Effects of bacterial activity on the saturated hydraulic conductivity of remolded loess

Abstract

Loess is widely used as a construction material in engineering projects on the Loess Plateau of China but is regarded as one of the most problematic soils due to water sensitivity. Hydraulic conductivity ( K ) is an essential parameter in assessment of water transfer in porous media. However, the influence of bacterial activity on the saturated permeability of remolded loess is still unclear and rarely being studied. We systematically studied the effects of Gram-negative bacterial activity (a common bacteria in the loess region) on the hydraulic properties of remolded loess, with the aim to better understand water flow through loess. A series of laboratory experiments were conducted to identify the variation in hydraulic conductivity under various bacterial inoculation and carbon constraint conditions. The results indicated that the K was relatively stable (0.14–0.16 m/d) at the initial phase and decreased by 81–93% reaching 0.01–0.05 m/d at the end of the experiments. The variation of hydraulic conductivity can be divided into the unaffected stage, linear reduction stage and stable stage. The growth of bacteria and the accumulation of extracellular polymeric substances occupied the pores between soil particles in the sample, which reduced the porosity and lead to the decrease of K . Higher available carbon contents and greater initial inoculum sizes speeded up the process of hydraulic conductivity reduction. The delayed effect was observed between rapid decline of hydraulic conductivity and exponential growth of bacteria. It should be highlighted that the reduction of hydraulic conductivity in the soil columns was mainly dependent on clogging in the inlet layer. Our findings provide a better insight into the change of hydraulic conductivity related to bacterial activities, and promote the reorganization of water cycle uncertainty and the prevention of engineering geological disasters in loess areas. • Saturated hydraulic conductivity decreased by 81–93% due to bacterial activities. • Delayed effect between linear reduction of hydraulic conductivity and exponential growth of bacteria. • Reduction of hydraulic conductivity is mainly due to bacterial growth in inlet layer.