Effect of treatment on the microstructural characteristics of bio-improved sand

Bio-chemical processes have recently become the core of several studies in the geoengineering field, where the emphasis is placed on new and emerging applications. Microbially Induced Calcite Precipitation has been introduced as an alternative cementation mechanism for soils, and it is studied for its potential to offer substantial and environment-friendly solutions for soil improvement. In this paper, we focus on the microstructural characteristics of the new geo-material, which is engineered after harnessing a treatment process including multiple injections of bacteria and chemical solutions, which ultimately induce the formulation of calcite particles inside the soil matrix. Obtaining detailed knowledge of the local structural properties of the precipitated nuclei is considered an essential factor because calcite particles affect the macroscopic mechanical response with the foreseen increase in strength and stiffness, which are attributed to the precipitated mass and its geometrical arrangement. The main objective of this study is to elucidate the role of the calcium chloride concentration on the yielded precipitation and clarify some of the predominant fabric characteristics of the new structure. For this purpose, microstructural observations with Scanning Electron Microscopy (SEM) were used. Here, calcite mesocrystals, i.e., aggregates of single particles, are identified as a distinct form of precipitates that provide the crucial grain-to-grain contact surfaces. The cemented samples were subsequently subjected to undrained triaxial shear. The results show the increase in peak strength at low strains, followed by a brittle fracture and post-yield softening.