The accumulation of soil organic carbon (SOC) and total nitrogen (TN) is easily accomplished by returning crop straw, which strongly affects the formation and pore structure of aggregates, especially in black soil. We returned maize straw at different rates (6,000, 9,000, 12,000 and 15,000 kg ha(-1)) for nine years to investigate its influence on the SOC and TN contents in the SOC fractions of aggregates by combining size and density fractionation. Their subsequent influences on pore morphology and size distribution characteristics were examined using X-ray micro-computed tomography scanning (mu CT). The results showed that returning straw significantly increased the contents of C and N in the SOC fractions of aggregates, especially at the return rates of 12,000 and 15,000 kg ha(-1), which in turn promoted aggregate formation and stability, and ultimately amended pore structure. The pore size>100 mu m, porosity (>2 mu m), and morphological characteristics (anisotropy, circularity, connectivity and fractal dimension) significantly increased, but the total number of pores significantly decreased (P<0.05). Our results indicated that the amendment of the pore morphology and size distribution of soil aggregates was primarily controlled by the higher contents of C and N in the density fractions of aggregates, rather than in the aggregate sizes. Furthermore, this pore network reconfiguration favored the storage of C and N simultaneously. The findings of this study offer valuable new insights into the relationships between C and N storage and the pore characteristics in soil aggregates under straw return.

Sandy soils are prone to engineering issues due to their high permeability and low cohesion in the natural environment. Therefore, eco-friendly reinforcement techniques are required for projects such as subgrade filling and soft soil foundation reinforcement to enhance their performance. This study proposes a synergistic reinforcement method that combines Enzyme-Induced Calcium Carbonate Precipitation with Glutinous rice slurry (G-EICP). The macroscopic mechanical properties and pore structure evolution of reinforced sand were systematically investigated through triaxial permeability tests, unconfined compressive strength (UCS) tests, and microstructural characterization based on Scanning Electron Microscope (SEM) and Micro- Computed Tomography (CT) tests. The results indicate that when the glutinous rice slurry volume ratio (VG) reaches 10%, the UCS of G-EICP-reinforced soil peaks at 449.2 kPa. The permeability coefficient decreases significantly with increasing relative density (Dr), VG, confining pressure (sigma 3), and seepage pressure (p). Microstructural analysis reveals that glutinous rice slurry may promote calcium carbonate crystal growth, potentially by providing nucleation sites, establishing a dual mechanism of skeleton enhancement and pore-throat clogging. The increased incorporation of glutinous rice slurry reduces the number of connected pores, lowers the coordination number, and elevates tortuosity, thereby inducing marked enhancements in both the strength and permeability of the treated soil compared to plain soil.

Three-dimensional printing (3DP) offers valuable insight into the characterization of natural rocks and the verification of theoretical models due to its high reproducibility and accurate replication of complex defects such as cracks and pores. In this study, 3DP gypsum samples with different printing directions were subjected to a series of uniaxial compression tests with in situ micro-computed tomography (micro-CT) scanning to quantitatively investigate their mechanical anisotropic properties and damage evolution characteristics. Based on the two-dimensional (2D) CT images obtained at different scanning steps, a novel void ratio variable was derived using the mean value and variance of CT intensity. Additionally, a constitutive model was formulated incorporating the proposed damage variable, utilizing the void ratio variable. The crack evolution and crack morphology of 3DP gypsum samples were obtained and analyzed using the 3D models reconstructed from the CT images. The results indicate that 3DP gypsum samples exhibit mechanical anisotropic characteristics similar to those found in naturally sedimentary rocks. The mechanical anisotropy is attributed to the bedding planes formed between adjacent layers and pillar-like structures along the printing direction formed by CaSO4$2H2O crystals of needle-like morphology. The mean gray intensity of the voids has a positive linear relationship with the threshold value, while the CT variance and void ratio have concave and convex relationships, respectively. The constitutive model can effectively match the stress-strain curves obtained from uniaxial compression experiments. This study provides comprehensive explanations of the failure modes and anisotropic mechanisms of 3DP gypsum samples, which is important for characterizing and understanding the failure mechanism and microstructural evolution of 3DP rocks when modeling natural rock behavior. (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/).

This paper investigates the effects of particle morphology (PM) and particle size distribution (PSD) on the micro-macro mechanical behaviours of granular soils through a novel X-ray micro-computed tomography (mu CT)-based discrete element method (DEM) technique. This technique contains the grain-scale property extraction by the X-ray mu CT, DEM parameter calibration by the one-to-one mapping technique, and the massive derivative DEM simulations. In total, 25 DEM samples were generated with a consideration of six PSDs and four PMs. The effects of PSD and PM on the micro-macro mechanical behaviours were carefully investigated, and the coupled effects were highlighted. It is found that (a) PM plays a significant role in the micro-macro mechanical responses of granular soils under triaxial shear; (b) the PSD uniformity can enhance the particle morphology effect in dictating the peak deviatoric stress, maximum volumetric strain, contact-based coordination number, fabric evolution, and shear band formation, while showing limited influences in the maximum dilation angle and particle-based coordination number; (c) with the same PSD uniformity and PM degree, the mean particle volume shows minimal effects on the macro-micro mechanical behaviours of granular soils as well as the particle morphology effects.

Fujian River sand (FJS) is a complex mixture of minerals and rock fragments shaped by the dynamic geological history of Fujian province, China. The macro-micro mechanical responses of FJS under triaxial shear were carefully investigated through the X-ray tomography-based in situ triaxial test. By utilising the particle tracking strategy with the signature of histograms of orientation, both intact and crushed FJS particles can be successfully recognised and tracked at different stages of axial strain. It is found that (a) smaller particles are more likely to crush than larger ones, and the crushed particles have more irregular particle shapes than the original set of particles; (b) the coordination number, fabric anisotropy, 3D rose map, and particle displacement are found to highly correlate to the phase transition point from volumetric contraction to dilation; (c) the sample deformation is found to be uniform at the early stage, and then it starts to spread from the boundaries to the inner part and finally develops into an inclined shear band; (d) locations of particle breakage within the granular assemblage show an overall sporadic and irregular pattern throughout the shearing process, which is not strongly correlated with the shear band that has developed, even at large strains.

Characterizing the effects of particle interaction and the influence of the fabric of granular materials is one of the primary challenges in studying the constitutive behavior of granular materials. The evolution of the fabric of granular materials and their response to applied stresses have been investigated extensively in the literature. Contact number is one of the most common metrics used to assess the evolution of the fabric of granular materials subjected to external loading. However, contact number is a limited metric as it incorporates only the effect of the particles in contact with a specific particle; it cannot be used to characterize the evolution of the fabric of granular materials at a mesoscale. A new metric that can incorporate the effect of particles in direct contact with a specific particle (as well as other particles within its vicinity) is much more powerful in characterizing the evolution of granular material fabric. Subgraph centrality (SC) is a complex network property that describes the change in the number of closed cycles in a network and represents a new metric for characterizing the contact network of the particles at the particle scale and mesoscale. 3D Synchrotron micro-computed tomography images (SMT) and SC were used to characterize the evolution of the fabric in five specimens, which were composed of two different types of silica sand particles subjected to axisymmetric triaxial loading. The effects of the specimens' initial density, confining pressure, kinematics of the particles, and particle morphology on the evolution of the contact network of the particles were investigated. The evolution of four node structures as one of the underlying fabric structures within the specimen was investigated to illustrate how the structure of the specimens was evolving and causing the change in the SC of the particles. Variation in the average SC of the specimens was correlated with their volumetric strain to demonstrate the relationship between the change in the contact structure of the particles and the constitutive behavior of sheared sand.