Field Monitoring of the Persistence of Microbially Induced Desaturation for Mitigation of Earthquake Induced Soil Liquefaction in Silty Soil

Abstract

Earthquake liquefaction hazards in silty soils are a critical problem in Portland, Oregon, and other areas around the world. Recent studies suggest that liquefaction mitigation using microbially induced desaturation (MID) may provide the capability to mitigate liquefaction potential beneath existing structures in a cost-effective manner. The objective of MID is to reduce earthquake-induced excess pore water pressure generation compared to saturated soil, and thereby reduce the potential for triggering liquefaction. A field study of liquefaction mitigation using MID was performed at two sites in Portland in the summer of 2019. Low-plasticity, liquefiable silts were treated with MID by injecting a treatment solution to stimulate native bacteria for a duration of four weeks. This paper presents monitoring data that evaluate the level of desaturation before, during and after the completion of the MID trials. Monitoring included crosshole pressure-wave velocity measurements, a vertical array of embedded sensors that measure water content and bulk electrical conductivity, pre- and post-treatment seismic cone penetration tests, and direct soil sampling of the treated soils. Monitoring at both sites indicated that liquefiable silts were successfully desaturated. The persistence of desaturation was monitored for 8 months post-treatment at one site and is ongoing at the other site. The monitoring data indicate that the induced desaturation has persisted through seasonal fluctuations of the ground water table since the end of treatment. These data, which document MID longevity, are particularly important to establish MID as a viable option for liquefaction mitigation of silty soils that threatens lifelines worldwide.