Biogrouting has been proposed for improving mechanical properties of soils and rocks, whose performance greatly depends on the location of biocement at pore-scale. To enhance the performance of biogrouting, many strategies were proposed, including the addition of assistants, controlling curing moisture degree, and flocculation of bacteria. Clay is one such assistant which has been proven to be effective, with an assumption of increasing active biocement, i.e. those located between soil particles. In this work, we employed microfluidics to directly observe whether clay minerals can certainly control the location of precipitates and how they function. First of all, the capacity of bentonite and kaolin for adsorbing bacteria were investigated. Then, the location of CaCO3 crystals with and without clay minerals were visually observed using microfluidics. Pore-filling ratios and CaCO3 ratios, which are closely related to permeability and strength of biocemented soils, were quantitatively analyzed from collected images. Finally, the effects of bentonite and kaolin and their dosages on the location of biocement were comprehensively discussed. The results demonstrated that the performance of bentonite and kaolin on adsorbing bacteria and regulating biocement location is distinct due to differences in the morphologies of clays. These findings can help us to improve biogrouting performance on soil stabilization and propose new strategies in various practical applications, such as CO2 sequestration, heavy metal remediation, and oil recovery enhancement, all with a foundational understanding of their mechanisms.

The reinforcement and repair materials for earthen sites have high requirements for strength, resistance to deterioration, and aesthetic coordination. In this study, the enzyme-induced carbonate precipitation (EICP) and the microbially induced carbonate precipitation (MICP) techniques were used to reinforce the earthen site soil. The applicability of EICP and MICP for stabilizing earthen sites soil was investigated through static contact angle tests, disintegration tests and colorimetry tests. In addition, the improvement of mechanical properties of biotreated earthen sites soil was examined by unconfined compression strength tests. The tests results show that MICP and EICP techniques could improve the mechanical characteristics and water-stability properties of the earthen sites soil. With the increase in cementing solution concentration, the effectiveness of EICP was enhanced, while the water-stability and hydrophobicity of MICP-treated soils increased first and then decreased due to the influences of organic matter and soluble salts. EICP and MICP techniques showed different performance in reinforcing effects on calcium carbonate content, shear wave velocity, unconfined compressive peak strength, total disintegration time, and static contact angle. This study is expected to contribute valuable insights to the conservation of earthen heritage site using bio-based methods.

Cementations bind sand/soil particles via physical and chemical interactions to form composite solids with macroscopic mechanical properties. While conventional cementation processes (e.g., silicate cement production, phosphate adhesive synthesis, and lime calcination) remain energy-intensive, bio-cementation based on ureolytic microbially induced carbonate precipitation (UMICP) has emerged as an environmentally sustainable alternative. This microbial-mediated approach demonstrates comparable engineering performance to traditional methods while significantly reducing carbon footprint, positioning it as a promising green technology for construction applications. Nevertheless, three critical challenges hinder its practical implementation: (1) suboptimal cementation efficiency, (2) uneven particle consolidation, and (3) ammonia byproduct emissions during ureolysis. To address these limitations, strategic intervention in the UMICP process through polymer integration has shown particular promise. This review systematically examines polymer-assisted UMICP (P-UMICP) technology, focusing on three key enhancement mechanisms: First, functional polymers boost microbial mineralization efficacy through multifunctional roles, namely microbial encapsulation for improved survivability, calcium carbonate nucleation site provision, and intercrystalline bonding via nanoscale mortar effects. Second, polymeric matrices enable homogeneous microbial distribution within cementitious media, facilitating uniform bio-consolidation throughout treated specimens. Third, selected polymer architectures demonstrate ammonium adsorption capabilities through ion-exchange mechanisms, effectively mitigating ammonia volatilization during urea hydrolysis. Current applications of P-UMICP span diverse engineering domains, including but not limited to crack repair, bio-brick fabrication, recycled brick aggregates utilization, soil stabilization, and coastal erosion protection. The synergistic combination of microbial cementation with polymeric materials overcomes the inherent limitations of pure UMICP systems and opens new possibilities for developing next-generation sustainable construction materials.

Granite saprolite (GS) slope failure is a common yet catastrophic phenomenon in South China. Although the impact of subtropical climate, characterized by high temperatures and heavy rainfall, is widely recognized, the effect of the capillary imbibition and drying (CID) process, which frequently occurs during the dry season, on the hydro-mechanical properties of GS and slope stability is largely overlooked. This research examines natural GS specimens with various degrees of weathering subjected to CID cycles. The study investigates the capillary imbibition (CI) process and the evolution of the soil's hydromechanical properties across CID cycles. The results indicate that the CI process in GS is fundamentally different from that in clays and sands. The aggregated structure of GS comprising numerous fissures and large pores plays a critical role. In addition, the CID cycles cause the hydro-mechanical degradation of GS, including a finer particle composition, decreased shear strength, and increased permeability and disintegration potential, where damage to soil cementation and fissure development are identified as critical factors. This investigation reveals new insights into the mechanical properties of GS that are essential for the development of effective landslide management strategies in South China. (c) 2025 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/).

The objective of the current study is to explore the effect of biostimulation treatment methods on the mechanical properties and microstructure characteristics of biocemented soil. Biostimulated microbially induced carbonate precipitation (MICP) is an eco-friendly and economical soil reinforcement measure. It relies on the stimulation of the urease-producing bacteria (UPB) in situ for the MICP process. Different biostimulation treatment methods involve different oxygen availability, stimulation solution content and distribution, and number of biostimulation treatments. There may be differences in the effect of UPB stimulation and biocementation when different biostimulation treatment methods are used. In this study, four biostimulation treatment methods, i.e., unsaturated single biostimulation treatment (USBT), unsaturated multiple biostimulation treatments (UMBT), saturated single biostimulation treatment (SSBT) and saturated-unsaturated-combined single biostimulation treatment (CSBT), were used to stimulate native UPB in soil columns, and then, the same cementation treatment was applied to the soil columns. Subsequently, the mechanical behavior and microstructural properties of the biocemented soil were investigated. The results indicated that the saturated single biostimulation treatment was more conducive to stimulating native UPB to induce CaCO3 precipitation. Samples subjected to the saturated single biostimulation treatment exhibited higher CaCO3 precipitation content (CCP), dry density, unconfined compressive strength (UCS) and lower permeability within the same cementation treatment cycle (NC). However, UCS was not only determined by CCP, but was also regulated by CaCO3 spatial distribution and precipitation pattern. This study could help guide the selection of biostimulation treatment methods.

Sandy hydrate reservoirs are considered an ideal target for the extraction of marine natural gas hydrates (NGH). However, engineering geological risks, including reservoir sand production and seabed subsidence during the extraction process, present a significant challenge. In 2019, China discovered a high-concentration sandy NGH reservoir with favorable commercial development potential in the Qiongdongnan Basin of the South China Sea, establishing the region as a key focus for future exploration and development efforts. A thorough comprehension of macro-meso mechanical properties of this specific sandy NGH reservoir is essential for the safe and efficient extraction of hydrates. In this study, a novel method is proposed to calculate hydrate saturation of hydrate-bearing sandy sediments (HBSS) with hexagonal close-packed state. A series of undrained biaxial compression with flexible boundary show that hydrate cementation enhances the strength of the sample. However, an excessively high hydrate saturation is likely to induce strain softening, whereas an increase in confining pressure helps to mitigate strain softening. Hydrate cementation promotes the formation of abundant force chains. The inhomogeneous displacement, sliding, and relative rotation of the particles are the primary factors contributing to the formation of X-shaped shear bands, which is related to cemented bond breakage. The primary cause of hydrate cementation failure is tensile stress failure. External loading induces force chains to undergo buckling, fracturing, and restructuring, which governs fabric development. The research outcomes offer novel insights into the inhomogeneous deformation and macro-meso mechanical properties of HBSS at the particle-scale.

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.

Infrastructure development often faces challenges due to soils with low bearing capacity, which can potentially cause instability and subsidence and threaten the safety of structures. Therefore, an efficient and environmentally friendly stabilization method is required. This study aims to evaluate the effectiveness of Microbial Induced Calcite Precipitation (MICP) in improving bearing capacity and soil strength through the formation of bacterial soil columns. This study employed a full-scale physical model test using 40 cm diameter and 200 cm deep soil columns filled with soil mixed with Bacillus subtilis, compacted, and cured for 56 days. The results showed significant improvements in the geotechnical characteristics of the soil, with CBR values increasing from 5.5% to over 12%, unconfined compressive strength reaching 345 kPa, and modulus of elasticity increasing to 12.5 MPa. Soil cohesion increased to 65 kPa, while internal friction angle increased from 10 degrees to 34 degrees. The novelty of this research is the application of MICP technology in the form of bacterial soil columns as an innovative, effective, and sustainable stabilization method to improve the mechanical properties of soft soils.

This paper presents the results of an experimental program aiming to explore the mechanical response of lightly cemented sands under different orientations of the principal stress axes using the hollow cylinder torsional apparatus. Two compositions of lightly cemented sands featuring the same porosity/volumetric cement content index, eta/Civ, but characterized by different cement content and sand density have been subjected to linear probing stress paths with orientations of the principal stress, alpha sigma, varying from 0 degrees to 90 degrees from the vertical specimen axis. All the tests have been carried out under drained conditions. It will be shown that despite the same value of eta/Civ, different soil strengths were recorded for the two cemented soil compositions. This may suggest that the relative contribution of the cementation and soil density may be affected by the orientation of principal stress axis during loading. The suitability of multiaxial strength criteria proposed for sand materials to reproduce the peak deviatoric strength of the lightly cemented sands is also investigated.

Well-graded granular materials are extensively used in the foundation layers of roads and railways. Excessive deformation developed in these layers under traffic-induced cyclic loading represents a major contributor to structural deterioration, while no viable methods are currently available for rehabilitating these layers without causing substantial disruption. In this context, biocementation holds promise as a non-disruptive solution, yet dedicated investigations have been lacking. This study, through a series of multi-stage cyclic triaxial tests, explores the feasibility and effectiveness of biocementation in improving the deformation and shakedown behaviour of a well-graded aggregate representative of typical granular materials used in road and railway foundations. The results show that both uncemented and biocemented aggregates exhibit distinct stable and unstable responses with increasing cyclic stress. Biocementation effectively enhances deformation resistance and elevates the shakedown limit in the stable regime, while it aggravates brittleness in the unstable regime. Further interpretation using the normalised stress ratio (NSR) reveals the existence of a unique critical NSR zone that separates stable and unstable regimes, independent of both cementation level and confining pressure. Microstructural characterisation elucidates multiple precipitation patterns. A cementation mechanism where small aggregate particles and CaCO3 crystals merge to form cementing bonds between large particles is postulated.