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For this reason, a nuclear power plant (NPP) will be constructed in Akkuyu in southern Turkey and the nuclear wastes generated by the Akkuyu NPP are ...
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Research paper

Evaluation of a sand bentonite mixture as a shaft/borehole sealing material Haluk Akgüna, Mustafa K. Koçkarb, a b



Geotechnology Unit, Department of Geological Engineering, Middle East Technical University, Ankara, Turkey Earthquake Engineering Implementation and Research Center, Gazi University, Ankara, Turkey

A R T I C L E I N F O

A B S T R A C T

Keywords: Compacted sand bentonite mixture Mechanical and hydrological evaluation Hydraulic conductivity Unconfined compressive strength Shear strength Waste disposal

The mechanical and hydrological characteristics of compacted sand bentonite mixtures with bentonite contents ranging from 5 to 40% were investigated in the laboratory in order to assess their use as a waste isolation material and to select an optimum sand bentonite mixture. Laboratory tests included compaction, compaction permeability, unconfined compression and direct shear tests which led to a recommendation to select a mixture with a bentonite content of 30% for the isolation of underground geological waste disposal repositories. This study complements the previous studies of the authors of this manuscript by determining the mechanical and hydrological properties of sand bentonite mixtures that possess bentonite contents > 30% to determine the geotechnical properties (i.e., unconfined compressive strength, Young's modulus, cohesion and angle of internal friction) and the mechanical behavior of these relatively high levels of bentonite mixtures for the first time in the literature.

1. Introduction According to the Turkish Electricity Generation and Transmission Company (TEGT, 1999), even if all the energy resources of Turkey were to be used in full capacity, energy shortage in Turkey seems inevitable by the year 2020. For this reason, a nuclear power plant (NPP) will be constructed in Akkuyu in southern Turkey and the nuclear wastes generated by the Akkuyu NPP are planned to be disposed in underground repositories situated in the near vicinity of the Akkuyu NPP and contained reliably in order to prevent contamination of the environment (International Atomic Energy Agency (IAEA, 1990; 1999; 2001; 2003). The potential Akkuyu NPP site is situated on the Mediterranean coast and lies 55 km WSW of Silifke in the Mersin province (Fig. 1). The NPP is planned to be constructed in the Çamalanı bay that is situated approximately 5 km south of Büyükeceli. The Silifke-Anamur highway provides access to the site. The Turkish Electric Authority (TEK), Earthquake Engineering Research Center of the Middle East Technical University (METU/ EERC), General Directorate of Mineral Research and Exploration (MTA), General Directorate of Electrical Survey Administration (EIEI) and State Hydraulic Works (DSİ) have performed geological and geotechnical investigations at the potential Akkuyu NPP site. The 1/1000 scale geological map of the potential Akkuyu NPP site that was prepared by MTA in 1985 encompasses three formations, namely, Akdere formation, Büyükeceli formation and Kırtıldağı formation. The Büyükeceli formation consists of dolomitic wackestone, quartzitic



sandstone, dolomite, wackestone, carbonate mudstone and limestone. The Akkuyu NPP site is situated mainly in the Akdere formation which generally consists of mudrocks, mudstone, shales and alternations of them. The Kırtıldağı formation that conformably lies on the Akdere formation and that is the uppermost formation of the site consists of carbonate mudrocks, sandy wackestone, grainstone, packstone and dolomites (METU EERC, 1984; Demirtaşlı, 1985; EIEI, 1985). Argillaceous formations, in particular the shale horizons of the Akdere formation, can be considered as host rock formations for the isolation of waste materials. The Akdere formation, particularly the shale and mudstone horizons of the Akdere formation are considered to be desirable for geological repositories in order to reliably contain nuclear waste. The shale horizon of the Akdere formation which is planned to host the nuclear waste generated from the Akkuyu NPP site possesses a mean hydraulic conductivity of about 9.0 × 10− 11 m/s (EIEI, 1985; METU EERC, 1984). Argillaceous formations represent very good conditions for hosting repositories for long-lived radioactive waste because of the low solubility of their clay constituents, low hydraulic conductivity, effective filtration capacity for colloids and large molecules, high sorption capacity for dissolved cations and self-sealing ability, provided that they do not contain continuous permeable layers of silt and sand (BENIPA, 2004; Pusch and Svemar, 2004). Hence, any waste material generated from the NPP site could be deposited within this formation. For these reasons, various research studies have been performed in National Underground Research Laboratories (URLs) on argillaceous formations as host rock candidates for the isolation of

Corresponding author. E-mail address: [email protected] (M.K. Koçkar).

https://doi.org/10.1016/j.clay.2017.12.043 Received 27 February 2017; Received in revised form 28 November 2017; Accepted 26 December 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Akgün, H., Applied Clay Science (2017), https://doi.org/10.1016/j.clay.2017.12.043

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Fig. 1. Location map of the Akkuyu nuclear power plant (NPP) site.

waste have been successfully operated in different geological environments, such as plutonic rock of the Canadian Shield in Canada (CTECH, 2002) and crystalline rock in Sweden (Aspo Hard Rock Laboratory; Pusch and Svemar, 2004). All penetrations and excavations (i.e., boreholes, shafts) in the vicinity of a underground geological nuclear waste repository are required to be sealed effectively to retard any radionuclide migration to the accessible environment (e.g., U.S. Nuclear Regulatory Commission, 1983, 1985). One of the most essential components of the underground repositories are the materials that are used to isolate the wastes from the environment where the materials are required to have a very low permeability, be compatible with the host rock, be mechanically stable, be resistant to destruction, and be chemically durable (IAEA, 1990, 1999, 2001, 2003; Pusch and Bergström, 1980; Pusch, 1994; Meyer and Howard, 1983; DOE/WIPP, 1995; Akgün, 2000). Sand bentonite mixtures are considered for sealing shafts that are situated in the vicinity of the Waste Isolation Pilot Plant (WIPP) site in southeastern New Mexico (e.g., US DOE/WIPP, 1995; Daemen and Ran, 1996) and as backfill and buffer materials for nuclear waste repositories in Canada, Sweden, Germany, Switzerland and France (e.g., IAEA, 1990; Coulon et al., 1987; Pusch, 1994). The objective of this study is to perform an experimental program to recommend an optimum sand bentonite mixture to be used for sealing geological repositories that contain nuclear waste. Mechanical and hydrological laboratory testing was performed to determine the Atterberg limits, specific gravity, optimum moisture content, maximum dry unit weight, total unit weight, hydraulic conductivity, unconfined

waste materials in France (Meuse/Haute Marne URL at Bure), Switzerland (Mont Terri URL at St. Ursanne), and other countries (Witherspoon and Bodvarsson, 2006). A conceptual design of an underground nuclear waste disposal repository, often referred to as a deep geological repository (DGR) that could be utilized as a model for the potential Akkuyu Nuclear Waste Disposal Site (NWDS) could entail the following components. The buffer material may consist of a compacted bentonite–sand mixture as stated for example by CTECH (2002). OECD (2000), SKB (2006) and ANDRA (2011) are some of the sources out of many that provide conceptual designs for underground nuclear waste disposal repositories and their components. The criteria will be in accordance with IAEA (2011). Concerning the desirable characteristics of clay rock formations for long-lived radioactive waste, Akkuyu NWDS conceptual design study of DGR could be similar to the deep geological repository for high-level radioactive waste in Opalinus Clay (claystone) at the Mont Terri test site in Switzerland and Callovo-Oxfordian argillites (argillaceous rock) at the Meuse/Haute-Marne test site in France (Witherspoon and Bodvarsson, 2006). It should be noted that the different site-specific geological conditions have led to different design and instrumentation of the URL's. However, the engineered barriers tested and analyzed in the URLs have a similar function and, despite some obvious differences, many of the solutions and techniques are believed to be applicable to disposal repository concepts in a variety of different rock types (Pusch, 2008). Hence, various URLs for studying the possibility of deep geological repository of long-lived radioactive 2

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compressive strength, Young's modulus, shear strength parameters of the compacted sand bentonite mixtures that contained 5 to 40% of bentonite by weight to recommend an optimum mixture that satisfied the regulatory requirements. This study complements the studies of Akgün et al. (2015, 2016) by determining the mechanical and hydrological properties of sand bentonite mixtures possessing bentonite contents > 30% to determine the geotechnical properties and the mechanical behavior of these relatively high levels of bentonite mixtures for the first time in the literature. An additional objective of this study is to differentiate the mechanical and hydrological characteristics of the mixtures with bentonite contents < 30% which show sandy behavior (i.e., that display the property of a cohesive material that strengthen the skeletal structure of the sand particles) from mixtures with bentonite contents > 30% which show the behavior of bentonite.

Table 2 Physical and index properties of the Çanbensan Na-bentonite (Çanbensan, 2017).

2. Mechanical and hydrological characterization of the sand bentonite mixtures

smectite as the main clay mineral group along with the quartz and feldspar group minerals. The major adsorbed cation of the smectite group was Na-smectite according to its interlayer spacing and 2θ angle values (Fig. 2(a)). Then, quantitative analysis implemented by the RIR method gave the relative percentage of Na-smectite as 75% based on the histogram given in Fig. 2(b). The sand utilized was uniform fine to medium clean sand with subangular quartz particles and a specific gravity (Gs) of 2.68 according to the ASTM D854-10 standard. The particle size distribution of the sand was determined by sieve analysis in accordance with ASTM D42263 (2007). The sand was classified as poorly graded (SP) fine to medium sand according to the Unified Soil Classification System (USCS; ASTM D2487-11). The coefficient of uniformity (Cu) and coefficient of curvature (Cc), defined as Cu = D60/D10 and Cc = D302/D10 ∗ D60 were calculated based on the grain size distribution curve. The soil classification parameters are presented in Table 3. The specific gravity of the sand bentonite mixtures with 5% to 40% bentonite content by weight was determined in accordance with ASTM D854-10. Table 4 presents the specific gravity (Gs) values of the sand bentonite mixtures that ranged from 2.676 to 2.712. A slight increase in the specific gravity of the sand bentonite mixtures with an increase in the bentonite content was observed as the specific gravity of bentonite is greater than that of sand. Komine and Ogata (1999) have reported similar trends. The compaction characteristics of the sand bentonite mixtures with bentonite contents ranging from 5 to 40% were determined at various molding water contents in accordance with ASTM D0698-12. According to the compaction test results, the maximum dry unit weights ranged between 17.23 kN/m3 and 17.76 kN/m3 corresponding to optimum moisture content values of 17.37% and 15.86%, respectively. The total unit weights ranged from 20.22 kN/m3 to 20.58 kN/m3. The maximum dry unit weight, optimum water content and total unit weight values are summarized in Table 4. It should be noted that the seals or plugs are used in the repository at different strategic positions and are a part of a general barrier concept. A basic seal/plug consists of a hydraulic sealing element made from bentonite and one or two bounding support elements (i.e., sand) that guarantee the mechanical stability of the hydraulic seal. These support elements have to withstand substantial loads caused by the swelling pressure of the bentonite and water pressure. It is difficult to assess and predict the evolution of pressure but stable conditions may require several tens of years. Especially the experiments that include the effects of saturation of sealing material are problematic because of the long time spans needed for these processes to reach equilibrium/ steady state. Before going in-situ, artificial saturation (pre-wetting) has to be used to realize the natural process but the evaluation of the results is quite problematic as the observed reactions (especially for coupled processes) may differ from those expected during natural re-saturation (Pusch and Svemar, 2004; Pusch, 2008). In this sense, considering the sealing performance of bentonite/sand mixtures that are emplaced with

Physical properties

The mechanical and hydrological characteristics of the compacted sand bentonite mixtures to be used as an isolation material in underground geological waste repositories were investigated by a laboratory testing program that included sieve analyses, specific gravity testing, index testing, standard compaction testing, compaction permeability testing, unconfined compressive strength testing and direct shear testing. Tables 1 and 2 give the chemical composition and the physical as well as the index properties of Çanbensan Na-bentonite that was used in this study. According to the ASTM D854-10 standard, the specific gravity of the Çanbensan Na-bentonite was determined to be 2.75 (Çanbensan, 2017). Qualitative and quantitative XRD analyses have been performed on Çanbensan Na-bentonite samples to assess which smectite minerals appear, what the smectite percentage is and what the major adsorbed cation is in the investigated bentonite. In the qualitative study, XRD analyses have been conducted both on unoriented and oriented bentonite specimens to detect clay and non-clay minerals. During these analyses, a Rigaku Miniflex II Desktop X-Ray Diffractometer with Ni filtered Cu Kα radiation and a graphite monochromator at 35 keV, 15 mA and with a scan speed of 2° per min was used. The clay minerals were determined by following the procedure defined by Chen (1977), Brown and Brindley (1980) and Moore and Reynolds Jr. (1997). The Xray patterns were recorded in random, natural (air-dried), ethylene glycolated and thermal treated (300 °C and 550 °C) conditions (Fig. 2a). During the quantitative study, XRD analyses have been conducted based on the peak intensities to obtain information about the relative amounts of clay minerals. The quantitative analysis was performed by using the Reference Intensity Ratio (RIR) method, which is the ratio of the highest peak intensity of a mineral to that of corundum (Al2O3) when mixed with corundum at a weight ratio of 1:1 (Chung, 1974). The RIR method implemented in the Rigaku PDXL software program has been applied (Rigaku, 2010). Considering both the qualitative and the quantitative study, the XRD diffractograms of the bentonite samples indicated the presence of Table 1 Chemical composition of the Çanbensan Na-bentonite (Çanbensan, 2017). Elements

Composition (%)

SiO2 Al2O3 Fe2O3 MgO Na2O CaO K2O TiO2

59–61 18–20 4–6 2.5–3.5 2–3 0.5–1.5 0.5–1.5 0.5–1.5

3

Color Methylene blue value Montmorillonite content Free swell amount Moisture content pH (6.5% slurry) Minimum temperature of handling

Yellow 320 mg/g > 75% 591% 9.5% (on the basis of dry product) 9.5 1 °C

Index properties (Atterberg consistency limits) Liquid Limit, wL (ASTM D4318-10, 2010) Plastic Limit, wP (ASTM D4318-10, 2010) Plasticity Index, IP (ASTM D4318-10, 2010)

373% 52.3% 321%

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Fig. 2. a) XRD diffractograms of the bentonite sample. From top to the bottom, the patterns are given for random, air-dried, ethylene glycolated, 300 °C and 550 °C heated samples, respectively (Sm: smectite, Q: quartz, P: plagioclase, F: feldspar), b) Histogram of the mineral percentages found by the quantitative XRD analyses.

Percentage (%)

(a)

Nasmectite

Mineral

(b) Unconfined compressive strength testing was performed on sand bentonite mixtures in accordance with ASTM D2166-06. Compacted specimens, which were prepared with a predetermined dry unit weight and water content, were used in this study. The requirement that the specimens should have a height-to-diameter ratio between 2.0 and 2.5

their optimum water content, the time for water saturation can be hundreds of years during which significant changes of the geotechnical properties can occur. This implies that the repository designers would have to pre-wet the mixture, which would require an extended study to determine the level of the pre-wetting required.

4

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The results summarized in Table 5 indicate that an increase in the bentonite content led to increased cohesion (c) and decreased angle of internal friction (ϕ). Rigid wall compaction permeameters were utilized to measure the hydraulic conductivities of compacted sand bentonite mixtures in accordance with ASTM D5856-95 (2007). De-aired and distilled water was used as the permeant. Each permeameter test was continued for up to two months until the compacted samples reached full saturation as was confirmed by water flowing out of the permeameter outlet portal. As suggested for example by Mitchell (1993), Haug and Wong (1992), Lambe and Whitman (1969), Dixon et al. (1985), Mitchell et al. (1965), Olson and Daniel (1981), Benson and Daniel (1990), since compacting specimens on the wet side of the optimum water contents leads to lower hydraulic conductivities, the sand bentonite mixtures were compacted at 2% above their optimum water contents. The hydraulic conductivity values of the sand bentonite mixtures presented in Table 5 indicated a decrease in hydraulic conductivity with increased bentonite content.

Table 3 Soil classification parameters of sand (from Akgün et al. (2015, 2016). Particle size distribution

Class

(ASTM D422-63, 2007) D10 = 0.23 mm D30 = 0.38 mm D60 = 0.65 mm Cc = 0.96 Cu = 2.82

USCS (ASTM D2487-11) SP, fine to medium sand

Table 4 Specific gravity (Gs) and standard Proctor compaction test results (maximum dry unit weight (γdmax), optimum moisture content (wopt) and total unit weight (γt) values) of the sand bentonite mixtures. Bentonite content (%)

Gs

γdmax (kN/m3)

wopt (%)

γt (kN/m3)

5 10 15 20 22.5 25 30 32.5 35 37.5 40

2.676 2.685 2.687 2.690 2.695 2.697 2.701 2.704 2.707 2.709 2.712

17.23 17.52 17.604 17.65 17.72 17.83 18.04 17.88 17.86 17.82 17.76

17.37 16.15 16.06 15.90 15.50 15.38 15.00 15.48 15.55 15.76 15.86

20.22 20.35 20.43 20.46 20.50 20.57 20.75 20.65 20.64 20.63 20.58

3. Evaluation of the test results and discussion Table 4 summarizes the maximum dry unit weight (γdmax) and optimum water content (wopt) values of the compacted sand bentonite mixtures. Plots of maximum dry unit weight vs. bentonite content, optimum moisture content vs. bentonite content and maximum dry unit weight vs. optimum moisture content (Fig. 3) and best-fit regression equations were generated (Eqs. (1)–(3)) in order to study the effect of the bentonite content on the compaction characteristics of sand bentonite mixtures. Fig. 3 and Eq. (1) through (3) indicate that as the bentonite content of the sand bentonite mixtures increases, the maximum dry unit weight increases (Fig. 3(a), Eq. (1)) and the optimum moisture content decreases (Fig. 3(b), Eq. (2)) up to a bentonite content of 30%. The opposite takes place after a bentonite content > 30%. The maximum dry unit weight and the optimum moisture content are inversely proportional (Fig. 3(c), Eq. (3)).

Table 5 Unconfined compressive strength (qu), Young's modulus (E), cohesion (c), angle of internal friction (ϕ) and hydraulic conductivity (k) of the sand bentonite mixtures with maximum dry unit weight results (γdmax) and bentonite contents ranging from 5 to 40%. It should be noted that data for 5-15% bentonite content is from Akgün et al. (2016) and data for 20-30% bentonite content is from Akgün et al. (2015). Bentonite content (%)

γdmax (kN/m3)

qu (kPa)

E (MPa)

c (kPa)

ϕ (°)

k (m/s)

5 10 15 20 22.5 25 30 32.5 35 37.5 40

17.23 17.52 17.604 17.65 17.72 17.83 18.04 17.88 17.86 17.82 17.76

81.60 102.0 198.4 399.3 455.4 550.2 672.2 693.4 711.1 733.1 767.1

15.19 19.03 29.30 38.18 57.32 90.87 108.9 114.1 128.1 135.2 141.0

19.00 28.34 35.14 44.61 54.13 64.98 88.97 108.2 127.1 145.1 160.1

31.02 29.14 26.83 24.15 22.36 21.65 20.93 19.64 18.83 18.04 16.30

6.18 × 10− 8 3.04 × 10− 8 8.73 × 10− 10 3.23 × 10− 10 6.76 × 10− 11 2.24 × 10− 11 9.81 × 10− 12 8.98 × 10− 12 8.22 × 10− 12 7.11 × 10− 12 5.67 × 10− 12

γdmax = −0.0009 (b/c)2 + 0.0562 (b/c) + 16.98, r 2 = 0.887

(1)

wopt = 0.0035 (b/c)2 − 0.1982 (b/c) + 18.14, r 2 = 0.892

(2)

γdmax = 1.56 (wopt )2 − 57.69 (wopt) + 548.2, r 2 = 0.937

(3)

where, γdmax is the maximum dry unit weight (kN/m ), wopt is the optimum water content (%), b/c is the bentonite content (%) and r2 is the coefficient of determination of the compacted sand bentonite mixtures. According to Komine (2010), the reason for the increase of the maximum dry density and the decrease of the optimum water content up to a bentonite content of 30% is because bentonite can act as a cohesive material to strengthen the skeletal structure of the sand particles contained in the mixture up to a bentonite content of 30%. In contrast, since a sand bentonite mixture possessing a bentonite content > 30% contains a relatively high amount of bentonite, the skeletal structure of the mixture is mainly dominated by the skeletal structure of the bentonite particles. Therefore, the compaction characteristics of a mixture with a bentonite content > 30% reflect the property of bentonite so that the maximum dry density decreases and the optimum water content increases for mixtures possessing a bentonite content > 30%. The opposite takes place for mixtures with less than or equal to 30% bentonite content, as shown in Table 4. Table 5 summarizes the unconfined compressive strength (qu) and Young's modulus (E) values of the compacted sand bentonite mixtures. The plots and the best-fit regression equations of unconfined compressive strength (qu) vs. bentonite content (Fig. 4(a), Eq. (4)), Young's modulus (E) vs. bentonite content (Fig. 4(b), Eq. (5)) indicate that the unconfined compressive strength and Young's modulus increase with increased bentonite content. The maximum rate of increase (i.e., slope) 3

was attempted to be satisfied by preparing specimens with a diameter (D) of 35 mm and a height (H) of 71 mm. A compression device capable of producing an axial strain rate of 1%/min was used to load the specimen until 15% strain was reached. The unconfined compressive strength and Young's modulus values increased with increased bentonite content (Table 5). Consolidated undrained (CU) direct shear tests were performed for the measurement of constant volume strength and stress-strain characteristics of the sand bentonite samples following one-dimensional consolidation by utilizing a constant rate of simple shear deformation loading in accordance with ASTM D6528-07. The compacted sand bentonite specimen was placed in a direct shear box cell and constrained axially between two parallel, rigid platens and laterally, so that its cross sectional area remained constant. Then, the specimen was allowed to consolidate one-dimensionally by loading it axially and maintaining each normal load increment until all of the excess pressure dissipated. A constant rate of displacement was used to shear the specimen and the resulting shear force was measured. This procedure was repeated for three different levels of normal stress. 5

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Optimum moisture content, wopt (%)

Maximum dry unit weight, γ dmax (kN/m3

H. Akgün, M.K. Koçkar

γ

Bentonite content, b/c (%)

Bentonite content, b/c (%)

Maximum dry unit weight, γ dmax (kN/m3

(a)

(b)

γ

Optimum moisture content, wopt (%)

(c)

Fig. 3. (a) Maximum dry unit weight (γd) vs. bentonite content (b/c), (b) optimum moisture content (wopt) vs. bentonite content and (c) maximum dry unit weight (γd) vs. optimum moisture content (wopt).

for the measured unconfined compressive strength (qu) took place between a bentonite content of 15% and 20% and was equal to about 40.2. That for the Young's modulus (E) took place between a bentonite content of 22.5% and 25% and was equal to about 13.4.

qu = 8.56 (b/c)1.246, r 2 = 0.948

(4)

E = 1.404 (b/c)1.236 , r 2 = 0.915

(5)

(7)

c = 3.0 x 106 ϕ−3.44 , r 2 = 0.975

(8)

where, c is cohesion (kPa), ϕ is angle of internal friction (°), b/c is the bentonite content (%) and r2 is the coefficient of determination. The increase of cohesion and the decrease of the angle of internal friction with increased bentonite content are attributed to the increase of the cohesive nature of the mixtures with increasing bentonite content. The maximum rate of increase (i.e., slope) for the measured cohesion took place between a bentonite content of 30% and 32.5% and was equal to about 7.7. The maximum rate of decrease for the measured angle of internal friction took place between a bentonite content of 20% and 22.5% and was equal to about 0.716. The results of the hydraulic conductivity tests summarized in Table 5 indicate a decrease in hydraulic conductivity with increased bentonite content. This result agrees well with the results of, for example, Kaya et al. (2006), Chapuis (1990, 2002), Kenney et al. (1992), Komine (2004), Tashiro et al. (1998) and Kaoser et al. (2006) who concluded that since bentonite by itself has a very low permeability, in the order of 10− 10 and 10− 12 m/s, the hydraulic conductivity values of the compacted sand bentonite mixtures are expected to decrease with an increased bentonite content of the mixture. Fig. 6 and Eq. (9) present hydraulic conductivity (k) of the sand bentonite mixtures as a function of bentonite content (b/c) which indicate that hydraulic conductivity is inversely proportional to the bentonite content.

where, qu is the unconfined compressive strength (kPa), E is the Young's modulus (MPa), b/c is the bentonite content (%) and r2 is the coefficient of determination of the compacted sand bentonite mixtures. The results obtained for Young's modulus versus bentonite content herein agree well with the results of Tripathi and Viswanadham (2012) who have measured the Young's modulus of sand bentonite mixtures with bentonite contents ranging from 5 to 25% and have determined that the Young's modulus increases with increased bentonite content. Table 5 summarizes the cohesion (c) and angle of internal friction (ϕ) values of the various mixtures. The plots and best-fit regression equations of cohesion vs. bentonite content, angle of internal friction vs. bentonite content and cohesion vs. angle of internal friction indicate that as the bentonite content increases, the cohesion increases (Fig. 5(a), Eq. (6)) and the angle of internal friction decreases (Fig. 5(b), Eq. (7)). The cohesion and angle of internal friction are inversely proportional (Fig. 5(c), Eq. (8)).

c = 14.09 e0.0616 (b/c) , r 2 = 0.996

ϕ = 34.33 e−0.018 (b/c) , r 2 = 0.986

(6) 6

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Unconfined compressive strength, qu (kPa

Fig. 4. (a) Unconfined compressive strength (qu) and (b) Young's modulus (E) as a function of bentonite content (b/ c) of the sand bentonite mixtures.

Bentonite content, b/c (%)

Young's modulus, E (MPa

(a)

Bentonite content, b/c (%)

(b) k = 8.0 x 10−4 (b/c)−5.16, r 2 = 0.946

(9)

reduction of hydraulic conductivity is less in sand bentonite mixtures containing > 20% bentonite because the voids are almost entirely filled by swollen bentonite. The results of the unconfined compressive strength, Young's modulus, cohesion and angle of internal friction as a function of bentonite content confirm the results of the compaction tests. These tests indicate that in mixtures with a bentonite content up to 30%, bentonite acts as a cohesive material to strengthen the skeletal structure of the sand particles whereas since mixtures with a bentonite content > 30% contain relatively high amount of bentonite, the skeletal structure of the mixtures are mainly dominated by the skeletal structure of the bentonite particles. Therefore, the characteristics of a mixture with a bentonite

where, k is hydraulic conductivity (m/s), b/c is the bentonite content (%) and r2 is the coefficient of determination. The maximum rate of decrease (i.e., slope) for the measured hydraulic conductivity took place between a bentonite content of 10% and 15% and was about 59. This result agrees with Komine (2004), De Magistris et al. (1998) and Sivapullaiah et al. (2000) who measured the hydraulic conductivity of sand bentonite mixtures with different proportions and concluded that the reduction of hydraulic conductivity is greatest for mixtures with a relatively low bentonite content (i.e., 5–20%) because the filling conditions of the voids change. The rate of 7

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Hydraulic Conductivity, k (m/s

Cohesion, c (kPa

H. Akgün, M.K. Koçkar

Bentonite content, b/c (%)

(a) Bentonite content, b/c (%)

Angle of internal friction, φ (o

Fig. 6. Hydraulic conductivity (k) of sand bentonite mixture as a function of bentonite content (b/c).

unconfined compressive strength of 5 to 30% was equal to about 23.62 whereas that between a bentonite content of 30 to 40% was equal to about 9.49, a decrease by a factor of 2.49. These values were 3.75 and 3.20 for Young's modulus which represents a decrease by a factor of 1.17, 2.80 and 7.12 for cohesion which represents an increase by a factor of 2.54, and, 0.404 and 0.463 for angle of internal friction, which represents a decrease by a factor of 1.15. The rate of increase in the cohesion was higher for mixtures with bentonite contents > 30% because of the cohesive nature of these mixtures. For mixtures with bentonite contents lower than 30%, the rate of increase in the unconfined compressive strength and Young's modulus was higher and the rate of decrease of the angle of internal friction was lower since in mixtures of these levels bentonite acted as a cohesive agent to strengthen the sand particles and mixtures with bentonite contents < 30% acted like sand. The Swedish (2000), the Republic of Turkey, Ministry of Environment and Forestry (2010), and the U.S. Environmental Protection Agency (2010) require that the hydraulic conductivity of a clay liner or a sand bentonite mixture for landfill liners should be lower than 1 × 10− 9 m/s. However, since isolation materials with hydraulic conductivities lower than 10− 11 m/s have been proposed for the isolation of radioactive waste (e.g., Radhakrishna and Chan, 1982, 1989; Westsik Jr. et al., 1981), a 30% compacted bentonite-sand mixture satisfies the condition for a proper isolation of radioactive waste. The hydraulic conductivity, unconfined compressive strength (qu), Young's modulus (E), cohesion (c) and angle of internal friction (ϕ) of the 30% compacted bentonite-sand mixture were determined to be 9.81 × 10− 12 m/s, 672.2 kPa, 108.9 MPa, 88.97 kPa and 21° (Table 5), respectively. The mechanical properties (e.g., qu, E, c and ϕ) of the sand bentonite mixtures may be utilized to determine the stability of the seals and to design the seals as described in Akgün (2000).

Bentonite content, b/c (%)

Cohesion, c (kPa

(b)

Angle of internal friction φ (o)

(c) Fig. 5. (a) Cohesion (c) vs bentonite content (b/c), (b) angle of internal friction (ϕ) vs. bentonite content and (c) cohesion vs. angle of internal friction of the sand bentonite mixtures.

4. Summary and conclusions This study assesses the mechanical and hydrological characteristics of compacted sand bentonite mixtures to evaluate their use as an isolation material in underground radioactive waste repositories. In order to select an optimum sand bentonite mixture, a variety of laboratory tests, such as compaction, compaction permeability, unconfined compression and shear strength tests were performed. According to the results of the laboratory tests, the specific gravity,

content > 30% reflect the property of bentonite and the opposite takes place for mixtures less than or equal to 30% which reflect the property of sand. This fact is confirmed by observing the rate of change (i.e., slopes) of the geotechnical parameters between a bentonite content of 5 to 30% versus that between 30 and 40%. The results presented in Table 5 indicate that the rate of decrease (i.e., slope) for the measured 8

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unconfined compressive strength, Young's modulus, cohesion increased and the angle of internal friction and hydraulic conductivity decreased with increased bentonite content of the sand bentonite mixture. As the bentonite content of the sand bentonite mixtures increased, the maximum dry unit weight increased and the optimum moisture content decreased up to a bentonite content of 30%. The opposite took place after a bentonite content > 30% which was attributed to the behavior of the sand bentonite mixture as a cohesive material to strengthen the skeletal structure of the sand particles contained in the mixture up to a bentonite content of 30%. In contrast, since a sand bentonite mixture possessing a bentonite content > 30%, especially a mixture with a bentonite content of 40%, contained a relatively high amount of bentonite, the skeletal structure of the mixture was mainly dominated by the skeletal structure of the bentonite particles. Therefore, the compaction characteristics of a mixture with a bentonite content > 30% reflected the property of bentonite so that the maximum dry density decreased and the optimum water content increased for mixtures possessing a bentonite content > 30%. The opposite took place for mixtures with less than or equal to 30% bentonite content which behaved as sandy soil. The results of the unconfined compressive strength, Young's modulus, cohesion and angle of internal friction as a function of bentonite content confirmed the results of the compaction tests from a mechanical behavior point of view. A 30% compacted bentonite-sand mixture satisfied the minimum condition for proper isolation of a radioactive waste repository leading to measured hydraulic conductivity, unconfined compressive strength, Young's modulus, cohesion and angle of internal friction values of 9.81 × 10− 12 m/s, 672.2 kPa, 108.9 MPa, 88.97 kPa and 21°, respectively. The mechanical properties (e.g., qu, E, c and ϕ) of the sand bentonite mixtures may be utilized to determine the stability of the seals and to design the seals as described in Akgün (2000).

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