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Influence of initial degree of saturation on swell pressures of compacted Barmer bentonite specimens
Annals of Nuclear Energy 80 (2015) 303–311
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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Influence of initial degree of saturation on swell pressures of compacted Barmer bentonite specimens Sudhakar M. Rao a,1, K. Ravi b,⇑ a b
Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India Geo-environmental Research Centre (GRC), Cardiff University, CF24 3AA, United Kingdom
a r t i c l e
i n f o
Article history: Received 18 February 2014 Received in revised form 13 October 2014 Accepted 1 February 2015 Available online 26 February 2015 Keywords: Bentonite Geological repository Microstructure Saturation Swell
a b s t r a c t Densely compacted bentonite or bentonite–sand mixture has been identified as suitable buffer in deep geological repositories as its exceptionally high swelling capacity enables tight contact between the waste canister and surrounding rock. The degree of saturation of the compacted bentonite buffer can increase upon ingress of groundwater from the surrounding rock mass or decrease from evaporation due to high temperature (50–210 °C) derived from the waste canister. Available studies indicate that the influence of initial moisture content or degree of saturation on the swell pressure or swell potential of compacted bentonites is unclear. Some studies suggest that initial degree of saturation has an influence, while others suggest that it does not have bearing on the swell pressure of compacted bentonites. This paper examines the influence of initial degree of saturation in montmorillonite voids (termed, Sr,MF) on swell pressure of compacted Barmer bentonite–sand mixtures (dry density range: 1.4–2 Mg/m3) from micro-structural considerations. The experimental results bring out that, constant dry density specimens that developed similar number of hydration layers upon wetting developed comparable swell pressures and were unaffected by variations in initial Sr,MF values. Comparatively, constant dry density specimens that developed dis-similar number of hydration layers upon wetting established different swell pressures and were responsive to variations in initial Sr,MF. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Disposal in deep geological repositories (DGR) is a widely accepted concept to dispose high level nuclear waste. The function of such a deep geological repository (DGR) is based on a system of successive superimposed barriers and consists of natural barrier, which is often considered to be either the hard crystalline rock or the clay rock and the engineered barrier system (EBS) that consist of the canister, buffer and back fill which isolates the potentially harmful radioactive waste from the biosphere (Dixon et al., 1985; Bucher and Vonmoos, 1989; Cho et al., 1999; SKB, 2006; Villar, 2007). Densely compacted bentonite or bentonite–sand mixture is considered as a suitable buffer as a part of EBS in a DGR due to its favourable, physico-chemical and engineering properties (Lloret et al., 2003; Pusch, 1994). The exceptionally high swelling ability of bentonite upon wetting is one such property as it helps the clay to maintain tight contact between the waste ⇑ Corresponding author. Tel.: +44 7448966538. E-mail addresses: [email protected] (S.M. Rao), [email protected] (K. Ravi). 1 Tel.: +91 8022932812. http://dx.doi.org/10.1016/j.anucene.2015.02.019 0306-4549/Ó 2015 Elsevier Ltd. All rights reserved.
canister and surrounding rock and seal any gaps, cracks or fissures which might be present. The degree of saturation of the compacted bentonite buffer may increase upon ingress of groundwater from the surrounding rock mass or decrease from evaporation due to increase in temperature (50–210 °C, Garcia et al., 2006; Duquette et al., 2008) of the waste canister. The degree of saturation has an important bearing on suction characteristics, swell potential or swell pressure and hydraulic conductivity of compacted bentonite (Nayak and Christensen, 1971; Lu and Likos, 2004; Masrouri et al., 2008). Dependency of soil suction on the degree of saturation is illustrated by the soil water characteristic curve (SWCC) developed for varying compaction condition and type of soils (Fredlund and Rahardjo, 1993; Fredlund and Xing, 1994; Barbour, 1998; Miao et al., 2002; Aubertin et al., 2003; Chen et al., 2006; Sun et al., 2010). Komine and Ogata (1994, 1996a, b), Delage et al. (1998), Lee et al. (1999), Komine (2004), Villar and Lloret (2008) and Wang et al. (2012) have examined the swell potential or swell pressure of bentonite or bentonite sand mixture as a function of density, initial water content or suction in the context of buffer performance in the DGR conditions. The above mentioned studies used Kunigel VI, FoCa clay, FEBEX, Jinmyeong and MX-80 bentonites
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for potential use in the DGR in Japan, Belgium, Spain, Korea and Sweden respectively. The studies bring out that dry density has a strong bearing on the swell pressure of compacted bentonites. However, the influence of initial moisture content or degree of saturation on the swell pressure or potential has not been well defined. Results of Komine and Ogata (1994) and Villar and Lloret (2008) indicate that the swell pressure of Kunigel VI and FEBEX bentonites is independent of initial water content for dry densities ranging from 1.4 to 2 Mg/m3; comparatively, swelling pressure of Jinmyeong and MX-80 bentonites is affected by initial water content or suction at densities ranging from 1.6 to 1.9 Mg/ m3 (Lee et al., 1999; Wang et al., 2012). Thus the dependency of initial moisture content or degree of saturation or suction does not follow a definite trend for compacted bentonites. This paper examines the influence of initial degree of saturation of montmorillonite voids on swell pressure of compacted bentonite–sand mixtures (dry density range: 1.4–2 Mg/m3) from micro-structural considerations. The bentonite clay used in the study is from Rajasthan, India (termed Barmer clay). Based on the evaluation of physico-chemical and engineering properties of several Indian bentonite clays for their suitability as buffer material in deep geological repository (Rao et al., 2008), Barmer bentonite was identified as suitable buffer material. The micro-structure of the bentonite specimens were examined in the compacted and wetted states by performing X-ray diffraction (XRD) measurements.
2. Experimental programme
ethylene glycol treatment (Greene-Kelly, 1952). JEOL X Ray Diffractometer (Model: JDX 8030) was used to obtain the XRD pattern (2h range, 3–60°; scanning rate 0.5°/min) and the minerals present were analyzed using X’pert high score software (PAN-alytical B.V., Almelo, Netherlands). The montmorillonite content in bentonite was determined by methylene blue adsorption method (Pusch, 2002). The total dissolved solid concentration of bentonite sand was estimated as per USDA procedure (USDA, 1954). 10 g of bentonite was remoulded with 50 mL of distilled water and equilibrated for 48 h. The pore fluid was extracted by centrifuging the slurry at 30,000 rpm (revolutions per minute). The extracted fluid was filtered through a 0.45 l filter (MF-Millipore Membrane filter paper; No: HAWP04700) and analysed for various cations and anions using a ICP-OES (Model: Thermo-ICAP 6500) and an ion chromatograph (model: Dionex ICS 2000). The cation and anion concentrations per unit mass of bentonite were derived from these results. The total specific surface area of bentonite powder was measured using Ethylene Glycol Monoethyl Ether (EGME) adsorption method (Cerato and Lutenegger, 2002). Triplicate measurements were performed with the powder specimens and the average of three measurements is reported. The percentage variation from the average for the powder specimens corresponded to 1% to 5%. The external surface area of the powder specimen was measured using the Brunauer, Emmett and Teller (BET) technique (Brunauer et al., 1938) that is based on nitrogen gas adsorption by the external surface of clay particles. Properties of bentonite and sand used are summarised in Table 1. The detailed characterisation of Barmer bentonite was earlier reported by Rao and Ravi (2013).
2.1. Materials Bentonite clay from Barmer District, Rajasthan, India and river sand from Karnataka were used in the study. A physical mixture comprising of 70% bentonite + 30% sand was selected for the study (termed bentonite enhanced sand – BES, after Stewart et al., 2003) as it is considered advantageous to deploy a mixture than pure bentonite to facilitate ease of handling, manufacturing and economic efficiency in the DGR (JNC, 2000). The index properties and cation exchange capacity of Barmer bentonite were determined as per BIS standards IS 2720 (Parts 3-5)-1987 and IS 2720 (Part XXIV)-1976, respectively. The concentrations of exchangeable calcium, magnesium, sodium and potassium ions in the ammonium acetate extract were determined using an inductively coupled plasma – optical emission spectrometer (ICP-OES Model: Thermo-ICAP 6500). The X-ray diffraction (XRD) pattern (Fig. 1) of bentonite was obtained after subjecting the specimen to
2.2. Preparation of compacted specimens Batches of 70% bentonite + 30% sand (on dry mass basis) mixture were remoulded with different volumes of distilled water (DW) to obtain mixtures with moisture contents ranging from 9 to 22%. The remoulded mixtures were stored in desiccator for 24 h to facilitate uniform moisture distribution and then statically compacted in single layer to dry densities of 1.4, 1.5, 1.75 and 2 Mg/m3 in stainless steel rings (diameter 60 mm and height 20 mm) by applying compaction stress of 35 MPa. The final thickness of the specimens corresponded to 8 mm. The dry density of the 1.4 Mg/m3 series specimens range from 1.38 to 1.41 Mg/m3, the dry density of the 1.5 Mg/m3 series specimens range from 1.48 to 1.50 Mg/m3, the dry density of the 1.75 Mg/m3 series specimens range from 1.72 to 1.77 Mg/m3 and of the 2 Mg/m3 series
Fig. 1. X ray diffraction pattern of Ethylene Glycol treated Barmer bentonite.
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 1 Properties of bentonite and sand used in the study. Property
Value
Bentonite Specific gravity Liquid limit Plastic limit Shrinkage limit Unified soil classification Cation exchange capacity Free salts concentration (1:5 extract) Total surface area External surface area (BET)
2.69 409% 42% 11% CH 80.00 meq/100 g 7.77 mg/g (1546 mg/L) 450 m2/g 20 m2/g
Sand Specific gravity Silt (75 lm – 2 lm) Sand Fine (75 lm – 425 lm) Medium (425 lm – 2 mm) Coarse (2 mm – 4.75 mm)
39% 54% 4%
Cu Cc Unified soil classification
3.33 0.83 SP
2.64 3%
before and after swell pressure tests were presented in Tables 2– 9. Tables 2–5 present oedometer specimen data before swell pressure measurements. At a given compaction condition, variations in measured specimen volume, water content and dry density for a given set of triplicate specimens range between 0.02–1.56%, 0.03–5.79% and 0.0–1.5% respectively. Likewise, variations in specimen volumes, water content and dry density after swell pressure test range between 0.03–2.26%, 0.05–3.14% and 0.0–2.04% respectively (Tables 6–9). The average of three measurements for any given compaction condition is used in the study (Table 10). Comparison of data in Tables 2–9 bring out that despite increase in water content from swell pressure mobilization, specimen volumes remain unaltered. 2.4. Calculation of effective clay dry density (ECDD) and effective montmorillonite dry density (EMDD) The average dry densities of the BES specimens (Table 10) were used to calculate the effective clay dry density (ECDD) and effective montmorillonite dry density (EMDD) according to equations (Kurosawa et al., 2006):
specimens range from 1.96 to 1.99 Mg/m3. Slight variations from target density were inevitable due to elastic deformations of the compacted clay on release of compaction stress.
ECDD ¼
qd;BES ð100 Rs Þ 100 qd;BES
EMDD ¼ 2.3. Swell pressure measurements
ð1Þ
Rs
qs
f m ECDD 1 ð1fqm ÞECDD
ð2Þ
others
The swell pressures of the compacted BES specimens were determined by a constant volume method using test apparatus and a procedure similar to that outlined by Komine (2004). The compacted BES specimens were inundated with distilled water in oedometer cells under constant volume condition. The swelling stress developed by the inundated specimen was measured using a load cell as function of time until equilibrium was attained. The specimens were inundated with double drainage condition with drainage paths on top and bottom of the specimens. The time required for attaining equilibrium swell pressure varied from 280 to 160 h depending upon the density and initial degree of saturation. The swelling stress at equilibrium represents the swell pressure. Following the attainment of swell pressure, the wetted specimens were dismantled from the oedometer cells and their moisture content and total volume were determined. Swell pressure measurements were performed in triplicate at each dry density and water content. The measured specimen volumes, water contents and dry density of triplicate specimens
where qd,BES is the average dry density of triplicate specimens with similar initial conditions, Rs is the fraction of sand in BES mixture, qs is the density of sand, fm is the fraction of montmorillonite in bentonite and qothers is the density of other minerals which is taken as 2.7 Mg/m3 (Kurosawa et al., 2006). The EMDD values were used to calculate the void ratio of the montmorillonite fraction in BES specimens according to the equation:
eEMDD ¼
GM qw 1 EMDD
ð3Þ
The degree of saturation of the montmorillonite fraction (Sr,MF) in the BES specimens was calculated as:
Sr;MF ¼
wGM eEMDD
ð4Þ
where GM is the specific gravity of montmorillonite which is taken as 2.74 (Lambe and Whitman, 1969).
Table 2 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 1.4 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.74 20.57 20.57
0.54 0.26 0.28
10.55 10.87 11.03
2.47 0.49 1.97
1.38 1.40 1.37
0.40 1.24 0.84
2
20.46 20.29 20.29
0.54 0.26 0.28
13.29 12.90 13.50
0.44 2.49 2.05
1.39 1.40 1.37
0.13 0.97 1.10
3
20.16 20.29 20.55
0.86 0.20 1.06
16.49 17.00 15.90
0.16 3.26 3.42
1.41 1.39 1.40
0.71 0.71 0.00
4
20.18 20.01 20.57
0.37 1.18 1.56
17.92 18.40 17.30
0.26 2.95 3.21
1.39 1.41 1.37
0.10 1.25 1.15
5
20.46 20.29 20.57
0.08 0.71 0.63
21.71 22.40 21.00
0.03 3.21 3.24
1.38 1.41 1.37
0.48 1.50 1.02
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 3 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 1.5 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.72 20.57 20.25
1.00 0.29 1.29
10.49 11.09 9.90
0.03 5.69 5.66
1.49 1.51 1.48
0.40 1.25 0.85
2
20.72 20.29 20.53
1.00 1.06 0.07
13.29 13.00 13.80
0.55 2.72 3.27
1.48 1.50 1.47
0.22 1.12 0.90
3
20.41 20.29 20.25
0.47 0.12 0.35
16.42 16.90 16.10
0.33 2.59 2.26
1.50 1.48 1.51
0.28 1.14 0.86
Table 4 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 1.75 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
1
20.18 20.57 20.25
0.76 1.18 0.42
2
20.41 20.29 20.25
3
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
8.69 9.02 9.50
4.22 0.54 4.76
1.77 1.75 1.76
0.59 0.58 0.01
0.47 0.12 0.35
10.49 10.80 10.10
0.26 3.22 3.47
1.73 1.74 1.75
0.57 0.00 0.57
20.14 20.02 20.25
0.02 0.58 0.57
13.03 13.80 12.80
1.34 4.45 3.11
1.75 1.76 1.74
0.00 0.57 0.57
4
20.14 20.29 20.25
0.44 0.34 0.11
12.64 13.04 12.50
0.68 2.46 1.78
1.73 1.74 1.73
0.19 0.38 0.19
5
20.16 20.29 20.25
0.37 0.30 0.07
15.48 15.98 15.00
0.03 3.18 3.15
1.72 1.74 1.73
0.58 0.58 0.00
Table 5 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 2.0 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
1
20.14 20.57 20.25
0.90 1.25 0.35
2
20.16 20.57 20.25
3
4
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
8.69 9.02 9.50
4.22 0.54 4.76
1.97 1.98 1.97
0.17 0.34 0.17
0.82 1.21 0.39
10.49 10.80 10.10
0.26 3.22 3.47
1.97 1.98 1.97
0.17 0.34 0.17
20.14 20.29 20.25
0.44 0.34 0.11
12.00 12.80 11.50
0.83 5.79 4.96
1.96 1.99 1.97
0.68 0.84 0.17
20.14 20.29 20.25
0.44 0.34 0.11
13.03 13.60 12.98
1.29 2.99 1.70
1.99 2.00 1.98
0.00 0.50 0.50
2.5. X-ray diffraction analysis XRD analysis of BES specimens in the compacted and wetted states were performed on 1 mm thin slices using JEOL X Ray Diffractometer (model JDX 8030) over 2h range of 3–33° (scanning rate 0.5°/min). This model of JEOL X Ray diffractometer has a provision to record XRD patterns of dry and moist specimens. 3. Results and discussions 3.1. Swell pressure of compacted BES specimens The montmorillonite fraction of BES specimens is responsible for moisture absorption during compaction and wetting in the
oedometer tests. Consequently, it was considered reasonable to calculate degree of saturation based on EMDD using Eqs. (1)–(4). The developed swell pressure values are correlated with Sr,MF (degree of saturation of montmorillonite voids) in subsequent figures. The gross dry density (qd), EMDD and Sr,MF of BES specimens are presented in Table 11. Fig. 2 plots the variation of swell pressure with initial Sr,MF values of BES specimens at dry densities of 1.4, 1.5, 1.75 and 2 Mg/m3. The EMDD values for gross dry densities of 1.4, 1.5, 1.75 and 2.0 Mg/m3 correspond to 0.92, 1.00, 1.27 and 1.57 Mg/m3, respectively. The swell pressures of the BES specimens are insensitive to initial degree of montmorillonite saturation at lower dry densities of 1.4 and 1.5 Mg/m3. In comparison, the swell pressures of denser specimens respond to variations in Sr,MF values.
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 6 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 1.4 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.63 20.66 20.68
0.15 0.03 0.12
42.34 42.95 42.10
0.29 1.15 0.85
1.39 1.39 1.38
0.24 0.24 0.48
2
20.63 20.66 20.40
0.30 0.48 0.78
44.09 42.00 44.00
1.68 3.14 1.47
1.38 1.38 1.39
0.32 0.02 0.34
3
20.33 20.39 20.66
0.64 0.36 1.00
51.26 52.00 51.40
0.57 0.87 0.29
1.40 1.40 1.39
0.24 0.24 0.48
4
20.63 20.10 20.96
0.31 2.26 1.94
38.79 39.20 38.50
0.10 0.95 0.85
1.41 1.41 1.39
0.53 0.33 0.86
5
20.35 20.38 20.68
0.59 0.45 1.04
39.17 40.30 39.00
0.81 2.05 1.24
1.39 1.38 1.40
0.02 0.97 0.99
Table 7 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 1.5 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.89 20.66 20.36
1.23 0.11 1.35
38.72 37.90 38.10
1.26 0.89 0.37
1.48 1.52 1.47
0.65 2.04 1.40
2
20.89 20.38 20.36
1.69 0.79 0.90
38.25 39.00 37.90
0.35 1.61 1.26
1.49 1.51 1.48
0.22 1.12 0.89
3
20.87 20.38 20.36
1.61 0.75 0.86
37.44 38.00 37.00
0.10 1.38 1.28
1.49 1.49 1.5
0.22 0.22 0.45
Table 8 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 1.75 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.36 20.38 20.67
0.55 0.41 0.97
24.28 23.80 24.60
0.21 1.76 1.55
1.76 1.75 1.76
0.19 0.38 0.19
2
20.03 20.11 20.39
0.70 0.35 1.05
29.68 30.01 29.40
0.05 1.05 1.00
1.75 1.76 1.75
0.19 0.38 0.19
3
20.03 20.38 20.39
1.16 0.57 0.59
31.69 32.30 31.04
0.05 1.97 2.01
1.74 1.75 1.73
0.00 0.57 0.57
4
20.31 20.38 20.39
0.24 0.11 0.13
28.48 29.02 27.90
0.05 1.94 1.99
1.76 1.75 1.76
0.19 0.38 0.19
5
20.34 20.38 20.39
0.16 0.07 0.09
35.18 34.80 35.50
0.05 1.02 0.97
1.73 1.75 1.73
0.38 0.77 0.38
The swell pressures of 1.75 Mg/m3 specimens reduce from 1359 to 799 kPa upon increase in Sr,MF from 22 to 30% and later became constant. Likewise, the swell pressures of 2 Mg/m3 specimens respond to increase in degree of saturation by reducing from 2087 to 783 kPa upon increase in Sr,MF from 31 to 49%. The final Sr,MF values after swell pressure measurements vary from 54 to 73%, 61 to 62%, 60 to 83% and 83 to 110% for 1.4, 1.5, 1.75 and 2 Mg/m3 series specimens, respectively. The variation in final Sr,MF values is attributed to variations in initial compaction conditions of the specimens. Slightly larger than 100%
degree of saturation were observed at the maximum dry density of 2.0 Mg/m3. Earlier researchers have observed that density of water adjacent to clay layers can deviate from unity as the structure of the water molecules are possibly disturbed by the physico-chemical interactions between inter-lamellar water and montmorillonite surfaces (Skipper et al., 1991; Hueckel, 1992; Mitchell, 1993; Ichikawa et al., 1999; Villar and Lloret, 2004). Villar and Lloret (2008) report that the density of inter-lamellar water in compacted bentonite specimens range from 1.0 to 1.35 Mg/m3. Slightly larger final Sr,MF values at 2.0 Mg/m3 may
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 9 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 2.0 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.03 20.38 20.39
1.16 0.57 0.59
23.48 24.00 22.90
0.09 2.30 2.39
1.98 1.99 1.97
0.00 0.51 0.51
2
20.34 20.11 20.11
0.76 0.39 0.37
27.00 27.60 26.70
0.38 1.85 1.47
1.99 1.99 1.99
0.00 0.00 0.00
3
20.31 20.38 20.39
0.24 0.11 0.13
26.45 27.00 26.01
0.15 1.94 1.80
1.97 1.98 1.97
0.17 0.34 0.17
4
20.31 20.38 20.14
0.17 0.52 0.69
31.69 32.10 31.40
0.12 1.16 1.04
1.98 1.99 1.97
0.00 0.51 0.51
Table 10 Average specimen volumes, water contents and dry densities of triplicate measurements. Volume of the specimen (cm3)
Water content (%)
Compacted state
After swell pressure test
Compacted state
After swell pressure test
Compacted state
After swell pressure test
1.4 Mg/m3
20.63 20.35 20.33 20.25 20.44
20.66 20.56 20.46 20.56 20.47
11 13 16 18 22
42 43 51 39 39
1.38 1.39 1.41 1.39 1.39
1.39 1.37 1.4 1.4 1.38
1.5 Mg/m3
20.51 20.51 20.32
20.64 20.55 20.54
10 13 16
39 38 37
1.49 1.48 1.50
1.48 1.49 1.49
1.75 Mg/m3
20.33 20.32 20.13 20.23 20.23
20.47 20.18 20.27 20.36 20.37
9 10 13 13 15
24 30 28 32 35
1.77 1.73 1.73 1.75 1.72
1.76 1.75 1.76 1.74 1.73
2 Mg/m3
20.32 20.33 20.23 20.23
20.27 20.18 20.36 20.28
9 10 12 13
23 27 26 32
1.97 1.97 1.96 1.99
1.98 1.99 1.97 1.99
Test series
Dry density (Mg/m3)
Table 11 Average dry densities, degree of saturation of montmorillonite (Sr,MF), c-axis spacing of the specimens before and after swell pressure test and the swell pressure values. Test seriesa
a b
Dry density of specimens/ EMDDa (Mg/m3)
Degree of saturation of montmorillonite (%)
Compacted state
After swell pressure test
Compacted state
1.4 (0.92) Mg/m3
1.38 1.39 1.41 1.39 1.39
1.39 1.37 1.40 1.40 1.38
(0.91) (0.89) (0.92) (0.92) (0.91)
15 19 24 25 31
59 59 73 55 54
1.5 (1.00) Mg/m3
1.49 (1.00) 1.48 (0.99) 1.50 (1.00)
1.48 (0.99) 1.49 (1.00) 1.49 (1.00)
17 21 27
1.75 (1.27) Mg/m3
1.77 1.73 1.73 1.75 1.72
(1.29) (1.25) (1.25) (1.27) (1.24)
1.76 1.75 1.76 1.74 1.73
(1.28) (1.27) (1.28) (1.26) (1.25)
2.0 (1.57) Mg/m3
1.97 1.97 1.96 1.99
(1.53) (1.53) (1.52) (1.55)
1.98 1.99 1.97 1.98
(1.54) (1.55) (1.53) (1.54)
(0.90) (0.91) (0.93) (0.91) (0.91)
Swell pressure (kPa)
Compacted state
After swell pressure test
244 305 315 287 252
13.00 13.90 14.90 15.35 16.60
21.10 21.00 21.20 20.80 20.85
62 62 61
416 427 395
13.10 (1.04) 14.00 (1.31) 15.10 (1.63)
21.20 (3.45) 21.30 (3.48) 20.80 (3.33)
22 25 30 32 36
60 73 71 76 83
1359 986 799 783 754
13.20 14.00 15.00 16.00 17.50
(1.07) (1.31) (1.61) (1.90) (2.35)
21.10 21.00 20.40 19.90 19.80
(3.42) (3.39) (3.21) (3.07) (3.04)
31 35 42 49
86 100 95 110
2087 1410 905 783
14.30 15.10 17.10 18.00
(1.40) (1.64) (2.23) (2.50)
22.50 22.00 21.70 21.30
(3.84) (3.69) (3.60) (3.48)
Values in parentheses correspond to dry density of montmorillonite (EMDD). Values in parentheses correspond to number of hydration layers.
After swell pressure test
c-Axis spacing (Å)/number of hydration layersb
(1.01) (1.28) (1.58) (1.71) (2.08)
(3.42) (3.39) (3.45) (3.33) (3.35)
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2500 1.4 Mg/m³ 1.5 Mg/m³
2000
Swell pressure (kPa)
1.75 Mg/m³ 2 Mg/m³
1500
1000
500
0 0
10
20
30
40
50
Intial degree of saturation of montmorillonite, S r,MF (%) Fig. 2. Variation of swell pressure with initial degree of saturation of montmorillonite at different dry densities.
possibly be a consequence disturbances in density of interlamellar water of montmorillonite. 3.2. Micro-structural investigations X-ray diffraction results of BES specimens were examined for the compacted and wetted conditions (Table 11). The compacted state or compacted specimen refers to specimens that have not been subjected to wetting; the wetted state or wetted specimen refers to compacted specimens that were wetted and allowed to attain equilibrium swell pressure under constant volume conditions. The c-axis spacing of 1.4 Mg/m3 dry density series specimens increased from 13.00 Å to 16.60 Å upon increase in Sr,MF from 15 to 31%. Similar observations were made for the 1.5, 1.75 and 2 Mg/m3 series specimens. The c-axis spacing of wetted specimens however did not show uniform trend. The c-axis spacing of 1.4 Mg/m3 series specimens expand similarly on wetting despite being characterized by varying initial degree of saturation values; the average caxis spacing of the wetted 1.4 Mg/m3 specimens corresponds to 20.98 Å (% variation from average ± 0.6%). A similar trend of results is observed for the 1.5 Mg/m3 series specimens that exhibit average c-axis spacing of 21.05 Å in the wetted state (% variation from average ± 0.95%). Comparatively, c-axis spacing of the denser specimens (1.75 and 2 Mg/m3) are sensitive to variations in initial degree of saturation in the wetted state. The water layer thickness in the first, second and third hydration layers of sodium-montmorillonite correspond to 3.03, 3.23 and 3.48 Å, respectively (Pusch and Yong, 2006). The c-axis spacing of oven dried Barmer bentonite specimen (essentially composed of sodium-montmorillonite) corresponds to 9.98 Å. Based on these data and c-axis spacing, the number of hydration layers in the compacted and wetted states of the BES specimens has been obtained (Table 11). Specimens compacted to dry density of 1.4 Mg/m3 develop 3.39 layers of water molecules in the wetted state (average value) at all initial Sr,MF values. Likewise specimens compacted to dry density of 1.5 Mg/m3 develop 3.41 layers of water molecules in the wetted state (average value) at all initial Sr,MF values. Despite developing similar number of hydration layers in the wetted state (3.39 and 3.41), the 1.4 and 1.5 Mg/m3 dry density series specimens, develop different swell pressures (average values: 283 and 413 kPa, respectively). The results indicate that compaction dry density influences the swell pressure when specimens are compacted differently. However, when specimens are compacted to same dry density (1.4 or 1.5 Mg/m3), the number of hydration layers developed upon wetting appear to influence the swell pressure.
The c-axis spacing and number of hydration layers of the denser specimens respond to wetting. The c-axis spacing and number of hydration layers in the wetted state reduce from 21.10 to 19.80 Å and from 3.42 to 3.04 respectively when initial Sr,MF values increase from 22 to 36% for the 1.75 Mg/m3 series specimens. Likewise, wetting reduces the c-axis spacing and number of hydration layers of the 2 Mg/m3 series specimens from 22.50 to 21.30 Å and from 3.84 to 3.48 at initial Sr,MF variations of 31 to 49%. Fig. 3 plots the variation of swell pressures with number of hydration layers in the wetted state for the 1.4, 1.5, 1.75 and 2 Mg/m3 series specimens. The 1.4 Mg/m3 series specimens exhibit similar swell pressures as they develop similar number of hydration layers on wetting, irrespective of the initial degree of saturation. The same holds true for the 1.5 Mg/m3 series specimens. The 1.75 and 2 Mg/m3 series specimens develop progressively larger hydration layers in the wetted state and consequently exhibit increasingly higher swell pressures with reducing initial Sr,MF values. That the initial degree of saturation of montmorillonite voids has bearing on number of hydration layers developed by denser (1.75–2 Mg/m3) but not looser (1.4–1.5 Mg/m3) Barmer specimens upon wetting needs investigations.
3.3. Practical implications of the study Based on the results of earlier researchers (Villar and Lloret, 2008; Komine and Ogata, 1994) and observations of the present study it appears beneficial to compact bentonite buffer to dry densities of 61.5 Mg/m3 as their swell pressures are expected to be unaffected by variations in degree of saturation. For satisfactory performance in deep geological repository environment, SKB (2006) recommends that bentonite buffer should develop minimum swell pressures of 1–2 MPa. Review of literature shows that compacted (dry density = 1.5 Mg/m3) FEBEX bentonite develop swell pressures >2 MPa (Villar and Lloret, 2008), while Kunigel VI (Komine and Ogata, 1994), Jimmyeong (Lee et al., 1999) and 70% Barmer bentonite + 30% sand (present study) develop swell pressure < 1 MPa at compaction dry density of 1.5 Mg/m3. The 1.75 and 2 Mg/m3 series BES specimens experience 44–62% decrease in swell pressure upon increase in degree of saturation from 22 to 36% and 31 to 49%, respectively. Hence, the possibility of reduction in swell pressure of densely compacted bentonite buffer (dry density > 1.5 Mg/m3) from increase in Sr,MF should be factored in during design and operation of deep geological repositories.
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Swell pressure (kPa)
2000
1.5 Mg/m³ 1.75 Mg/m³ 2 Mg/m³
1500
1000
500
0 3.00
3.10
3.20
3.30
3.40
3.50
3.60
3.70
3.80
3.90
4.00
Number of hydration layers in saturated state
Fig. 3. Variation of swell pressure with number of hydration layers in the saturated state.
4. Conclusions The influence of initial degree of saturation on swell pressures of compacted Barmer bentonite + sand specimens was tied up with their dry densities. The swell pressures of 1.4 and 1.5 Mg/m3 dry density series specimens were unaffected by variations in initial degree of saturation of montmorillonite voids (Sr,MF). Comparatively, swell pressures of denser specimens (1.75 and 2 Mg/m3) responded to variations in initial Sr,MF values. XRD measurements demonstrated that when specimens belonging to common density series developed similar number of hydration layers on wetting they exhibited similar swell pressures and vice versa. The former was true for 1.4 and 1.5 Mg/m3 series specimens and latter for specimens belonging to 1.75 and 2 Mg/m3 series. When specimens developed similar number of hydration layers on wetting, the compaction dry density determined the swell pressure. For example, swell pressures of specimens that developed 3.8 hydration layers in the wetted state followed the sequence: 2.0 Mg/m3 > 1.75 Mg/ m3 > 1.5 Mg/m3 > 1.4 Mg/m3. Results of the study also indicate that the possibility of swell pressure reduction from increase in Sr,MF should be considered during design and operation of bentonite buffer compacted at dry densities > 1.5 Mg/m3. References Aubertin, M., Mbonimpa, M., Bussiere, B., Chapius, R.P., 2003. A model to predict the water retention curve from basic geotechnical properties. Can. Geotech. J. 40, 1104–1122. Barbour, S.L., 1998. Nineteenth canadian geotechnical colloquium: the soil-water characteristic curve: a historical perspective. Can. Geotech. J. 35, 873–894. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60 (2), 309–319. Bucher, F., Vonmoos, M.M., 1989. Bentonite as a contaminant barrier for the disposal of highly radioactive waste. Appl. Clay Sci. 4 (2), 157–177. Cerato, A.B., Lutenegger, A.J., 2002. Determination of surface area of fine-grained soils by the ethylene glycol monoethyl ether (EGME) method. Geotech. Test. J. 25 (3). http://dx.doi.org/10.1520/GTJ11087J. Chen, B., Qian, L., Ye, W., Cui, Y., Wang, J., 2006. Soil water characteristic curves of Gaomiaozi bentonite. Chin. J. Rock Mech. Eng. 25 (4), 788–794. Cho, W.J., Lee, J.O., Chun, K.S., 1999. Basic physicochemical and mechanical properties of domestic bentonite for use as a buffer material in a high level radioactive waste repository. J. Korean Nucl. Soc. 31 (6), 39–50. Delage, P., Howat, M.D., Cui, Y.J., 1998. The relationship between suction and swelling properties in a heavily compacted unsaturated clay. Eng. Geol. 50, 31– 48. Dixon, D.A., Gray, M.N., Thomas, A.W., 1985. A study of the compaction properties of potential clay-sand buffer mixtures for the use in nuclear fuel waste disposal. Eng. Geol. 21 (3–4), 247–255. Duquette, D.J., Latanision, R.M., Di Bella, C.A.W., Kirstein, B.E. 2008. Corrosion issues related to disposal of high-level nuclear waste in the yucca mountain repository. In: NACE International, CORROSION 2008, Conference and Expo, New Orleans, Louisiana.
Fredlund, D.G., Rahardjo, H., 1993. Soil Mechanics for Unsaturated Soils. John Wiley and Sons Inc., New York. Fredlund, D.G., Xing, A., 1994. Equations for soil water characteristic curve. Can. Geotech. J. 31, 521–532. Garcia, S.G., Jonsson, M., Wold, S., 2006. Temperature effect on the stability of bentonite colloids in water. J. Colloid Interf. Sci. 298, 694–705. Greene-Kelly, R., 1952. A test for montmorillonite. Nature 170, 1130–1131. Hueckel, T., 1992. Water–mineral interaction in hygromechanics of clays exposed to environmental loads: a mixture-theory approach. Can. Geotech. J. 29, 1071– 1086. Ichikawa, Y., Kawamura, K., Nakano, M., Kitayama, K., Kawamura, H., 1999. Unified molecular dynamics and homogenization analysis for bentonite behaviour: current results and future possibilities. Eng. Geol. 54, 21–31. IS 2720, Part 4, 1987. Methods of test for soils: grain size analysis. Bureau of Indian Standards, New Delhi. IS 2720, Part 5, 1987. Methods of test for soils: determination of liquid and plastic limit. Bureau of Indian Standards, New Delhi. IS 2720, Part XXIV, 1976. Methods of test for soils: determination of cation exchange capacity. Bureau of Indian Standards, New Delhi. IS 2720, Part 3, 1987. Methods of test for soils: Determination of specific gravity. Bureau of Indian Standards, New Delhi. JNC, 2000. H12: Project to Establish the Scientific and Technical Basis for HLW Disposal in Japan: Project Overview Report and Supporting Reports I, II, III. Japan Nuclear Cycle Development Institute, 295. Komine, H., 2004. Simplified evaluation for swelling characteristics of bentonites. Eng. Geol. 71, 265–279. Komine, H., Ogata, N., 1994. Experimental study on swelling characteristics of compacted bentonite. Can. Geotech. J. 31, 478–490. Komine, H., Ogata, N., 1996a. Predicting swelling characteristics of bentonites. J. Geotechnical. Geoenviron. Eng. ASCE 130, 818–829. Komine, H., Ogata, N., 1996b. Prediction for swelling characteristics of compacted bentonite. Can. Geotech. J. 33, 11–22. Kurosawa, S., Tanaka, S., James, S.C., Yui, M., 2006. The effect of montmorillonite partial density on the role of colloid filtration by a bentonite buffer. J. Nucl. Sci. Tech. 43 (5), 605–609. Lambe, T.W., Whitman, R.V., 1969. Soil Mechanics. John Wiley and Sons Inc., New York. Lee, J.O., Cho, W.J., Kang, C.H., Chun, K.S. 1999. Swelling and hydraulic properties of Ca-bentonite for the buffer of a waste repository. In: Proceedings of Symposium on Technologies for the Management of Radioactive Waste from Nuclear Power Plants and Back End Nuclear Fuel Cycle Activities. Taejon, Republic of Korea. Lloret, A., Villar, M.V., Sanchez, M., Gens, A., Pintado, X., Alonso, E.E., 2003. Mechanical behaviour of heavily compacted bentonite under high suction changes. Geotechnique 53 (1), 27–40. Lu, N., Likos, W.J., 2004. Unsaturated Soil Mechanics. John Wiley and Sons Inc., New Jersey. Masrouri, F., Bicalho, K.V., Kawai, K., 2008. Laboratory hydraulic testing in unsaturated soils. Geotech. Geol. Eng. 26, 691–704. Miao, L., Liu, S., Lai, Y., 2002. Research of soil–water characteristics and shear strength features of Nanyang expansive soil. Eng. Geol. 65, 261–267. Mitchell, J.K., 1993. Fundamentals of Soil Behavior, second ed. John Wiley and Sons Inc., New York. Nayak, N.V., Christensen, R.W., 1971. Swelling characteristics of compacted, expansive soils. Clays Clay Miner. 19, 251–261. Pusch, R., 1994. Waste disposal in rock. In: Developments in Geotechnical Engineering, 76. Elsevier, Amsterdam, p. 490. Pusch, R., 2002. The buffer and backfill handbook. Part 1: definitions, basic relationships and laboratory methods. SKB Technical Reports, TR-02-20.
S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Pusch, R., Yong, R.N., 2006. Micro structure of Smectite Clays and Engineering Performance. Taylor and Francis, New York. Rao, S.M., Ravi, K., 2013. Hydro-mechanical characterization of Barmer 1 bentonite from Rajasthan, India. Nucl. Eng. Des. 265, 330–340. Rao, S.M., Kachroo, T.A., Allam, M.M., Joshi, M.R., Acharya, A. 2008. Geotechnical characterization of some indian bentonites for their use as buffer material in geological repository. In: Proceedings of 12th International Conference Of International Association for Computer. Methods and Advances in Geomechanics (IACMAG). Goa, India, pp. 2106–2114. SKB, 2006. Long-term safety for KBS-3 repositories at Forsmark and Laxemar – a first evaluation. SKB Technical Reports. TR-06-09. Skipper, N.T., Refson, K., McConnell, J.D.C., 1991. Computer simulation of interlayer water in 2:1 clays. J. Chem. Phys. 94 (11), 7434–7445. Stewart, D.I., Studds, P.G., Cousens, T.W., 2003. The factors controlling the engineering properties of bentonite-enhanced sand. Appl. Clay Sci. 23 (1–4), 97–110.
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Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Influence of initial degree of saturation on swell pressures of compacted Barmer bentonite specimens Sudhakar M. Rao a,1, K. Ravi b,⇑ a b
Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India Geo-environmental Research Centre (GRC), Cardiff University, CF24 3AA, United Kingdom
a r t i c l e
i n f o
Article history: Received 18 February 2014 Received in revised form 13 October 2014 Accepted 1 February 2015 Available online 26 February 2015 Keywords: Bentonite Geological repository Microstructure Saturation Swell
a b s t r a c t Densely compacted bentonite or bentonite–sand mixture has been identified as suitable buffer in deep geological repositories as its exceptionally high swelling capacity enables tight contact between the waste canister and surrounding rock. The degree of saturation of the compacted bentonite buffer can increase upon ingress of groundwater from the surrounding rock mass or decrease from evaporation due to high temperature (50–210 °C) derived from the waste canister. Available studies indicate that the influence of initial moisture content or degree of saturation on the swell pressure or swell potential of compacted bentonites is unclear. Some studies suggest that initial degree of saturation has an influence, while others suggest that it does not have bearing on the swell pressure of compacted bentonites. This paper examines the influence of initial degree of saturation in montmorillonite voids (termed, Sr,MF) on swell pressure of compacted Barmer bentonite–sand mixtures (dry density range: 1.4–2 Mg/m3) from micro-structural considerations. The experimental results bring out that, constant dry density specimens that developed similar number of hydration layers upon wetting developed comparable swell pressures and were unaffected by variations in initial Sr,MF values. Comparatively, constant dry density specimens that developed dis-similar number of hydration layers upon wetting established different swell pressures and were responsive to variations in initial Sr,MF. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Disposal in deep geological repositories (DGR) is a widely accepted concept to dispose high level nuclear waste. The function of such a deep geological repository (DGR) is based on a system of successive superimposed barriers and consists of natural barrier, which is often considered to be either the hard crystalline rock or the clay rock and the engineered barrier system (EBS) that consist of the canister, buffer and back fill which isolates the potentially harmful radioactive waste from the biosphere (Dixon et al., 1985; Bucher and Vonmoos, 1989; Cho et al., 1999; SKB, 2006; Villar, 2007). Densely compacted bentonite or bentonite–sand mixture is considered as a suitable buffer as a part of EBS in a DGR due to its favourable, physico-chemical and engineering properties (Lloret et al., 2003; Pusch, 1994). The exceptionally high swelling ability of bentonite upon wetting is one such property as it helps the clay to maintain tight contact between the waste ⇑ Corresponding author. Tel.: +44 7448966538. E-mail addresses: [email protected] (S.M. Rao), [email protected] (K. Ravi). 1 Tel.: +91 8022932812. http://dx.doi.org/10.1016/j.anucene.2015.02.019 0306-4549/Ó 2015 Elsevier Ltd. All rights reserved.
canister and surrounding rock and seal any gaps, cracks or fissures which might be present. The degree of saturation of the compacted bentonite buffer may increase upon ingress of groundwater from the surrounding rock mass or decrease from evaporation due to increase in temperature (50–210 °C, Garcia et al., 2006; Duquette et al., 2008) of the waste canister. The degree of saturation has an important bearing on suction characteristics, swell potential or swell pressure and hydraulic conductivity of compacted bentonite (Nayak and Christensen, 1971; Lu and Likos, 2004; Masrouri et al., 2008). Dependency of soil suction on the degree of saturation is illustrated by the soil water characteristic curve (SWCC) developed for varying compaction condition and type of soils (Fredlund and Rahardjo, 1993; Fredlund and Xing, 1994; Barbour, 1998; Miao et al., 2002; Aubertin et al., 2003; Chen et al., 2006; Sun et al., 2010). Komine and Ogata (1994, 1996a, b), Delage et al. (1998), Lee et al. (1999), Komine (2004), Villar and Lloret (2008) and Wang et al. (2012) have examined the swell potential or swell pressure of bentonite or bentonite sand mixture as a function of density, initial water content or suction in the context of buffer performance in the DGR conditions. The above mentioned studies used Kunigel VI, FoCa clay, FEBEX, Jinmyeong and MX-80 bentonites
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for potential use in the DGR in Japan, Belgium, Spain, Korea and Sweden respectively. The studies bring out that dry density has a strong bearing on the swell pressure of compacted bentonites. However, the influence of initial moisture content or degree of saturation on the swell pressure or potential has not been well defined. Results of Komine and Ogata (1994) and Villar and Lloret (2008) indicate that the swell pressure of Kunigel VI and FEBEX bentonites is independent of initial water content for dry densities ranging from 1.4 to 2 Mg/m3; comparatively, swelling pressure of Jinmyeong and MX-80 bentonites is affected by initial water content or suction at densities ranging from 1.6 to 1.9 Mg/ m3 (Lee et al., 1999; Wang et al., 2012). Thus the dependency of initial moisture content or degree of saturation or suction does not follow a definite trend for compacted bentonites. This paper examines the influence of initial degree of saturation of montmorillonite voids on swell pressure of compacted bentonite–sand mixtures (dry density range: 1.4–2 Mg/m3) from micro-structural considerations. The bentonite clay used in the study is from Rajasthan, India (termed Barmer clay). Based on the evaluation of physico-chemical and engineering properties of several Indian bentonite clays for their suitability as buffer material in deep geological repository (Rao et al., 2008), Barmer bentonite was identified as suitable buffer material. The micro-structure of the bentonite specimens were examined in the compacted and wetted states by performing X-ray diffraction (XRD) measurements.
2. Experimental programme
ethylene glycol treatment (Greene-Kelly, 1952). JEOL X Ray Diffractometer (Model: JDX 8030) was used to obtain the XRD pattern (2h range, 3–60°; scanning rate 0.5°/min) and the minerals present were analyzed using X’pert high score software (PAN-alytical B.V., Almelo, Netherlands). The montmorillonite content in bentonite was determined by methylene blue adsorption method (Pusch, 2002). The total dissolved solid concentration of bentonite sand was estimated as per USDA procedure (USDA, 1954). 10 g of bentonite was remoulded with 50 mL of distilled water and equilibrated for 48 h. The pore fluid was extracted by centrifuging the slurry at 30,000 rpm (revolutions per minute). The extracted fluid was filtered through a 0.45 l filter (MF-Millipore Membrane filter paper; No: HAWP04700) and analysed for various cations and anions using a ICP-OES (Model: Thermo-ICAP 6500) and an ion chromatograph (model: Dionex ICS 2000). The cation and anion concentrations per unit mass of bentonite were derived from these results. The total specific surface area of bentonite powder was measured using Ethylene Glycol Monoethyl Ether (EGME) adsorption method (Cerato and Lutenegger, 2002). Triplicate measurements were performed with the powder specimens and the average of three measurements is reported. The percentage variation from the average for the powder specimens corresponded to 1% to 5%. The external surface area of the powder specimen was measured using the Brunauer, Emmett and Teller (BET) technique (Brunauer et al., 1938) that is based on nitrogen gas adsorption by the external surface of clay particles. Properties of bentonite and sand used are summarised in Table 1. The detailed characterisation of Barmer bentonite was earlier reported by Rao and Ravi (2013).
2.1. Materials Bentonite clay from Barmer District, Rajasthan, India and river sand from Karnataka were used in the study. A physical mixture comprising of 70% bentonite + 30% sand was selected for the study (termed bentonite enhanced sand – BES, after Stewart et al., 2003) as it is considered advantageous to deploy a mixture than pure bentonite to facilitate ease of handling, manufacturing and economic efficiency in the DGR (JNC, 2000). The index properties and cation exchange capacity of Barmer bentonite were determined as per BIS standards IS 2720 (Parts 3-5)-1987 and IS 2720 (Part XXIV)-1976, respectively. The concentrations of exchangeable calcium, magnesium, sodium and potassium ions in the ammonium acetate extract were determined using an inductively coupled plasma – optical emission spectrometer (ICP-OES Model: Thermo-ICAP 6500). The X-ray diffraction (XRD) pattern (Fig. 1) of bentonite was obtained after subjecting the specimen to
2.2. Preparation of compacted specimens Batches of 70% bentonite + 30% sand (on dry mass basis) mixture were remoulded with different volumes of distilled water (DW) to obtain mixtures with moisture contents ranging from 9 to 22%. The remoulded mixtures were stored in desiccator for 24 h to facilitate uniform moisture distribution and then statically compacted in single layer to dry densities of 1.4, 1.5, 1.75 and 2 Mg/m3 in stainless steel rings (diameter 60 mm and height 20 mm) by applying compaction stress of 35 MPa. The final thickness of the specimens corresponded to 8 mm. The dry density of the 1.4 Mg/m3 series specimens range from 1.38 to 1.41 Mg/m3, the dry density of the 1.5 Mg/m3 series specimens range from 1.48 to 1.50 Mg/m3, the dry density of the 1.75 Mg/m3 series specimens range from 1.72 to 1.77 Mg/m3 and of the 2 Mg/m3 series
Fig. 1. X ray diffraction pattern of Ethylene Glycol treated Barmer bentonite.
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 1 Properties of bentonite and sand used in the study. Property
Value
Bentonite Specific gravity Liquid limit Plastic limit Shrinkage limit Unified soil classification Cation exchange capacity Free salts concentration (1:5 extract) Total surface area External surface area (BET)
2.69 409% 42% 11% CH 80.00 meq/100 g 7.77 mg/g (1546 mg/L) 450 m2/g 20 m2/g
Sand Specific gravity Silt (75 lm – 2 lm) Sand Fine (75 lm – 425 lm) Medium (425 lm – 2 mm) Coarse (2 mm – 4.75 mm)
39% 54% 4%
Cu Cc Unified soil classification
3.33 0.83 SP
2.64 3%
before and after swell pressure tests were presented in Tables 2– 9. Tables 2–5 present oedometer specimen data before swell pressure measurements. At a given compaction condition, variations in measured specimen volume, water content and dry density for a given set of triplicate specimens range between 0.02–1.56%, 0.03–5.79% and 0.0–1.5% respectively. Likewise, variations in specimen volumes, water content and dry density after swell pressure test range between 0.03–2.26%, 0.05–3.14% and 0.0–2.04% respectively (Tables 6–9). The average of three measurements for any given compaction condition is used in the study (Table 10). Comparison of data in Tables 2–9 bring out that despite increase in water content from swell pressure mobilization, specimen volumes remain unaltered. 2.4. Calculation of effective clay dry density (ECDD) and effective montmorillonite dry density (EMDD) The average dry densities of the BES specimens (Table 10) were used to calculate the effective clay dry density (ECDD) and effective montmorillonite dry density (EMDD) according to equations (Kurosawa et al., 2006):
specimens range from 1.96 to 1.99 Mg/m3. Slight variations from target density were inevitable due to elastic deformations of the compacted clay on release of compaction stress.
ECDD ¼
qd;BES ð100 Rs Þ 100 qd;BES
EMDD ¼ 2.3. Swell pressure measurements
ð1Þ
Rs
qs
f m ECDD 1 ð1fqm ÞECDD
ð2Þ
others
The swell pressures of the compacted BES specimens were determined by a constant volume method using test apparatus and a procedure similar to that outlined by Komine (2004). The compacted BES specimens were inundated with distilled water in oedometer cells under constant volume condition. The swelling stress developed by the inundated specimen was measured using a load cell as function of time until equilibrium was attained. The specimens were inundated with double drainage condition with drainage paths on top and bottom of the specimens. The time required for attaining equilibrium swell pressure varied from 280 to 160 h depending upon the density and initial degree of saturation. The swelling stress at equilibrium represents the swell pressure. Following the attainment of swell pressure, the wetted specimens were dismantled from the oedometer cells and their moisture content and total volume were determined. Swell pressure measurements were performed in triplicate at each dry density and water content. The measured specimen volumes, water contents and dry density of triplicate specimens
where qd,BES is the average dry density of triplicate specimens with similar initial conditions, Rs is the fraction of sand in BES mixture, qs is the density of sand, fm is the fraction of montmorillonite in bentonite and qothers is the density of other minerals which is taken as 2.7 Mg/m3 (Kurosawa et al., 2006). The EMDD values were used to calculate the void ratio of the montmorillonite fraction in BES specimens according to the equation:
eEMDD ¼
GM qw 1 EMDD
ð3Þ
The degree of saturation of the montmorillonite fraction (Sr,MF) in the BES specimens was calculated as:
Sr;MF ¼
wGM eEMDD
ð4Þ
where GM is the specific gravity of montmorillonite which is taken as 2.74 (Lambe and Whitman, 1969).
Table 2 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 1.4 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.74 20.57 20.57
0.54 0.26 0.28
10.55 10.87 11.03
2.47 0.49 1.97
1.38 1.40 1.37
0.40 1.24 0.84
2
20.46 20.29 20.29
0.54 0.26 0.28
13.29 12.90 13.50
0.44 2.49 2.05
1.39 1.40 1.37
0.13 0.97 1.10
3
20.16 20.29 20.55
0.86 0.20 1.06
16.49 17.00 15.90
0.16 3.26 3.42
1.41 1.39 1.40
0.71 0.71 0.00
4
20.18 20.01 20.57
0.37 1.18 1.56
17.92 18.40 17.30
0.26 2.95 3.21
1.39 1.41 1.37
0.10 1.25 1.15
5
20.46 20.29 20.57
0.08 0.71 0.63
21.71 22.40 21.00
0.03 3.21 3.24
1.38 1.41 1.37
0.48 1.50 1.02
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 3 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 1.5 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.72 20.57 20.25
1.00 0.29 1.29
10.49 11.09 9.90
0.03 5.69 5.66
1.49 1.51 1.48
0.40 1.25 0.85
2
20.72 20.29 20.53
1.00 1.06 0.07
13.29 13.00 13.80
0.55 2.72 3.27
1.48 1.50 1.47
0.22 1.12 0.90
3
20.41 20.29 20.25
0.47 0.12 0.35
16.42 16.90 16.10
0.33 2.59 2.26
1.50 1.48 1.51
0.28 1.14 0.86
Table 4 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 1.75 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
1
20.18 20.57 20.25
0.76 1.18 0.42
2
20.41 20.29 20.25
3
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
8.69 9.02 9.50
4.22 0.54 4.76
1.77 1.75 1.76
0.59 0.58 0.01
0.47 0.12 0.35
10.49 10.80 10.10
0.26 3.22 3.47
1.73 1.74 1.75
0.57 0.00 0.57
20.14 20.02 20.25
0.02 0.58 0.57
13.03 13.80 12.80
1.34 4.45 3.11
1.75 1.76 1.74
0.00 0.57 0.57
4
20.14 20.29 20.25
0.44 0.34 0.11
12.64 13.04 12.50
0.68 2.46 1.78
1.73 1.74 1.73
0.19 0.38 0.19
5
20.16 20.29 20.25
0.37 0.30 0.07
15.48 15.98 15.00
0.03 3.18 3.15
1.72 1.74 1.73
0.58 0.58 0.00
Table 5 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens in compacted state (specimens of 2.0 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
1
20.14 20.57 20.25
0.90 1.25 0.35
2
20.16 20.57 20.25
3
4
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
8.69 9.02 9.50
4.22 0.54 4.76
1.97 1.98 1.97
0.17 0.34 0.17
0.82 1.21 0.39
10.49 10.80 10.10
0.26 3.22 3.47
1.97 1.98 1.97
0.17 0.34 0.17
20.14 20.29 20.25
0.44 0.34 0.11
12.00 12.80 11.50
0.83 5.79 4.96
1.96 1.99 1.97
0.68 0.84 0.17
20.14 20.29 20.25
0.44 0.34 0.11
13.03 13.60 12.98
1.29 2.99 1.70
1.99 2.00 1.98
0.00 0.50 0.50
2.5. X-ray diffraction analysis XRD analysis of BES specimens in the compacted and wetted states were performed on 1 mm thin slices using JEOL X Ray Diffractometer (model JDX 8030) over 2h range of 3–33° (scanning rate 0.5°/min). This model of JEOL X Ray diffractometer has a provision to record XRD patterns of dry and moist specimens. 3. Results and discussions 3.1. Swell pressure of compacted BES specimens The montmorillonite fraction of BES specimens is responsible for moisture absorption during compaction and wetting in the
oedometer tests. Consequently, it was considered reasonable to calculate degree of saturation based on EMDD using Eqs. (1)–(4). The developed swell pressure values are correlated with Sr,MF (degree of saturation of montmorillonite voids) in subsequent figures. The gross dry density (qd), EMDD and Sr,MF of BES specimens are presented in Table 11. Fig. 2 plots the variation of swell pressure with initial Sr,MF values of BES specimens at dry densities of 1.4, 1.5, 1.75 and 2 Mg/m3. The EMDD values for gross dry densities of 1.4, 1.5, 1.75 and 2.0 Mg/m3 correspond to 0.92, 1.00, 1.27 and 1.57 Mg/m3, respectively. The swell pressures of the BES specimens are insensitive to initial degree of montmorillonite saturation at lower dry densities of 1.4 and 1.5 Mg/m3. In comparison, the swell pressures of denser specimens respond to variations in Sr,MF values.
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 6 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 1.4 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.63 20.66 20.68
0.15 0.03 0.12
42.34 42.95 42.10
0.29 1.15 0.85
1.39 1.39 1.38
0.24 0.24 0.48
2
20.63 20.66 20.40
0.30 0.48 0.78
44.09 42.00 44.00
1.68 3.14 1.47
1.38 1.38 1.39
0.32 0.02 0.34
3
20.33 20.39 20.66
0.64 0.36 1.00
51.26 52.00 51.40
0.57 0.87 0.29
1.40 1.40 1.39
0.24 0.24 0.48
4
20.63 20.10 20.96
0.31 2.26 1.94
38.79 39.20 38.50
0.10 0.95 0.85
1.41 1.41 1.39
0.53 0.33 0.86
5
20.35 20.38 20.68
0.59 0.45 1.04
39.17 40.30 39.00
0.81 2.05 1.24
1.39 1.38 1.40
0.02 0.97 0.99
Table 7 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 1.5 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.89 20.66 20.36
1.23 0.11 1.35
38.72 37.90 38.10
1.26 0.89 0.37
1.48 1.52 1.47
0.65 2.04 1.40
2
20.89 20.38 20.36
1.69 0.79 0.90
38.25 39.00 37.90
0.35 1.61 1.26
1.49 1.51 1.48
0.22 1.12 0.89
3
20.87 20.38 20.36
1.61 0.75 0.86
37.44 38.00 37.00
0.10 1.38 1.28
1.49 1.49 1.5
0.22 0.22 0.45
Table 8 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 1.75 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.36 20.38 20.67
0.55 0.41 0.97
24.28 23.80 24.60
0.21 1.76 1.55
1.76 1.75 1.76
0.19 0.38 0.19
2
20.03 20.11 20.39
0.70 0.35 1.05
29.68 30.01 29.40
0.05 1.05 1.00
1.75 1.76 1.75
0.19 0.38 0.19
3
20.03 20.38 20.39
1.16 0.57 0.59
31.69 32.30 31.04
0.05 1.97 2.01
1.74 1.75 1.73
0.00 0.57 0.57
4
20.31 20.38 20.39
0.24 0.11 0.13
28.48 29.02 27.90
0.05 1.94 1.99
1.76 1.75 1.76
0.19 0.38 0.19
5
20.34 20.38 20.39
0.16 0.07 0.09
35.18 34.80 35.50
0.05 1.02 0.97
1.73 1.75 1.73
0.38 0.77 0.38
The swell pressures of 1.75 Mg/m3 specimens reduce from 1359 to 799 kPa upon increase in Sr,MF from 22 to 30% and later became constant. Likewise, the swell pressures of 2 Mg/m3 specimens respond to increase in degree of saturation by reducing from 2087 to 783 kPa upon increase in Sr,MF from 31 to 49%. The final Sr,MF values after swell pressure measurements vary from 54 to 73%, 61 to 62%, 60 to 83% and 83 to 110% for 1.4, 1.5, 1.75 and 2 Mg/m3 series specimens, respectively. The variation in final Sr,MF values is attributed to variations in initial compaction conditions of the specimens. Slightly larger than 100%
degree of saturation were observed at the maximum dry density of 2.0 Mg/m3. Earlier researchers have observed that density of water adjacent to clay layers can deviate from unity as the structure of the water molecules are possibly disturbed by the physico-chemical interactions between inter-lamellar water and montmorillonite surfaces (Skipper et al., 1991; Hueckel, 1992; Mitchell, 1993; Ichikawa et al., 1999; Villar and Lloret, 2004). Villar and Lloret (2008) report that the density of inter-lamellar water in compacted bentonite specimens range from 1.0 to 1.35 Mg/m3. Slightly larger final Sr,MF values at 2.0 Mg/m3 may
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S.M. Rao, K. Ravi / Annals of Nuclear Energy 80 (2015) 303–311 Table 9 Measured specimen volumes, gravimetric water contents and dry densities of the triplicate specimens after swell pressure test (specimens of 2.0 Mg/m3 series). Test number
Volume of the specimens (cm3)
Percentage variation from average value
Water content (%)
Percentage variation from average value
Dry density (Mg/m3)
Percentage variation from average value
1
20.03 20.38 20.39
1.16 0.57 0.59
23.48 24.00 22.90
0.09 2.30 2.39
1.98 1.99 1.97
0.00 0.51 0.51
2
20.34 20.11 20.11
0.76 0.39 0.37
27.00 27.60 26.70
0.38 1.85 1.47
1.99 1.99 1.99
0.00 0.00 0.00
3
20.31 20.38 20.39
0.24 0.11 0.13
26.45 27.00 26.01
0.15 1.94 1.80
1.97 1.98 1.97
0.17 0.34 0.17
4
20.31 20.38 20.14
0.17 0.52 0.69
31.69 32.10 31.40
0.12 1.16 1.04
1.98 1.99 1.97
0.00 0.51 0.51
Table 10 Average specimen volumes, water contents and dry densities of triplicate measurements. Volume of the specimen (cm3)
Water content (%)
Compacted state
After swell pressure test
Compacted state
After swell pressure test
Compacted state
After swell pressure test
1.4 Mg/m3
20.63 20.35 20.33 20.25 20.44
20.66 20.56 20.46 20.56 20.47
11 13 16 18 22
42 43 51 39 39
1.38 1.39 1.41 1.39 1.39
1.39 1.37 1.4 1.4 1.38
1.5 Mg/m3
20.51 20.51 20.32
20.64 20.55 20.54
10 13 16
39 38 37
1.49 1.48 1.50
1.48 1.49 1.49
1.75 Mg/m3
20.33 20.32 20.13 20.23 20.23
20.47 20.18 20.27 20.36 20.37
9 10 13 13 15
24 30 28 32 35
1.77 1.73 1.73 1.75 1.72
1.76 1.75 1.76 1.74 1.73
2 Mg/m3
20.32 20.33 20.23 20.23
20.27 20.18 20.36 20.28
9 10 12 13
23 27 26 32
1.97 1.97 1.96 1.99
1.98 1.99 1.97 1.99
Test series
Dry density (Mg/m3)
Table 11 Average dry densities, degree of saturation of montmorillonite (Sr,MF), c-axis spacing of the specimens before and after swell pressure test and the swell pressure values. Test seriesa
a b
Dry density of specimens/ EMDDa (Mg/m3)
Degree of saturation of montmorillonite (%)
Compacted state
After swell pressure test
Compacted state
1.4 (0.92) Mg/m3
1.38 1.39 1.41 1.39 1.39
1.39 1.37 1.40 1.40 1.38
(0.91) (0.89) (0.92) (0.92) (0.91)
15 19 24 25 31
59 59 73 55 54
1.5 (1.00) Mg/m3
1.49 (1.00) 1.48 (0.99) 1.50 (1.00)
1.48 (0.99) 1.49 (1.00) 1.49 (1.00)
17 21 27
1.75 (1.27) Mg/m3
1.77 1.73 1.73 1.75 1.72
(1.29) (1.25) (1.25) (1.27) (1.24)
1.76 1.75 1.76 1.74 1.73
(1.28) (1.27) (1.28) (1.26) (1.25)
2.0 (1.57) Mg/m3
1.97 1.97 1.96 1.99
(1.53) (1.53) (1.52) (1.55)
1.98 1.99 1.97 1.98
(1.54) (1.55) (1.53) (1.54)
(0.90) (0.91) (0.93) (0.91) (0.91)
Swell pressure (kPa)
Compacted state
After swell pressure test
244 305 315 287 252
13.00 13.90 14.90 15.35 16.60
21.10 21.00 21.20 20.80 20.85
62 62 61
416 427 395
13.10 (1.04) 14.00 (1.31) 15.10 (1.63)
21.20 (3.45) 21.30 (3.48) 20.80 (3.33)
22 25 30 32 36
60 73 71 76 83
1359 986 799 783 754
13.20 14.00 15.00 16.00 17.50
(1.07) (1.31) (1.61) (1.90) (2.35)
21.10 21.00 20.40 19.90 19.80
(3.42) (3.39) (3.21) (3.07) (3.04)
31 35 42 49
86 100 95 110
2087 1410 905 783
14.30 15.10 17.10 18.00
(1.40) (1.64) (2.23) (2.50)
22.50 22.00 21.70 21.30
(3.84) (3.69) (3.60) (3.48)
Values in parentheses correspond to dry density of montmorillonite (EMDD). Values in parentheses correspond to number of hydration layers.
After swell pressure test
c-Axis spacing (Å)/number of hydration layersb
(1.01) (1.28) (1.58) (1.71) (2.08)
(3.42) (3.39) (3.45) (3.33) (3.35)
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2500 1.4 Mg/m³ 1.5 Mg/m³
2000
Swell pressure (kPa)
1.75 Mg/m³ 2 Mg/m³
1500
1000
500
0 0
10
20
30
40
50
Intial degree of saturation of montmorillonite, S r,MF (%) Fig. 2. Variation of swell pressure with initial degree of saturation of montmorillonite at different dry densities.
possibly be a consequence disturbances in density of interlamellar water of montmorillonite. 3.2. Micro-structural investigations X-ray diffraction results of BES specimens were examined for the compacted and wetted conditions (Table 11). The compacted state or compacted specimen refers to specimens that have not been subjected to wetting; the wetted state or wetted specimen refers to compacted specimens that were wetted and allowed to attain equilibrium swell pressure under constant volume conditions. The c-axis spacing of 1.4 Mg/m3 dry density series specimens increased from 13.00 Å to 16.60 Å upon increase in Sr,MF from 15 to 31%. Similar observations were made for the 1.5, 1.75 and 2 Mg/m3 series specimens. The c-axis spacing of wetted specimens however did not show uniform trend. The c-axis spacing of 1.4 Mg/m3 series specimens expand similarly on wetting despite being characterized by varying initial degree of saturation values; the average caxis spacing of the wetted 1.4 Mg/m3 specimens corresponds to 20.98 Å (% variation from average ± 0.6%). A similar trend of results is observed for the 1.5 Mg/m3 series specimens that exhibit average c-axis spacing of 21.05 Å in the wetted state (% variation from average ± 0.95%). Comparatively, c-axis spacing of the denser specimens (1.75 and 2 Mg/m3) are sensitive to variations in initial degree of saturation in the wetted state. The water layer thickness in the first, second and third hydration layers of sodium-montmorillonite correspond to 3.03, 3.23 and 3.48 Å, respectively (Pusch and Yong, 2006). The c-axis spacing of oven dried Barmer bentonite specimen (essentially composed of sodium-montmorillonite) corresponds to 9.98 Å. Based on these data and c-axis spacing, the number of hydration layers in the compacted and wetted states of the BES specimens has been obtained (Table 11). Specimens compacted to dry density of 1.4 Mg/m3 develop 3.39 layers of water molecules in the wetted state (average value) at all initial Sr,MF values. Likewise specimens compacted to dry density of 1.5 Mg/m3 develop 3.41 layers of water molecules in the wetted state (average value) at all initial Sr,MF values. Despite developing similar number of hydration layers in the wetted state (3.39 and 3.41), the 1.4 and 1.5 Mg/m3 dry density series specimens, develop different swell pressures (average values: 283 and 413 kPa, respectively). The results indicate that compaction dry density influences the swell pressure when specimens are compacted differently. However, when specimens are compacted to same dry density (1.4 or 1.5 Mg/m3), the number of hydration layers developed upon wetting appear to influence the swell pressure.
The c-axis spacing and number of hydration layers of the denser specimens respond to wetting. The c-axis spacing and number of hydration layers in the wetted state reduce from 21.10 to 19.80 Å and from 3.42 to 3.04 respectively when initial Sr,MF values increase from 22 to 36% for the 1.75 Mg/m3 series specimens. Likewise, wetting reduces the c-axis spacing and number of hydration layers of the 2 Mg/m3 series specimens from 22.50 to 21.30 Å and from 3.84 to 3.48 at initial Sr,MF variations of 31 to 49%. Fig. 3 plots the variation of swell pressures with number of hydration layers in the wetted state for the 1.4, 1.5, 1.75 and 2 Mg/m3 series specimens. The 1.4 Mg/m3 series specimens exhibit similar swell pressures as they develop similar number of hydration layers on wetting, irrespective of the initial degree of saturation. The same holds true for the 1.5 Mg/m3 series specimens. The 1.75 and 2 Mg/m3 series specimens develop progressively larger hydration layers in the wetted state and consequently exhibit increasingly higher swell pressures with reducing initial Sr,MF values. That the initial degree of saturation of montmorillonite voids has bearing on number of hydration layers developed by denser (1.75–2 Mg/m3) but not looser (1.4–1.5 Mg/m3) Barmer specimens upon wetting needs investigations.
3.3. Practical implications of the study Based on the results of earlier researchers (Villar and Lloret, 2008; Komine and Ogata, 1994) and observations of the present study it appears beneficial to compact bentonite buffer to dry densities of 61.5 Mg/m3 as their swell pressures are expected to be unaffected by variations in degree of saturation. For satisfactory performance in deep geological repository environment, SKB (2006) recommends that bentonite buffer should develop minimum swell pressures of 1–2 MPa. Review of literature shows that compacted (dry density = 1.5 Mg/m3) FEBEX bentonite develop swell pressures >2 MPa (Villar and Lloret, 2008), while Kunigel VI (Komine and Ogata, 1994), Jimmyeong (Lee et al., 1999) and 70% Barmer bentonite + 30% sand (present study) develop swell pressure < 1 MPa at compaction dry density of 1.5 Mg/m3. The 1.75 and 2 Mg/m3 series BES specimens experience 44–62% decrease in swell pressure upon increase in degree of saturation from 22 to 36% and 31 to 49%, respectively. Hence, the possibility of reduction in swell pressure of densely compacted bentonite buffer (dry density > 1.5 Mg/m3) from increase in Sr,MF should be factored in during design and operation of deep geological repositories.
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Swell pressure (kPa)
2000
1.5 Mg/m³ 1.75 Mg/m³ 2 Mg/m³
1500
1000
500
0 3.00
3.10
3.20
3.30
3.40
3.50
3.60
3.70
3.80
3.90
4.00
Number of hydration layers in saturated state
Fig. 3. Variation of swell pressure with number of hydration layers in the saturated state.
4. Conclusions The influence of initial degree of saturation on swell pressures of compacted Barmer bentonite + sand specimens was tied up with their dry densities. The swell pressures of 1.4 and 1.5 Mg/m3 dry density series specimens were unaffected by variations in initial degree of saturation of montmorillonite voids (Sr,MF). Comparatively, swell pressures of denser specimens (1.75 and 2 Mg/m3) responded to variations in initial Sr,MF values. XRD measurements demonstrated that when specimens belonging to common density series developed similar number of hydration layers on wetting they exhibited similar swell pressures and vice versa. The former was true for 1.4 and 1.5 Mg/m3 series specimens and latter for specimens belonging to 1.75 and 2 Mg/m3 series. When specimens developed similar number of hydration layers on wetting, the compaction dry density determined the swell pressure. For example, swell pressures of specimens that developed 3.8 hydration layers in the wetted state followed the sequence: 2.0 Mg/m3 > 1.75 Mg/ m3 > 1.5 Mg/m3 > 1.4 Mg/m3. Results of the study also indicate that the possibility of swell pressure reduction from increase in Sr,MF should be considered during design and operation of bentonite buffer compacted at dry densities > 1.5 Mg/m3. References Aubertin, M., Mbonimpa, M., Bussiere, B., Chapius, R.P., 2003. A model to predict the water retention curve from basic geotechnical properties. Can. Geotech. J. 40, 1104–1122. Barbour, S.L., 1998. Nineteenth canadian geotechnical colloquium: the soil-water characteristic curve: a historical perspective. Can. Geotech. J. 35, 873–894. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60 (2), 309–319. Bucher, F., Vonmoos, M.M., 1989. Bentonite as a contaminant barrier for the disposal of highly radioactive waste. Appl. Clay Sci. 4 (2), 157–177. Cerato, A.B., Lutenegger, A.J., 2002. Determination of surface area of fine-grained soils by the ethylene glycol monoethyl ether (EGME) method. Geotech. Test. J. 25 (3). http://dx.doi.org/10.1520/GTJ11087J. Chen, B., Qian, L., Ye, W., Cui, Y., Wang, J., 2006. Soil water characteristic curves of Gaomiaozi bentonite. Chin. J. Rock Mech. Eng. 25 (4), 788–794. Cho, W.J., Lee, J.O., Chun, K.S., 1999. Basic physicochemical and mechanical properties of domestic bentonite for use as a buffer material in a high level radioactive waste repository. J. Korean Nucl. Soc. 31 (6), 39–50. Delage, P., Howat, M.D., Cui, Y.J., 1998. The relationship between suction and swelling properties in a heavily compacted unsaturated clay. Eng. Geol. 50, 31– 48. Dixon, D.A., Gray, M.N., Thomas, A.W., 1985. A study of the compaction properties of potential clay-sand buffer mixtures for the use in nuclear fuel waste disposal. Eng. Geol. 21 (3–4), 247–255. Duquette, D.J., Latanision, R.M., Di Bella, C.A.W., Kirstein, B.E. 2008. Corrosion issues related to disposal of high-level nuclear waste in the yucca mountain repository. In: NACE International, CORROSION 2008, Conference and Expo, New Orleans, Louisiana.
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