Experimental investigations on thermo-hydro-mechanical properties of compacted GMZ01 bentonite-sand mixture using as buffer materials

Highlights

• Sand content can inhibit the maximum swelling pressure of sand-bentonite mixture effectively.

• The hydraulic conductivity of the mixture increased with increasing sand content.

• The increase of sand content could induce significant increase of thermal conductivity of the mixture.

• Sand content in the mixture should not be more than 30% in view point of THM properties.

Abstract

Bentonite-sand mixture has been proposed as engineered barriers for high-level waste disposal in many countries. For investigation of the thermo-hydro-mechanical properties of compacted GMZ01 bentonite-sand mixture, swelling pressure, hydraulic and thermal conductivity tests were conducted in this paper. Results show that addition of sand will increase the thermal conductivity of the mixture. However, depending on the dry density and water content of the specimen, the increasing rate of thermal conductivity becomes stable or even decreases with further increase of sand content. The addition of sand will also impede the development of swelling pressure while the inhibiting effect becomes weak as the sand content increases. In the meantime, with increasing sand content, the hydraulic conductivity of the mixture will increase significantly, especially at higher sand contents. Analysis shows that, sand content should be kept below 38% and 39% to guarantee the swelling pressure above 1.0 MPa and the hydraulic conductivity lower than 1.0e − 12 m/s, respectively. The most effective sand content in the view of heat dissipation of buffer materials is < 30%. Therefore, the preferential option of sand content in the mixture should not be > 30% in view point of thermal-hydro-mechanical properties.

Introduction

Deep geological storage is a widely accepted technology for disposal of high-level radioactive wastes. The function of the repository in geological formations relies on a multi-barrier system that typically comprises the vitrification, metal waste canister, buffer/backfill materials and natural geological formations. According to the functional requirements of the multiple-barrier system, bentonite-based materials have been chosen by several countries as buffer materials due to, 1) the proper swelling capacity to seal the structural cracks on hydration and 2) the low permeability and high absorption capacity to impede the possible leakage of radioactive substance. Therefore, researches related to the bentonite-based materials have been widely performed during the past decades (Sivapullaiah et al., 2000, Agus, 2005, Ye et al., 2007, Komine, 2010, Yong et al., 2010, Sun et al., 2014, Ye et al., 2014).

The pure bentonite owns extremely high plasticity and a tendency towards pelletization during water-soil mixing process, which will induce inhomogeneous wetting of the bentonite and hinder the blocking ability of the buffer material (Cheng et al., 2008). Especially, owning to the low thermal conductivity of the pure bentonite, the heat generated by radioactive decay in the canister will accumulate and increase the temperature in the repository. The excessively high temperature can induce the conversion of montmorillonite (the main constituent of bentonite) to illite and the increase of liquid gasification-induced water vapor pressure, which threatens the stability and functionality of the repository (Inoue et al., 1987, Cui et al., 2012). As an optimal solution to these problems, quartz sand has been preferentially adopted and mixed with bentonite to improve the workability and the heat-conductivity of the buffer/backfill materials. Investigations have been extensively conducted on sand-bentonite mixtures related to design, construction and integrity evaluations of the repository (JNC members, 1999, Komine, 2010, Sun et al., 2009, Sun et al., 2014, Cui et al., 2012, Zhang et al., 2012).

Experimental researches showed that as the sand content increased, the maximum dry density of the bentonite-sand mixture increased and the corresponding optimum water content decreased, in the meantime, the liquid limit and plastic limit decreased as well. This indicated that the addition of sand could improve the compaction characteristic and the workability of the materials (Zhang et al., 2012). Pakbaz and Khayat (2004) evaluated the effect of sand addition on the strength of bentonite-sand mixtures. Results indicated that the unconfined strength of the mixture increased until its sand content increased to 50% and turned to decrease thereafter. However, the shear strength obtained from triaxial tests increased with increasing sand contents. Swelling characteristics of bentonite-sand mixtures investigated by Cui et al. (2012) showed that, the maximum swelling pressure exponentially decreased with increasing sand content. Relationship between the maximum swelling strain and the initial dry density of the sand-bentonite mixtures could be fitted by a straight line. The slope of the fitted line had a downward trend with sand content increased from 0% to 50%, which proved that the addition of sand reduced the effect of dry density on the swelling potential. Sun et al., 2009, Sun et al., 2014 performed swelling pressure, swelling and collapse deformation tests and concluded that the addition of sand decreased the swelling capacity of the mixtures. The void ratio increased with increasing bentonite content in the mixtures at a same effective vertical pressure. In a double logarithmic scale, a linear relationship between the void ratio and the vertical pressure applied can be observed. Hydraulic characteristics investigated by Komine (2004) suggested that the addition of sand, as well as the decrease of dry density, will increase the hydraulic conductivity significantly, especially for specimen with higher sand contents due to the insufficient filling of voids by the bentonite. Similar conclusions obtained by Cho et al. (2000) indicate that there was a linear relationship between the logarithm of hydraulic conductivity and sand content. Investigations on the thermal characteristic of the bentonite-sand mixture show that the thermal conductivity of bentonite could be improved by mixing with sand, while the increasing rate became stable with further increasing sand content (Zhuang et al., 2005). This conclusion is consistent to the results reported by Börgesson et al. (1994) and Ould-Lahoucine et al. (2002). Cho et al. (2011) also found that the increase of sand content, dry density and water content could cause an increase in the thermal conductivity. With this observation, they proposed a single expression to predict the thermal characteristic of the mixture.

Therefore, comprehensive and comparative knowledge of the thermo-hydro-mechanical properties of the bentonite-sand mixtures for using as buffer materials should be investigated in further researches (Komine and Ogata, 1999, Komine, 2004, Komine, 2010). In this work, tests involved in swelling pressure, hydraulic conductivity and thermal conductivity with consideration of the influences of sand content, dry density and initial water content were conducted on GMZ01 bentonite-sand mixtures. In the meantime, based on the test results, the preferential option of sand content in the mixtures for using as buffer materials was recommended in view point of thermal-hydro-mechanical properties.