Shear wave velocity based prediction of CaCO3 content and UCS in MICP-treated soils with different particle sizes

As an emerging soil reinforcement technology, microbial-induced carbonate precipitation (MICP) requires in-situ testing to assess its field performance. Shear wave velocity (Vs) is a viable in-situ method. However, a clear predictive model linking its variations with the CaCO3 content (Cm) and unconfined compressive strength (UCS) of MICP-treated soil—considering particle size as a variable—remains to be established, constraining broader applications of this approach. This study conducts MICP treatment tests on mixtures of excavated soil (silty sand) and surplus sludge (elastic silt), using the mean particle size (d50) as the indicator of particle size. In addition, by varying the sludge proportion (Ps), the impact of particle size on the Vs, Cm, and UCS is examined. During the MICP process, an increase in Ps reduces the increase rate of Cm and Vs. Moreover, Vs stabilizes about 10 h earlier than Cm at a given Ps. This is probably attributed to a shift in CaCO3 precipitation modes from significant strength/stiffness enhancement for soil mixtures to minor contribution. After MICP treatment, Vs, Cm, and UCS decrease linearly with increasing Ps. Following this, Vs-based prediction models for Cm and UCS are respectively developed by considering the particle size. The prediction models are proven to accurately forecast Cm and UCS in MICP-treated soils with different d50 values. This advancement enhances in-situ monitoring techniques that employ Vs to evaluate MICP cementation effectiveness.