Grain-Scale Tensile and Shear Strengths of Glass Beads Cemented by MICP

This study explores the mechanical behavior of glass bead pairs cemented by microbial induced calcite precipitation (MICP) when subjected to tensile or shear loading. The mineral precipitation habit and contact area are also examined using X-ray computed tomography (X-ray CT). Examination of the failure surfaces reveals three distinctive failure modes: debonding failure at the precipitate-grain interface, internal failure within the precipitate, and mixed failure. The internal failure mode appears dominant when the calcite content (CC) of the bonded glass bead pair is greater than 17%, and it results in the smallest strengths: $\sim 8\mathrm{kPa}$ in tension and $\sim 7\mathrm{kPa}$ in shear. When CC is less than 17%, the debonding failure mode is mostly observed, and the debonding failure leads to the greatest strengths: $\sim 35\mathrm{kPa}$ in tension and $\sim 13\mathrm{kPa}$ in shear. The mixed failure mode occurs when $11\%<\mathrm{CC}<20\%$, partly overlapping with the other two modes. The average tensile strength is greater than the average shear strength in all modes. The X-ray CT images demonstrate that the deposition of calcium carbonate first begins by coating the grain surface, and later shifts toward preferential precipitation at the grain contacts as the CC increases. Therefore, the relationship between contact radius and CC is bounded by the grain-coating and meniscus-filling models when CC < 20%; however, at a greater CC this relationship is bounded by the meniscus-filling and flat torus-filling models. This study presents unprecedented grain-scale mechanical responses associated with MICP-treated granular materials, which can be further extended to advance the understanding of the interplay between grain-scale cementation and the mechanical response of MICP-treated specimens, as well as the simulation of cemented soil behavior using discrete element modeling.