A long-term issue that has hampered the efficient operation of heavy-haul tracks is the migration of fluidized fines from the shallow soft subgrade to the overlying ballast, i.e., mud pumping. This paper presents a series of undrained cyclic triaxial tests where realistic cyclic loading conditions were simulated at low confining pressure that is typical of shallow subgrade beneath a ballast track. Subgrade soil specimens with a low-plasticity index collected from a field site with recent history of mud pumping were tested at frequencies from 1.0 to 5.0 Hz and a cyclic stress ratio (CSR) from 0.1 to 1.0. The experimental results indicate that under adverse loading conditions of critical cyclic stress ratio (CSR_c) and frequency, there is upward migration of moisture and the finest particles towards the specimen top and this causes the uppermost part of the soil specimen to soften and fluidize. Conversely, a smaller value of CSR tends to maintain stability of the specimen despite the increasing number of loading cycles. It is noteworthy that for any given combination of CSR and frequency, the relative compaction has a significant influence on the cyclic behaviour of the soil and its potential for fluidization.

Increasing hydraulic gradients and associated seepage in a soil foundation accompanied by a reduction in effective stress, degradation of soil stiffness, and diminished internal stability contribute to adverse conditions in engineered earth structures, including dams and transport infrastructure. Although much attention has been drawn into these geotechnical challenges, most previous analytical and experimental studies could not properly capture the detailed response of fluid and soil particles, especially the localized or microscopic fluid–soil perspectives. In this regard, this paper aims to apply a numerical approach to analyze the response of a soil–fluid system under increasing hydraulic gradients. Soils with different gradation properties and porosities are created using the discrete element method (DEM), which is then coupled with computational fluid dynamics (CFD) based on Navier-Stokes equations. This numerical investigation reveals different stages in the development of hydraulic critical state, that is, from localized erosion (e.g., piping) to overall heave and fluidization. The transformation of fluid and particle characteristics, such as particle migration, the erosion rate, and hydraulic conductivity associated with porosity when soil approaches critical state, is discussed in detail. Micromechanical degradation within the contact network and the associated reduction in effective stress of soil due to an increasing hydraulic gradient are also analyzed in this study. A number of key factors that govern the soil response, such as friction, porosity, and grain uniformity, are addressed through numerical investigations. This study demonstrates acceptable numerical predictions for hydraulic behavior and erosion rates that are in good agreement with previous experimental data.

Enhancing the use of natural fibre for geotechnical purposes has attracted greater attention in the past decade, mainly because of the tangible benefits that this green approach would bring to our sustainable infrastructure developments. While this topic has been subjected to often sceptical review or discussions, they usually focus on narrow aspects such as soil reinforcement, resulting in a lack of thorough assessment over different aspects and applications. The current paper hence aims to not only provide a more balanced review between theoretical concepts and practical perspectives, but also to link different functions of natural fibre that would facilitate design effectiveness. Three major geotechnical purposes of natural fibre in terms of the practice are identified and discussed, i.e., (i) soil reinforcement; (ii) enhanced drainage for soil consolidation; and (iii) filtration, separation and erosion controls. In these distinct applications, natural fibres, despite being used in different forms such as geotextiles, drains and individual fibres, often give significant contributions to improving soil structures, resulting in greater stabilization of the entire system. The key unique feature of natural fibres is their ability to generate biological bonding with soil media (i.e., biodegradation associated with reinforcement), while substantially improving the tensile strength of the soil structure, thus providing larger resistance to mud pumping, liquefaction, internal instability and erosion. Apart from successful findings and applications in practice, main challenges that are currently hampering the wider application of natural fibres will be addressed in this paper.

Previous studies indicate that particle shape plays an important role in the hydraulic conductivity (k) of granular materials, often represented through the Kozeny–Carman (KC) concept. Several recent studies have improved the accuracy of the KC approach using the particle-size distribution (PSD) to estimate the specific surface area of particles but overly simplifying the effect of particle shape. This current study innovatively adopts the micro-computed tomography technique to compute particle shape parameters of different granular materials (e.g. glass beads, sand and crushed gravel) and then incorporate these parameters into the KC equation to estimate k more accurately, which is then validated with experimental data. The results indicate that k varies significantly according to different particle shapes even if the same mean porosity and PSD are retained. Particles that are less spherical and rounded have a larger fluid–particle contact area (i.e. larger shape factor), hence a smaller hydraulic conductivity. The study suggests a shape factor of 1·28–1·52 for natural sand and 1·84–2·1 for crushed sand and gravel can be used for KC method to estimate k while a porosity-dependent equation is proposed to estimate the tortuosity for different shaped materials.