Filling efficiency of mono-sized pellets for sealing boreholes: Identifying optimal borehole-to-pellet diameter ratio via laboratory experiments

Bentonite pellets are one of the practical options for sealing deep investigation boreholes. To show that there are proper and improper sizes of the pellets that could lead to high or low as-filled dry density when filling a borehole with mono-sized pellets, a set of idealized laboratory experiments was performed. Mono-sized spherical particles (mock-up pellets) were filled in cylindrical containers (mock-up boreholes), and their filling efficiency in terms of average inter-particle porosity was investigated for a wide range of borehole-to-pellet diameter ratios. The results indicated that improper selection of the particle size could lead to an average inter-particle porosity twice as large as when a proper particle size was used. This difference in the average porosity could lead to a two-fold difference in the hydraulic conductivity after the pellets had finished swelling. Although the finding was obtained with mono-sized particles/pellets, it provided a clear insight into how the pellet size could affect the filling efficiency if D / d is improperly chosen, which could, in turn, strongly affect the effectiveness of the seal. Based on the findings, a guide for selecting proper pellet size, or at least to avoid improper pellet size, was provided when the use of mono-sized pellets is considered.