Biogrouting has been proposed for improving mechanical properties of soils and rocks, whose performance greatly depends on the location of biocement at pore-scale. To enhance the performance of biogrouting, many strategies were proposed, including the addition of assistants, controlling curing moisture degree, and flocculation of bacteria. Clay is one such assistant which has been proven to be effective, with an assumption of increasing active biocement, i.e. those located between soil particles. In this work, we employed microfluidics to directly observe whether clay minerals can certainly control the location of precipitates and how they function. First of all, the capacity of bentonite and kaolin for adsorbing bacteria were investigated. Then, the location of CaCO3 crystals with and without clay minerals were visually observed using microfluidics. Pore-filling ratios and CaCO3 ratios, which are closely related to permeability and strength of biocemented soils, were quantitatively analyzed from collected images. Finally, the effects of bentonite and kaolin and their dosages on the location of biocement were comprehensively discussed. The results demonstrated that the performance of bentonite and kaolin on adsorbing bacteria and regulating biocement location is distinct due to differences in the morphologies of clays. These findings can help us to improve biogrouting performance on soil stabilization and propose new strategies in various practical applications, such as CO2 sequestration, heavy metal remediation, and oil recovery enhancement, all with a foundational understanding of their mechanisms.

The reinforcement and repair materials for earthen sites have high requirements for strength, resistance to deterioration, and aesthetic coordination. In this study, the enzyme-induced carbonate precipitation (EICP) and the microbially induced carbonate precipitation (MICP) techniques were used to reinforce the earthen site soil. The applicability of EICP and MICP for stabilizing earthen sites soil was investigated through static contact angle tests, disintegration tests and colorimetry tests. In addition, the improvement of mechanical properties of biotreated earthen sites soil was examined by unconfined compression strength tests. The tests results show that MICP and EICP techniques could improve the mechanical characteristics and water-stability properties of the earthen sites soil. With the increase in cementing solution concentration, the effectiveness of EICP was enhanced, while the water-stability and hydrophobicity of MICP-treated soils increased first and then decreased due to the influences of organic matter and soluble salts. EICP and MICP techniques showed different performance in reinforcing effects on calcium carbonate content, shear wave velocity, unconfined compressive peak strength, total disintegration time, and static contact angle. This study is expected to contribute valuable insights to the conservation of earthen heritage site using bio-based methods.

The objective of the current study is to explore the effect of biostimulation treatment methods on the mechanical properties and microstructure characteristics of biocemented soil. Biostimulated microbially induced carbonate precipitation (MICP) is an eco-friendly and economical soil reinforcement measure. It relies on the stimulation of the urease-producing bacteria (UPB) in situ for the MICP process. Different biostimulation treatment methods involve different oxygen availability, stimulation solution content and distribution, and number of biostimulation treatments. There may be differences in the effect of UPB stimulation and biocementation when different biostimulation treatment methods are used. In this study, four biostimulation treatment methods, i.e., unsaturated single biostimulation treatment (USBT), unsaturated multiple biostimulation treatments (UMBT), saturated single biostimulation treatment (SSBT) and saturated-unsaturated-combined single biostimulation treatment (CSBT), were used to stimulate native UPB in soil columns, and then, the same cementation treatment was applied to the soil columns. Subsequently, the mechanical behavior and microstructural properties of the biocemented soil were investigated. The results indicated that the saturated single biostimulation treatment was more conducive to stimulating native UPB to induce CaCO3 precipitation. Samples subjected to the saturated single biostimulation treatment exhibited higher CaCO3 precipitation content (CCP), dry density, unconfined compressive strength (UCS) and lower permeability within the same cementation treatment cycle (NC). However, UCS was not only determined by CCP, but was also regulated by CaCO3 spatial distribution and precipitation pattern. This study could help guide the selection of biostimulation treatment methods.

Infrastructure development often faces challenges due to soils with low bearing capacity, which can potentially cause instability and subsidence and threaten the safety of structures. Therefore, an efficient and environmentally friendly stabilization method is required. This study aims to evaluate the effectiveness of Microbial Induced Calcite Precipitation (MICP) in improving bearing capacity and soil strength through the formation of bacterial soil columns. This study employed a full-scale physical model test using 40 cm diameter and 200 cm deep soil columns filled with soil mixed with Bacillus subtilis, compacted, and cured for 56 days. The results showed significant improvements in the geotechnical characteristics of the soil, with CBR values increasing from 5.5% to over 12%, unconfined compressive strength reaching 345 kPa, and modulus of elasticity increasing to 12.5 MPa. Soil cohesion increased to 65 kPa, while internal friction angle increased from 10 degrees to 34 degrees. The novelty of this research is the application of MICP technology in the form of bacterial soil columns as an innovative, effective, and sustainable stabilization method to improve the mechanical properties of soft soils.

Well-graded granular materials are extensively used in the foundation layers of roads and railways. Excessive deformation developed in these layers under traffic-induced cyclic loading represents a major contributor to structural deterioration, while no viable methods are currently available for rehabilitating these layers without causing substantial disruption. In this context, biocementation holds promise as a non-disruptive solution, yet dedicated investigations have been lacking. This study, through a series of multi-stage cyclic triaxial tests, explores the feasibility and effectiveness of biocementation in improving the deformation and shakedown behaviour of a well-graded aggregate representative of typical granular materials used in road and railway foundations. The results show that both uncemented and biocemented aggregates exhibit distinct stable and unstable responses with increasing cyclic stress. Biocementation effectively enhances deformation resistance and elevates the shakedown limit in the stable regime, while it aggravates brittleness in the unstable regime. Further interpretation using the normalised stress ratio (NSR) reveals the existence of a unique critical NSR zone that separates stable and unstable regimes, independent of both cementation level and confining pressure. Microstructural characterisation elucidates multiple precipitation patterns. A cementation mechanism where small aggregate particles and CaCO3 crystals merge to form cementing bonds between large particles is postulated.

Biocemented soils present a promising sustainable alternative to traditional Portland cement and asphalt in road embankment construction and remediation. However, the cyclic loading experienced by transportation infrastructures like roads over extended periods explicitly leads to performance degradation. Biocementation, achieved through Microbially Induced Calcite Precipitation (MICP) using ureolytic bacteria or Enzyme-Induced Calcite Precipitation (EICP) with urease enzymes, precipitates calcium carbonate (calcite) as a bonding agent within the soil matrix. Despite the environmental appeal of biocemented soils, their durability under cyclic and repeatable loads remains relatively unexplored. This paper investigates the modulus degradation of biocemented sand subjected to cyclic loading, considering various strain amplitudes and confinement levels. The experimental program involves subjecting two distinct specimens-one uncemented and the other cemented-to three confinement levels (50, 100, and 200 kPa). Each specimen undergoes incremental torque amplitudes to elucidate stiffness behavior across a spectrum of strain levels. Additionally, resilient modulus estimates are obtained for different strain levels, and a critical strain threshold is identified. The primary objective of this research is to unveil fatigue susceptibility criteria, offering crucial insights into the performance of biocemented soils. By doing so, this study contributes to the advancement of sustainable and durable infrastructural solutions, particularly in the context of road construction and maintenance.

Biocementation is an emerging field within geotechnical engineering that focuses on harnessing microbiological activity to enhance the mechanical properties and behavior of rocks. It often relies on microbial-induced carbonate precipitation (MICP) or enzyme-induced carbonate precipitation (EICP) which utilizes biomineralization by promoting the generation of calcium carbonate (CaCO3) within the pores of geomaterials (rock and soil). However, there is still a lack of knowledge about the effect of porosity and bedding on biocementation in rocks from a mechanistic view. This experimental study investigated the impact of porosity and bedding orientations on the mechanical response of rocks due to biocementations, using two distinct biocementation strategies (MICP and EICP) and characteristically low porosity but interbedded rocks (shale) and more porous but non-bedded (dolostone) rocks. We first conducted biocementation treatments (MICP and EICP) of rock samples over a distinct period and temperature. Subsequently, the rock strength (uniaxial compressive strength, UCS) was measured. Finally, we analyzed the preand post-treatment changes in the rock samples to better understand the effect of MICP and EICP biocementations on the mechanical response of the rock samples. The results indicate that biocementations in dolostones can improve the rock mechanical integrity (EICP: +58% UCS; MICP: +25% UCS). In shales, biocementations can either slightly improve (EICP: +1% UCS) or weaken the rock mechanical integrity (MICP: -39% UCS). Further, results suggest that the major controlling mechanisms of biogeomechanical alterations due to MICP and EICP in rocks can be attributed to the inherent porosity, biocementation type, and bedding orientations, and in few cases the mechanisms can be swelling, osmotic suction, or pore pressurization. The findings in this study provide novel insights into the mechanical responses of rocks due to MICP and EICP biocementations.

Historically, cow dung has been widely used as a biostabilizer in earth building, although the scientific research on this subject is still limited. The available research provides evidence of the positive effects of this bioaddition on earthen blocks and plasters, as it improves their physical and mechanical properties and durability in water contact. The present research does not aim to characterize biostabilized earthen mortars or to explain the interaction mechanisms between the earth and cow dung components, because this topic has already been investigated. Instead, it aims to investigate strategies to optimize the collection and processing of cow dung so as to optimize their effects when used in earth-plastering mortars, as well as considering the effects of using them fresh whole, dry whole, and dry ground (as a powder); the effects of two different volumetric proportions of cow dung addition, 20% and 40% (of the earth + added sand); the effects of 72 h (fermentation-humid curing) before molding the biostabilized mortar; the influence of the cow diet; and the potential of reusing cow dung stabilized mortars. The results show that as the freshness of the cow dung increases, the mortar's durability increases under water immersion, as well as the mechanical and adhesive strength. Collecting cow dung fresh and drying (composting) it in a plastic container is more efficient than collecting cow dung that is already dry on the pasture. The cow diet and the use of dry (composted) cow dung, whole or ground into a powder, does not result in a significant difference. A 72 h period of humid curing fermentation increases the adhesive strength and durability under water. The proportion of 40% promotes better durability under water, but 20% offers greater mechanical and adhesive strength. Finally, cow dung addition does not reduce the reusability of the earth mortar. The new mortar obtained by remixing the mortar with water presents increased properties in comparison to the original reference mortar with no cow dung addition. Therefore, the contributions of this research are innovative and important, offering technical support in the area of biostabilized earth-plastering mortars. Furthermore, it is emphasized that cow dung addition can be optimized as an efficient traditional solution to increase the mechanical resistance, but especially to increase the durability of earth mortars when in contact with water. This effect is particularly important for communities lacking financial resources, but also reveals the possibility of using eco-efficient waste instead of binders obtained at high firing temperatures.

Microbially induced carbonate precipitation (MICP) utilizing a urease active bioslurry is an ecofriendly method that can improve soil strength. However, the micromechanisms, such as ion diffusion, production rate of CaCO3, porosity, and permeability of pile reinforced by bioslurry, require further investigation. In this study, both biopile model tests and a coupled fluid-flow, solute transport and biochemical reactive model were conducted to analyze the mechanical property and biocementation mechanism of pile formed by urease active bioslurry. Results showed that the simulated CaCO3 content along the biopile length after 120 h grouting was close to test results. The UCS of the biopile decreased from 3.44 MPa to 0.88 MPa and the CaCO3 content decreased from 13.5% to 9.1% with increasing depth. The largest reduction in CaCO3 content was observed in the middle part of the biopile as the CaCO3 crystals in the upper part hindered the downward transport of the cementation solution. The morphology of CaCO3 crystals was influenced by cementation solution concentration, as evidenced by the predominance of spherical vaterite crystals in the upper part of the biopile and rhomboidal calcite crystals in the middle and lower parts. During the grouting process, the concentration of calcium ions and urea decreased, while the ammonium ion levels increased with depth due to the utilization of calcium ions and urea for CaCO3 precipitation and ammonium ion production. The production rate of CaCO3 first increased rapidly to reach a peak value and then decreased. The porosity and permeability demonstrated both linear and nonlinear decreasing trends as the CaCO3 concentration increased. The largest reduction in porosity and permeability, reaching 20% and 58% in the biopile top.

Soil biocementation through microbially induced carbonate precipitation (MICP) is a promising technique for improving soil behavior in a nondisruptive manner, particularly for rehabilitation and retrofitting applications. Previous studies characterizing the shear behavior of biocemented soils have concentrated on poorly graded sands, whereas research on well-graded gravelly soils, which are extensively used in shallow geotechnical structures, has been lacking. Mohr-Coulomb strength parameters have been predominately employed to interpret the macromechanical effects of biocementation, but the previously reported findings show significant contradictions. In this study, a well-graded aggregate, representative of commonly used well-graded gravelly soils, was biocemented and subjected to monotonic drained triaxial compression. The test results show remarkable improvements in shear behavior, with the observed changes in stress-strain responses, strength and stiffness development, and stress dilatancy agreeing with those reported for biocemented sands as well as conventional cemented soils. Relatively low cementation levels can effectively rectify the mechanical performance caused by poor compaction to that seen at optimal levels, demonstrating the feasibility and potential of biocementation for improving soils of this type. Detailed analysis of the results reveals the decisive role of cementing bonds and their degradation in causing behavioral changes at different shearing stages. The theories of bonded structure and force-chain evolution are used to explain the preyielding observations, while an analytical approach capable of quantifying the evolution of different strength components is presented for postyielding macromechanical characterization. Conversely to the inference drawn from the strength parameters, the largest improvement is found in the frictional rather than the dilative and cohesive components of strength. Further analysis reveals the commonality of the macromechanical effects of biocementation, density, and confinement, and a unique relationship between macromechanical composition and peak stress ratio emerges.