Collaborative Research: Efficacy and Durability of Microbially Induced Desaturation to Mitigate Liquefaction in Fine-grained Soils

Portland State University, Portland OR

Investigators

Abstract

Earthquake liquefaction is the severe loss of soil strength due to earthquake shaking in saturated soils, which can cause significant infrastructure damage from large ground deformations and bearing capacity loss. Major infrastructure in the US is on silt soils vulnerable to liquefaction, including many fuel tanks along the Columbia River in Portland, Oregon. However, there are few feasible methods to mitigate liquefaction of silt soils beneath infrastructure due to the high cost and, in many cases, the invasive nature of existing ground improvement methods. This award will examine the effectiveness and durability of microbially induced desaturation (MID) to mitigate liquefiable silt soils. MID injects into the ground a treatment solution that stimulates native denitrifying microbes. The primary product of the denitrification reaction is nitrogen gas, which reduces soil saturation. A small reduction in saturation is known to substantially increase liquefaction resistance in sands. However, the effectiveness and longevity of MID in silts remain as major unknowns. This research addresses these unknowns through laboratory experiments, field testing, and theoretical modeling. This award will also engage underrepresented students from an all-girls high school through research internships. The project seeks to understand fundamental soil-water-gas interactions to evaluate the potential for MID to mitigate liquefaction of fine-grained liquefiable materials. This research specifically aims to (i) examine changes to the cyclically induced excess pore water pressure due to MID to prevent liquefaction triggering, (ii) examine the persistence of biogas on time-scales relevant to civil infrastructure, and (iii) link spatial and temporal saturation changes in the field to the fundamental physics governing gas mobility and longevity. The effects of MID on liquefaction triggering will be examined through laboratory cyclic tests considering both changes in pore fluid compressibility and soil skeleton damage. Gas persistence will be investigated by examining the formation of gas in fine-grained soils through laboratory experiments, gas diffusion and redistribution in layered soils with bench-scale experiments, and the effects of groundwater flow in fine-grained stratified soils through a field-scale experiment. Theory-based gas transport models to examine soil resaturation will be developed and validated with the project dataset. This work will generate a rich data set and understanding of soil-water-gas interactions required to move MID from an abstract ground improvement method to one that can be assessed as a practicable long-term ground improvement method. This project is jointly funded by the Engineering for Civil, Mechanical and Manufacturing Innovation (CMMI) Division and the Established Program to Stimulate Competitive Research (EPSCoR). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.