Predicting the Tensile and Unconfined Compressive Strength of MICP-Treated Sands

Microbially induced carbonate precipitation (MICP) has been studied over the last 20 years as a promising biomediated alternative to enhance soil mechanical performance. Multiple studies have investigated different MICP treatment techniques and their application to various coarse-grained soils. Results of these works show the evolution of transmission properties (e.g., ultrasound waves and permeability) and the increase in strength measured by direct shear, uniaxial compressive strength (UCS), and triaxial testing, among others. These analyses have enriched our understanding of the capabilities of MICP, yet there is a lack of predictability on the efficiency of MICP treatments on the treated soil strength. This study uses an extensive data set of triaxial results collected from the literature and a new experimental campaign to propose a framework that predicts the evolution of the tensile and compressive strengths in MICP-treated sands based on the calcium carbonate content achieved and the untreated soil index properties. The investigation starts with an analysis of the influence of multiple soil characteristics on the final mechanical strength of MICP-treated sands using data from the literature. This preliminary analysis uncovers a gap in the data on the treatment of angular sands. To fill this gap, we conducted experiments using highly angular sand in which the MICP treatment strategy was varied to obtain varying levels of cementation. The complete data set was then used to assess the validity of an analytical model that builds on previous analyses to predict the tensile strength of MICP-treated sands. Our model results are remarkably consistent with all published data sets. Furthermore, we also use the full data set to calibrate the beta factor from a previously proposed model to predict the UCS values, as a function of final cemented porosity and the untreated index properties of the soil. Our research provides a robust framework for practicing engineers to use for the prediction of the mechanical enhancement of MICP-treated sands, based on the mass of calcium carbonate precipitated and the untreated soil index properties. This is a critical step toward the use of biomediated soil enhancement techniques in practice, where prediction of MICP-treated strengths will be a critical input for engineering design.