive wastes and high-level radioactive wastes. Low-level radioactive wastes consisting of contaminated * Corresponding author. Tel.: +22-2576-4290. E-mail address: [email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017
doi:10.1016/j.proeng.2017.05.195
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
working wears, reactor water treatment residues, equipment and tools amount to approximately 90 % of the total volume of wastes. Although having a higher share volume, the total radioactivity emitted by such wastes is only 1 %. The maximum share of the radioactivity is emitted from the high-level radioactive wastes which constitute primarily of the spent fuel rods from the nuclear power stations and the waste materials from the fuel processing plants. These high-level nuclear waste contribute 3-4 % to the total nuclear waste generated. Disposal of both the high-level and low-level nuclear waste in geological repositories has gained significant attention amongst the scientific and industrial community across the globe. The schematic of the disposal facility of low level and high level nuclear waste is shown in Fig. 1. A major concern in the disposal activities is the possibility of the interaction between the radionuclides and the nearby environment. Interaction through leaks can be prevented by isolating and sealing the repositories after the completion of disposal [15]. In order to achieve high degree of reliance, the material/s used for the isolation should bear high mechanical stability in coherence with the host rocks. Additionally, the isolation material should be chemically inert to avoid any chemical reactions which could lead to deterioration of the strength. Materials possessing low permeability would ensure minimum transfer of the radionuclides thereby preventing contamination [6, 8, 11, 14]. Countries such as Sweden, Switzerland, Canada, Germany, and France have studied the efficacy of sand and bentonite mixture as a possible backfill and buffer material [9, 12, 14]. Several studies have been performed to optimize the amount of bentonite to be added in the sand-bentonite mixture. Dixon et al. [7] studied the compaction properties of potential clay-sand mixture and found that a mixture of containing equal amounts of sand and clay (i.e. Na bentonite or illite) by mass displayed highest strength. Akgün et al. [8] have assessed the geotechnical performance of bentonite-sand mixture with a bentonite content varying from 15 to 30 % for nuclear waste isolation at potential Akkuyu nuclear waste disposal site, southern Turkey. They recommended that 30 % of bentonite in bentonite-sand mixture is an optimum amount which can be used in the disposal facilities. Thus, the bentonite-sand mixture is a key material for repository system and it governs the overall behavior of whole barrier system. Therefore, a proper and scientific understanding of the geotechnical behavior of the mixture is essential for the assessment of the repository system. In the present study, geotechnical properties like specific gravity, compaction, unconfined compression, direct shear and falling head permeability were investigated for bentonite-sand mixtures possessing 10 to 50 % bentonite by weight to carrying out a series of experiments. The attention was given to obtain optimum amount of bentonite used in bentonite-sand content for nuclear waste disposal. The obtained experimental data were used to develop relations between geotechnical parameters and bentonite content of the bentonite-sand mixtures.
Fig. 1. Schematic of the disposal facilities for (a) low level radioactive waste; (b) high level radioactive waste [10].
387
388
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
2. Materials and Methods Commercially available bentonite clay from Kutch, India was used in the present study. The X-ray diffraction analysis revealed that the used bentonite clay consist of 72.0 % montmorillonite (Fig. 2). The available literatures describe that the bentonite clay containing more than 48 % montmorillonite is suitable as an artificial barrier against nuclear waste [10]. The sand used in this study was a type of river sand and observed to be uniform fine to medium grains. It was found from the X-ray diffraction analysis that the used sand is mainly composed of quartz (99.5 %).The geotechnical properties of bentonite clay and sand are tabulated in Table 1. All the tests in this study were performed on samples of bentonite-sand mixture with bentonite content of 10 %, 20 %, 30 %, 40 % and 50 % by weight. The geotechnical laboratory tests include specific gravity, standard proctor compaction, hydraulic conductivity, unconfined compressive strength and direct shear strength test were performed to optimize the amount of bentonite in bentonite-sand mixture used for sealing purpose in nuclear waste repositories.
Fig. 2. X-ray diffraction patterns of (a) Bentonite; (b) Sand used in mixture of bentonite-sand.
Table 1. Basic physical properties of potential buffer materials Properties
Sodium Bentonite
Sand
Specific gravity
2.77
2.63
Clay (< 2 micron) (%)
81.0
0
Silt (2 micron - 0.06 mm) (%)
18.0
9.0
0.06-0.2 mm (%)
1.0
15.0
0.2-1.0 (%)
0
61.0
1-2 mm (%)
0
13.0
> 2 mm (%)
0
2.0
Liquid limit (%)
399.0
0
Plastic limit (%)
31.0
0
Plasticity index (%)
368.0
0
Sand
389
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
3. Results and Discussion 3.1. Specific gravity The specific gravity of the bentonite-sand mixtures was calculated by water pycnometer as per ASTM D854 [5]. A summary of the specific gravity of various bentonite-sand mixtures is tabulated in Table 2. A best fit regression line for specific gravity corresponding to bentonite content is shown in Fig. 3. A linear regression equation for specific gravity as a function of bentonite content is demonstrated by Eq. 1. Increase in the amount of bentonite leads to a slight increase in the specific gravity. Akgün et al. [8] explained that the increment in the specific gravity of bentonite-sand mixtures are due to mixing of bentonite and because of higher specific gravity of bentonite than the sand. Komine and Ogata [10] have also observed similar type of trend.
SG 0.0012u BC 2.66
(1)
where, SG is the specific gravity and BC is the bentonite content (%) of the bentonite-sand mixture.
Fig. 3. Specific gravity as a function of the amount of bentonite in the bentonite-sand mixtures.
3.2. Standard compaction parameters Bentonite-sand mixture with bentonite contents ranging from 10-50 % by weight were compacted at varied water content as per ASTM D698 [4]. The standard proctor compaction curves were plotted for all the mixtures and the maximum dry unit weight corresponding optimum moisture content were determined (Fig. 4a). As per compaction results, the dry unit weight ranged from 15.57 to 17.32 kN/m3 corresponding to optimum moisture content 19 to 14 % respectively (Table 2). The linear regression equation for maximum dry unit weight (Eq. 2) and polynomial regression equation for optimum moisture content (Eq. 3) were generated by analysing best fit regression lines. Fig. 4b gives best fit regression plots for maximum dry unit weight and optimum moisture content as a function of bentonite content. It depicts that as the bentonite content of the bentonite-sand mixture increases, the maximum dry unit weight increases and the optimum moisture content decreases up to about 30 % of bentonite content. Above 30 % of the bentonite doses, the optimum moisture content remains constant, it may be because of completion of intergranular pores of sand by fine grains of bentonite. Similar type of study was carried out by Akgün et al. [8] and has been reported that the maximum dry unit weight increases with increasing bentonite content whereas, optimum moisture content decreases with the increasing doses of bentonite in bentonite-sand mixtures.
J d max
0.0463u BC 15.05
(2)
390
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
OMC
0.005 u ( BC ) 2 0.43 u BC 23
(3)
where, the maximum dry unit weight (kN/m3), OMC is the optimum moisture content (%) and BC is the bentonite content (%) of bentonite-sand mixture.
Fig. 4. (a) Compaction curves at different proportion of bentonite; (b) Maximum dry unit weight and OMC versus bentonite content.
3.3. Unconfined compressive strength and Young’s modulus The unconfined compressive strength test was performed [2] on a cylindrical bentonite-sand mixture samples (i.e. 38 mm diameter and 76 mm height) by increasing the axial load until the samples fail by either reaching a maximum load or attaining 15 % axial strain. Young’s modulus is measured as the ratio of stress along an axis over the strain along that axis in the range of elastic behaviour. The test results of unconfined compressive strength and Young’s modulus are summarized in Table 2. The best fit regression equations (Eq. 4; Eq. 5) and plots (Fig. 5a; Fig. 5b) depict that unconfined compressive strength and Young’s modulus increases with increasing bentonite content.
UCS 16.24 u BC 42.31
E
0.031u ( BC ) 2 0.68 u BC 9.16
(4) (5)
where, UCS is the unconfined compressive strength (kPa), E is the Young’s modulus (MPa) and BC is the bentonite content (%) of bentonite-sand mixture.
Fig. 5. (a) Unconfined compressive strength (UCS) versus bentonite content; (b) Young’s modulus versus bentonite content.
391
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
3.4. Shear strength parameters Determination of the friction angle and cohesion of bentonite-sand mixture were carried out using consolidated undrained (CU) direct shear test as per ASTM D 6528 [1]. The samples were allowed to consolidate during loading axially. Then, the mixed samples were subjected to shearing at a constant rate of displacement and the resulting shearing force was measured. The test was repeated for three different normal stress levels and corresponding shear stresses were measured. Then, the cohesion and angle of internal friction values of the various mixtures were determined by joining the line of three coordinates in normal and shear stresses. The slope and intercept of the connected line on stress coordinates give measure of the angle of internal friction and cohesion, respectively. The summary of cohesion and angle of internal friction values corresponding to bentonite content of bentonite-sand mixtures are tabulated in Table 2. The polynomial best fit equations (Eq. 6; Eq. 7) and plots (Fig. 6a; Fig. 6b) show that an increase in the bentonite content of the bentonite–sand mixtures led to increased cohesion and decreased angle of internal friction.
C
0.028u ( BC) 2 3.44 u BC 8.62
(6)
I
0.007 u ( BC) 2 0.72 u BC 34.56
(7)
where, C is the cohesion (kPa), of bentonite-sand mixture.
I is the angle of internal friction (degree) and BC is the bentonite content (%)
Fig. 6. (a) Cohesion versus bentonite content; (b) Angle of internal friction versus bentonite content.
3.5. Hydraulic conductivity The hydraulic conductivity of the bentonite-sand mixtures was studied under falling head per-meameter according to ASTM D5856-95 [3]. It was determined using Eq. 8.
k
aL h ln 1 A(t 2 t1 ) h2
(8)
where, k is hydraulic conductivity, and are heads at time and, respectivel, is the cross-sectional area of the of the soil specimen of length and is the area of burette used to measure the head loss. The hydraulic conductivity of the bentonite-sand mixture is given in Table 2. The best fit exponential equation was obtain by simple regression plot between bentonite content and hydraulic conductivity (Eq. 9) (Fig. 7). The result indicates that the hydraulic conductivity of bentonite-sand mixture decreases with increasing the bentonite content. Akgün et al. [8] observed similar type of behaviour. Gray et al. [13] have also suggested that the compacted clay-
392
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
sand mixture may possess lower hydraulic conductivity than the compacted clay. So, the bentonite-sand mixture should be considered as the potential buffer material used for the isolation of nuclear wastes.
k
4 u 10 7 u e 0.216 BC
(9)
where, k is the hydraulic conductivity (m/sec) and BC is the bentonite content (%) of bentonite-sand mixture.
Fig. 7. Hydraulic conductivity versus bentonite. Table 2. Summary of the geotechnical properties of bentonite-sand mixtures. Bentonite content (%)
SG
Ȗd (kN/m3)
OMC (%)
UCS (kPa)
E (MPa)
C (kPa)
Ԅ (°)
k(m/sec)
10
2.68
15.57
19
190.4
21.4
26.4
28.1
6.23E-08
20
2.689
15.82
17
315.2
31.6
39.2
23.3
2.54E-09
30
2.705
16.54
14
625.4
56.2
78.5
19.2
6.48E-10
40
2.712
16.95
14
714.1
93.2
81.4
18.1
5.14E-11
50
2.726
17.32
14
803.2
119.2
92.5
17
8.78E-12
4. Conclusion This study is performed to evaluate the geomechanical behaviour of bentonite-sand mixtures for its utilization in underground radioactive waste repositories as isolation material. The geotechnical laboratory tests such as specific gravity, compaction, unconfined compression, direct shear and falling head permeability were performed to select an optimum amount of bentonite and sand for designing and constructing a better nuclear waste disposal facility. The results of the executed tests designate that an increase in specific gravity, maximum dry density, unconfined compressive strength, Young’s modulus, cohesion were observed with increasing proportion bentonite in bentonite-sand mixtures whereas decreasing trends were found for the optimum moisture content, angle of internal friction and hydraulic conductivity with the addition of bentonite in bentonite-sand mixture. Thus, the results of the present study support that the 50 % bentonite in bentonite-sand mixture satisfied the optimum condition for proper isolation of radioactive waste repository. However, the optimum amount is depends on the type of bentonite and sand used in bentonite-sand mixture. References [1] ASTM D 6528, Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Cohesive Soils, Philadelphia, USA, 2000. [2] ASTM D2166, Standard Test Method for Unconfined Compressive Strength of Cohesive Soil, Philadelphia, USA, 2013.
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393 [3] ASTM D5856-95, Standard Test Method for Measurement of Hydraulic Conductivity of Porous Material Using a Rigid-Wall, CompactionMold Permeameter, Philadelphia, USA, 2007. [4] ASTM D698, Standard test methods for laboratory compaction characteristics of soil, Philadelphia, USA, 2012. [5] ASTM D854, Standard test methods for specific gravity of soil solids by water Pycnometer, Philadelphia, USA, 2014. [6] D. Meyer, J.J. Howard, Evaluation of clays and clay minerals for application to repository sealing, ONWI-486, Office of Nuclear Waste Isolation, Battelle Memorial Institute, 1983. [7] D.A. Dixon, M.N. Gray, A.W. Thomas, A study of the compaction properties of potential clay-sand buffer mixtures for use in nuclear fuel waste disposal, Eng. Geol. 21(3-4) (1985) 247-255. [8] H. Akgün, M. Ada, M.K. Koçkar, Performance assessment of a bentonite–sand mixture for nuclear waste isolation at the potential Akkuyu Nuclear Waste Disposal Site, southern Turkey, Environ. Earth Sci. 73(10) (2015) 6101-6116. [9] H. Coulon, A. Lajudie, P. Debrabant, R. Atabek, M. Jorda, R. Andre-Jehan, Choice of French clays as engineered barrier components for waste disposal, Mater. Res. Symp. Proc. 84 (1987) 813-824. [10] H. Komine, N. Ogata, Experimental study on swelling characteristics of sand-bentonite mixture for nuclear waste disposal, Soils Found 39(2) (1999) 83–97. [11] IAEA, Scientific and technical basis for the geological disposal of radioactive wastes, Tech Reps Series No. 413, International Atomic Energy Agency, Vienna, 2003. [12] IAEA, Sealing of underground repositories for radioactive wastes, Tech Reps Series No. 319, International Atomic Energy Agency, Vienna, 1990. [13] M.N. Gray, S.C.H. Cheung, D.A. Dixon, The influence of sand content on swelling pressures and structure developed by statically compacted Na bentonite, Report 7825, Atomic Energy of Canada Limited, Mississauga, Ont, 1984, pp. 1–24. [14] R. Pusch, Waste disposal in rock, Dev. Geotech. Eng. 76 (1994) 490. [15] U.S. Environmental Protection Agency, Environmental radiation protection standards for management and disposal of spent nuclear fuel, high-level and transuranic radioactive wastes, CFR40, Part 191, Washington, DC, 1995.
393
ScienceDirect Procedia Engineering 191 (2017) 386 – 393
Symposium of the International Society for Rock Mechanics
Experimental Evaluation of Geomechanical Behaviour of Bentonite-Sand Mixture for Nuclear Waste Disposal L.K. Sharmaa*, Rajesh Singha,b, M. Ahmada, R.K. Umraoa, T.N. Singha a
Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai- 400076, India b Department of Geology, University of Lucknow, Lucknow- 226007, India
Abstract Nuclear waste disposal and spent nuclear fuel are a major concern in most countries. International Atomic Energy Agency (IAEA), the world's central intergovernmental forum for scientific and technical co-operation in the nuclear field, recommended the deep geological disposal for high level radioactive spent fuel. For such disposal, selection of the host rock and the barrier system may be different, but almost all the programs are considering an engineered barrier. A mixture of sand and bentonite is primarily selected as a possible artificial buffer for thermal disintegration that surrounds and protects the individual nuclear waste canisters. In this study, the behavior of geotechnical properties was observed for various bentonite-sand mixing ratios (10-50 %). The geomechanical experiments such as specific gravity, compaction, unconfined compression, direct shear and falling head permeability were executed to define an optimum amount of bentonite and sand proportion for designing and constructing a better nuclear waste disposal facility. The addition of bentonite increases the geotechnical parameters such as maximum dry density, specific gravity, unconfined compressive strength, Young’s modulus and cohesion. However, the addition leads to a decrease in the optimum moisture content, angle of internal friction and the hydraulic conductivity of sand mixture. Optimum performance of the bentonite-sand mixture was observed when the mixture contained equal amounts of sand and bentonite. © Published by Elsevier Ltd. This ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017. Peer-review under responsibility of the organizing committee of EUROCK 2017 Keywords: Bentonite; sand; nuclear waste; geomechanical behaviour; permeability
1. Introduction On the basis of waste’s origin, radioactive nuclear wastes can be roughly classified into two groups as low-level radioactive wastes and high-level radioactive wastes. Low-level radioactive wastes consisting of contaminated * Corresponding author. Tel.: +22-2576-4290. E-mail address: [email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017
doi:10.1016/j.proeng.2017.05.195
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
working wears, reactor water treatment residues, equipment and tools amount to approximately 90 % of the total volume of wastes. Although having a higher share volume, the total radioactivity emitted by such wastes is only 1 %. The maximum share of the radioactivity is emitted from the high-level radioactive wastes which constitute primarily of the spent fuel rods from the nuclear power stations and the waste materials from the fuel processing plants. These high-level nuclear waste contribute 3-4 % to the total nuclear waste generated. Disposal of both the high-level and low-level nuclear waste in geological repositories has gained significant attention amongst the scientific and industrial community across the globe. The schematic of the disposal facility of low level and high level nuclear waste is shown in Fig. 1. A major concern in the disposal activities is the possibility of the interaction between the radionuclides and the nearby environment. Interaction through leaks can be prevented by isolating and sealing the repositories after the completion of disposal [15]. In order to achieve high degree of reliance, the material/s used for the isolation should bear high mechanical stability in coherence with the host rocks. Additionally, the isolation material should be chemically inert to avoid any chemical reactions which could lead to deterioration of the strength. Materials possessing low permeability would ensure minimum transfer of the radionuclides thereby preventing contamination [6, 8, 11, 14]. Countries such as Sweden, Switzerland, Canada, Germany, and France have studied the efficacy of sand and bentonite mixture as a possible backfill and buffer material [9, 12, 14]. Several studies have been performed to optimize the amount of bentonite to be added in the sand-bentonite mixture. Dixon et al. [7] studied the compaction properties of potential clay-sand mixture and found that a mixture of containing equal amounts of sand and clay (i.e. Na bentonite or illite) by mass displayed highest strength. Akgün et al. [8] have assessed the geotechnical performance of bentonite-sand mixture with a bentonite content varying from 15 to 30 % for nuclear waste isolation at potential Akkuyu nuclear waste disposal site, southern Turkey. They recommended that 30 % of bentonite in bentonite-sand mixture is an optimum amount which can be used in the disposal facilities. Thus, the bentonite-sand mixture is a key material for repository system and it governs the overall behavior of whole barrier system. Therefore, a proper and scientific understanding of the geotechnical behavior of the mixture is essential for the assessment of the repository system. In the present study, geotechnical properties like specific gravity, compaction, unconfined compression, direct shear and falling head permeability were investigated for bentonite-sand mixtures possessing 10 to 50 % bentonite by weight to carrying out a series of experiments. The attention was given to obtain optimum amount of bentonite used in bentonite-sand content for nuclear waste disposal. The obtained experimental data were used to develop relations between geotechnical parameters and bentonite content of the bentonite-sand mixtures.
Fig. 1. Schematic of the disposal facilities for (a) low level radioactive waste; (b) high level radioactive waste [10].
387
388
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
2. Materials and Methods Commercially available bentonite clay from Kutch, India was used in the present study. The X-ray diffraction analysis revealed that the used bentonite clay consist of 72.0 % montmorillonite (Fig. 2). The available literatures describe that the bentonite clay containing more than 48 % montmorillonite is suitable as an artificial barrier against nuclear waste [10]. The sand used in this study was a type of river sand and observed to be uniform fine to medium grains. It was found from the X-ray diffraction analysis that the used sand is mainly composed of quartz (99.5 %).The geotechnical properties of bentonite clay and sand are tabulated in Table 1. All the tests in this study were performed on samples of bentonite-sand mixture with bentonite content of 10 %, 20 %, 30 %, 40 % and 50 % by weight. The geotechnical laboratory tests include specific gravity, standard proctor compaction, hydraulic conductivity, unconfined compressive strength and direct shear strength test were performed to optimize the amount of bentonite in bentonite-sand mixture used for sealing purpose in nuclear waste repositories.
Fig. 2. X-ray diffraction patterns of (a) Bentonite; (b) Sand used in mixture of bentonite-sand.
Table 1. Basic physical properties of potential buffer materials Properties
Sodium Bentonite
Sand
Specific gravity
2.77
2.63
Clay (< 2 micron) (%)
81.0
0
Silt (2 micron - 0.06 mm) (%)
18.0
9.0
0.06-0.2 mm (%)
1.0
15.0
0.2-1.0 (%)
0
61.0
1-2 mm (%)
0
13.0
> 2 mm (%)
0
2.0
Liquid limit (%)
399.0
0
Plastic limit (%)
31.0
0
Plasticity index (%)
368.0
0
Sand
389
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
3. Results and Discussion 3.1. Specific gravity The specific gravity of the bentonite-sand mixtures was calculated by water pycnometer as per ASTM D854 [5]. A summary of the specific gravity of various bentonite-sand mixtures is tabulated in Table 2. A best fit regression line for specific gravity corresponding to bentonite content is shown in Fig. 3. A linear regression equation for specific gravity as a function of bentonite content is demonstrated by Eq. 1. Increase in the amount of bentonite leads to a slight increase in the specific gravity. Akgün et al. [8] explained that the increment in the specific gravity of bentonite-sand mixtures are due to mixing of bentonite and because of higher specific gravity of bentonite than the sand. Komine and Ogata [10] have also observed similar type of trend.
SG 0.0012u BC 2.66
(1)
where, SG is the specific gravity and BC is the bentonite content (%) of the bentonite-sand mixture.
Fig. 3. Specific gravity as a function of the amount of bentonite in the bentonite-sand mixtures.
3.2. Standard compaction parameters Bentonite-sand mixture with bentonite contents ranging from 10-50 % by weight were compacted at varied water content as per ASTM D698 [4]. The standard proctor compaction curves were plotted for all the mixtures and the maximum dry unit weight corresponding optimum moisture content were determined (Fig. 4a). As per compaction results, the dry unit weight ranged from 15.57 to 17.32 kN/m3 corresponding to optimum moisture content 19 to 14 % respectively (Table 2). The linear regression equation for maximum dry unit weight (Eq. 2) and polynomial regression equation for optimum moisture content (Eq. 3) were generated by analysing best fit regression lines. Fig. 4b gives best fit regression plots for maximum dry unit weight and optimum moisture content as a function of bentonite content. It depicts that as the bentonite content of the bentonite-sand mixture increases, the maximum dry unit weight increases and the optimum moisture content decreases up to about 30 % of bentonite content. Above 30 % of the bentonite doses, the optimum moisture content remains constant, it may be because of completion of intergranular pores of sand by fine grains of bentonite. Similar type of study was carried out by Akgün et al. [8] and has been reported that the maximum dry unit weight increases with increasing bentonite content whereas, optimum moisture content decreases with the increasing doses of bentonite in bentonite-sand mixtures.
J d max
0.0463u BC 15.05
(2)
390
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
OMC
0.005 u ( BC ) 2 0.43 u BC 23
(3)
where, the maximum dry unit weight (kN/m3), OMC is the optimum moisture content (%) and BC is the bentonite content (%) of bentonite-sand mixture.
Fig. 4. (a) Compaction curves at different proportion of bentonite; (b) Maximum dry unit weight and OMC versus bentonite content.
3.3. Unconfined compressive strength and Young’s modulus The unconfined compressive strength test was performed [2] on a cylindrical bentonite-sand mixture samples (i.e. 38 mm diameter and 76 mm height) by increasing the axial load until the samples fail by either reaching a maximum load or attaining 15 % axial strain. Young’s modulus is measured as the ratio of stress along an axis over the strain along that axis in the range of elastic behaviour. The test results of unconfined compressive strength and Young’s modulus are summarized in Table 2. The best fit regression equations (Eq. 4; Eq. 5) and plots (Fig. 5a; Fig. 5b) depict that unconfined compressive strength and Young’s modulus increases with increasing bentonite content.
UCS 16.24 u BC 42.31
E
0.031u ( BC ) 2 0.68 u BC 9.16
(4) (5)
where, UCS is the unconfined compressive strength (kPa), E is the Young’s modulus (MPa) and BC is the bentonite content (%) of bentonite-sand mixture.
Fig. 5. (a) Unconfined compressive strength (UCS) versus bentonite content; (b) Young’s modulus versus bentonite content.
391
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
3.4. Shear strength parameters Determination of the friction angle and cohesion of bentonite-sand mixture were carried out using consolidated undrained (CU) direct shear test as per ASTM D 6528 [1]. The samples were allowed to consolidate during loading axially. Then, the mixed samples were subjected to shearing at a constant rate of displacement and the resulting shearing force was measured. The test was repeated for three different normal stress levels and corresponding shear stresses were measured. Then, the cohesion and angle of internal friction values of the various mixtures were determined by joining the line of three coordinates in normal and shear stresses. The slope and intercept of the connected line on stress coordinates give measure of the angle of internal friction and cohesion, respectively. The summary of cohesion and angle of internal friction values corresponding to bentonite content of bentonite-sand mixtures are tabulated in Table 2. The polynomial best fit equations (Eq. 6; Eq. 7) and plots (Fig. 6a; Fig. 6b) show that an increase in the bentonite content of the bentonite–sand mixtures led to increased cohesion and decreased angle of internal friction.
C
0.028u ( BC) 2 3.44 u BC 8.62
(6)
I
0.007 u ( BC) 2 0.72 u BC 34.56
(7)
where, C is the cohesion (kPa), of bentonite-sand mixture.
I is the angle of internal friction (degree) and BC is the bentonite content (%)
Fig. 6. (a) Cohesion versus bentonite content; (b) Angle of internal friction versus bentonite content.
3.5. Hydraulic conductivity The hydraulic conductivity of the bentonite-sand mixtures was studied under falling head per-meameter according to ASTM D5856-95 [3]. It was determined using Eq. 8.
k
aL h ln 1 A(t 2 t1 ) h2
(8)
where, k is hydraulic conductivity, and are heads at time and, respectivel, is the cross-sectional area of the of the soil specimen of length and is the area of burette used to measure the head loss. The hydraulic conductivity of the bentonite-sand mixture is given in Table 2. The best fit exponential equation was obtain by simple regression plot between bentonite content and hydraulic conductivity (Eq. 9) (Fig. 7). The result indicates that the hydraulic conductivity of bentonite-sand mixture decreases with increasing the bentonite content. Akgün et al. [8] observed similar type of behaviour. Gray et al. [13] have also suggested that the compacted clay-
392
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393
sand mixture may possess lower hydraulic conductivity than the compacted clay. So, the bentonite-sand mixture should be considered as the potential buffer material used for the isolation of nuclear wastes.
k
4 u 10 7 u e 0.216 BC
(9)
where, k is the hydraulic conductivity (m/sec) and BC is the bentonite content (%) of bentonite-sand mixture.
Fig. 7. Hydraulic conductivity versus bentonite. Table 2. Summary of the geotechnical properties of bentonite-sand mixtures. Bentonite content (%)
SG
Ȗd (kN/m3)
OMC (%)
UCS (kPa)
E (MPa)
C (kPa)
Ԅ (°)
k(m/sec)
10
2.68
15.57
19
190.4
21.4
26.4
28.1
6.23E-08
20
2.689
15.82
17
315.2
31.6
39.2
23.3
2.54E-09
30
2.705
16.54
14
625.4
56.2
78.5
19.2
6.48E-10
40
2.712
16.95
14
714.1
93.2
81.4
18.1
5.14E-11
50
2.726
17.32
14
803.2
119.2
92.5
17
8.78E-12
4. Conclusion This study is performed to evaluate the geomechanical behaviour of bentonite-sand mixtures for its utilization in underground radioactive waste repositories as isolation material. The geotechnical laboratory tests such as specific gravity, compaction, unconfined compression, direct shear and falling head permeability were performed to select an optimum amount of bentonite and sand for designing and constructing a better nuclear waste disposal facility. The results of the executed tests designate that an increase in specific gravity, maximum dry density, unconfined compressive strength, Young’s modulus, cohesion were observed with increasing proportion bentonite in bentonite-sand mixtures whereas decreasing trends were found for the optimum moisture content, angle of internal friction and hydraulic conductivity with the addition of bentonite in bentonite-sand mixture. Thus, the results of the present study support that the 50 % bentonite in bentonite-sand mixture satisfied the optimum condition for proper isolation of radioactive waste repository. However, the optimum amount is depends on the type of bentonite and sand used in bentonite-sand mixture. References [1] ASTM D 6528, Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Cohesive Soils, Philadelphia, USA, 2000. [2] ASTM D2166, Standard Test Method for Unconfined Compressive Strength of Cohesive Soil, Philadelphia, USA, 2013.
L.K. Sharma et al. / Procedia Engineering 191 (2017) 386 – 393 [3] ASTM D5856-95, Standard Test Method for Measurement of Hydraulic Conductivity of Porous Material Using a Rigid-Wall, CompactionMold Permeameter, Philadelphia, USA, 2007. [4] ASTM D698, Standard test methods for laboratory compaction characteristics of soil, Philadelphia, USA, 2012. [5] ASTM D854, Standard test methods for specific gravity of soil solids by water Pycnometer, Philadelphia, USA, 2014. [6] D. Meyer, J.J. Howard, Evaluation of clays and clay minerals for application to repository sealing, ONWI-486, Office of Nuclear Waste Isolation, Battelle Memorial Institute, 1983. [7] D.A. Dixon, M.N. Gray, A.W. Thomas, A study of the compaction properties of potential clay-sand buffer mixtures for use in nuclear fuel waste disposal, Eng. Geol. 21(3-4) (1985) 247-255. [8] H. Akgün, M. Ada, M.K. Koçkar, Performance assessment of a bentonite–sand mixture for nuclear waste isolation at the potential Akkuyu Nuclear Waste Disposal Site, southern Turkey, Environ. Earth Sci. 73(10) (2015) 6101-6116. [9] H. Coulon, A. Lajudie, P. Debrabant, R. Atabek, M. Jorda, R. Andre-Jehan, Choice of French clays as engineered barrier components for waste disposal, Mater. Res. Symp. Proc. 84 (1987) 813-824. [10] H. Komine, N. Ogata, Experimental study on swelling characteristics of sand-bentonite mixture for nuclear waste disposal, Soils Found 39(2) (1999) 83–97. [11] IAEA, Scientific and technical basis for the geological disposal of radioactive wastes, Tech Reps Series No. 413, International Atomic Energy Agency, Vienna, 2003. [12] IAEA, Sealing of underground repositories for radioactive wastes, Tech Reps Series No. 319, International Atomic Energy Agency, Vienna, 1990. [13] M.N. Gray, S.C.H. Cheung, D.A. Dixon, The influence of sand content on swelling pressures and structure developed by statically compacted Na bentonite, Report 7825, Atomic Energy of Canada Limited, Mississauga, Ont, 1984, pp. 1–24. [14] R. Pusch, Waste disposal in rock, Dev. Geotech. Eng. 76 (1994) 490. [15] U.S. Environmental Protection Agency, Environmental radiation protection standards for management and disposal of spent nuclear fuel, high-level and transuranic radioactive wastes, CFR40, Part 191, Washington, DC, 1995.
393