Microscopic investigation on bonding fracture of biocemented sand from novel in situ brazil splitting tests

The failure mechanism of biocemented sand has been largely speculated based on scanning electron microscopy images of biocemented sand after mechanical tests. However, some breakage patterns cannot be directly observed, such as the separation of CaCO₃ crystals and the separation of CaCO₃ crystals and sand surfaces. A study of the breakage process of biocemented sand at microscale is essential for understanding the mechanism of biocementation. In this work for the first time, a series of in situ splitting tensile tests were conducted using a tensile & compression module installed inside an SEM machine so that the breakage process of a biocemented sample under loading could be observed directly. The results showed that no breakage was observed before peak stress indicating an elastic–plastic deformation stage. Once the peak stress was reached, the stress fluctuated around the peak stress. At the end of the tests, two kinds of breakage of biocement were observed, i.e., calcite-calcite breakage and calcite-silica breakage. The detailed breakage seems also affected by the form of CaCO₃. Furthermore, besides increasing active precipitates, the improvements in the strength of biocement and the bonding strength between biocement and the sand surface were suggested to improve the performance of MICP for stabilizing sand. The findings of microscale in situ observation during mechanical tests can also provide basic evidence for modeling debonding process of biotreated soils using constitutive models or numerical simulation methods.