Evaluation of Shearing Behavior of Inorganic Microfiber-Reinforced Bentonite for Engineered Barrier Materials
Authors: Calaunan Jose Maria Ferdinand , Feng Yuan , Eun Jongwan , Kim Seunghee , Kim Yong-Rak 
Published: September 2023

ABSTRACT
The deep geological repository is the preferred disposal method for high-level radioactive waste after nuclear energy production. In the geological repository, one of the vital components of the engineered barrier system (EBS) is the buffer material that holds the radioactive waste-containing canister in place. However, it is known that the EBS could be readily dried near the canister due to heat emitted from the spent fuel inside the canister, which could generate cracks in the EBS. Inorganic microfiber can be an alternative to mitigate such cracks and, thus, enhance the performance of the engineered buffer material. This study investigated the shearing behavior of glass fiber-reinforced bentonite through a series of direct-simple shear tests on the bentonite samples. From this laboratory experiment, shear strength parameters such as cohesion and friction angle were evaluated for both peak and residual strengths. Furthermore, the normalized shear stress and stress paths were analyzed. At both peak and residual states, the cohesion consistently reduced as the fiber content was increased. Higher cohesion was observed at the residual behavior than at the peak behavior. In the case of peak strength, the friction angle decreased from 0.0% to 1.0% fiber content, then vastly increased for 2.0% and 3.0% fiber content. Meanwhile, the friction angle continuously increased with increasing fiber content for the residual strength.

INTRODUCTION
Recent studies have shown that the saturation of engineered barrier system (EBS) in the high-level radioactive waste (HLRW) repository can be a prolonged process due to the heat generation and thermo-osmosis from spent fuel (Villar et al., 2012; Sánchez et al., 2016). Due to this slow saturation, heat-induced dry-out near the canister and subsequent generation of desiccation cracks in the buffer could lead to preferential pathways in the EBS. One of the methods to address these critical issues is to add fibers in the bentonite as a mechanical reinforcement for soil. On the other hand, several types of organic and inorganic fiber reinforcement, including polymers, minerals, glass, and synthetic materials, have been used to enhance clay properties in other geotechnical and geoenvironmental applications. These include utilizing sand-bentonite-carbon fiber mixtures as borehole backfilling material, polypropylene fiber with sand for the enhancement of bearing capacity, and polypropylene fiber and lime admixture for ground improvement (Joshi et al., 2004; Yetimoglu et al., 2005; Cai et al., 2006; Chaduvula et al., 2017; Liu et al., 2019). Although in a saturated condition, desiccation cracks might not be present in the bentonite, resulting mainly in the reduction of swelling pressure of the material.. On another note, microfibers can increase the tensile force to improve the shear strength of the soil. Particularly, inorganic fibers such as glass, carbon, and basalt fibers are proven to be helpful, especially for situations with exposure to high temperatures, due to their heat resistance (Ishikawa, 2014).