Microbially induced calcite precipitation (MICP) is an environmentally conscious ground-improvement method that can enhance the engineering properties of granular soils through the precipitation of calcium carbonate (CaCO3) on soil particle surfaces and contacts. Although numerous studies have shown the ability of biocementation to improve the liquefaction resistance of loose sands, the effects of light cementation levels on undrained cyclic behaviors have remained relatively unexplored. A series of undrained monotonic and cyclic direct simple shear tests were performed to examine the effect of light biocementation (ΔVs<100 m/s and CaCO3 contents <0.9%) on the liquefaction triggering and post-triggering behavior of loose Ottawa F-65 sand subjected to varying loading magnitudes [cyclic stress ratio (CSR)=0.1 to 0.3]. Results suggest that the presence of light biocementation can significantly improve the liquefaction triggering resistance of loose sands, with log-linear increases in the number of cycles required to trigger liquefaction, which consistently correlated with cementation-induced Vs increases. Despite these remarkable pretriggering improvements, almost no improvements were observed in post-triggering strain accumulation and postcyclic reconsolidation behaviors, with Vs measurements indicating that small-strain improvements were largely erased following shearing events.

With the recent advances in the biogeotechnics field and specifically microbially induced calcite precipitation (MICP), cone penetration testing (CPT) has become a valuable tool to overcome the challenges associated with intact sampling of improved soils, evaluate the spatial extent and magnitude of the applied MICP treatment, and assess the consequential improvement of engineering properties. Although the CPT cone tip resistance (qc) can effectively monitor the improvement of densified clean sands, no relationship exists to estimate cementation and strength parameters in MICP-treated sands. This paper proposes a relationship between the apparent cohesion (c) stemming from the MICP-induced cementation bonds at particle contacts and the change in tip resistance (qc) in initially loose sands. To develop a broadly useful correlation, available experimental CPT data in biocemented soils were used to guide computation simulations using a direct axisymmetric model of cone penetration in biocemented sands. The CPT numerical model uses the finite-difference method with a rezoning algorithm for large-deformation problems along with the Mohr-Coulomb constitutive model. The biocemented sand was characterized by Mohr-Coulomb strength parameters and an elastic shear modulus informed by shear-wave velocity measurements (Vs). The correlation parameters of interest were identified (c, qc, and Vs), and results of the numerical simulations were validated against available experimental data. Once validated, the numerical simulations were extended to different initial conditions, and the trends between parameters of interest were analyzed and interpreted. Results from the simulations are consistent with experimental data and show an increase in the cone tip resistance as the cementation level increases. The cementation level is modeled through apparent cohesion and the shear stiffness model parameters, which both increase as the cementation level increases. A linear relationship is proposed between the apparent cohesion and the change in cone tip resistance as a function of the confining stress.