Micromechanical Properties of Biocemented Shale Soils Analyzed Using Nanoindentation Test

Biocemented shale soils are composite materials, and their micromechanical properties are difficult to measure using common macromechanical tests. In this work, nanoindentation tests were conducted to study the effects of microstructures, crystal types of calcium carbonate, and locations of test points on the micromechanical properties of biocemented shale soils. The transformation process of crystal types of calcium carbonate during biogrouting was revealed using scanning electron microscopy and X-ray diffraction imaging techniques. The results showed that a significant amount of voids within the cementing body were generated due to the death of bacillus pasteurii. Vaterite and calcite were both detected in the same sample, where the vaterite gradually changed into calcite during the biogrouting process. The nanoindentation tests were carried out in locations with different surrounding conditions, and the micromechanical properties of biocemented soils were obtained through analysis of the load-displacement curves. The relationships between fracture toughness, elastic modulus, and hardness were found to be linear based on our test results. The established empirical relationships help understand the correlations between micromechanical properties and also provide a useful method for the characterization of fracture toughness of biocemented shale soils.