Anisotropic hydro-mechanical behavior of full-scale compacted bentonite-sand blocks

Compacted bentonite-sand blocks have been proposed to construct buffer barriers in high-level radioactive waste (HLW) repositories in order to isolate wastes. The bentonite-based blocks are usually manufactured by uniaxial compression in mould and exhibit anisotropic behavior, which may influence the buffer performance. This current paper aims at investigating the anisotropic hydro-mechanical behavior of full-scale compacted bentonite-sand blocks. The 1/12 block rings were axially compressed in a mould with 170 and 475 mm in radiuses and 100 mm in height, and the density distribution was examined by computed tomography (CT). The block's engineering performance was monitored in different directions, including the drying shrinkage and crack development during the storage, and the swelling pressure and hydraulic conductivity during hydration. The blocks' surface shrinkage strain was measured by the digital image correlation (DIC), and the surface cracks were analyzed by an image-processing technique. In addition, subsamples were excavated from replicate blocks to measure their hydraulic conductivity and swelling pressure in different directions. The blocks showed anisotropic properties, such as the less shrinkage strain and higher swelling pressure in the vertical load direction, and the appearance of nearly horizontal cracks. In contrast, the hydraulic conductivity of the blocks was similar in the vertical and horizontal directions. The horizontally oriented smectite plates and bentonite aggregations, as verified by the scanning electron microscope (SEM) images and development of swelling pressure, contributed to blocks' anisotropic behavior. On the other hand, the sand and incompressible impurities created preferential flow paths for their isotropic permeation.