Evaluation of MICP Treatment Level Accounting for Changes in Seismic Site Response and Liquefaction Potential

Microbial induced carbonate precipitation (MICP) has gained popularity in the last decade as a biologically based ground improvement technique because it can improve the mechanical behavior of sandy soils. Previous studies of the behavior of MICP-treated sand subjected to cyclic loading have focused on liquefaction mitigation effects, while changes in the improved site’s dynamic response have received rather limited attention. We evaluate MICP treatment approaches for sandy soil considering the need to mitigate liquefaction triggering and the potential for ground motion amplification due to changes in the soil’s engineering behavior. The absence of strain-dependent shear modulus and damping ratio characterization for MICP-treated soils represents a major challenge when investigating potential site effects. We assumed modulus reduction and damping (MRD) curves for MICP-treated soils from the literature based on similarities in initial shear modulus and cyclic stress response. One dimensional, equivalent-linear site response analyses are performed at the Treasure Island site in the San Francisco Bay area, which liquefied in the 1989 Loma Prieta earthquake. Untreated and MICP-treated soils are considered, and their responses compared. The outcropping rock ground motions recorded from the 1989 Loma Prieta earthquake at two nearby stations were selected as input for the numerical analyses. In addition, the effect of MICP on the liquefaction resistance of the profile was considered as a design objective. The evaluation of treatment approaches included the level of cementation and targeted treatment layers as the main parameters. Treatment recommendations that account for the simultaneous mitigation of liquefaction triggering and potential for ground motion amplification at the MICP-treated site are provided.