Estimation of pore fluid pH effect on Skempton’s pore pressure parameter Bw

ABSTRACT: Since near-surface aerobic biologic processes produce CO2, the amount of biogenic CO2 is expected to influence pore fluid compressibility. Consequently, pore water pressure development during events that deform a soil mass that holds microorganisms that produce CO2 rapidly is also expected to depend on CO2 production, as will be the liquefaction susceptibility of such soils. However, bacterial metabolic processes, life cycle, and CO2 production affect pore fluid pH as well. That, in turn, would affect CO2 solubility. The relationship between Skempton’s pore water pressure parameter, Bw – the ratio of pore water pressure rise during rapid deformation of soil mass to the corresponding incremental mean normal stress – and the amount of CO2 production is expected to be pH-dependent. A pH-dependent relationship between Bw and CO2 amount has been derived here from theoretical considerations. The relationship was validated against the data from a series of closed-system laboratory experiments on loose sand that hosted two species of aerobic soil bacteria, Lysinibacillus sp. (DRG3) and Bacillus megaterium (RB-05). These species were minimally sustained within pore space in such a manner that one of three distinct metabolic pathways was triggered; one involving the production of exocellular metabolic substances (EPS), while the other two involving the production of EPS and calcite in the presence and absence of urea. Amounts of CO2, pH, and the corresponding values of Bw were tracked over up to 72 hours for these cases. Bw estimates from the theoretical relationship derived herein were found to agree reasonably with experimentally obtained values.