Properties of bentonite, mainly used as buffer and/or backfill materials, will evolve with time due to thermo-hydro-mechanical-chemical (THMC) processes, which could deteriorate the long-term integrity of the engineered barrier system. In particular, degradation of the backfill in the evolution processes makes it impossible to sufficiently perform the safety functions assigned to prevent groundwater infiltration and retard radionuclide transport. To phenomenologically understand the performance degradation to be caused by evolution, it is essential to conduct the demonstration test for backfill material under the deep geological disposal environment. Accordingly, in this paper, we suggest types of tests and items to be measured for identifying the performance evolution of backfill for the Deep Geological Repository (DGR) in Korea, based on the review results on the performance assessment methodology conducted for the operating license application in Finland. Some of insights derived from reviewing the Finnish case are as follows: 1) The THMC evolution characteristics of backfill material are mainly originated from hydro-mechanical and/or hydrochemical processes driven by the groundwater behavior. 2) These evolutions could occur immediately upon installation of backfill materials and vary depending on characteristics of backfill and groundwater. 3) Through the demonstration experiments with various scales, the hydro-mechanical evolution (e.g. advection and mechanical erosion) of the backfill due to changes in hydraulic behavior could be identified. 4) The hydro-chemical evolution (e.g. alteration and microbial activity) could be identified by analyzing the fully-saturated backfill after completing the experiment. Given the findings, it is judged that the following studies should be first conducted for the candidate backfill materials of the domestic DGR. a) Lab-scale experiment: Measurement for dry density and swelling pressure due to saturation of various backfill materials, time required to reach full saturation, and change in hydraulic conductivity with injection pressure. b) Pilot-scale experiment: Measurement for the mass loss due to erosion; Investigation on the fracture (piping channel) forming and resealing in the saturation process; Identification of the hydro-mechanical evolution with the test scale. c) Post-experiment dismantling analysis for saturated backfill: Measurement of dry density, and contents of organic and harmful substances; Investigation of water content distribution and homogenization of density differences; Identification of the hydro-chemical evolution with groundwater conditions. The results of this study could be directly used to establishing the experimental plan for verifying performance of backfill materials of DGR in Korea, provided that the domestic data such as facility design and site characteristics (including information on groundwater) are acquired.

PURPOSES : In this study, to improve the quality and construction performance of backfill materials for road excavation and restoration, the basic properties of rapid-hardening materials and stone sludge are analyzed to propose an optimal mix design. METHODS : To utilize rapid-hardening materials and stone sludge as controlled low-strength materials for pipeline construction in downtown areas, specimens were prepared for each compounding condition of fast-hardening materials. Flow, slump, and compressive strength tests were performed at various setting times. Subsequently, the physical and mechanical characteristics of the rapid-hardening backfill material for each mixing factor were analyzed. RESULTS : When ultrafast hardening cement and stone sludge are used, a setting time exceeding 30 min is required for a water–binder (W/B) ratio of 200% or higher. Considering the economic feasibility of ultrafast hardening cement, a W/B of 300% is considered the most suitable when high-performance superplasticizer and retarders are mixed. A flow test was performed on the rapid-hardening backfill material and the results show that if the mixing time exceeds 10 min, then the fluidity decreases rapidly, which necessitates a higher amount of superplasticizer. When ultrafast hardening cement is used, the initial strength (based on 4 h) is 0.7 MPa or higher for all formulations at a W/B ratio of 300%, and the compressive strength decreases slightly as the amount of superplasticizer is increased by 0.1%. CONCLUSIONS : Based on the fluidity and strength of the backfill material, which is composed of a rapid-hardening material and stone sludge, the most optimal performance is achieved when ultrafast-hardening cement with a W/B ratio of 300% is used in addition to a highperformance fluidizing agent 0.3% (wt./B) and retarder 0.2% (wt./B).

Backfill is one of the main components of engineered barrier in a high-level waste repository. The material selection of the backfill determines the barrier performance of the backfill. Overseas, its related research has been carried out mainly in Sweden, Finland, Canada, and Japan. However, Korea has recently started backfill research, and it is urgent to select a potential material for establishing the concept of backfill material and conducting backfill research. This study reviews NEA report, potential materials for overseas backfill research, advantages and disadvantages of single and mixed backfill materials, cases of license applications in Finland and Sweden for the selection of potential materials for backfill in Korea’s high-level waste repository. The review results indicated that it is reasonable to carry out backfill research according to the following plan: Both single and mixed materials are considered as potential materials for backfill research; experiments and performance studies are conducted with these materials; and, based on the results, a potential material or candidate material for the backfill suitable for the HLW repository in Korea is determined. For this plan, the single material is tentatively selected, as in Sweden, as bentonite with a montmorillonite content of about 40-50%. Then, if the selection criteria for montmorillonite content are determined through experiments and performance studies, we determine the final potential backfill material. As for the mixed backfill material, the bentonite/crushed rock mixture seems to be more advantageous than the bentonite/sand mixture considering the disposing problem of crushed rock generated from tunnel excavation and economic feasibility through its recycling. It is thought that the bentonite used in the bentonite/crushed rock mixture should have a higher montmorillonite content than bentonite used as a single backfill material since the crushed rock acts as an inert material in the mixture. The results of this study can be used as basic data for selecting the backfill material to be applied to the high-level waste repository in Korea, and can be used as a guideline for selecting the potential material required for backfill experiments and performance studies to be carried out in the future.

After analyzing the engineering characteristics of backfill soil, we are trying to figure out whether or not satisfy the compaction criteria. For that purpose, Case of design and construction were investigated. I have also evaluted the effect of backfill materials engineering properties on structures according to degree of compaction.