This study examines a triaxial testing system for unsaturated subgrade fillers, utilizing a high-suction tensiometer and photogrammetry to more accurately simulate and analyze their mechanical behavior. Digital image correlation (DIC) technology is combined with non-contact photogrammetry, employing a multi-ray tracing method to reconstruct the 3D model of the sample and monitor its volume changes. Real-time matric suction is measured using a high-suction tensiometer, avoiding traditional suction control methods and enabling a more accurate reproduction of deformation and suction changes in unsaturated soil samples under natural conditions. This study further analyzes key parameters, such as specific volume change, suction change, and shear failure state, under varying moisture content and stress conditions, with parameter calibration for mechanical behavior performed using the BBM model. This system significantly reduces traditional experimental time, offering a new tool for studying the mechanical behavior of unsaturated subgrade fillers, with substantial theoretical value and practical application potential.

To determine the effects of root volume density on the mechanical behaviour of sand, drained and undrained triaxial compression tests were conducted on sand with root volume densities of 0.8%, 1.2%, 1.6%, 2.0%, and 2.4% under different confining pressures. Higher root content formed a denser and more uniform root network in the soil, enabling more roots to mobilize tensile stress, share external loads, and limit volumetric deformation. This enhanced the root-soil composite strength, reduced volumetric strain under drained conditions, and decreased excess pore water pressure under undrained conditions. The roots made a more pronounced contribution to the soil shear strength under lower confining pressures and undrained conditions. Specifically, with increasing confining pressure, the increment in the inherent soil strength far exceeded that in the additional strength provided by the roots. Under undrained conditions, the roots enhanced the soil strength by bearing part of the external loads and preventing the development of excess pore water pressure. Furthermore, the critical state line of a root-soil composite depended on the stress path. Since roots are non-granular materials and their mechanical reinforcement effect varies under different stress paths. Additionally, the roots enhanced liquefaction resistance of the sand by raising the initial effective stress required for triggering static liquefaction and the critical state effective stress. The greater the root volume density was, the stronger the liquefaction resistance of the sand.

Gassy soil is prevalent in coastal regions, and the presence of gas bubbles can significantly alter the mechanical properties of soil, potentially leading to various marine engineering geological hazards. In this study, a series of triaxial tests were conducted on fine-grained gassy soils under different consolidation pressures (pc'), stress paths, and initial pore water pressures (uw0). These tests were also used to verify the applicability of a newly proposed constitutive model. According to the test results, the response to excess pore pressure and the stress-strain relationship of fine-grained gassy soils strongly depend on the initial pore water pressure (uw0), with the degree of variation being influenced by the consolidation pressure (pc') and stress path. As uw0 decreases, the undrained shear strength (cu) of fine-grained gassy soils gradually increases, and this is lower under the reduced triaxial compression (RTC) path compared to the conventional triaxial compression (CTC) path, which can be attributed to the destruction of the pore structure due to an increase in gas volume. The newly proposed model accurately predicts the pore pressure and stress-strain relationship of fine-grained gassy soils at low consolidation pressures (pc'), but it falls short in predicting the mechanical behavior during shear progression under high pc' or the RTC path. Although the model effectively predicts the excess pore pressure and deviator stress at the shear failure point (axial strain = 15%), further improvement is still required.

This study presents some consolidated undrained triaxial compression (CU) tests of sand-low plastic silt (ML) mixtures, with ML contents of 0 %, 10 %, 20 %, 30 %, 40 %, and 50 %. The tests were performed on each mixture at three effective consolidation stresses (ECSs) of 50, 100, and 150 kPa. Triaxial testing equipment equipped with submersible local linear variable differential transformers (LVDTs) was employed to obtain accurate non-linear stiffness responses of the tested specimens over the course of the test. The testing results showed that the minimum and maximum void ratios (e min and e max ) of the specimens decreased until 20 % ML additions and then increased. Increasing the ECS of the test increased the deviatoric stress, contractive volumetric response and secant modulus (Eu) of all mixtures. Increasing the ML content at a given ECS decreased the deviatoric stress of the mixtures. The ML additions increased the excess pore water pressure (PWP) of the mixtures. The sand with low ML contents (0, 10, and 20 %) exhibited an initial contractive behaviour, followed by a dilative response. However, sand mixed with 30, 40, and 50 % ML were dominated by contractive response. The Euvalues of sand decreased with the ML additions. Consequently, these suggest that sand grains can retain their dilative nature and stability when the ML contents are low (i.e., sand-dominated soil matrix). However, when ML dominates the soil matrix, the mixtures exhibited a dominant contractive response with decreasing mean effective stress in their stress paths.

This study used triaxial tests to examine the impact of the root diameter of Cunninghamia lanceolata (Chinese fir) on the mechanical behavior of sand, including stress-strain development, strength, volumetric strain, and failure envelope. It also revealed the reinforcement mechanisms of roots with different diameters based on root-soil interactions. The results showed the following: (1) The addition of roots significantly enhanced sand strength and reduced volumetric deformation. The average peak strength increased by 31.8%, while the average peak volumetric strain decreased by 34.3%. (2) Roots provided additional cohesion and increased the friction angle of the sand, causing the failure envelope to shift upward and deflect. (3) Smaller-diameter roots improved the mechanical properties of sand more significantly, leading to higher peak strength, shear strength parameters, and smaller volumetric deformation. As the root diameter increased from 1 mm to 5 mm, the peak strength ratio decreased from 1.78 to 1.13, and the peak volumetric strain increased from 0.48 to 0.79. (4) Smaller-diameter roots, which form denser networks, allowing more roots to resist loads, and have a higher elastic modulus providing greater tensile stress, also possess higher tensile strength and critical sliding tensile stress, making them less likely to fail, thereby making the mechanical reinforcement of sand more significant.

This paper addresses the cyclic behaviour and stiffness degradation of subgrade soils subjected to stress-controlled cyclic loading, with particular emphasis on soils that are prone to mud pumping or subgrade instability. With continuous passage of trains over weak, saturated, low-plastic subgrade foundations, the finer fraction of the soils tends to fluidise (i.e., behave like a fluid) and migrate upwards, thereby, fouling the ballast and hindering the long-term performance of the rail track infrastructure. This leads to significant costs associated with annual track maintenance. Through a series of undrained cyclic triaxial testing varying the cyclic stress ratio (CSR, representing the axle loads) and loading frequency (simulating train speeds), the authors noted a significant upward migration of finer fraction coupled with internal moisture redistribution within the failed specimens. Further analysis revealed the instability of specimens was caused by early softening behaviour, and it is accompanied by a sharp reduction in the specimen stiffness. To tackle this, the stiffness was evaluated in terms of axial dynamic modulus and strain energy per cycle was evaluated to better understand the fluidisation behaviour. A novel quasi-linear relationship between threshold residual strain and number of cycles is proposed to serve as a practical guide.

The majority of the world's railways are on ballasted track, which consists of rails attached to sleepers, supported by a granular layer (ballast) lying on the natural ground. The repeated passing of trains results in a gradual deterioration of track alignment, leading to the need for periodic maintenance of the ballast. Following a number of maintenance cycles, the ballast is considered degraded, and the track is renewed. Conventional track renewal is costly and potentially unsustainable, as it requires quarrying of fresh ballast, increasing the railway's carbon footprint. A better understanding of the mechanical properties of used ballast, particularly how its stiffness compares to that of fresh ballast, could inform more extensive reuse of ballast. Previous research has demonstrated that the stiffness of granular materials is greatly influenced by their particle shape, size and surface characteristics. This project investigates the difference in stiffness in the vertical and horizontal directions between fresh and used ballast using advanced triaxial tests of 1/3rd scaled material. Fresh-scaled ballast can be readily sourced. Used scaled ballast was created by abrading fresh scaled ballast using a previously established procedure, which resulted in grain characteristics that closely mimic those of ballast recovered during track renewals after 30 years of use. The results from the advanced triaxial tests show that the used scaled ballast in this project had a greater stiffness in the vertical direction than the fresh scaled ballast. Additionally, the horizontal stiffness generally also remained higher for used ballast compared to fresh, suggesting that life-expired ballast has the potential to be reused.

Stress path is a key factor affecting the particle breakage of calcareous sand. In this study, the effects of stress path variations on calcareous sand particle breakage were investigated through triaxial compression tests across four distinct stress paths. Additionally, the gradation evolution of calcareous sand during particle breakage was analyzed. Furthermore, the correlation between the total input energy and characteristic particle size was investigated through energy dissipation analysis. The results indicated that the relative breakage index increases gradually with an increase in the maximum deviatoric stress and final volumetric strain, irrespective of the stress path. However, the dilatancy of calcareous sand is related to the relative breakage index as well as the stress path. Notably, the relationship between the relative breakage index and the total input energy can be represented using a power function. A gradation evolution model was formulated based on the results of energy dissipation analysis, and its validity was verified. The results confirmed the model's effectiveness in predicting the particle breakage evolution in both single-gradation and continuous-gradation calcareous sand specimens, accounting for the effects of the various stress paths.

Reinforcement of soils with fibers generally increases the mechanical properties of the fiber-reinforced soil (FRS) system. However, published literature is limited to investigating the undrained response of clay and synthetic fibers, with few studies targeting natural clay and natural fibers under drained conditions. There is a need to study the response of fiber-reinforced clay systems under drained conditions to assess long-term stability. This paper investigated the drained shear strength and durability of clays reinforced with natural hemp fibers using isotropically consolidated drained triaxial tests, in which the fiber content, confining pressure, and compaction water content were varied. Results showed that the incorporation of hemp fibers improved the deviatoric stress at failure by up to 60%, which increased the drained cohesion and friction angle of the FRS by 7-10 kPa and 3-7 degrees, respectively. The increase in cohesive intercept was not affected by the compaction water content, while the increase in friction angle was pronounced in specimens compacted at optimum water content (w = 18%). Durability tests showed that the improvement in strength due to hemp fibers diminishes after 3 weeks of curing prior to drained testing, indicating the dramatic negative impact of degradation of natural fibers on the mechanical performance of fiber-reinforced clay and the need for industrial treatment of the fiber.

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.