Unfrozen water content (θu) in weak expansive soil during freezing is a primary factor influencing uneven frost heave deformation of high-speed railway subgrades in western China. However, the mechanism underlying θu evolution and its prediction modeling in weak expansive soil remains unexplored. This study employs nuclear magnetic resonance technology to investigate the influence of swelling potential on the θu in weak expansive soils and to elucidate the underlying mechanisms. Based on thermodynamic principles, a theoretical prediction model incorporating the swelling potential parameter is thus established. The results indicated that the freezing process can be categorized into three stages, with a pronounced phase change occurring between −2 and −4 °C. Increasing swelling potential from low potential to high potential in weak expansive soil leads to a reduction in the freezing point by 2 °C, and the θu at −4 °C improves by 7.3
This study investigates the effectiveness and underlying mechanisms of bio-carbonation of reactive magnesia cement (RMC) for the improvement and stabilization of dispersive loess. A comprehensive experimental program, including pinhole tests, crumb tests, unconfined compressive strength tests, and multiple microstructural characterization techniques, was conducted to systematically evaluate the soil dispersivity, mechanical performance, and water stability under varying urea concentrations and RMC contents. The results demonstrate that bio-carbonation treatment can rapidly and significantly reduce soil dispersivity. The unconfined compressive strength of the treated soil increased dramatically, accompanied by pronounced enhancements in stiffness and water stability. Microstructural analyses reveal that the carbonation products are dominated by hydromagnesite, which predominantly occurs in the form of platy crystals. These hydromagnesite crystals effectively cement and encapsulate soil particles, fill interparticle pores, and promote the formation of a dense and well-bonded soil structure. Furthermore, the results indicate that urea concentration and RMC content jointly govern the efficiency of dispersivity mitigation and mechanical reinforcement achieved through the bio-carbonation process. This study confirms the effectiveness of bio-carbonation as a high-efficiency and low-carbon technique for the stabilization and reinforcement of dispersive loess, providing valuable insights and practical guidance for engineering ground improvement and ecological protection in loess regions.
Microbial Induced Carbonate Precipitation (MICP) offers significant potential for strengthening calcareous sand foundations in island reclamation. However, controlling bacterial transport and uniformity within complex three-dimensional (3D) seepage fields remains a critical challenge. This study investigates bacterial transport under single-pipe grouting and double-pipe circulation modes by integrating 3D physical model tests with two-phase flow numerical simulations. Physical experiments revealed a competition mechanism between density-driven gravitational settling and pressure-driven radial advection. The reliability of the numerical model in predicting fluid migration was validated by correlating measured normalized penetration resistance with simulated normalized bacterial concentration. The comprehensive parametric study revealed a key finding for single-pipe grouting, increasing the injection rate markedly enhances radial driving forces. This increase, in turn, expands the treatment radius and improves vertical uniformity. Treatment efficacy, however, proved insensitive to the thickness of the overlying clay layer. For the double-pipe circulation cases, an effective hydraulic threshold for pipe spacing exists, beyond which the treatment zone becomes disconnected. Furthermore, adopting an asymmetric flow strategy, with a suction rate higher than the injection rate, effectively mitigates fluid accumulation near the injection, thereby significantly improving the treatment uniformity in the inter-well region. These findings provide a theoretical basis for optimizing MICP grouting in calcareous sand foundations.
The alkali-silica reaction (ASR) poses a persistent durability threat in concrete structures containing reactive aggregates. This study combines experimental testing with a calibrated numerical modeling framework to evaluate the synergistic mitigation of ASR through combined use of fly ash and rice husk ash (RHA) as supplementary cementitious materials (SCMs). Aggregates from the Chichali and Lumshiwal formations classified as highly and moderately reactive, respectively, were characterized by Petrographic, X-ray Diffraction, and Scanning Electron Microscopy analyses, and incorporated into mortar bars following ASTM Standard. Experimental results revealed that individual additions of FA or RHA reduced expansion relative to the control; however, the binary blend containing 20
The micromorphology of crystal fragments in tuff significantly influences its macroscopic mechanical properties and fracture behavior. To investigate this effect, a rectangular grain model (RGM) is proposed to characterize approximately rectangular or angular crystal fragments in tuff, addressing the limitation of the conventional grain-based model, which has limited flexibility in representing mineral particle shapes. Using the discrete element method, this study examines how crystal fragment content, shape, and orientation affect rock strength and crack propagation. The results indicate that variations in crystal fragment content influence the mechanical behavior and failure patterns of tuff by altering material heterogeneity. Changes in the shape and orientation of crystal fragments affect the mechanical response by modifying the number and spatial arrangement of contact points along cemented interfaces; an increase in the length of cemented interfaces along the long-axis direction of crystal fragments promotes tensile failure. When the long-axis cemented interfaces tend to align with the macroscopic fracture direction, the initiation, coalescence, and propagation of through-going cracks are facilitated. The results provide a mesoscale reference for understanding the mechanical behavior of tuffs with similar crystal-fragment morphologies.
