Investigation on hydraulic properties of compacted GMZ bentonite used as buffer / backfill material

During the past decades, GMZ bentonite has been widely investigated for its use as buffer/backfill materials in China. Based on a comprehensive review of the former studies, achievements on experimental and theoretic works on the hydraulic aspects of compacted GMZ bentonite with consideration of temperature effects are presented in this paper. Water retention property of compacted GMZ bentonite depends on constraint conditions. Temperature effects on water-retention depend on constraint conditions and suction. The hysteresis behaviour is not obvious. Based on the test results, a revised water retention model was developed for considering the temperature effect. The saturated hydraulic conductivity of the densely compacted GMZ bentonite increases as dry density and temperature increases. A revised model, which considers temperature influence on water viscosity and the effective flow cross-sectional area of porous channels, for prediction of saturated hydraulic conductivity have been developed and verified. The unsaturated hydraulic conductivity of confined densely compacted GMZ bentonite samples decreases first and then increases with suction decrease from an initial value of 80 MPa to zero. With consideration of temperature effects and microstructure changes, a revised model for prediction of unsaturated hydraulic conductivity of compacted GMZ01 bentonite was proposed.


Introduction
Bentonite has generally been chosen as buffer and backfill material for construction of engineered barrier in deep geological repository because of its high swelling capacity, low permeability, micro-porous structure and good sorption properties.This clay-based engineering barrier has multiple purposes of providing mechanical stability for the waste canister (by absorbing stresses and deformations); serving as a buffer around it; sealing discontinuities in the emplacement boreholes and drifts; and delaying the water infiltration from the host rock.During the long-term operation of the repository, the engineered clay barrier and adjacent host rock will be subjected to the heating effect of the nuclear waste, to various associated hydraulic and mechanical phenomena that interact in a complex way.Therefore, the hydraulic property of the compacted bentonite is one of the key properties for the design of such a disposal system (Dixon et al., 1987;Nachabe, 1995;Liu and Wen, 2003;Ye et al., 2013;2014a,b;Villar, 2000;2002;Komine, 2004).Furthermore, compacted bentonite is initially unsaturated, and will therefore be subjected to hydration from the surrounding rock, triggering further coupled thermo-hydro-mechanical (THM) phenomena.
Chinese high-level radioactive waste repository program was launched in the middle of 1980s (Wang et al., 2006).It was suggested that a high-level radioactive waste repository will be built in the middle of the 21st Century.Beishan in Gangsu Province has been selected as the potential site for construction of the first repository in China.Gaomiaozi (GMZ) bentonite has been considered as a possible material for the construction of engineered barrier in the recent Chinese program of radioactive waste disposal (Liu et al., 2001;Ye et al. 2009a).
In this paper, based on a comprehensive review of the former studies, the achievements on experimental and theoretic works on hydraulic properties of GMZ bentonite are presented.

Basic properties of GMZ bentonite
Gaomiaozi (GMZ) bentonite originates from Inner Mongolia, China, 300 km northwest from Beijing.There are 160 million tons bentonite (with 120 million tons Na-Bentonite) reserves in the deposit, which occupies a mining area about 72 km 2 .The deposit was formed in the later Jurassic period (Liu et al., 2001).GMZ Bentonite is bedded, with a soapy texture and waxy appearance.The Cation Exchange Capacity (CEC) of GMZ bentonite is showed in Table1 (Ye et al., 2010).Some basic physical properties of GMZ bentonite are presented in Table 2. Results in Tab. 1 and Tab. 2 show that GMZ bentonite has high montmorillonite content, which gives it a high Cations Exchange Capacity (CEC = 77.30meq/100g), a large plasticity index (Ip = 239), and a large specific surface area (S = 570 m 2 /g).

Water retention properties
Using the vapour phase technique and the osmotic technique, the WRCs of both the confined and unconfined GMZ bentonite specimens were measured at room temperature (20ºC) by Chen et al. (2006) (Fig. 1).The result in Fig. 1 shows that, starting from the initial suction of 4.2 MPa, the difference between different confining conditions becomes less significant as the suction increases.However, for suctions lower than 4 MPa, the confined specimen gives much lower water content: at the lowest suction (0.013 MPa) measured the difference in water content is as large as 140%.
The water retention curves obtained under temperatures following the wetting path for the unconfined GMZ01 bentonite specimens with a dry density of 1.70 g/cm 3 are presented in Fig. 2. It is observed that temperature effect on the water retention capacity is insignificant.The water retention curves of confined compacted GMZ bentonite following wetting path at different temperatures are presented in Fig. 3. Curves in Fig. 3 show that, for confined specimens, the water retention capacity decreases as temperature rises.Moreover, this temperature influence strongly depends on suction.When the suction is close to zero (0.01 MPa), the temperature effect is hardly observed.A possible explanation of this observation is that the lateral swelling stress applied to the compacted bentonite sample DOI: 10.1051/ , 9 E 2016 increases with suction decrease under confined conditions.Meanwhile, decreasing suction could not make more water moving into large pores, as most of the pores have already been filled with water.
Comparison between the water retention curves shown in Figs. 2 and 3 reveals that temperature effects on water retention capacity strongly depends on the constraint conditions.This conclusion agrees well with the observations reported by Villar and Lloret (2004).

Fig. 4. Hysteresis behavior of unconfined GMZ bentonite at 20 and 40 ºC temperatures (Wan et al., 2015)
The hysteresis curves of unconfined compacted GMZ bentonite following wetting / drying cycles at 20 and 40 ºC are shown in Fig. 4. It is observed that the hysteresis behavior becomes less obvious with temperature increase, in agreement with the observation reported by Ye et al. (2009b).This indicates that, as temperature increases, the water retention capacity and the hysteresis decrease regardless of the confining conditions.Fig. 4 also shows that the influence of temperature on the hysteresis behavior depends on the suction level: the higher the temperature, the less obvious the hysteresis behavior.
For modelling the soil water retention behavior of compacted GMZ bentonite with consideration of temperature effects, based on a comprehensive review of literatures (Brooks and Where w 0 sat (T)= 0.25; ζ=0.00018;T 0 =273.4.By fitting the SWRC measured at 20°C using equation (1), for the confined compacted GMZ01 bentonite with a dry density of 1.7 Mg/m 3 , parameters m and n are determined, equal to 0.5864 and 0.8086, respectively.
With equation ( 1), the SWRCs of compacted GMZ01 bentonite at 60°C and 80°C were calculated and compared to the measured ones.Comparison demonstrates that the revised model can satisfactorily describe the soil water retention properties of the confined compacted GMZ01 bentonite changing with temperature.
Where k s is saturated hydraulic conductivity, k in intrinsic hydraulic conductivity, η water viscosity, T is temperature, ρ w is water density and g is acceleration of gravity.
Calculation shows that the proposed model (eq.( 2)) can well describe the temperature influences on the saturated hydraulic conductivity of the compacted GMZ bentonite.
It should be noted that the values measured during the first 600 h in Fig. 5 probably correspond to a period when the sample was still unsaturated.

Unsaturated hydraulic conductivity
Using a self-designed cell, Ye et al. (2009a) measured the unsaturated hydraulic conductivity of GMZ bentonite with a dry density of 1.7 g/cm 3 under confined conditions at ambient temperature.With the instantaneous profile method, the unsaturated hydraulic conductivities measured at different locations along the height of the specimen evolving with suction are presented in Fig. 6. Results in Fig. 6 indicate that, when suction is reduced from the starting point about 80 MPa to zero, the measured unsaturated hydraulic conductivity firstly decreases from 2E-14 m/s at 80 MPa suction, then increases from 7E-15 m/s at 70 MPa suction, and 10E-13  Comparison of results in Figs. 6 and 7 reveals that, for a same suction, the unsaturated hydraulic conductivity of compacted GMZ bentonite under unconfined conditions is higher than that of under confined conditions.This is possibly induced by the difference in the mechanism of microstructural changes during hydration under different confining conditions.
With self-made apparatus, Ye et al. (2009b; 2012) measured the unsaturated hydraulic conductivity of the confined GMZ specimens at 20ºC, 40ºC and 60ºC, which were compared in Fig. 8. Results show that under confined conditions, the unsaturated hydraulic conductivity of the highly compacted GMZ bentonite increases with temperature rise.This temperature effect can be explained by changes of water viscosity (Duley and Domingo, 1943;Hopmans and Dane, 1986;Constantz and Murphy, 1991), water density, and to some extent the intrinsic hydraulic conductivity (Villar et al., 2005).Among which, the water viscosity has been found to be the most significant factor (Delage et al., 2000).
Moreover, the rate of change also decreases as temperature increases.The temperature effect becomes more significant at higher suctions (above 20 MPa).In the range of lower suctions (less than 20 MPa), it is observed that the lower the suction, the less the temperature effect.The possible explanation is that for lower suctions the moisture absorbed by the bentonite is mainly associated with microstructure changes and the temperature effect on the microstructure is not significant.
Where, k s (T) is saturated hydraulic conductivity of compacted GMZ bentonite with consideration of temperature effects, which can be expressed by equation (2).Parameter A is the influencing factor of microstructure changes, B is the influence coefficient of the effective degree of saturation on the variation of void ratio.For the compacted GMZ01 bentonite specimen tested: B = 0.515.Based on the measured unsaturated hydraulic conductivity of densely compacted GMZ01 bentonite with a dry density of 1.7 Mg/m 3 under confined conditions at temperatures 40°C and 60°C in Fig. 8

Conclusions
Based on a review of the former studies, achievements on experimental and theoretic results obtained on the hydraulic behaviour of compacted GMZ bentonite are presented in this paper.
The influences of constraint conditions on water retention properties of GMZ bentonite depends on the range of suction.Temperature effects depends on constraint conditions and suction.Under unconfined conditions, the effect of temperature at high suctions is different from that at low suctions.The hysteresis behaviour of unconfined compacted bentonite is not obvious on the whole, even it was found reduced with temperature increase.A revised water retention model was developed for considering the temperature effect.Comparison between the calculated results and the experimental data shows the relevance of the model developed in describing the water retention properties of compacted GMZ bentonite at different temperatures under confined conditions.
The saturated hydraulic conductivity of the GMZ bentonite decreases as dry density increases and increases as temperature increases.With consideration of both the temperature influence on water viscosity and the effective flow cross-sectional area of porous channels, the proposed model can satisfactorily account for the temperature effects.
The unsaturated hydraulic conductivity of GMZ bentonite firstly decreases and then increases with suction decrease from an initial value of 80 MPa to zero.This phenomenon could be attributed to the large pore clogging due to exfoliation process.The unsaturated hydraulic conductivity of compacted GMZ bentonite under unconfined conditions is higher than that under confined conditions.Under confined conditions, the unsaturated hydraulic conductivity of the highly compacted GMZ bentonite increases with temperature rise.Moreover, the changing rate depends on suction.

Fig. 5 .
Fig. 5. Evolution of saturated hydraulic conductivity of the compacted GMZ bentonite with time at temperatures (Ye et al., 2013) According to the test results, the following equation was proposed for prediction of the saturated hydraulic conductivity of the compacted GMZ bentonite considering temperature influences on both water viscosity and effective flow cross area (apparent porosity) in its porous channels (Ye et al., 2013).      273 -T 0.034688 -exp 001517 .0 0002601 .0 )) 273 ( 0054 .0 703 .45 exp( ) (   at zero suction.The decrease can be attributed to the large pore clogging due to the possible exfoliation process (Ye et al., 2009a).

Fig. 6 Fig. 7
Fig. 6 Evolutions of hydraulic conductivity of the GMZ bentonite at different heights of the specimen with suction under confined conditions (Ye et al., 2009a) Niu (2008) tested the unsaturated hydraulic conductivity of the densely compacted GMZ bentonite under unconfined conditions at room temperature.Results (Fig.7) show that the unsaturated hydraulic conductivity is in a larger range of 1.0E-12 and 1.0E-15 m/s and it develops gently with suction except for fluctuations both at the initial and the final stages of the experiment.

Fig. 8
Fig. 8 Evolution of unsaturated hydraulic conductivity with suction for the confined GMZ bentonite at temperatures (Ye et al., 2012) According to the test results, a revised fractal model for prediction of unsaturated hydraulic conductivity of compacted bentonite with consideration of temperature effects and microstructure changes was developed by Ye et al. (2014b).