Traditional soil stabilizers, such as cement and lime, typically entail substantial energy consumption and environmental pollution. In contrast, bio-enzyme has emerged as a promising alternative, aligning with the imperatives of sustainable development, cost-effectiveness, and environmental friendliness. Bio-enzymes are primarily one or more protein molecules that catalyzes chemical reactions in the soil to form a cementing bond that stabilizes the soil structure and reduces the soil’s affinity for water. Currently, a plethora of studies on bio-enzyme have been conducted by scholars worldwide, yet there remains a notable absence of the systematic organization and comprehensive review of these findings. This study offers a thorough examination of bio-enzyme technology, encompassing its biochemical properties, mechanisms, the engineering properties of stabilized soil, bio-enzymatic composites, and its engineering applications. And current trends and future prospects of bio-enzyme are also scrutinized. This forward-looking study indicates that bio-enzyme functions through mechanisms such as cation exchange, specific binding, and surfactants, among others to diminish the electric double layer thickness and hydrophilicity of soil, consequently enhancing engineering properties of soil. And the improvement performance can be influenced by various factors, including soil properties, enzyme dosage, specificity, and sample preparation, etc. It is also noted that the composites of bio-enzyme with conventional stabilizers tend to enhance improvement performance more efficiently. The engineering applications of bio-enzyme have demonstrated its superiority over traditional stabilizers in soil improvement. However, the performance of treated soils with available bio-enzyme remains limited, highlighting the necessity for extracting novel bio-enzyme form plants/animals and determining its mechanisms and engineering mechanical properties. It is also essential to develop more bio-enzymatic composites and conduct application in-situ to develop relevant standards and application guidelines.
Many past studies have investigated the Unconfined Compressive Strength (UCS) behaviour of MICP-treated sandy soils and developed empirical relationships to predict strength improvement. While the UCS of MICP-treated soils can be affected by many factors such as chemical concentration, temperature, and biochemistry, it has been found that particle size ($d_{10}$) is one of the important contributing factors, but most of the previously published studies have not considered it. This study applied MICP on three different variants of Adelaide Industrial (AI) clean sands with different grain size distributions to evaluate the effect on UCS and Calcium Carbonate ($CaCO_{3}$) precipitation. To better understand the influence of particle size, this study also collected literature data on UCS, $CaCO_{3}$ content and soil grading properties. A numerical method was used to interpolate the distribution of the combined data (literature and experimental) in 3D space to establish a clear correlation between UCS, $CaCO_{3}$ content (CC) and the soil grading properties. So, contour plots were generated between UCS, CC, and $d_{10}$ and other soil grading properties. Where the 2D and 3D plots could not clearly present the influence of $d_{10}$ on the variation of UCS and CC, contour plots presented the distribution rather clearly. The contour plots showed a visible trend in the variation of UCS and CC for $d_{10}$ and Coefficient of Curvature ($C_{c}$), but not for Coefficient of Uniformity ($C_{ u }$).
This study validates the feasibility of extracting calcium and phosphorus from kitchen waste bones for crude enzyme-induced calcium phosphate precipitation (EICPP) under both normal and microgravity conditions. The experimental results demonstrate no significant differences in the degree of reaction and characteristics of precipitation between these environments. By leveraging local resources, reducing material transport costs, and addressing waste management challenges, this research underscores the potential for extraterrestrial construction, thereby enhancing sustainability in space environments. These findings offer promising insights for the application of space biocementation, particularly during the expansion phase of human settlements.
Despite the growing interest in microbially induced carbonate precipitation (MICP) for geotechnical applications, reports on meter-scale MICP trials for soil improvement remain limited, and controlling and predicting cementation efficiency on a large-scale is even more scarce. This study presented a meter-scale improvement of a poorly-graded sand (initial dry density: 1581 kg/m3, porosity: 40 %) through MICP in a cylindrical cell (diameter: 1 m; thickness: 15 cm) using a radial flow injection strategy, which involves injecting fluids radially from a single well located at the center while maintaining a constant hydraulic head at the outer boundary. Nine cycles of a two-phase MICP treatment were applied: Phase 1- injection of 0.7 pore volumes (PVs) of bacterial solution and 1-L water pulse; Phase 2- injection of 1.4 PVs of 0.5 mol/L cementing solution in two stages (i) 0.7 PV injection two hours after the bacteria were injected, and (ii) a further 0.7 PV injection the following morning after an overnight static reaction period. We observed non-uniform CaCO3 precipitations along the distance from the central well and over the depth, which was induced by the decreasing flux towards the outer boundary under the radial flow pattern, along with influences from layered soil packing and hydraulically induced flow channels. CaCO3 precipitation with distance from the central well follows a symmetric Gaussian-type distribution, with sufficient cementation to retrieve full-length cores occurring near the midpoint between the central well and the outer boundary. The unconfined compressive strengths of the full-length cores were in the range of 1.2-6.8 MPa with CaCO3 contents of 0.08-0.17. Our study suggests that cementation level under radial flow conditions is controllable on a large scale and highly dependent on the injection volume of both bacteria and rinsing water pulse. The study provides a solid baseline for predicting and controlling CaCO3 distribution in large-scale MICP soil improvement using a two-phase radial injection approach.
Global climate change has exacerbated extreme weather events, such as intense rainfall and heat waves, resulting in the deterioration of geotechnical earthen structures. To address the urgent need for sustainable development, eco-friendly solutions are being explored, with vegetation emerging as a vital natural engineer. Despite the potential of vegetation, traditional practices often limit its role to aesthetics, overlooking the engineering benefits of plant roots. This paper introduces the new interdisciplinary field of eco-geotechnics, which integrates soil mechanics, ecology, botany, and atmospheric sciences, etc. to enhance geotechnical infrastructure. By focusing on atmosphere-plant-soil interactions, this review highlights how plants contribute to the stability of earthen infrastructure through root reinforcement and hydrological benefits. This paper also reviews recent advancements in constitutive modelling of vegetated soils, particularly focusing on a novel eco-unsaturated soil model. It discusses experimental testing of vegetated soils and their wide applications. Critical research gaps are identified in terms of the effects of extreme weather on root systems, soil cracking dynamics, ecological restoration in contaminated areas, and the synergistic effects of vegetation with sustainable soil stabilisers. Additionally, the use of smart monitoring techniques based on a combination of remote sensing and machine learning is proposed to assess vegetation-soil interactions in real-time. By integrating ecological and geotechnical processes, a comprehensive framework is recommended for future research directions in eco-geotechnics, which will ultimately facilitate the development of resilient engineering solutions that can withstand the challenges posed by climate change. The insights gained will be invaluable for improving the sustainability of geotechnical practices and enhancing the resilience of infrastructures in a changing climate.
Although biochar is widely recognized for enhancing various soil properties, its impact on soil erosion resistance remains unclear and sometimes shows contradictory results. The main objective of this study is to quantify the effects of corn-cob biochar amendment, both with and without erosion control blankets (ECB), as well as the influence of biochar/compost incubation time on erosion resistance of a silty sand. The study also investigates the effects of biochar on Atterberg limits, shear strength, and thermal conductivity. As biochar content increases from 0 % to 20 %, the liquid limit (LL), plastic limit (PL), and shrinkage limit (SL) rise by 8 %-10 %, suggesting that biochar-amended soil (BAS) retains more water without losing strength. The addition of biochar has minimal impact on the shear strength of BAS at lower normal stresses (<45 kPa) but reduces its thermal conductivity by about 70 %. Submerged jet erosion tests show that biochar alone increases soil erosion in BAS. However, when combined with ECB and vegetation, erosion is significantly reduced (up to 39 %). Overall, this study underscores the importance of utilizing biochar in combination with ECB and such vegetation as ruzi grass to mitigate soil erosion in the silty sand.
Microbial induced calcium carbonate precipitation (MICP) technology is widely used for reinforcement in geotechnical engineering due to its low cost, simple process, strong applicability and lack of secondary pollution. However, the presence of clay particles in silt increases the compressibility and decreases the permeability of soil, complicating the even distribution of slurry into soil pores. Therefore, it is necessary to develop a treatment technology which is suitable for silty soil sites, achieving effective solidification using MICP. This study examines three treatment techniques, including grouting, immersing and mixing methods, to solidify silt material. The strength characteristics of the solidified soil were analyzed by using unconfined compression tests. Results show that the mixing method provides the highest strength, followed by the grouting method, with the immersion method yielding the lowest strength. The uniformity of the solidified samples was assessed by determining calcium carbonate content, X-ray diffraction tests, and mercury injection tests. The MICP samples made by using immersing and grouting methods exhibited inhomogeneity in both radial and longitudinal directions. For the immersing method, calcium carbonate content decreased, pore volume increased, and the degree of cementation worsened progressively from the outer layer to the inner layer. For grouting method, the same phenomenon occurs from the bottom (grouting point) to the top. In contrast, the MICP samples with mixing method showed good homogeneity in all spatial directions. This study provides guidance and optimization strategies for applying MICP technology in silty soil sites.
Microbially Induced Calcite Precipitation (MICP) is an effective ground improvement technique for mitigating liquefaction-induced ground deformations. However, limited research has explored its application for reducing shallow foundation settlement in liquefiable soils. Understanding the extent of the area requiring MICP treatment beneath a foundation is critical to minimizing settlement. This study aims to evaluate the impact of the improvement area of MICP-treated blocks on mitigating liquefaction-induced settlements of shallow foundations using a series of 1 g shake table model tests. The dimensions of the treated blocks were determined based on the Boussinesq load distribution chart and treated to achieve a shear wave velocity of 250 m/s. Scaled shake table tests were conducted, modeled after a large-scale shake table experiment. The testing setup included three soil layers with different relative densities, with a shallow foundation placed on the surface crust. MICP-treated blocks of varying sizes were placed beneath the foundation. The results demonstrated that when the MICP-treated block was configured as either L×B× 1.25B or 1.5L× 1.5B×B - where L and B are the length and width of the foundation - resulted in substantial improvements, with reductions of 80% in foundation settlement and 98 % in foundation tilting. Notably, the L×B× 1.25B configuration achieved performance similar to the 1.5L× 1.5B×B, while reducing the treated volume by 44%. Furthermore, the results emphasize the importance of optimizing the MICP-treated area to effectively mitigate liquefaction, providing valuable insights into the practical application of MICP for improving the performance of shallow foundations in liquefiable soils.