This study introduces a novel, interdisciplinary method that merges fundamental geomechanics with computer vision to develop an advanced hybrid feature-aided Digital Volume Correlation (DVC) technique. This technique is specifically engineered to measure and compute the full-field strain distribution in fine-grained soil mixtures. A clay-sand mixture specimen composed of quartz sand particles and kaolinite was created. Its mechanical properties and deformation behaviour were then tested using a mini-triaxial apparatus, combined with micro-focus X-ray Computed Tomography (mu CT). The CT slices underwent image processing for denoising, segmentation of distinct phases, reconstruction of sand particles, and feature extraction within the soil specimen. The proposed approach incorporated a two-step particle tracking method, which initially uses particle volume and surface area features to establish a preliminary matching list for a reference particle and then use the Iterative Closest Point (ICP) method for precise target particle matching. The soil specimen's initial displacement field was then mapped onto the DVC method's grid, and further refined through subvoxel registration via a three-dimensional inverse compositional Gauss-Newton algorithm. The proposed method's effectiveness and efficiency were validated by accurately calculating the displacement and strain fields of the soil mixture sample, and comparing the results with those from a traditional DVC method. Given the soil's compositional and microstructural characteristics, these image-matching techniques can be integrated to create a versatile, efficient, and robust DVC system, suitable for a variety of soil mixture types.

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/).

Soil-fluid retention (SFR) is considered a fundamental unsaturated property with a wide application from transient flow analysis, and strength prediction to constitutive modeling. Previous research has led to a consensus that soil microstructure can significantly affect retention properties. Furthermore, the soil microstructure is subject to changes due to variations in the sampling method and pore fluid chemistry. Therefore, this study aims to explore the influence of multiple factors, including sample preparation technique, dissolved salt concentration and cation type, on soil-fluid retention and volumetric behavior in a systematic manner. The results of tests conducted by employing the axis translation and filter paper methods are interpreted in detail based on some complementary scanning electron microscopy and micro-CT scan experiments. Furthermore, the repeatability of the test results is validated through some control tests. The results reveal a higher retention capacity for reconstituted samples compared with the compacted ones independent of the solute concentration. In contrast, both the SFR and shrinkage are suppressed by the addition of sodium chloride to the pore fluid independent of the sample preparation method. It is also found that the SFR capability of potassium cation is less than that of sodium cation at the same concentration. The SFRs are quantified and compared in terms of air entry/air expulsion suction, hydraulic hysteresis and desorption/adsorption rates. The interpretation of the results in light of the diffuse double-layer theory is supported by microstructural observation and provides insights into the SFR of lean clays exposed to saline environments with different types of salt species.

Determining the evolution characteristics of cracks and shear bands is important for understanding the damage mechanisms of geotechnical materials. However, there are few experimental studies on the mesoscopic evolution characteristics of cracks in shear bands. In this study, micro -chromatography (micro -CT) is used to evaluate the triaxial test processes of granite residual soil samples under confining pressures of 50 kPa, 100 kPa, 200 kPa and 300 kPa. Then, advanced digital volume correlation (DVC) is used to obtain three-dimensional strain fields and shear bands at different test loading stages, the evolution characteristics of cracks inside and outside the shear band are further measured basis on this. The results indicate that, if the maximum gradient region of the crack distribution in the initial state of the sample can form a trend surface similar to the interface , then the shear band may be generated in this region. After the formation of the shear band, the penetration degrees of the shear bands are similar to an S curve with the increasing axial strain. The crack volume densities of the shear bands are obviously smaller than those of whole specimens. The crack volume densities on the shear bands first decrease rapidly and then increase gradually with increasing axial strain, and the increasing rate decreases with increasing confining pressure. Among the various crack types on the shear bands, the volume contents of brand-new cracks increase the fastest and exceed half, indirectly reflecting the severe deformation characteristics on the shear bands.

While the fabric of soil can significantly influence its behaviour, the effect of varying fabric parameters on the subgrade shear response is still not well understood. This study creates soil specimens with different fabrics which are then captured through X-ray microscopic-computed tomography scanning and quantified by image processing techniques. A comprehensive laboratory investigation is conducted to understand how the soil fabric affects its monotonic and cyclic shear behaviour. The results indicate that the consolidation method creates a more homogeneous fabric with mainly small-to-medium interconnected pores, whereas the compaction technique creates significantly large and mostly inter-aggregate pores with lower connectivity. In this regard, the consolidated specimens exhibit an elastic-perfectly plastic behaviour, while the compacted specimens show strain-hardening transformation during isotropic monotonic shearing. Under anisotropic conditions, the compacted specimens exhibit a greater strain softening response and excess pore pressure than the consolidated specimens because they have a weaker fabric. Furthermore, the compacted specimens show a smaller threshold strain at a lower critical number of cycles due to the collapse of large pores. These current findings prove the decisive role that soil fabric plays in determining the shear response and failure of subgrade soils.

Sandy saline soils are widely distributed and commonly experience seasonal or long-term freezing, yet the freezing process in these soils is rarely studied. This research utilized in situ X-ray computed tomography (CT) to visualize pore-scale freezing processes in sandy saline soils under various initial water and salt contents. Micron-resolution observations of pore ice and unfrozen water produced new insights into the preferential orientation and grouping of pore ice crystals, intersections between different pore ice crystal groups causing anisotropic behavior, decreasing pore ice crystal size under faster freezing rates, and the formation of interconnected networks of both pore ice and unfrozen water upon freezing. From a thermodynamic perspective, the salt content in the unfrozen liquid is dependent on the local temperature as described by water-salt phase diagram. Furthermore, the local volume ratio between unfrozen water and pore ice reflects the initial salt content and salt mass transfer occurring due to both diffusion and fluid flow processes. This work improves the understanding of complex freezing phenomena in sandy saline soils through high-resolution evidence of crystallization patterns, transformation mechanisms, and coupled heat-mass transfer.