This paper presents the results from two model open-ended pipe pile installations in sand and subsequent particle-scale analysis using X-ray computed tomography. The experiments were performed in a large half-cylindrical calibration chamber with imaging capabilities. During testing, digital images were captured and later analysed using digital image correlation (DIC) to obtain displacement and strain fields in the sand domain. After installation, sand around, below and inside the pile was impregnated using an epoxy resin. The epoxy-impregnated samples collected from the calibration chamber were scanned using X-ray computed tomography and, from the reconstructed three-dimensional (3D) volumes, the sand fabric was determined. Kinematics obtained from two-dimensional (2D) DIC analyses of the experiment provide insights on the underlying mechanisms leading to changes in sand fabric due to pile installation. Results show that interparticle contact normal unit vectors tend to have vertical orientations below the pile base, but sub-horizontal orientations after entering the sand plug. This rotation of contact normal directions occurs in a region immediately below the pile base where the sand is being partially forced into the pile. Overall, sand particles tend to rearrange so that contact normal orientations match the direction of compressive principal strains near the model pile.
The mechanical behaviour of macro-porous collapsible rocks cannot be fully represented by classical stress-based criteria, such as the Hoek–Brown model, which neglects failure induced by structural collapse under isotropic confinement (zero deviatoric stress). In this study, an energy-based failure criterion and a corresponding generalised flow rule are introduced to characterise the strength and collapse behaviour of macro-porous volcanic rocks. The proposed model defines a closed failure surface that incorporates multiple stress paths, allowing the prediction of the shear strength of macro-porous rocks within a unified energy framework. The criterion was validated through a comprehensive laboratory experimental programme, including uniaxial, triaxial and isotropic compression tests performed on undisturbed pyroclastic samples from the Canary Islands, as well as a dataset comprising Italian tuffs. The results demonstrate that porosity and bonding control the isotropic collapse pressure and shear strength, and that the model provides excellent agreement with experimental data. Validation using Italian tuffs confirms its robustness and applicability. The proposed approach offers a physically consistent tool for assessing the stability of macro-porous rocks in volcanic regions.
A fundamental problem in theoretical soil mechanics, and one with an interesting history, involves determining the lateral force per unit length required to translate a rigid circular pile (or other cylindrical object) through a body of rigid–plastic cohesive soil. A dimensionless factor α is typically used to specify the shear strength of the pile–soil interface relative to that of the soil. Previous studies using the method of characteristics, or slip-line method, have succeeded in establishing an exact solution for the special case of a fully rough pile (α=1). The slip-line solution developed in this paper is shown to be exact (i.e. to give strict lower- and upper-bound collapse loads that coincide) over the full range of α, including the complementary special case of a perfectly smooth pile (α=0). The exact value of the lateral bearing capacity factor N varies from 9·20117 to 11·9400 as α varies from 0 to 1. To verify that the incomplete slip-line stress field can be extended throughout the soil in a manner that is both statically and plastically admissible, over the full range of α, traditional analytical extension methods employing slip lines and stress discontinuities are supplemented by a novel numerical stress field extension technique based on finite-element limit analysis.
Understanding the link between the evolution of fabric and elastic wave velocity in sand subjected to shear is of critical significance in engineering practice. To achieve this objective, an innovative multiscale experimental campaign was conducted by integrating triaxial compression with simultaneous wave velocity measurements and X-ray computed tomography. Discrete digital volume correlation (DDVC) was employed to measure particle kinematics and combined with contact detection algorithms to characterise contact-based fabric. The results revealed clear correlations of elastic wave propagation, fabric evolution and strain localisation development during shearing. Three distinct stages of the shear process were observed. Based on effective medium theory, the stress normalised wave velocity was found to positively correlate with the coordination number contributed in the wave propagation direction after accounting for contact normal anisotropy and void ratio, exhibiting a relatively unified relationship. This study provides experimental evidence for a strong correlation between normalised wave velocity and selected fabric descriptors, offering a potential basis for improving the calibration and validation of discrete-element simulations of sheared granular media.
Ballast degradation under high-energy impact loads, such as that caused by wheel–rail irregularities and stiffness variations in railway tracks, significantly compromises track stability and exacerbates maintenance and repair costs. This study investigates the coupled effect of rubber granules and geogrid reinforcement on enhancing ballast performance under impact loading. A series of large-scale impact tests was conducted using a custom-designed drop hammer apparatus. The results show that when rubber granules are used in combination with geogrids, the two exhibit a synergistic effect, mitigating the deformation of ballast, optimising load transmission and striking a balance between deformation control and structural stability. A coupled discrete-element–finite-difference method (coupled DEM–FDM) numerical model was developed and validated against experimental data to investigate the rubber–ballast interaction from a micromechanical perspective. The simulation results confirm that rubber granules decrease the coordination number and reduce interparticle contact forces, leading to a more homogeneous force distribution, and play a key role in energy dissipation, while geogrids restrict particle movement and mitigate structural deformation. These findings offer a novel reinforcement strategy for railway tracks, enhancing ballast longevity and track resilience under dynamic loading conditions.
