Microorganisms have been essential in the natural world for millions of years, contributing significantly to environmental interaction. It has been disoverd that some bacteria are potential in geotechnical and environmental engineering due to their outstanding ability of biomineralization. Therefore, how to train bacteria as special and professional “workers” for biomineralization is increasingly a key topic in related research fields. This article briefly introduces the methods that are commonly utilized to improve the environmental adaptability and mineralization efficiency of bacteria, including microbial domestication, microbial mutation breeding, microbial targeted screening, and bio-stimulation, which make great implications to advance the field of biomineralization.

As eco-friendly methods, microbial induced carbonate precipitation (MICP) method was used to reinforce the calcareous sand in the South China Sea in this paper. The durability characteristics and deterioration mechanism of MICP-reinforced calcareous sand under various environment factors were investigated synthetically based on the unconfined compressive strength, mass loss rate and microscopic morphology in laboratory and field experimental study. Results show that, the unconfined compressive strength value of the sample is only 35.19 % of the initial strength, while the mass loss rate is about 6.69 % after 30-days of field marine environment erosion. MICP-reinforced calcareous sand shows the strongest resistance to temperature cycles, followed by dry-wet cycles, coupling effect of temperature and dry-wet cycle and salt spraying with drying cycles. MICP-reinforced calcareous sand exhibits the worst resistance to the field marine conditions, but the integrity of the sample could still be maintained after 30-days of field tests. The deterioration mechanism of MICP-reinforced calcareous sand is consistent under the various environmental cycles. First, the weakly cemented calcium carbonate crystals on the sample surface fall off, and then the hard-shell layer on the sample surface became weaker under various erosion. Finally, the internal cemented structure of the sample was gradually destroyed. The results indicated the utilization value of the MICP method in ocean engineering, but it is necessary to enhance the performance of the MICP-reinforced calcareous sand to ensure its protective effect after a certain environmental impact cycle.

Enzyme-induced carbonate precipitation (EICP) has emerged promising in various geotechnical applications, and has been presented as an alternative to the traditional cementitious materials-based ground improvement method. However, the study on mechanical properties and disintegration behavior of EICP-reinforced sea sand subjected to drying-wetting cycles are limited. This study investigated the mechanical properties and disintegration behavior of EICP-reinforced sea sand against the impact of drying-wetting (D-W) cycles. The uniaxial compressive strength (UCS) tests were performed to discuss the effect of drying-wetting cycles on the mechanical behavior of EICP-treated sea sand. The disintegration tests were conducted on EICP-treated sea sand to investigate the disintegration resistance of bio-cemented samples with various cementation levels. The microstructures of samples before and after disintegration were examined to disclose the disintegration mechanisms of EICP-reinforced sea sand. D-W cycles significantly affect the mechanical properties of EICP-reinforced sea sand, with UCS decreasing by 63.7% after undergoing 15 D-W cycles. The disintegration resistance index of specimens with a lower cementation level decreases significantly under the effect of D-W treatment. The higher disintegration resistance of specimens with higher cementation can be attributed to more crystals with better crystallinity formed in the contact point between sand particles within specimen. The crystals formed by soybean husk urease are mainly calcite and the crystallinity of spherical calcites would gradually change into larger rhombic calcite with further bio-grouting. The crystal with poor crystallinity is susceptible to the effect of D-W treatment, resulting in the obvious disintegration of EICP-reinforced sea sand. Overall, this study is expected to provide useful guidance on the long-term stability and drying-wetting disintegration mechanisms of EICP-reinforced sea sand.

Compromised integrity of cementitious materials can lead to potential geo-hazards such as detrimental fluid flow to the wellbore (borehole), potential leakage of underground stored fluids, contamination of water aquifers, and other issues that could impact environmental sustainability during underground construction operations. The mechanical integrity of wellbore cementitious materials is critical to prevent wellbore failure and leakages, and thus, it is imperative to understand and predict the integrity of oilwell cement (OWC) and microbial-induced calcite precipitation (MICP) to maintain wellbore integrity and ensure zonal isolation at depth. Here, we investigated the mechanical integrity of two cementitious materials (MICP and OWC), and assessed their potential for plugging leakages around the wellbore. Further, we applied Machine Learning (ML) models to upscale and predict near-wellbore mechanical integrity at macro-scale by adopting two ML algorithms, Artificial Neural Network (ANN) and Random Forest (RF), using 100 datasets (containing 100 observations). Fractured portions of rock specimens were treated with MICP and OWC, respectively, and their resultant mechanical integrity (unconfined compressive strength, UCS; fracture toughness, K_s) were evaluated using experimental mechanical tests and ML models. The experimental results showed that although OWC (average UCS = 97 MPa, K_s = 4.3 MPa·√m) has higher mechanical integrity over MICP (average UCS = 86 MPa, K_s = 3.6 MPa·√m), the MICP showed an edge over OWC in sealing microfractures and micro-leakage pathways. Also, the OWC can provide a greater near-wellbore seal than MICP for casing-cement or cement-formation delamination with relatively greater mechanical integrity. The results show that the degree of correlation between the mechanical integrity obtained from lab tests and the ML predictions is high. The best ML algorithm to predict the macro-scale mechanical integrity of a MICP-cemented specimen is the RF model (R² for UCS = 0.9738 and K_s = 0.9988; MAE for UCS = 1.04 MPa and K_s = 0.02 MPa·√m). Similarly, for OWC-cemented specimen, the best ML algorithm to predict their macro-scale mechanical integrity is the RF model (R² for UCS = 0.9984 and K_s = 0.9996; MAE for UCS = 0.5 MPa and K_s = 0.01 MPa·√m). This study provides insights into the potential of MICP and OWC as near-wellbore cementitious materials and the applicability of ML model for evaluating and predicting the mechanical integrity of cementitious materials used in near-wellbore to achieve efficient geo-hazard mitigation and environmental protection in engineering and underground operations.

Microbially induced carbonate precipitation (MICP) is a promising technique to enhance the geotechnical properties of geomaterial either by strengthening via biocementation or reducing the hydraulic conductivity via bioclogging. This rate of modification mainly depends on the amount, and nature of biomineral precipitated and it is influenced by various environmental, chemical, and microbial factors. Given this, the present study aims to investigate the effect of biochemical conditions such as concentration of biomass and chemical reagents on the amount and nature of biomineral and its impact on the strength and permeability of biomodified sand. For this, the two microbes i.e., Sporosarcina pasteurii and isolated Proteus species at three different initial concentrations and chemical reagents by varying 0.1-1 molar of urea and calcium were considered. The amount and microstructural behavior of biomineral in different biochemical conditions concluded that the governing mechanism differs for both biocementation and bioclogging under identical MICP treatment. The strength enhancement or biocementation is dependent on the size of the biomineral precipitated whereas the reduction in permeability or bioclogging is mainly dominated by the amount of biomineral. The optimum value of biochemical conditions i.e., 10⁸ cells/ml of biomass and 0.25 M concentration of cementation reagents was chosen to further evaluate the effect of equal MICP treatment on the biocementation and bioclogging of sands having different grain sizes. The study infers that not the absolute size of the biomineral but the relative size of soil grain and biomineral influence the linkage between the soil particles and hence affect the strength of biomodified soil.

This paper reviews and analyzes recent research development on bio-cementation for soil stabilization and wind erosion control. Bio-cement is a type of cementitious materials by adopting natural biological processes for geotechnical and construction applications. Bio-cementation is usually achieved through microbially- or enzyme-induced carbonate precipitation (MICP or EICP). The use of soybean urease can be a cost-effective solution for carbonate precipitation and bio-cementation, which is named SICP. The produced calcium carbonate can cement soil particles and bring considerable strength improvement to soils. In this paper, the mechanisms and recent development on the technology optimization are reviewed first. The optimization of bio-cementation involves 1) altering the treatment materials and procedures such as using lysed cells, low pH, the salting-out technique; and 2) using cheap and waste materials for bio-cement treatment and bacterial cultivation. The objectives are to improve treatment uniformity and efficiency, use bio-cement in more scenarios such as fine-grain soils, and reduce costs and environmental impacts, etc. Studies on the mechanical behaviour and wind erosion performances of bio-cemented soil show that the wind erosion resistance can be improved significantly through the bio-cement treatment. In addition, the use of optimized method and additives such as xanthan gum and fibers can further enhance the strength, treatment uniformity or ductility of the bio-cemented soils. Attention should be paid to wind forces with saltating particles which have much stronger destructive effect than pure wind, which should be considered in laboratory tests. Field studies indicate that bio-cement can improve soil surface strength and wind erosion resistances effectively. Besides, local plants can germinate and grow on bio-cemented soil ground with low-concentration treatments.

Reasonable control of rainwater infiltration rate can ensure that soil slope will not fail due to rapid infiltration of rainwater in heavy rainfall, and at the same time, more rainwater can be infiltrated in light rainfall to meet the water demand of animals and plants. In this study, based on microbial-induced calcium carbonate precipitation (MICP) technique, a controllable bio-method for rainfall infiltration of soil slope was proposed. To have a comprehensive understanding the relationship among the rainwater infiltration control capacity, biocement treated soil permeability, slope angle and rainfall intensity, a series of physical modelling experiments of rainfall diversion on slopes with three types of soils and three slope angles were carried out in the presence of various rainfall intensities. Experimental results indicated that the proposed bio-method had the ability of controlling rainwater infiltration in term of varying rounds of biocement spraying treatment. In general, it was found that the rainwater infiltration decreases with the increase in slope angle and rainfall intensity. In the worst case of smallest slope angle (15°) and lightest rainfall intensity (n = 50 mm/h), more than 82.6%, 92.2% and 84.4% of rainwater were prevented from infiltration into the MICP treated natural sand, fine sand and medium sand, respectively, while the untreated soils were not able to prevent the rainwater infiltration at all. The corresponding maximum local uniaxial compressive strength for the MICP treated natural sand, fine sand and medium sand, respectively, were found to be 2.3 MPa, 2.0 MPa, 2.6 MPa, whereas the flexural stresses were 0.46 MPa, 0.33 MPa, 0.67 MPa, which could resist rainfall droplet impact force. Overall, the proposed bio-method showed good rainwater infiltration control capacity and high bearing strength against the impact of heavy rainfalls, suggesting a good potential to mitigate the rainfall-induced landslides.

Loess is widely distributed all over the world, covering about 10% of the land surface on earth. China is one of the countries with the most serious loess soil erosion in the world, especially the loess plateau. This is mainly related to the poor water stability and mechanical properties of the loess. A new coupling method of bio-cementation (Microbially Induced Calcite Precipitation: MICP) and sand additive to improve the hydro-mechanical behavior of loess was proposed. The feasibility, coupling improvement mechanism and the effects of sand content, bio-cement treatment cycle and cementation solution (CS) concentration were investigated through a series of tests. The results indicated that the proposed method was effective to improve the water stability and structure strength of loess. The coupling improvement performance were positively related to the sand content. When the sand content was 40%, compared to bio-cement treatment, the coupling treatment was 9 times deeper in treatment depth, 3.5 times stronger in peak structure strength, and the sum slaking rate was less than half. The coupling improvement mechanism can be attributed to the form of the double layers including hard crust and cemented layer. With the addition of sand, the thickness, structure strength and water stability of the double layers increased. The main reason is that there were more interfacial voids between sand particles and loess particles, increasing the permeability of loess and treatment depth, forming more amount of calcium carbonates. Based on the experimental condition in this study, 1.0 M of CS concentration was the optimal spaying strategy to improve the hydro-mechanical properties of loess.