Influences of salt solution concentration and vertical stress during saturation on the volume change behavior of compacted GMZ01 bentonite

    Influences of salt solution concentration and vertical stress during saturation on the volume change behavior of compacted GMZ01 bentonite Feng Zhang, Wei-Min Ye, Yong-Gui Chen, Bao Chen, Yu-Jun Cui PII: DOI: Reference:

S0013-7952(16)30092-8 doi: 10.1016/j.enggeo.2016.04.010 ENGEO 4262

To appear in:

Engineering Geology

Received date: Revised date: Accepted date:

11 April 2015 27 March 2016 13 April 2016

Please cite this article as: Zhang, Feng, Ye, Wei-Min, Chen, Yong-Gui, Chen, Bao, Cui, Yu-Jun, Influences of salt solution concentration and vertical stress during saturation on the volume change behavior of compacted GMZ01 bentonite, Engineering Geology (2016), doi: 10.1016/j.enggeo.2016.04.010

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Influences of salt solution concentration and vertical

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stress during saturation on the volume change behavior of

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compacted GMZ01 bentonite

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Feng Zhang a, Wei-Min Yea,b,*, Yong-Gui Chena, Bao Chen a, Yu-Jun Cui a,c a. Department of Geotechnical Engineering, College of civil engineering, Tongji University,

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Shanghai 200092

Tongji University, Shanghai 200092

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b. Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education,

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c. Laboratoire Navier, Ecole des Ponts ParisTech, France

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Prof. Weimin YE

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Corresponding author

Tel.: +86 21 6598 3729

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Fax: +86 21 6598 2384 E-mail: [email protected]

ACCEPTED MANUSCRIPT Abstract: Investigation of the effects of salt solution concentration and vertical stress during saturation on the volume change behavior of compacted bentonite is of great

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importance for the assessment of the behavior of engineering barrier in deep

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geological repository for disposal of high-level radioactive waste. In this study, oedometer tests were conducted on densely compacted GMZ01 bentonite specimens with an initial dry density of 1.70 Mg/m3, which experienced swelling under different

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vertical stresses with infiltration of de-ionized water or NaCl solutions at different

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concentrations. The effects of salt solution concentration and vertical stress during saturation on the volume change of GMZ01 bentonite specimens were investigated in

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terms of swelling strain, elastic compressibility parameter, plastic compressibility

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parameter and yield stress. Results show that as the concentrations of salt solutions

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increase, the swelling strain, the elastic compressibility parameterand the plastic compressibility parameter decrease, while the yield stress increases. However, the

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vertical stress during saturation can only affect the swelling strain and the yield stress. Based on the test results, empirical equations with consideration of salt solution concentration and vertical stress during saturation effects were proposed allowing the prediction of swelling strain, plastic compressibility parameter and yield stress. The calculated results are in good agreement with the experimental ones. Key words: clay; salt solution concentration; swelling; compressibility parameters; yield stress

ACCEPTED MANUSCRIPT 1 Introduction Geological repositories are planned to be built at great depths of about 500 m to

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1000 m below the ground surface for disposal of high-level radioactive waste (HLW)

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(Wang et al., 2006; Ye et al., 2012). Due to its high swelling capacity, low hydraulic conductivity, good sorption properties, adequate mechanical resistance, etc., compacted bentonite has been considered as potential buffer/backfill materials for

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construction of engineering barriers in geological repositories in many countries

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(Pusch, 1977; Dixon and Gray, 1985; Radhakrishna et al., 1989; Lloret et al., 2003; Cuisinier and Masrouri, 2005; Ye et al., 2009).

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In practice, bentonite is pre-compacted in bricks and placed in layers around the

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canister. With absorption of groundwater with different chemical components from

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the surroundings, the compacted bentonite bricks will swell under certain vertical stresses induced by the gravity of upper brick layers and the pressures caused by

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swelling of the adjacent compacted bentonite bricks to fill in the voids and cracks in the repository. Along with the sealing and formation of the artificial barrier, compression pressure on the compacted bentonite bricks is expected to be as high as 9.0-16.0 MPa (Tripathy and Schanz, 2007). Meanwhile, during the long-term operation of an underground repository, due to the interactions between the buffer/backfill materials and the surrounding geological formations, the concrete structures and groundwater, as well as the buffer/backfill materials, the canister, etc., the groundwater salinity could reach a higher value (above 2.0 M) and play a drastic role on the performance of the bentonite barrier (Castellanos et al., 2008). The

ACCEPTED MANUSCRIPT investigation of the effects of salt solution concentration and vertical stress during saturation on the volume change behavior of compacted bentonite is thus of great

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importance for the assessment of the behavior of the barrier in radioactive waste

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repository.

The swelling characteristics of bentonites have been widely investigated (Di Maio, 1996; Musso et al., 2003; Castellanos et al., 2008; Siddiqua et al., 2011; Zhu et

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al., 2015). The experimental results show that the swelling properties can be

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influenced by many factors, such as initial dry density, initial vertical stress during saturation, chemical composition of pore water, etc.

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For investigating the influence of salt solution concentration on the

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compressibility of bentonite, most of the experimental works focus on the behavior of

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specimens reconstituted from slurries and tested under saturated conditions (Bolt, 1956; Barbour and Fredlund, 1989; Di Maio, 1996; Di Maio et al., 2004). Castellanos

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et al. (2008) observed from the compressibility tests on the FEBEX bentonite that, after swelling under 0.5 MPa followed by loading under oedometric conditions up to 2 MPa, the specimens saturated with high-salinity solutions are less deformable and consolidate more rapidly than those saturated with low-salinity solutions. However, the influence of the vertical stress during saturation on the compressibility of soils is rarely reported in literature. Most of experimental works focused on the influences of initial condition, initial dry density and grain size distribution effects, etc. (Priyanto et al., 2008a; Nowamooz and Masrouri, 2009; Tang et al., 2011). From a practical point of view, it is necessary to investigate the effect of

ACCEPTED MANUSCRIPT the vertical stress during saturation on the compressibility of soils The Chinese HLW disposal program was launched in the mid-1980s. According

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to the preliminary long-term plan, the first deep geological repository will be built and

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put into operation in the middle of the 21st Century (Wang et al., 2006; Ye et al., 2014a). Up to now, Beishan, near the ancient Silk Road, in Gansu province, northwest China, has been chosen as a potential site for the construction of the Chinese deep

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geological repository. Gao-Miao-Zi (GMZ) bentonite has been selected as a potential

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buffer/backfill material. In this regard, preliminary researches have been conducted on the swelling, mechanical, hydraulic and thermal properties of GMZ bentonite (Ye et

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al., 2009; Ye et al., 2010; Ye et al., 2012; Ye et al., 2014b; Zhu et al., 2013). Results

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show that GMZ bentonite is a good buffer/backfill material for its relatively high

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thermal conductivity (Ye et al., 2010), low water permeability (Ye et al., 2014a), high swelling capacity (Zhu et al., 2013) etc. Related fieldwork has been doing in Beishan

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including geological and hydro-geological investigations. Current results show that the total dissolved solids (TDS), which is rich in Na+, in the groundwater in Yemaquan, Beishan area, ranges from 2g/L to 80g/L (Guo et al., 2001). In this study, oedometer tests were conducted on specimens, which experienced swelling under vertical stresses with circulation of salt solutions (see Fig.1). The effects of salt solution concentration and vertical stress during saturation on the volume change behavior of GMZ01 bentonite specimens were investigated in terms of swelling strain, elastic compressibility parameter, plastic compressibility parameter and yield stress. Empirical equations considering salt solution concentration and

ACCEPTED MANUSCRIPT vertical stress during saturation were proposed allowing the evaluation of the above parameters.

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2 Experimental investigations

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2.1 Test apparatus

The experimental apparatus (Fig. 1) developed by Ye et al. (2014b) for swelling strains and compression tests with infiltration of salt solutions was employed. It

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includes an oedometer cell (Fig. 1a) and a load frame (Fig. 1b). Detailed information

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about the device can be found in Ye et al. (2014b). 2.2 Materials

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GMZ Bentonite is a sodium bentonite, which was taken from GaoMiaoZi deposit

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in the Inner Mongolia Autonomous Region, 300 km northwest from Beijing, China

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(Ye et al., 2009). The GMZ01 bentonite tested is a gray powder. Some of its basic properties are reported in Table 1 (Wen, 2006).

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2.3 Test procedures

2.3.1 Specimen preparation According to the target dimensions of 50mm in diameter and 10mm in height, as well as a target dry density of 1.70Mg/m3, GMZ01 bentonite powder at its hygroscopic water content (11.17%) was carefully weighted and poured into an oedometer ring. Static vertical load was applied using a digital frame at a constant rate of 0.4 kN/min. When the vertical stress reached 30 MPa, the desired dry density of 1.70 Mg/m3 was obtained. The compaction was then stopped and the maximum load was kept for 1 h.

ACCEPTED MANUSCRIPT 2.3.2 The swelling strain and compression tests The compacted GMZ01 Bentonite specimen was put into the oedometer cell

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(Fig.1) and a defined vertical stress was applied, which was lower than the

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compaction stress. Thus, the specimen was in an over-consolidated state. Then, de-ionized water or NaCl solution was put in contact with the specimen for saturation under the given vertical stress from the bottom. Air bubbles were expelled from the

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upper outlet of the specimen. The vertical displacements of the specimen were

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recorded. When the swelling strain reached a steady state, compression tests were conducted in a conventional way by successive loading to a maximum value of about

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42 MPa (Ye et al., 2014b). The degree of saturation was carefully measured at the end

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of swelling stage on part parallel samples. The value was found to be about 100%.

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The procedures mentioned above were carried out on different specimens under different vertical stresses with different solution concentrations. A total of 13 tests

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listed were conducted (see Table 2). For all the odometer tests, a displacement rate of 0.01 mm/24h was used as criterion of stabilization (Ye et al., 2014b). All the tests were performed at a controlled ambient temperature of 20 ± 1℃.

3 Results and discussion 3.1 Influences of solution concentration and vertical stress during saturation on swelling 3.1.1 Test results

ACCEPTED MANUSCRIPT Note that the swelling curve during the saturation phase was analyzed in Ye et al. (2014b). Only the final swelling strains measured under different vertical stresses

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during the saturation with infiltration of salt solutions with different concentrations

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are given in Fig. 2. It is observed from Fig. 2(a) that the swelling strain decreases as the concentration of salt solution increases, and the gradient decreases significantly when the concentration of salt solution is higher than 1 M. This observation is

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consistent with the results reported by Castellanos et al. (2008) and Rao et al. (2006).

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The measured swelling strains of GMZ01 bentonite specimens and their corresponding vertical stresses during saturation are plotted in a semi-logarithmic

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plane. Results in Fig. 2(b) show that a linear relationship can be identified. This

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observation confirms the result reported by Castellanos et al. (2008). It should be

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noted that more points are needed for GMZ01 bentonite to well define the linearity. 3.1.2 Fitting of swelling strains to empirical expressions

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Inspired by the relationship between the elastic compressibility parameter and the pore water chemistry proposed by Loret et al. (2002), the following equation was proposed for the description of the swelling strain() developing with salt solution concentration,

 (c)  1 tanh( 3

x  x dw x sat  x dw

)  2

(1)

where1, 2 and 3 are constants, x is the concentration ratio c/csat of the infiltration solution. Fig. 2(a) shows that a significant change of swelling strain can be observed when the concentration is low. The parameters 1, 2 and 3 can be obtained from the

ACCEPTED MANUSCRIPT following equation, which is similar to the method proposed by Loret et al. (2002).

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 sat   dw x sat  x dw d   , ,  2   dw 1  sat   dw d dw tanh( 3 )

(2)

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tanh( 3 )



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where sat and dw are the swelling strains measured at the concentration ratio 1 and 0,

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respectively.

The influence of vertical stress during saturation was considered by multiplying

x  x dw

x sat  x dw

)  2)

(3)

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   ( p)(1 tanh( 3

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a stress function (p) in Eq. (1),

For determination of the stress function (p), one set of vertical stress during

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saturation and their corresponding swelling strains were selected as reference values

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(pref and ref). The ratios (p/pref and /ref) (where p and  are the other vertical stresses

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during saturation and the corresponding swelling strains, respectively) were calculated and a relationship between the vertical stress ratio (p/pref) and the swelling strain ratio

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(/ref) was established (Fig. 3). The vertical stress during saturation of 0.22 MPa was selected as reference value. Then, the vertical stress ratios and the corresponding swelling strain ratios can be calculated and their relationship is presented in Fig. 3. With all the results in Fig. 3, the stress function can be fitted.

 ( p)  a ln( x)  b

(4)

where a and b are constants related to stress, x is the vertical stress ratio. At the same time, parameters 1, 2 and 3 in Eq. (1) were calculated using Eq. (2) to be -0.1821, 0.3037 and 2.7044, respectively.

ACCEPTED MANUSCRIPT Substituting Eq. (4) and parameters 1, 2 and 3 into Eq. (3), the relationship between the swelling strain and the concentration ratio can be determined. Fig. 4

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shows that the calculated results are in good agreement with the experimental ones.

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For further verification of the proposed method, the test results of FEBEX bentonite conducted by Castellanos et al. (2008) were analyzed in Figs. 5 and 6. Again, a good agreement was obtained between the calculations and the

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measurements.

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3.2 Influences of concentration and vertical stress during saturation on compressibility

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3.2.1 Experimental results

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The compression curves of GMZ01 Bentonite specimens hydrated with

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de-ionized water or NaCl solution with different concentrations under vertical stresses of 0.12 MPa, 0.22 MPa, 0.5 MPa and 1.45 MPa are presented in Fig. 7. Influence of salt solution concentration on the compressibility parameters

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1)

The determination of the elastic compressibility parameter , plastic compressibility parameterand yield stress P0 of GMZ01 bentonite specimens hydrated with de-ionized water under different initial vertical stresses is illustrated in Fig. 8. The notations adopted for these parameters are the same as in the BBM Model elaborated by Alonso et al. (1990) and Lloret et al. (2003). All the compressibility parameters obtained after saturation under different vertical stresses at different concentrations of solutions are shown in Fig. 9. It can be observed that the plastic compressibility parameter decreases as the

ACCEPTED MANUSCRIPT concentration of salt solution increases (Fig. 9(a)). A possible explanation is that the plastic compressibility parameter  is mainly governed by the deformation of soil

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aggregates and the macro-pore of inter-aggregate type (Tang et al., 2008). The

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increase of concentration of salt solution can be considered as the increase of osmotic suction inside the specimen (Rao and Shivananda, 2005), which can induce soil consolidation characterized by formation of aggregates and inter-aggregate pores.

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This is also why the hydraulic conductivity of soil increases with the increases of

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concentration of salt solution (Zhu et al., 2013; Ye et al., 2014b). As the deformability of aggregates decreases with the increase of osmotic suction, the plastic

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compressibility parameter decreases with the increase of concentration of salt

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solution. This suction effect is similar to the matric suction effect on the

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compressibility of unsaturated soils (Alonso et al., 1990; Cui and Delage, 1996). When the concentration of salt solution is higher than 1 M, the plastic compressibility

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parameter is almost constant, this observation is consistent with the results reported in literatures (Loret et al., 2002; Gajo et al., 2002). The curves of yield stress versus salt concentration are presented in Fig. 9(b). It can be observed from Fig. 9(b) that the yield stress increases as the concentration increases. This ‘chemical hardening’ behavior is similar to the suction-hardening effect reported by other researchers (Lloret et al., 2003; Tang et al., 2008; Nowamooz and Masrouri, 2009). This can be explained by the fact that the specimen saturated with salt solution of high concentration has less swelling. As a result, the specimen is denser and the yield stress is higher.

ACCEPTED MANUSCRIPT 2) Influence of vertical stress during saturation on compressibility parameters The influence of vertical stress during saturation on the compressibility

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parameters of compacted GMZ01 bentonite specimens is shown in Fig. 10(a). It can

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be found that the vertical stress has negligible influence on the compressibility parameters. This is expected results in the light of the discussion above about the influence of salt concentration on the plastic compressibility parameter.

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It is interesting to find that there is a linear relationship between the vertical

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stress during saturation and the yield stress for GMZ01 specimens saturated with de-ionized water and 1 M NaCl solution (see Fig. 10(b)). Fig. 10(b) also shows that

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the yield stress decreases as the vertical stress during saturation decreases. This can be

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explained by the more swelling under lower vertical stress, which results in a

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softening behavior of specimen.

3.2.2 Fitting of compressibility parameter to empirical expressions

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1) Plastic compressibility parameter Similar to the form of Eq. (3), Eq. (5) was proposed for the description of the influences of salt solution concentration and vertical stress during saturation on the plastic compressibility parameter :

λ   ( p)(1 tanh(3

x  x dw x sat  x dw

)  2 )

(5)

where 1、2 and 3 are constants, x is the concentration ratio c/csat of the infiltration solutions. Note that the vertical stress during saturation has almost no influence on the plastic compressibility parameter, the stress function pis equal to 1. The parameters 1, 2 and 3 can be determined to be 0.04918, 0.13468 and -3.765,

ACCEPTED MANUSCRIPT respectively. 2) Yield stress

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For the description of suction effects on yield stress, Alonso et al. (1990)

 ( 0 )

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proposed Eq. (6): ) P* ( P0  Pc ( 0 )  ( s ) Pc

(6)

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Considering the similarity between the suction-hardening and the chemical hardening, Eq. (7) was adopted for assessing the effect of salt solution concentration

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on yield stress:

 ( 0 )

(7)

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P0* (  ( c ) ) P  Pref ( ) Pref

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de-ionized water.

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where Pref is a reference stress, P0* is the yield stress of specimen infiltrated with

For accounting for the influence of vertical stress on the yield stress P, a stress

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function P(p) was added in Eq. (7),  ( 0 )

P0* (  ( c ) ) P  P( p) Pref ( ) Pref

(8)

Taking the vertical stress of 0.22 MPa as a reference value, the parameters Pref and P0* can be determined to be 0.51 and 0.6047, respectively. Correspondingly, the relationship between the vertical stress ratio (p/pref) and the yield stress ratio (P/Pref) can be determined and presented in Fig. 12. The stress function Ppwith the vertical stress ratio can then be fitted.

P( p)  0.8328x  0.1382 (9) Substituting Eq. (9) and values of parameters Pref and P0*into Eq. (8), the

ACCEPTED MANUSCRIPT relationships between the yield stress and the concentration ratio can be determined.

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Fig. 13 shows that the calculated results are in agreement with the experimental ones.

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4 Conclusions

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In this study, oedometer tests were conducted on densely compacted GMZ01 bentonite specimens with an initial dry density of 1.7 Mg/m3 after swelling under

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different vertical stresses. De-ionized water and NaCl solution at several concentrations were used. The influences of salt solution concentration and vertical

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stress during saturation on the volume change of compacted GMZ01 Bentonite were

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investigated in terms of the swelling strain, elastic and plastic compressibility

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parameters and yield stress. Based on these experimental results, empirical equations were proposed and verified. The following conclusions can be drawn.

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The swelling strains of the GMZ01 bentonite specimens decreases as the concentration of salt solution or the vertical stress during saturation increases.

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The plastic compressibility parameter decreases as the concentration of salt solution increases. The vertical stress applied during saturation has negligible influence on the plastic compressibility parameter. The yield stress increases as the concentrations of salt solutions or the vertical stress during saturation increases.

Acknowledgements The authors are grateful to China Atomic Energy Authority (Project: [2011]1051) and the National Natural Science Foundation of China (Project: 41030748 and

ACCEPTED MANUSCRIPT 41422207) for the financial support. The authors also wish to acknowledge the support of the European Commission via the Marie Curie IRSES project GREAT -

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Geotechnical and geological Responses to climate change: Exchanging Approaches

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and Technologies on a world-wide scale (FP7-PEOPLE-2013-IRSES- 612665).

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List of figures,

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Fig. 1 Schematic view of the test apparatus (Ye et al., 2014b)

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Fig. 2 Final swelling strain as a function of (a) concentration (b) vertical stress during saturation

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Fig. 3 Swelling strain ratio against vertical stress ratio during saturation

Fig.4 Relationship between the swelling strain and the concentration ratio

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Fig.5 Swelling strain ratio against vertical stress ratio during saturation for the results of Catellanos et al. (2008)

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Fig.6 Swelling strains at the end of saturation under different vertical stresses measured by

D

Castellanos et al. 2008 and calculated with Eq. 3

TE

Fig.7 Compression curves of specimens saturated with solutions under different vertical stresses Fig. 8 Determination of compressibility parameters ,  and P0 of specimens saturated under

CE P

different vertical stresses with de-ionized water

stresses

AC

Fig. 9 Compressibility parameters of specimens saturated with solutions under different vertical

Fig. 10 Influence of the vertical stress during saturation on the compressibility parameters Fig. 11 Plastic compressibility parameters against concentration ratio Fig. 12 Yield stress ratio against vertical stress ratio during saturation Fig. 13 Comparisons of the measured yield stresses to the ones calculated using Eq. 8

Table1. Basic properties of GMZ01 bentonite (Wen, 2006) Property

Description

Liquid limit（%）

276

Plastic limit（%）

37

Total specific surface area（m2/g）

597

Cation exchange capacity（mmol/100g）

IP

2.66

SC R

Specific gravity of soil grain

T

ACCEPTED MANUSCRIPT

77.3

Na+（43.36）, Ca2+（29.14）,

Main exchanged cation（mmol/100g）

NU

Mg2+（12.33）, K+（2.51）

MA

Montmorillonite（75.4%） Quartz（11.7%） Feldspar（4.3%） Cristobalite（7.3%）

AC

CE P

TE

D

Main minerals

Table2. Parameters for compression tests vertical stresses during saturation

infiltration solutions

ACCEPTED MANUSCRIPT 0.12MPa

De-ionized water De-ionized water

IP

T

0.5M NaCl 1 M NaCl

SC R

0.22MPa

2 M NaCl

NU

4 M NaCl

MA

De-ionized water

CE P

TE

D

0.5MPa

0.5M NaCl 1 M NaCl 2 M NaCl 4 M NaCl

De-ionized water

AC

1.45MPa

1 M NaCl

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Highlights

1. Oedometer test was conducted on specimen swollen under vertical pressures

IP

T

2. Swelling strain decreases as the concentration or vertical pressure increases

AC

CE P

TE

D

MA

NU

SC R

3. Compressibility decreases as the concentration or vertical pressure increases