The strength, deformation, and hydraulic properties of geomaterials, which constitute embankments, vary with fine fraction content. Therefore, numerous research studies have been conducted regarding the effects of fine fraction content on the engineering properties of geomaterials. Howe ver, there have only been a few studies in which the effects of fine fraction content on the soil skeletal structure have been quantitatively evaluated and related to compaction and mechanical properties. In this study, mechanical tests were conducted on geomaterials with various fine fraction contents to evaluate their compaction and mechanical properties focusing on the soil skeletal structure and void distribution. Furthermore, an internal structural analysis of specimens using X-ray computed tomography (CT) images was conducted to interpret the results of mechanical tests. As a result, it was discovered that the uniaxial compressive strength increased with fine fraction content, and the maximum uniaxial compressive strength was observed at a low water content, not at the optimum water content. Additionally, the obtained CT images revealed that large voids, which could ser ve as weak points for maintaining strength, decreased in volume, and small voids were evenly distributed within the specimens, resulting in a more stable soil skeletal structure.

Earthen construction is one of the earliest and most ubiquitous forms of building. Compressed stabilized earth blocks (CSEBs) combine compressed components including inorganic soil, water, and a stabilizer such as Portland cement, and can achieve greater strength than other earthen construction methods. Typically, site-specific soil comprises the bulk material in CSEB construction, which minimizes the quantity of construction materials that need to be provided from off-site and motivates this type of building material for remote locations. However, onsite manufacturing and innate soil variability increase the variability of CSEB mechanical properties compared to more standardized building materials. This study characterizes the effects of varying mix compositions and initial compressions on the density, compressive strength, and variability of compressed stabilized earth cylinders (CSECs) created from sandy soil. CSEC samples comprising nine mix compositions and four levels of initial compression provide data for the (i) statistical evaluation of strength, density, and variability and (ii) development of predictive equations for density and compressive strength, with R2 values of 0.90 and 0.89, respectively.

This study investigates the effectiveness of deep soil mixing (DSM) in enhancing the strength and modulus of organic soils. The research evaluates how varying cement types, binder dosages, water-to-cement (w/c) ratios, and curing durations affect the mechanical properties of two different organic soils that were used; natural soil from the Golden Horn region of Istanbul with 12.4% organic content, and an artificial soil created from a 50/50 mixture of Kaolin clay and Leonardite, which has an acidic pH due to high organic content. The specimens were cured for four durations, ranging from seven days to one year. The testing program included mechanical testing; Unconfined Compression Tests (UCS), Ultrasonic Pulse Velocity (UPV) measurements, and chemical analyses; XRay Fluorescence (XRF) and Thermogravimetric analyses (TGA). The UCS tests indicated that higher binder dosages and extended curing durations significantly improved the strength. Higher w/c ratios resulted in decreased strength. Long curing durations resulted in strength values which were four times the 28-day strength values. This amplified effect of strength gain in longer durations was evaluated through Curing time effect index, (fc). The results were presented in terms of cement dosage effect, effect of cement type, effect of total water/cement ratio (wt/c), standard deviation values, E50 values and curing time effect index (fc) values respectively. Results of UPV tests were used to develop correlations between strength and ultrasonic pulse velocities. Quantitative evaluations were made using the results of XRF and TGA analyses and strength. Significant amount of data was produced both in terms of mechanical of chemical analyses.

This study investigated the hydraulic and mechanical behaviors of unsaturated coarse-grained railway embankment fill materials (CREFMs) using a novel unsaturated large-scale triaxial apparatus equipped with the axis translation technique (ATT). Comprehensive soil-water retention and constant-suction triaxial compression tests were conducted to evaluate the effects of initial void ratio, matric suction, and confining pressure on the properties of CREFMs. Key findings reveal a primary suction range of 0-100 kPa characterized by hysteresis, which intensifies with decreasing density. Notably, the air entry value and residual suction are influenced by void ratio, with higher void ratios leading to decreased air entry values and residual suctions, underscoring the critical role of void ratio in hydraulic behavior. Additionally, the critical state line (CSL) in the bi-logarithmic space of void ratio and mean effective stress shifts towards higher void ratios with increasing matric suction, significantly affecting dilatancy and critical states. Furthermore, the study demonstrated that the mobilized friction angle and modulus properties depend on confining pressure and matric suction. A novel modified dilatancy equation was proposed, which enhances the predictability of CREFMs' responses under variable loading, particularly at high stress ratios defined by the deviatoric stress over the mean effective stress. This research advances the understanding of CREFMs' performance, especially under fluctuating environmental conditions that alter suction levels. (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 license (http://creativecommons.org/licenses/by/4.0/).

The granular and natural characteristics of soil introduce size effects to its deformation and strength properties. Therefore, investigating the phenomenon of strain localisation in soil requires a multi-scale characterisation. This study examined the intrinsic scale patterns in samples with different sizes of reinforcing particles through triaxial compression tests. Additionally, the formation mechanism of microscopic shear bands was investigated using numerical simulation methods. Drawing from the soil cell model theory, the average strain energy release coefficient was introduced to validate the transformation of the overall strain energy of the specimen after reaching the peak stress. This reflects the progressive initiation and competitive process of multiple bands. The results indicate that samples with different sizes and types of reinforcing particles exhibit various failure patterns, including single-type, 'x'-shaped, 'v'-shaped, parallel and others. The soil exhibits size effects, with the ratio of intrinsic scale to particle size decreasing as the size of reinforcing particles increases. Prior to the stress peak, non-elastic dissipation energy begins to increase, indicating the initiation of plastic deformation in the soil. Localised strain zones are activated, and after the peak, there is a sharp increase in stress within the shear bands, accompanied by rebound outside the band.

Coastal regions often face challenges with the degradation of cementitious foundations that have endured prolonged exposure to corrosive ions and cyclic loading induced by environmental factors, such as typhoons, vehicular traffic vibrations, and the impact of waves. To address these issues, this study focused on incorporating Nano-magnesium oxide (Nano-MgO) into cemented soils to investigate its potential impact on the strength, durability, corrosion resistance, and corresponding microstructural evolution of cemented soils. Initially, unconfined compressive strength tests (UCS) were conducted on Nano-MgO-modified cemented soils subjected to different curing periods in freshwater and seawater environments. The findings revealed that the addition of 3% Nano-MgO effectively increased the compressive strength and corrosion resistance of the cemented soils. Subsequent dynamic cyclic loading tests demonstrated that Nano-modified cemented soils exhibited reduced energy loss (smaller hysteresis loop curve area) under cyclic loading, along with a significant improvement in the damping ratio and dynamic elastic modulus. Furthermore, employing an array of microscopic analyses, including nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and scanning electron microscopy (SEM), revealed that the hydration byproducts of Nano-MgO, specifically Mg(OH)2 and magnesium silicate hydrates, demonstrated effective pore space occupation and enhanced interparticle bonding. This augmentation markedly heightened the corrosion resistance and durability of the cemented soil.

The artificial ground freezing (AGF) method is a frequently-used reinforcement method for underground engineering that has a good effect on supporting and water-sealing. When employing the AGF method, the mesoscopic damage reduces the strength of the frozen sandy gravel and consequently affects the bearing capacity of the frozen curtain. However, a few studies have been conducted on the mesoscopic damage of artificial frozen sandy gravel, which differs from fine-grained soil due to its larger gravel size. Therefore, based on triaxial compression tests and CT scanning tests, this paper investigates both the mesoscopic damage mechanism and variations in artificial frozen sandy gravels. The findings indicate that there are contact pressures between gravel tips within the frozen sandy gravel, with damage primarily concentrated around these gravels during incompatible deformation within a four-phase medium consisting of ice, water, soil, and gravel. Furthermore, numerical simulation validates that failure typically initiates at delicate contact surfaces between gravel and soil particles. For instance, when the axial strain reaches 8%, the plastic strain at the location of gravel contact reaches 4.6, which significantly surpasses most of the surrounding plastic strain zones measuring around 1.3. Additionally, the maximum local stress within the soil sample is as high as 48 MPa. This failure event is distinct from viscoplastic failure observed in frozen fine-grained soil or brittle failure seen in frozen rock. The findings also indicate that the mesoscopic damage is about 0.3 when the axial strain is 10%. The study's findings can serve as a valuable guide for developing finite element models to assess damage caused by freezing in sandy gravel using AGF method.

The complex distribution characteristics of root-soil composites pose challenges in understanding their mechanical behaviour during conservation tillage. This study aims to analyse mechanical parameters of root-soil composites at different soil depths, considering root distribution, and establish an empirical critical state model. Three layers were defined based on root density distribution: Shallow Aggregated Root Zone (SARZ: 0-60 mm), Middle Enriched Root Zone (MERZ: 60-150 mm), and Deep Extended Root Zone (DERZ: 150-210 mm). Triaxial tests revealed varying shear strengths, with MERZ exhibiting the highest and SARZ the lowest. The Duncan-Chang model parameters, initial modulus of deformation, and initial Poisson's ratio were significantly influenced by soil depth, mirroring shear strength trends. An empirical formula incorporating soil layer depth into the Duncan-Chang model was proposed. Critical state stress ratios for SARZ and MERZ were determined as 0.93 and 1.11, respectively, quantifying their relationship with soil depth and root distribution. This study provides theoretical and parameter support for understanding the failure mechanism of root-soil composites.

Fault fracture zones, characterized by high weathering, low strength, and a high degree of fragmentation, are common adverse geological phenomena encountered in tunneling projects. This paper performed a series of large-scale triaxial compression tests on the cohesive soil-rock mixture (SRM) samples with dimensions of 500 mm x 1000 mm to investigate the influence of rock content P-BV (20, 40, and 60% by volume), rock orientation angle alpha, and confining pressure on their macro-mechanical properties. Furthermore, a triaxial numerical model, which takes into account P-BV and alpha, was constructed by means of PFC3D to investigate the evolution of the mechanical properties of the cohesive SRM. The results indicated that (1) the influence of the alpha is significant at high confining pressures. For the sample with an alpha of 0 degrees, shear failure was inhibited, and the rock blocks tended to break more easily, while the samples with an alpha of 30 degrees and 60 degrees exhibited fewer fragmentations. (2) P-BV significantly affected the shear behaviors of the cohesive SRM. The peak deviatoric stress of the sample with an alpha of 0 degrees was minimized at lower P-BV (60%). Based on these findings, an equation correlating shear strength and P-BV was proposed under consistent alpha and matrix strength conditions. This equation effectively predicts the shear strength of the cohesive SRM with different P-BV values.

Organic soil is usually required to be improved/treated before engineering construction, especially in cold regions due to deterioration introduced by freeze-thaw cycle. In this study, cement-and-fly ash is adopted as agents to stabilise the organic soil. A photogrammetric method is proposed to accurately reconstruct the surface of these cement-and-fly ash-treated organic soils and measure the volume before and after freeze-thaw cycles (F-T-C). Meantime, unconfined compression (U-C) test was performed to evaluate the performance of these specimens after different numbers of F-T-C, and the influence of organic content on soil behaviour was also investigated. These results indicated that an increase in the cement content enhanced the resistance of the organic soils against volume change before and after F-T-C. A proper adoption of cement-and-fly ash significantly improves the unconfined compression strength (UCS) of organic soils subjected to different numbers of F-T-C. The strength of treated organic soil continuously decreased with increasing content of organic. A model was also established to predict soil stress-strain curves with consideration of the number of F-T-C and volumetric changes after the F-T-C.