Direct use of naturally occurring microbes for soil improvement has recently gained attention due to their ubiquitous and versatile characteristics in subsurface soil. Microbes produce soft and sticky extracellular polymeric substances (or biopolymers) that are known to alter the hydrological characteristics of soils; however, the mechanisms and extent of such soft biopolymers in altering soil erosion resistance remain scarcely explored. This study explored the role of microbial biopolymers in soil erosion resistance. The surface erosion resistance of sandy soils was evaluated by using a hybrid erosion function apparatus, in which the model bacteria Leuconostoc mesenteroides were stimulated to produce an insoluble biopolymer. The results revealed that the microbial biopolymer formation increased the critical shear stress and surface erosion resistance, which the researchers attributed to the increased cohesion by grain-coating biopolymer slimes and the reduced seepage flows due to pore clogging. This study provides baseline but promising results on how microbially grown biopolymers can be used to improve soil erosion resistance.

This study investigated the P and S wave responses and permeability reduction during bacterial biopolymer formation in unconsolidated porous media. Column experiments with fine sands, where the model bacteria Leuconostoc mesenteroides were stimulated to produce insoluble biopolymer, were conducted while monitoring changes in permeability and P and S wave responses. The bacterial biopolymer reduced the permeability by more than 1 order of magnitude, occupying~10% pore volume after 38 days of growth. This substantial reduction was attributed to the bacterial biopolymer with complex internal structures accumulated at pore throats. S wave velocity (V S ) increased by more than~50% during biopolymer accumulation; this indicated that the bacterial biopolymer caused a certain level of stiffening effect on shear modulus of the unconsolidated sediment matrix at low confining stress conditions. Whereas replacing pore water by insoluble biopolymer was observed to cause minimal changes in P wave velocity (V P ) due to the low elastic moduli of insoluble biopolymer. The spectral ratio analyses revealed that the biopolymer formation caused a~50-80% increase in P wave attenuation (1/Q P ) at the both ultrasonic and subultrasonic frequency ranges, at hundreds of kHz and tens of kHz, respectively, and a~50-60% increase in S wave attenuation (1/Q S ) in the frequency band of several kHz. Our results reveal that in situ biopolymer formation and the resulting permeability reduction can be effectively monitored by using P and S wave attenuation in the ultrasonic and subultrasonic frequency ranges. This suggests that field monitoring using seismic logging techniques, including time-lapse dipole sonic logging, may be possible. Key Points:• This study investigated P and S wave responses of bacterial biopolymer clogging in porous media • Biopolymer accumulation causes the increases in P and S wave attenuation at tens to hundreds of kHz and several kHz, respectively • In situ bacterial biopolymer clogging can be effectively monitored by using variations of P and S wave attenuation and S wave velocityCorrespondence to:

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Soft viscoelastic biological products such as biopolymers and biofilms have recently garnered significant interest as alternative biogrout materials for ground improvement because of their nontoxic and biodegradable characteristics. However, the impact of soft gel-like viscoelastic pore fillers on the undrained response of treated soils remains poorly understood. This study involves undrained triaxial compression tests with concurrent shear wave velocity measurements of loose contractive sands treated with soft gelatin. The specimens experience two distinct loading-gelation sequences, either consolidation before gelation (CbG) or confinement after gelation (CaG). Results reveal that the shear wave velocity can be used as an indicator of the effective stress carried by the granular skeleton. The inclusion of the viscoelastic biopolymer hinders the contractive tendency, diminishes postpeak softening, and increases the undrained shear strength of loose contractive sands. These effects become more pronounced for stiffer biopolymers because they provide an enhanced skeletal support against chain buckling and contraction. The presence of biopolymers increases the normalized undrained shear strength from S u =σ 0 o ¼ ∼0.1 to ∼1.4, particularly at low effective confining stress. The biopolymers alter the terminal state in the p 0 -q-e space. Therefore, critical states should be reconsidered for biopolymer-treated sands. The confinement-gelation sequence affects the effective stress supported by the granular frame and thus has pronounced effects on the undrained shear strength. This suggests the potential use of viscoelastic pore fillers as an effective treatment of loose sands prone to liquefaction.