As urbanization progresses, the demand for high-rise buildings and underground spaces is growing, and the need for firm geotechnical construction materials, efficient excavation methods, accurate testing instruments, and innovative geotechnical engineering theories and technologies is increasing. By investigating the phenomena of strengthening and toughening in nature, hydrophobic and ice-phobic, friction anisotropy and drilling as well as excavation, etc, researchers have found that organisms have distinctive external morphology and organization. By imitating the external morphology, structural characteristics or movement mechanism of organisms, novel ideas, new principles, and innovative theories can be provided for the innovation and sustainable development of geotechnical engineering. This paper mainly expounds on the bio-inspired application in geotechnical engineering from three perspectives: geo-materials, geotechnical components, and drilling & excavation equipment, and lists typical application cases. In conclusion, this paper presents a summary and prospects of bio-inspired geotechnical engineering, offering fundamental insights for future research.

Biocementation-based soil improvement is an emerging ground treatment method in geotechnical engineering that has garnered extensive attention over the past two decades. One of the challenges associated with this method revolves around the uniformity of biocementation, a crucial factor closely tied to bio-grouting technology. The traditional biotreatment methods, the two-phase method and the one-phase method, suffer from the issue of non-uniform biocementation. Consequently, in recent years, various improved grouting technologies have been proposed to address this concern by aiding bacterial adsorption and controlling carbonate precipitation. This paper reviews the mechanisms and grouting processes employed in these enhanced bio-grouting technologies. Additionally, the challenges of implementing these grouting technologies in real-world applications are also thoroughly discussed.

Calcareous sand is the main fill material for island reclamation projects, but untreated calcareous sand might not be used as a reclamation fill due to its poor mechanical properties. Microbial induced calcite precipitation (MICP) was directly used to consolidate calcareous sands. One-dimensional sand column tests were conducted to identify the optimized solutions and to investigate the effects of cement solution concentration, relative density, and consolidation frequencies on the permeability and mechanical properties of MICP-treated calcareous sands. Finally, three-dimensional model tests were carried out to investigate the effective consolidation range of microbially treated calcareous sands. The results show that the MICP-treated calcareous sand shows a reduction in the permeability of the sample, while the calcium carbonate cementation and its filling effect improves the mechanical properties of the soil. The one-dimentional test results show that the effective values for cement solution concentration, relative density, and consolidation frequencies range from 0.5 mol/L to 1.5 mol/L, 30%-70%, and 5-15 times. The consolidation frequencies have the greatest influence on the permeability and strength properties of the treated calcareous sand. A quadratic polynomial regression model for permeability and strength was established through response surface analysis, and the regression model proved to be highly accurate and reliable through testing. In three-dimentional tests, the consolidation range tends to move downwards in a trapezoidal shape, showing a "big bottom and small top" pattern, with a consolidation range of approximately 34 times the diameter of the pipe. This study serves as a reference for selecting consolidation parameters for subsequent tests and applications of MICP-treated calcareous sands.

Vegetation has been used as a means in geotechnical engineering for soil improvement and erosion control. This paper aims to present a state-of-the-art review and future prospective on soil improvement and reinforcement with plants, mainly from a perspective of plant mechanical effects. The mechanics of roots and root-soil composite are reviewed with regard to experiments, including root mechanical tests, direct shear tests, pullout tests and triaxial tests. Various factors influencing root reinforcement are characterized and discussed to explain root-soil interactions and related soil strengthening mechanisms. Considering cost and efficiency, extreme climates, and the conflicting mechanisms of plant growth and soil improvement, researchers have introduced nature-based water-soluble polymers (WSPs) into soil improvement to promote vegetation establishment and provide additional binding strength between soil particles. Despite the benefits, existing related researches and concepts are scarce, and there is still a significant knowledge gap in the coupling effect of WSP and plants for soil improvement. The review indicates that the combination of vegetation and WSP has the potential to create “trade-off” and “complementarity” for progressive soil improvement. Finally, new research topics in the field of soil improvement with plants are identified in the review.

The aim of this study is to disclose the feasibility of improving the thermal conductivity and mechanical strength of quartz sand steel slag mixtures treated by enzyme-induced carbonate precipitation (EICP). In this work, the effects of steel slag content (SSC) and number of treatment cycle (N) on the thermal conductivity and mechanical strength of EICP-treated specimens were investigated. The immersion method was adopted for specimen preparation. The thermal conductivity was measured by transient plane source method (TPS) and the unconfined compressive strength (UCS) was obtained through a uniaxial compression test. Moreover, the SEM test was conducted to obtain the morphology and deposition characteristics of calcium carbonate crystals. The result shows that the thermal conductivity and UCS of EICP-treated sands increase before decreasing as the SSC increases. Consequently, the maximum values of thermal conductivity and UCS are 1.28 W/(m⊡K) and 6.31 MPa, respectively, corresponding to the optimal parameter of 20% SSC at 12 N. The optimal thermal conductivity and UCS increase by 367% and 137%, respectively, compared to that of EICP-treated sand with no addition of steel slag. The SEM analysis indicates that the spherical calcium carbonate exists in the range of 0-20% SSC, whereas there is mainly amorphous calcium carbonate when the SSC varies from 40% to 80%. It also demonstrates that the UCS is more sensitive to the variation of calcium carbonate content than that of thermal conductivity.

Inspired by nature, the design and synthesis of novel biomimetic materials are gradually attracting the attention of scientists. Biomimetic materials with excellent performance are widely applied in medical health, industrial production, agricultural planting, aerospace, etc. As a natural porous biomass material, diatomite has the advantages of high porosity, low bulk density, stable chemical property and large surface area. Benefiting from these advantages, it is of great importance to treat diatomite as bionic substrate to synthesize diatomite biomimetic materials, which can be endowed good structure stability and natural mechanical property. It is an ideal option for crystal growth and uniform dispersion of nanostructures, to improve the agglomeration and high cost of nanomaterials. This review briefly introduces our recent achievements on diatomite biomimetic materials in different application fields. In view of its excellent optical, thermal, chemical and mechanical property, diatomite biomimetic materials have shown extensive application potential in various fields of science and engineering, which include catalysis, corrosion protection, microwave adsorption, super-hydrophobicity, pollutant adsorption, energy storage, etc. It demonstrates that diatomite biomimetic materials with different functional properties can be synthesized by diverse chemical means and preparation methods for different application. By composed of inorganic nanomaterial hybrid, this diatomite biomimetic materials display a three-dimensional network structure with diatomite morphology. The design and synthesis of diatomite biomimetic materials provide more potential bionic categories for different applications, which can accelerate the development of low-cost and high-performance biomimetic materials.

Microbial-induced carbonate precipitation (MICP) technique has been adopted in geotechnical engineering widely. In this study, the effect of drying-wetting cycles on MICP-recycled shredded coconut coir (RSC) reinforced calcareous sand was studied, and the deterioration mechanism under drying-wetting cycles was revealed. Test results indicated that drying-wetting cycles exert an important influence on the durability of MICP-RSC reinforced specimens. With the increase of drying-wetting cycles N, the specimens demonstrated significant increase in mass loss rate and critical void ratio, decrease in maximum shear modulus, peak strength and toughness. Furthermore, an increase in the initial relative density reduced the deterioration of MICP-RSC reinforced specimens exposed to drying-wetting cycles. Higher initial relative density of the specimen correlates with an increased maximum shear modulus, peak stress and toughness, a decreased in permeability and critical void ratio. Microanalysis revealed that the generated calcium carbonate adhering to sand particles and RSC gradually dropped off with the increase of N, weakened cementation, and led to the deterioration of MICP-RSC reinforced specimens, which is consistent with the deterioration characteristics under drying-wetting cycles.

Heavy metal contamination of soil and water is one of the most prominent environmental issues worldwide. Through bioaccumulation and biomagnification of the food chain, heavy metals can be enriched hundreds of times and eventually enter the human body, posing a major threat to human health. Biomineralization has the greatest potential to become an efficient and environmentally friendly heavy metal remediation technology and has received much attention in recent decades. This review summarizes the latest progress of biomineralization technology on carbonate precipitation and phosphate precipitation in heavy metal remediation. Both microorganisms (including bacteria and fungi) and enzymes can induce carbonate and phosphate precipitation, converting the free heavy metal ions into insoluble salts. However, the mechanisms of the heavy metal remediation are significantly different. For example, urea hydrolysis, which occurs intracellularly when urease-producing bacteria (UPB) are used, is the most commonly used mechanism for carbonate precipitation based bioremediation. In contrast, phosphate solubilization by either enzymes or organic acids secreted by phosphate solubilizing bacteria (PSB) is extracellular, and both soluble and insoluble phosphorus can be decomposed by PSB. Moreover, some influencing factors such as the different species of microorganism, heavy metals and some environmental conditions that may affect the bioremediation of heavy metals were also summarized in this paper. The challenges of biomineralization based heavy metal remediation are also discussed. Based on the reviews of previous studies, a comprehensive understanding of heavy metal removal through microorganism can be increased, and thus promotes the applications of biomineralization technology in the treatment of large-scale heavy metal contaminated sites.