Experimental Investigation on Small-Strain Dynamic Properties and Unconfined Compressive Strength of Gyeongju Compacted Bentonite for Nuclear Waste Repository

Abstract

Gyeongju bentonite is a Ca-bentonite and a highly compacted buffer material primarily considered in Korea, as a part of an engineered barrier system (EBS) of high-level radioactive waste (HLW) repository. During the repository lifetime, the buffer material should guarantee the safe disposal of HLW against any external mechanical impact, including seismic activity. Therefore, the dynamic deformation properties of buffer material is crucial to validate its lifetime effectiveness and performance. Even though there are some researches to investigate the dynamic properties of the bentonite buffer, there has been no researches for the Korean bentonite buffer. For this reason, the paper aims to evaluate the dynamic properties at small-strain of Gyeongju bentonite and correlate the small-strain deformation properties with unconfined compressive strength using seismic and static laboratory tests. Three sets of compacted cylindrical bentonite specimens with varying dry densities (1.50 g/cm3 to 1.77 g/cm3) were produced using a cold isostatic pressing technique under three different pressing loads. Free-free resonant column (FFRC) tests and unconfined compressive (UC) tests were performed. The FFRC tests measured deformation properties at small strain, including unconstrained and constrained compression velocities, Young’s modulus (Emax), constrained modulus (Max), material damping ratio (Dmin), and Poisson’s ratio. The UC tests evaluated the compressive strength and secant modulus (E50) at large-strain. In this study, it was found that the deformation properties (Emax and Mmax) and unconfined compressive strength increase with the increase in a design rage of dry density. Poisson’s ratio measured at constrained boundary yields relatively constant value close to 0.4 for specimens at a range of dry density higher than 1.5 g/cm3. In addition, it was found that the elastic moduli have an almost linear relationship with the unconfined compressive strength. Finally, empirical equations between elastic moduli and unconfined compressive strength for Gyeongju compacted bentonite are proposed for a Koeran design guideline.