Recently, the biostimulation has received attention due to its sustained mineralization, environmental adaptability and lower cost. In the current study, a series of isotropic consolidated undrained triaxial shear (CU) tests were performed on biocemented soil treated through biostimulation approach to examine the effect of cementation levels on the undrained shear behaviors. The test results demonstrate that the biocementation generated by the biostimulation approach can improve the shear behaviors remarkably, with the observed changes in stress-strain relationship, pore water pressure, stress path, stiffness development, and strength parameters. The variations of the strength parameters, i.e., effective cohesion and effective critical state friction angle, with increasing cementation treatment cycles can be well fitted by an exponential function and a linear function, respectively, while the variation of the effective peak-state friction angle is relatively small. The increased shear strength, stiffness, effective cohesion, and strain softening phenomenon of biocemented soils are related to the densification, increased particle surface roughness, and raised interparticle bonding caused by biostimulation approach. The liquefaction index decreases with the increase in cementation treatment cycles, especially at lower initial mean effective stress (100 and 200 kPa), indicating that the biostimulation approach may be a viable method for anti-liquefaction of soil.

The disposal of tailings in a safe and environmentally friendly manner has always been a challenging issue. The microbially induced carbonate precipitation (MICP) technique is used to stabilise tailings sands. MICP is an innovative soil stabilisation technology. However, its field application in tailings sands is limited due to the poor adaptability of non-native urease-producing bacteria (UPB) in different natural environments. In this study, the ultraviolet (UV) mutagenesis technology was used to improve the performance of indigenous UPB, sourced from a hot and humid area of China. Mechanical property tests and microscopic inspections were conducted to assess the feasibility and the effectiveness of the technology. The roles played by the UV-induced UPB in the processes of nucleation and crystal growth were revealed by scanning electron microscopy imaging. The impacts of elements contained in the tailings sands on the morphology of calcium carbonate crystals were studied with Raman spectroscopy and energy-dispersive X-ray spectroscopy. The precipitation pattern of calcium carbonate and the strength enhancement mechanism of bio-cemented tailings were analysed in detail. The stabilisation method of tailings sands described in this paper provides a new cost-effective approach to mitigating the environmental issues and safety risks associated with the storage of tailings.

Microbial induced carbonate precipitation (MICP) is a promising method for improving the performance of geotechnical engineering materials. However, there has been limited research on the creep characteristics of expansive soil treated with MICP. Therefore, this study investigated the improvement of consolidation creep characteristics of expansive soils using the MICP method through one-dimensional consolidation creep tests. The microstructure of the treated soil was examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The results indicate that the MICP method effectively enhances the resistance of expansive soil to creep deformation. Compared to untreated expansive soil, the creep deformation of the treated soil decreased by 3.85%, 22.62%, and 18.40% for cementation solution contents of 50 mL, 100 mL, and 150 mL, respectively. Additionally, the creep curve of the improved expansive soil exhibits significant nonlinear characteristics. The creep process of the improved expansive soil can be divided into three stages: instantaneous deformation, decay creep, and stable creep. SEM images and XRD patterns reveal that the calcium carbonate precipitates generated during the MICP process can wrap, cement, and fill the voids between soil particles, which is the fundamental reason why the MICP method improves the deformation resistance of expansive soil. On the basis of the creep test results, a fractional-order creep model for MICP-treated expansive soil was established. Compared to traditional integer-order creep model, the fractional creep model can more accurately describe the entire process of consolidation creep of expansive soil improved by MICP method. The findings of this study provide a theoretical basis for analyzing the deformation of MICP-treated expansive soil under long-term loads.

Microbially induced carbonate precipitation (MICP) is an eco-friendly technique for weak soil reinforcement. In this study, Sporosacina pasteurii was used to strengthen silty sand after multigradient domestication in an artificial seawater environment. The efficiency of MICP was investigated by carrying out a series of macroscopic and microscopic tests on biocemented silty sand specimens. It was found that the salt ions in seawater impacted bacterial activity. The best activity of the bacterial solution in the seawater environment was achieved after five-gradient domestication, which was approximately 8% lower than that in the deionized water environment. The significant effects of domesticated bacteria on silty sand reinforcement were demonstrated by the content of precipitated carbonate and the unconfined compressive strength (UCS) of the treated specimens. The seawater positively impacted the MICP procedure due to the roles of calcium and magnesium ions, indicated by the X-ray diffraction spectra. The scanning electron microscopy (SEM) results showed that carbonate precipitations distributed primarily on the surfaces and near the contact points of the soil particles, contributing to the soil strength. The cementation solution concentration and injection rate significantly influenced the content and distribution of carbonate precipitations and UCS of the biocemented silty sand, and the values corresponding to good reinforcement efficiency were 1.0 mol/L and 1.0 mL/min, respectively. The results of consolidated undrained triaxial tests showed that the mechanical properties of treated specimens were influenced by biocementation cycles. It was found that the stress-strain behavior of biocemented samples changed from strain hardening to strain softening when the number of reinforcement cycles increased. The peak strength of silty sand was increased by 1.9-3 times after 5 times MICP treatment. The effect of biocementation cycles on the shear strength parameters could be represented by relating the effective friction angle and effective cohesion of biocemented silty sand to the carbonate content.

Loess exhibits high sensitivity to water, rendering it susceptible to strength loss and structural destruction under hydraulic effects of rainfall, irrigation and groundwater. As an emerging soil improvement technology, microbial induced carbonate precipitation (MICP) stands out for its cost-effectiveness, efficiency, and environmental sustainability. In this study, hydroxypropyl methylcellulose (HPMC) was innovatively introduced into the MICP process to improve the strength and water stability of loess, and a set of unconfined compressive strength (UCS), direct shear, laser particle size analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were conducted. The results show that HPMC-modified MICP is able to generate a novel structural matrix combining organic and inorganic elements, significantly enhancing the strength, stiffness, and ductility of loess. HPMC protects loess from water erosion by forming viscous membranes on the surfaces of soil particles and calcium carbonate crystals. Increasing HPMC content can augment membrane viscosity, which is conducive to stabilizing the loess structure, but it has the negative effect of reducing inter-particle friction through increasing membrane thickness. As the HPMC content increased to 0.6%, the strength loss of loess under high water content decreased. These findings are expected to provide critical support for the engineering application of HPMC-modified MICP in loess improvement.

The impact of four distinct calcium sources on the microbial solidification of sand in the Kashi Desert, Xinjiang, was investigated. A wind tunnel test over a 60-day period revealed the cracking behavior of four different complex calcium nutrient solutions. By comparing the bearing capacity and the results from dry-wet cycling and freeze-thaw cycle tests, it was concluded that the sample treated with calcium gluconate exhibited superior sand fixation performance, whereas the sample treated with calcium acetate showed weaker sand fixation effects. The microstructure of the treated sand samples was analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Elemental analysis was conducted via energy dispersive spectroscopy (EDS), and functional groups were identified through Fourier transform infrared spectroscopy (FTIR). These experimental findings hold significant implications for soil remediation, pollutant removal in soil, enhancement of soil fertility, and desert soil stabilization.

Bio-mediated ground improvement techniques, including Microbial Induced Calcite Precipitation (MICP) and Enzyme Induced Calcite Precipitation (EICP) treatment methods, are extensively being employed nowadays in a variety of construction projects as newly emerging sustainable and environmentally-friendly approaches to enhance the mechanical properties and durability characteristics of earthen composites. The intrinsic brittleness of MICP- and EICP-treated soils, however, considerably limits their applications in practical geotechnical engineering. Fiber reinforcement has been widely acknowledged as an efficient solution to overcome such challenges and augment the ductility of biologically stabilized soils. Accordingly, there is growing attention to integrating natural and synthetic fibers into bio-based composites, opening up exciting possibilities for improved performance and versatility in different civil engineering applications. This review aims to examine the current state of research on utilizing fiber additives to enhance the effectiveness of MICP and EICP treatment methods in an attempt to provide an in-depth insight into the effects of fiber type, content, and length as well as the underlying mechanisms of fiber interactions within the porous structure of such treated soils. The applications of fiberreinforced bio-cemented soils, their limitations, and the major challenges encountered in practice, as well as the potential areas of interest for future research and the key factors to be considered when selecting suitable fiber for optimal soil treatment using MICP/EICP, are all critically elaborated and discussed. By synthesizing the current research findings, the study provides engineers with a valuable resource to guide the development and optimization of fiber-reinforced MICP and EICP techniques for effective soil improvement and stabilization. Based on the findings of all relevant studies in the literature, a comprehensive cost-performance-balance analysis is conducted aiming to serve as a useful guideline for researchers and practitioners interested in applying fibers in various construction projects or other related applications where either MICP or EICP technique is being utilized as the main soil stabilization approach.

Microbial-induced calcite precipitation (MICP) is an environmentally friendly treatment method for soil improvement. When combined with carbon fiber (CF), MICP can enhance the liquefaction resistance of sand. In this study, the effects of CF content (relative to the sand weight of 0%, 0.2%, 0.3%, and 0.4%) on the liquefaction resistance of MICP-treated silica and calcareous sand were investigated. The analysis was conducted using bacterial retention test, cyclic triaxial (CTX) test, LCD optical microscope, and scanning electron microscopy (SEM). The results showed that with the increase in CF content, the bacterial retention rate increased. Additionally, the cumulative cycles of axial strain to 5%, excess pore water pressure to initial liquefaction, as well as strength and stiffness, all increased with higher CF content. This trend continued up to the CF content of 0.2% for silica sand and 0.3% for calcareous sand, beyond which the cumulative cycles began to decrease. The great mechanical system of CF, calcite, and sand particles was significantly strengthened after MICP-treated. However, the reinforced calcite did not completely cover the CF, and excess CF hindered the connection between sand grains. The optimal amount of CF in silica and calcareous sands were 0.2% and 0.3%. This study provides valuable guidance for selecting the optimal CF content in the future MICP soil engineering.

The microbial-induced calcite precipitation (MICP) technique has been developed as a sustainable methodology for the improvement of the engineering characteristics of sandy soils. However, the efficiency of MICP-treated sand has not been well established in the literature considering cyclic loading under undrained conditions. Furthermore, the efficacy of different bacterial strains in enhancing the cyclic properties of MICP-treated sand has not been sufficiently documented. Moreover, the effect of wetting-drying (WD) cycles on the cyclic characteristics of MICP-treated sand is not readily available, which may contribute to the limited adoption of MICP treatment in field applications. In this study, strain-controlled consolidated undrained (CU) cyclic triaxial testing was conducted to evaluate the effects of MICP treatment on standard Ennore sand from India with two bacterial strains: Sporosarcina pasteurii and Bacillus subtilis. The treatment durations of 7 d and 14 d were considered, with an interval of 12 h between treatments. The cyclic characteristics, such as the shear modulus and damping ratio, of the MICP-treated sand with the different bacterial strains have been estimated and compared. Furthermore, the effect of WD cycles on the cyclic characteristics of MICP-treated sand has been evaluated considering 5-15 cycles and aging of samples up to three months. The findings of this study may be helpful in assessing the cyclic characteristics of MICP-treated sand, considering the influence of different bacterial strains, treatment duration, and WD cycles. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Microbially induced calcium carbonate precipitation (MICP) is an emerging ecofriendly microbial engineering technique that utilizes urease-producing microorganisms to enhance the mechanical properties of soils. Sporosarcina pasteurii (S. pasteurii) stands out among these microorganisms as an efficient urease producer. However, field trials often lead to less-than-optimal experimental outcomes due to the presence of native soil microbes. To evaluate the impact of indigenous microorganisms on the effectiveness of MICP at the site, bacteria isolated from natural soil, classified of on-site low-ureolysis and high-ureolysis bacteria (OSLUB and OSHUB, respectively), were combined with S. pasteurii to conduct MICP experiments both in microfluidic chips and sand columns. Analysis covered the bacterial population, urease activity, pH changes, calcium carbonate crystal count and volume, as well as the unconfined compressive strength (UCS) of reinforced samples. Experimental results revealed that combining OSLUB with S. pasteurii led to a reduction in bacterial activity of 74% to 84% by 120 h, resulting in an approximately 60% decrease in the chemical conversion rate and the UCS of MICP-treated soils was 60% lower than the S. pasteurii. However, when OSHUB is mixed with S. pasteurii, although there is a reduction in bacterial activity by 49% to 54% by the 120-h mark, the decrease remains less pronounced than the activity decrease observed in S. pasteurii alone, which is 64%. Consequently, the rates of calcium carbonate chemical conversion were enhanced by 9% to 45%, and the UCS of the reinforced sand columns showed a slight improvement relative to the control group. This research highlights the distinct impacts of OSLUB and OSHUB on the efficiency of MICP on location. The main difference between OSLUB and OSHUB lies in their respective effects on pH levels following mixing. OSLUB tends to decrease the pH level gradually in the combined bacterial environment, while OSHUB, in contrast, increases the pH level over time in the same setting. The maintenance of both high bacterial activity and high precipitation rates is crucially dependent on pH levels, highlighting the importance of these findings for enhancing MICP efficiency in field applications. Strategies that either diminish the presence of OSLUB while augmenting that of OSHUB, or that sustain a relatively high pH level, could be valuable. These avenues promise significant improvements and merit further investigation in future studies.