Field Trials and Long-Term Monitoring of Microbially-Induced Desaturation for the Treatment of Liquefiable Silty Soils

Abstract

Earthquake liquefaction hazards in silty soils are a critical problem in Portland, Oregon, and other areas around the world. This is a particular problem for existing facilities founded on liquefiable soils, for which there exists no cost-effective liquefaction mitigation solution at the present time. It is essential from both a seismic safety and a seismic resilience perspective to be able to mitigate potentially liquefiable soils underneath existing structures, and to do so in a cost-effective manner. Recent studies suggest that liquefaction mitigation using microbially-induced desaturation (MID) may provide this capability. 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. Monitoring at both sites indicated that the liquefiable soils were successfully desaturated. The persistence of desaturation was monitored for eight months post-treatment at one site and is ongoing at the other site (20 months so far). 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 for use not only in silty soils, but also underneath existing structures for which there is no other mitigation method available.