Evaluation on Compression Wave Velocities and Moduli of Gyeongju Compacted Bentonite

경주 압축 벤토나이트의 압축파속도와 탄성계수 산정 연구

Abstract

Gyeongju bentonite is a buffer material primarily considered in Korea and it is highly compacted as a part of an engineered barrier system (EBS) of high-level radioactive waste repository. The compacted bentonite undergoes swelling stress by groundwater penetration and thermal stress by decay heat from a canister. Therefore, the mechanical properties of the compacted bentonite buffer material is crucial for the performance assessment of EBS. This paper aims to evaluate deformation properties of Gyeongju compacted bentonite using seismic methods. Two sets of compacted bentonite specimens were prepared having dry densities of $1.59g/cm^3$ and $1.75g/cm^3$ with water contents of 10.6% and 8.7%. Free-free resonant column tests were performed to measure constrained and unconstrained compression wave velocities. With the measured wave velocities, Young's modulus ($E_{max}$) and constrained modulus ($M_{max}$), material damping ratio ($D_{min}$), and Poisson's ratio at small strain were determined. As results, this paper evaluates the deformation properties of Gyeongju compacted bentonite and compares them with the results of previous researches.

국내 고준위폐기물처분장 공학적방벽(EBS)의 일부가 되는 완충재로 경주 벤토나이트가 우선적으로 고려되고 있다. 압축벤토나이트는 지하수침투로 인한 팽윤압과 처분용기에서 발산되는 열응력을 경험한다. 따라서 EBS의 성능평가를 위해서 역학적 물성의 산정이 중요하다. 본 논문은 탄성파를 이용하여 경주 압축벤토나이트의 변형특성 측정을 목표로 하였다. 두 개의 $1.59g/cm^3$와 $1.75g/cm^3$의 건조밀도를 가지는 압축벤토나이트 시편을 제작하였고, 자유단-자유단 공진주시험을 수행하여 구속압축파속도와 비구속압축파속도를 측정하였다. 측정된 압축파속도를 이용하여 미소변형에서의 탄성계수($E_{max}$), 구속탄성계수($M_{max}$), 감쇠비($D_{min}$), 포아송비를 측정하였다. 그 결과로 경주 압축벤토나이트의 변형특성을 산정 제시하여 선행연구 결과들과 비교 분석하였다.

Fig. 1. Engineered Barrier System

Fig. 2. X–ray diffraction patterns of the KJ-I and KJ-II powders (Yoon et al., 2018a)

Fig. 3. Typical string-suspended specimen and manual longitudinal excitation for V_c and Vp measurements: (a) Schematic diagram, and (b) photo

Fig. 4. Typical frequency response curve of FFRC for GJ-B

Fig. 5. Half-power bandwidth method

Fig. 6. Direct travel time measurement (Trial 3, using 10-mm diameter ball on GJ-A)

Fig. 7. Frequency response curves of (a) GJ-A, and (b) GJ-B

Fig. 8. Velocity measurements, compared to Spanish Bentonite with minor sandy size filler constituents montmorillonite > 95 volume %, ρ_d = 1.66 g/cm³ ; V_p evaluation of Tisato and Marelli. (2013)

Fig. 10. Comparison with damping ratio of Vucetic & Dobry (1991) and Kim and Choo (2001)

Fig. 11. Comparison with Poisson’s ratio of this study with Cho et al. (1999) and Eloranta (2012)

Fig. 9. Comparison with Young’s Modulus of (a) Kunigel VI & MX-80, and (b) KJ-I of Cho et al. (1999)

Table 1. Quantitative XRD analysis for mineral constituents of KJ-I and KJ-II powders (Yoon et al., 2018a)

Table 2. Properties of compacted bentonite specimens

Table 3. Young’s modulus of compacted bentonite

Table 4. Poisson’s ratio of compacted bentonite

Table 5. Summary of the compressive seismic velocities and moduli of this study