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.

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.

Natural cementation of rock debris is a spontaneous geochemical process that plays an important role in geotechnical stabilization. The focus of this study is to analyze the natural cementation phenomenon in mudslide-prone areas using mineralogical and biological methods. We analyzed the formation of the natural cementation phenomenon by studying its mineral composition, elemental endowment distribution, mechanical properties, and community structure. Similarly, simulated cementation experiments of rock debris by carbonate mineralizing bacteria were carried out in the laboratory to assess the feasibility of biomineralization in the stabilization of rock and soil. The results show that the natural cementation of rock debris in mudslide-prone areas is caused by the formation of calcite under chemical action, and microorganisms also contribute to it; this cementation has multiple environmental protection significance, including improving the compressive properties of rock debris (up to 2.58 Mpa), slowing down or preventing the occurrence of geologic hazards such as slumps, landslides, etc., and significantly decreasing the migratory properties of heavy metal ions and its ecological risks. Laboratory simulation conditions showed that carbonate mineralizing bacteria were enabled to utilize the Ca2+ provided by weathering to achieve rapid cementation of the rock debris, which played an important role in the increase of their compressive strength and the improvement of their pore parameters. This study provides a theoretical basis for future engineering applications of biomineralization technology.

Microbially induced calcite precipitation (MICP) is a promising technology for soil improvement, where the treated soil can be regarded as the structural one. In this study, a micromechanics-based model is proposed to investigate the mechanical behaviors of inherently anisotropic MICP-cemented sand, which consists of a hexagonal close-packed (HCP) particle assembly (2D) composed of bonded elliptical particles with same size. A size-dependent bond failure criterion is adopted to define the microscopic mechanical reactions between the particles to model the nonlinear characteristics of the soil. Based on the homogenization theory and lattice model, the stress-strain relationship, strength criteria, and corresponding macroscopic mechanical parameters with respect to microscopic parameters for MICP-cemented sand are derived and verified by DEM simulation based on the regularly arranged particle assembly. The effects of key parameters, including cement content, initial void ratio, inherent anisotropy, and confining pressure, on the mechanical behaviors of MICP-cemented sand is investigated in detail, and the good agreement between the theoretical solution and laboratory test results validates the applicability of the theoretical solution for analyzing MICP-cemented sand.

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 application of microorganisms to improve the mechanical properties of soil is a new developing research area. A new native bacteria extracted from soil was introduced for the biological improvement of soil geotechnical parameters. The isolate was identified as Acinetobacter calcoaceticus S1. Sporosarcina pasteurii was used as a positive control. Direct shear tests were performed on the nontreated soil and soils treated with bacteria to determine the shear strength, adhesion and angle of internal friction. The treatment period was 40 days. The shear wave velocity was measured.The results showed that the untreated sample had relatively constant shear strength, but the shear strength of the treated soils increased significantly. The soil treated with A. calcoaceticus had greater shear strength. The angle of internal friction increased for the treated soils with A. calcoaceticus (39.3%) and S. pasteurii (28.6%). The greatest cohesion was found for soil treated with A. calcoaceticus, reaching 0.66 and 0.56 kg/cm2 for S. pasteurii. The shear wave velocity in the treated soils increased significantly. The results confirmed the ability of native A. calcoaceticus to improve soil geotechnical parameters. Calcium carbonate precipitation fills the voids between soil particles and forms a gel, which makes effective connections between soil particles and makes them coalesce and grow larger.

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.

The inclusion of calcite precipitates (CaCO3) in soft soil can improve the mechanical properties. Understanding the variability in sand stiffness due to heterogeneous precipitates is crucial for stiffness evaluation and prediction. A novel discrete element-Monte Carlo (DE-MC) method was proposed to quantify the sand stiffness variability induced by stochastic distributions of calcite precipitates, specifically focusing on shear wave velocity (Vs) as an indicator of soil stiffness. A total of 1972 samples were constructed to simulate stochastic spatial distributions of calcite precipitates. Through joint stochastic analysis, the preferential paths formed by calcite clusters were identified as significant contributors to Vs variability. The normalized connectivity per unity distance contact weight (Cd,n) exhibited the most correlated relation with Vs. Two weight selection methods were applicable for using Cd,n to characterize and predict Vs. The results suggest that the DE-MC method has the potential to assess the variability in sand stiffness quantitatively.

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.

Microbial-induced calcite precipitation (MICP) is a promising, sustainable, and environmentally friendly ground improvement technique. This study examined the effectiveness of molasses (MS) as a broth medium compared to nutrient broth (NB). Sporosarcina pasteurii was used in a 0.5 M cementation solution with pore volumes (PV) of 0.50, 0.75, and 1 PV in biotreatment cycles of 9 and 18 days. Mechanical properties of biotreated samples were assessed through unconfined compressive strength (UCS) and split tensile strength (STS) tests, while calcite content, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to interpret biocementation. NB-treated samples exhibited significantly higher strength and calcite content than MS-treated samples. The durability of biotreated samples under 6, 12, and 18 freeze-thaw (FT) cycles revealed that the FT cyclic process affects the mechanical and physical characteristics of biotreated samples. Samples treated with higher PV and for a longer duration exhibited higher strength and durability. The mass losses in NB and MS samples were 7-14.5% and 15-32%, respectively, after 18 FT cycles. Overall, NB samples exhibit higher strength and durability than MS samples. While MS proved less effective as a broth medium compared to NB for the MICP process, its cost-effectiveness and abundant availability make it a promising choice for the MICP process.