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.

Enzyme-induced carbonate precipitation (EICP) is an appealing bio-cementation technology for soil improvement in geotechnical engineering. This study investigated the bio-reinforcement efficacy of sword bean crude urease (SWCU)-mediated EICP and the enhancement effect of various additives on it. A set of sand column specimens with different bio-cementation levels were prepared. Magnesium chloride, sucrose, xanthan gum, sisal fiber, calcite seeds, and skim milk powder were adopted for comparison. Bio-reinforcement efficacy was evaluated by mechanical properties. SWCU possessed a similar to 127% higher specific activity than entry-level commercial urease while saving over 2000 times the enzyme cost. All specimens treated with SWCU-mediated EICP presented excellent moldability and uniformity for one-time treatment. UCS increased exponentially with bio-cementation level due to the uniformly growing CaCO3 content and crystal size. UCS of similar to 1.8 MPa was achieved in a single treatment using 60 g/L SWCU and 3.0 M urea-CaCl2. SWCU exhibited a superior bio-reinforcement efficiency over soybean crude urease, commercial urease, and bacterial urease, since higher soil strength was achieved at lower CaCO3 content. Magnesium chloride showed the most significant enhancement effect, implying an extensive application prospect of SWCU-mediated EICP in seawater environments. The absence of wet strength, markedly elevated dry strength, and notably higher stiffness and hardness at low stress (load) phase indicated that xanthan gum would be more suitable for windbreak and sand fixation in arid/semi-arid environments. Sisal fiber could also effectively improve soil mechanical properties; however, the labor and time costs caused by its premixing with soil should be considered additionally in practical applications.

Enzyme-induced carbonate precipitation (EICP) has emerged as an environment-friendly solution for soil improvement. As a composite material, it is challenging to determine the micromechanical properties of EICP-reinforced sand using common macromechanical tests. In this work, a systematic study was conducted to determine the micromechanical properties of EICP-reinforced sand. The development of the micromechanical properties obtained from indentations along the route of sand particle-CaCO3-sand particle was examined. The width of the interfacial transition zone (ITZ) in EICP-reinforced sand was investigated. The effect of the reaction environment on ductility (i.e., the ratio of elastic modulus over hardness) of CaCO3 was investigated. The experimental results have identified that the width of ITZ in EICP-reinforced sand ranges from 0 to 180 mu m, which is significantly influenced by the crystal crystallinity or crystal morphology of CaCO3. The presence of porous media (i.e., sand particles) leads to the decrease in impurity content in the crystal formation environment, resulting in the lower ductility of CaCO3 accordingly. The mean value of fracture toughness of CaCO3 precipitation was identified to be the lowest one among sand particles, CaCO3 precipitation, and sand particles-CaCO3 interface. The lowest fracture toughness of CaCO3 indicating the failure of biocementation is derived from the CaCO3-CaCO3 breakage.

Enzyme-induced carbonate precipitation (EICP) has emerged as an innovative soil stabilization technology to precipitate CaCO3 by catalyzing urea decomposition. Although extensive efforts have been made to increase the calcium carbonate content (CCC) formed in the EICP process for the better biocementation effect, the cementability and micromechanical properties of CaCO3 are rarely known. A study of the cementitious characteristics and micromechanical properties of CaCO3 precipitates with different mixing percentages of crystal morphology is essential for soil improvement. In the present study, ultrasonic oscillation tests and nanoindentation tests were performed to investigate the cementability and micromechanical properties of CaCO3 precipitate. The results show that the cementability and micromechanical properties of CaCO3 precipitate are related to the composition of the crystal morphology. A high content of calcite is beneficial to improve the adhesion of calcium carbonate precipitate. Calcite has better mechanical properties (elastic modulus, hardness and ductility) than vaterite, and the presence of vaterite can significantly affect the measured value of mechanical properties in nanoindentation tests. The ductility of CaCO3 precipitate induced by crude soybean urease (CSU) is higher than that of CaCO3 precipitate induced by commercially available pure enzyme, suggesting that commercially available pure enzyme can be replaced by CSU for cost-effective field-scale engineering applications. This work can provide insight into optimizing the properties of CaCO3 precipitate from the micro-scale. (c) 2024 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/).

Enzymatically induced carbonate precipitation (EICP) is an emerging and eco-friendly technology, which is considered a green alternative to traditional cement in soil stabilization. When stabilizing soil use one-phase grouting method, the activity of urease is often adjusted, leads to changes in the composition and cementitious characteristics of CaCO3. Previous studies primarily focused on the mechanical properties of solidified soil samples, while the production and cementitious characteristics of CaCO3 influenced by the control of urease activity is rarely discussed. This study investigated production and cementitious characteristics of CaCO3 under different reaction environment (pH adjustment, temperature adjustment, and addition of inhibitors), during which the urease activity tests, pH tests, CaCO3 production tests and ultrasonic oscillation tests are conducted. Meanwhile, the morphological characteristics and mineral composition of CaCO3 are revealed through Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS) tests and X-ray Diffraction (XRD) test. The results demonstrate that all three one-phase grouting methods can delay the production of CaCO3 at early-stage of EICP, while the pH should be maintained above 4 to prevent significant urease deactivation. The CaCO3 generated in EICP mainly consists of calcite and vaterite, and the size of CaCO3 increases with the urease activity increased. The cementitious characteristics of CaCO3 is mainly determined by the percentage composition of calcite and vaterite, where higher vaterite content results in weaker cementitious characteristics. This study provides insights for evaluating the cementitious characteristics of CaCO3, which is beneficial for guiding the promotion and application of one-phase grouting method.

Enzyme induced carbonate precipitation (EICP) is a new bio-cementation technique that utilizes plant-sourced urease to catalyze urea degradation and reaction with calcium iron, resulting in the formation of calcium carbonate (CaCO3) for soil improvement. EICP has considerable promise for novel and sustainable engineering applications such as soil strengthening, pollutant remediation, and other in situ field applications. In this study, the effect of EICP on the geotechnical characteristics of expansive soil is examined. A series of laboratory tests have been performed with an optimal concentration ratio of 0.75 mol/L. The outcomes of the compaction experiment indicated a slight increment in the dry density of the expansive soil from 15.78 to 16.71 kN/m3.Further, it diminished the optimal moisture content of the soil, decreasing it from 22.3 to 18.5%. The utilization of EICP improves the soil mechanical characteristics, reducing swelling pressure by 80% and increasing the UCS, cohesion, friction angle, unsoaked and soaked CBR by 66%, 44%, 49%, 441%, and 430%, approximately. Additionally, it leads to a significant decrease in soil permeability, approximately 63%. Moreover, SEM and XRD analysis confirmed the presence of CaCO3 content in the treated soil. The experimental findings indicated that the EICP method holds promise in enhancing expansive soil within engineering projects.

Enzyme-induced carbonate precipitation (EICP) is an attractive bio-geotechnical technique for soil improvement. As promising alternatives to commercial ureases, legume ureases crudely extracted from primary agricultural products can provide remarkable cost savings. This study investigated the bio-cementation effect of legume ureases with different protein contents on pore-scale, mechanical, and hydraulic properties of EICP-treated sand and revealed the causes, mechanisms, and effects of the bio-clogging induced by high protein level-legume urease. Urease centrifugal liquids of sword bean (JU), pigeon pea (PU), and soybean (SU) were prepared at equal activity of 10 mM/min for sand bio-cementation. Mechanical properties were analyzed based on CaCO3 content and soil strength. Pore-features were revealed by mercury intrusion porosimetry and scanning electron microscopy, and permeability was measured to evaluate the hydraulic properties. Results showed that JU and PU with lower protein content were more effective in multi-cycle EICP-treatments, since denser bio-cemented sands with higher strengths were obtained while being vertically uniform in CaCO3 distribution and pore structure. Conversely, the high protein level of SU induced uneven bio-cementation and surface bio-clogging, resulting in bad mechanical properties, such as low strength and a destruction pattern of bottom collapse. Bio-clogging virtually eliminated the effectiveness of subsequent EICP-treatments. SU exhibited an advantage over JU and PU in reducing soil permeability, as a dramatically lower permeability was achieved at a lower treatment cycle. Comprehensive analysis concluded that the high protein level, salting-out, different precipitation rate between protein and CaCO3, and limited soil filtration capacity were the key reasons for bio-clogging induced by SU.

Subgrade stability is a key factor that influences the long-term performance of the pavement structure under repeated traffic loading. Enzyme-induced carbonate precipitation (EICP) has been recently explored as a bioinspired solution to improve the mechanical properties of sandy soils. This study evaluates the potential of EICP to cement silica sand as a treated subgrade in pavement applications. The study investigated the mechanical performance of EICP-treated sand compacted at varying initial degrees of saturation and treatment cycles through repeated loading triaxial (RLT) and unconfined compressive strength (UCS). The results show that specimens having a lower initial EICP degree of saturation facilitated efficient carbonate precipitation bridging between sand grains, resulting in higher resilient modulus and UCS. Furthermore, surface percolation of EICP cementing solution resulted in a further increase in carbonate content and treated soil UCS and resilient modulus (Mr). A maximum UCS and Mr values of about 700 kPa and 165 MPa, respectively, were achieved, which implies the possibility of using EICP-treated sand as a treated subgrade layer or subbase to reduce the base layer thickness or improve the pavement structure performance. The Scanning electron microscopy (SEM) images further explained the influence of the carbonate crystal shape and morphology on the treated soil strength and modulus. Finally, simulations executed using AASHTOWare Pavement ME Design software show the potential of utilizing EICP for improving subgrade for pavement structures, with a significant reduction in base layer thickness while maintaining the same rutting and fatigue performance. Hence, demonstrating the potential of EICP in enhancing subgrade properties for pavement structures.

Enzyme-induced carbonate precipitation (EICP) is an emanating, eco-friendly and potentially sound technique that has presented promise in various geotechnical applications. However, the durability and microscopic characteristics of EICP-treated specimens against the impact of drying-wetting (D-W) cycles is under-explored yet. This study investigates the evolution of mechanical behavior and pore characteristics of EICP-treated sea sand subjected to D-W cycles. The uniaxial compressive strength (UCS) tests, synchrotron radiation micro-computed tomography (micro-CT), and three-dimensional (3D) reconstruction of CT images were performed to study the multiscale evolution characteristics of EICPreinforced sea sand under the effect of D-W cycles. The potential correlations between microstructure characteristics and macro-mechanical property deterioration were investigated using gray relational analysis (GRA). Results showed that the UCS of EICP-treated specimens decreases by 63.7% after 15 D-W cycles. The proportion of mesopores gradually decreases whereas the proportion of macropores increases due to the exfoliated calcium carbonate with increasing number of D-W cycles. The microstructure in EICP-reinforced sea sand was gradually disintegrated, resulting in increasing pore size and development of pore shape from ellipsoidal to columnar and branched. The gray relational degree suggested that the weight loss rate and UCS deterioration were attributed to the development of branched pores with a size of 100-1000 mm under the action of D-W cycles. Overall, the results in this study provide a useful guidancee for the long-term stability and evolution characteristics of EICPreinforced sea sand under D-W weathering conditions. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting 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/).