This research presents a simple, noninvasive, and cost effective approach where geoelectric measurements are employed to characterize gap-graded granular soils. Variations in electrical conductivity are measured using a four-point impedance analyzer and a custom-designed sample holder, which enabled geoelectric parameters to be linked to the fines content and particle shape characteristics of gap-graded soils. Experiments were conducted on sand-gravel and glass bead mixtures, representing irregular and spherical particle shapes, respectively. For each mixture, fines content was varied from 0% to 100% in 20% increments. X-ray mu CT imaging was employed to quantify the shape parameters of the constituent materials. The combination of X-ray mu CT imaging with geoelectric measurements is a particularly novel element of the experimental methodology. From the measured electrical conductivity data, three key geoelectric parameters were defined: formation factor, surface conductivity, and cementation exponent. These parameters are essential for understanding the electrical properties of the soil mixtures and were correlated to the porosity, specific surface area, and particle shape characteristics of the gap-graded mixtures. The surface conductivity was directly linked to the specific surface area of the mixture, and was able to distinguish between fines-dominated behavior and coarse-dominated behavior of gap-graded soils. The formation factor showed the expected inverse relationship with porosity, as per Archie's model, and exhibited the same trend as the relationship between porosity and fines content for gap-graded soils. Most significantly, the formation factor was able to identify the threshold fines content for both the sand-gravel and glass bead mixtures. Additionally, the cementation exponent clearly distinguished between the irregular and spherical shape characteristics of both mixtures. These results highlight the potential of geoelectric measurements to characterize gap-graded soils and the proposed preliminary classification serves as a foundation to further develop these capabilities for a wider array of soils.

The deformation of a soil mass induced by filtration is closely associated with the displacement of filter particles. To gain quantitative particle-scale insights to filter particle settlement, transparent soil imaging was employed during filtration experiments. The experiments considered a finer base layer underlying a coarser filter layer, where filtration was triggered with upward seepage flow. A range of filter-base size ratios that covered the complete spectrum of filtration behaviours were considered, along with various levels of seepage flow to investigate the combined influence of geometric characteristics and hydraulic loading. A novel particle detection algorithm was proposed to obtain geometric information of filter particles from the images acquired for the transparent soil assemblies. The filter particle settlement was quantitatively investigated with the particle detection algorithm, and was found to be positively correlated with the inlet flow velocity and the size ratio of the assembly, which was consistent with expectations. While the filter susceptibility has been conventionally evaluated by the size ratio, variability in local filter susceptibility was observed, which was correlated to the local pore geometry of the filter. Local pore geometry was quantitatively characterised by a newly proposed cumulative linear filter porosity associated with detected filter particles that were obtained from the transparent soil images. The observation of local susceptibility highlights the importance of investigating the internal erosion process at the particle scale, and the unique capability of transparent soil imaging in the quantitative investigation of particle behaviour.

In this study, we present a new integrated experimental approach to investigate simultaneously the electrical spectral induced polarization (SIP), mechanical, hydraulic and chemical properties of synthetic clayey soil mixed with different types and quantities of organic matter. It addresses knowledge gaps that aim to advance SIP as a non-destructive analysis tool for soils. We used an inorganic clay as a proxy for clayey soil with a moderate cation exchange capacity to achieve more realistic test conditions since most studies use sand. Three organic matter (OM) types with contrasting properties, biosolids, peat and sugar cane residue, broaden the range of organic carbon materials that have been tested. Our study demonstrates a strong relationship between the imaginary part of the complex conductivity and the total organic carbon content of the soil-OM mixtures. It indicates that the relationships depend on the degree of aromaticity, with the slope angle increasing as the degree of aromaticity of the OM increases. Hence, the quantity of OM, as well as its chemical structure, plays a key role in SIP response. Interestingly, these relationships are independent of soil water saturation and bulk density. These findings are of paramount importance for enabling field-scale applications and confirm the potential of SIP as a non-invasive tool for monitoring and characterizing soil in situ.

Hydrogeochemical processes contribute to long-term alterations in key physical properties of a porous medium, including porosity, tortuosity, and permeability, making it essential to understand their evolution and address clogging-dominated problems in hydrogeological systems such as acid rock drainage treatment and aquifer storage and recovery. However, accurately simulating extreme cases of evolving pore space presents challenges due to the inherent heterogeneity and nonlinear reactions in a porous medium. In response, this study introduces a comprehensive model that integrates the effects of tortuosity on permeability and surface area on reactivity during oxidative precipitation of Fe(II) in a porous medium. Benchmark simulations include an innovative permeability–tortuosity–porosity model accounting for Fe precipitation, as well as the occurrence of complete clogging from localized precipitation, which leads to a reduction of permeability and outflow. The outcomes demonstrate complete pore clogging when Fe(II) concentration reaches 10 mmol/L and a significant decrease in outflow at a Fe(II) concentration of 100 mmol/L. The model’s predictions provide detailed insights into the evolution of the pore matrix during hydrogeochemical reactions and support the development of regional engineering-scale models for applications in mining, agriculture, and environmental management.

Particle shape is a fundamental characteristic that governs the multiscale mechanical behaviors of coarse granular geotechnical materials, profoundly influencing properties such as strength, stability, deformation, and particle breakage. However, there is a notable lack of comprehensive reviews that explore the multiscale effects of particle shape, spanning micro to macro levels, on the breakage behavior of geotechnical materials. This study aims to bridge this gap by systematically analyzing macroscopic experimental data and microscopic observations, complemented by advanced numerical simulations and data analysis techniques. Based upon the analysis and synthesis of data, it is found that particle shape plays a critical role not only in macroscopic material properties such as strength, stability, and deformation behavior, but also in the mechanical interactions between particles at the microscopic level. Irregularly shaped particles generate localized, concentrated forces at contact points, especially at fracture keys, leading to complex fracture processes and diverse fragmentation mechanisms. These particles influence force chain networks by enhancing structural anisotropy and amplifying local stress fluctuations, which affects the stability and crack formation within the material. Conversely, spherical particles exhibit more uniform stress distribution, resulting in higher compressive strength and improved material stability. The study further highlights that the fragmentation patterns of irregular particles are more complex, with breakage modes ranging from splitting to explosive failure, resulting in a broader range of fragment sizes. Although a deeper understanding of the effects of particle shape has been achieved, further refinement is needed in developing shape-sensitive constitutive models and simulating multi-physics coupling (e.g., hydro-mechanical). This study explores future research possibilities and emphasizes the importance of factoring particle shape into the construction and adaptation of geotechnical materials.

In this paper, we present a novel approach to study the electrical properties of intact rock by combining impedance and vector network analyzer measurements in the frequency range from 5 Hz to 3 GHz. For the first time, this study enables electrical characterization of the exact same rock sample over nine orders of magnitude in frequency range, utilizing a coaxial cell apparatus with specifically prepared rock samples. Three types of rocks (basalt, sandstone and granite) with low to intermediate porosity (12.24%, 16.9% and 7.49%, respectively) were characterized at varying saturation levels. The relaxation behavior of the samples was quantified using an advanced phenomenological model incorporating both the dielectric constant and electrical conductivity in the form of Debye and Cole-Cole representations. Across the frequency range, three distinct relaxation processes were identified: a high-frequency water process and two interfacial processes related to interactions between the aqueous pore solution and mineral particles (membrane relaxation and Maxwell-Wagner effects). Additionally, an apparent direct current conductivity was observed. This comprehensive broadband analysis represents a further step toward the in situ characterization of rocks using electromagnetic field measurement methods and demonstrates advancements in both methodology and understanding of rock properties compared to existing studies.

In the mixing zone, where submarine groundwater carrying ferrous iron [Fe(II)] meets seawater with dissolved oxygen (DO), the oxidative precipitation of Fe(II) occurs at the pore scale (nm~μm), and the resulting Fe precipitation significantly influences the seepage properties at the Darcy scale (cm~m). Previous studies have presented a challenge in upscaling fluid dynamics from a small scale to a large scale, thereby constraining our understanding of the spatiotemporal variations in flow paths as porous media evolve. To address this limitation, this study simulated subsurface mixing by injecting Fe(II)-rich freshwater into a DO-rich saltwater flow within a custom-designed syringe packed with glass beads. Micro-computed tomography imaging at the representative elementary volume scale was utilized to track the development of Fe precipitates over time and space. Experimental observations revealed three distinct stages of Fe hydroxides and their effects on the flow dynamics. Initially, hydrous Fe precipitates were characterized by a low density and exhibited mobility, allowing temporarily clogged pathways to intermittently reopen. As precipitation progressed, the Fe precipitates accumulated, forming interparticle bonding structures that redirected the flow to bypass clogged pores and facilitated precipitate flushing near the syringe wall. In the final stage, a notable reduction in the macroscopic capillary number from 3.0 to 0.05 indicated a transition from a viscous- to capillary-dominated flow, which led to the construction of ramified, tortuous flow channels. This study highlights the critical role of high-resolution imaging techniques in bridging the gap between pore-scale and continuum-scale analyses of multiphase flows in hydrogeochemical processes, offering valuable insights into the complex groundwater–seawater mixing.

In slurry shield tunnelling, the penetration of the supporting bentonite suspension must be reduced to a critical value to ensure safety and cost-efficiency during construction. Aiming to measure bentonite suspension penetration, this study adopted the spatial time domain reflectometry (spatial TDR) technique. Although traditional TDR can detect point-wise changes in bentonite suspension concentration of pore fluid, this technique has rarely been extended to spatial profile detection. Spatioscale tests with a flat ribbon cable TDR sensor demonstrated the potential of TDR waveform analysis for determining penetration depth. Relationships between penetration depth and waveform characteristics were established. The travel time specified by the dual tangents method decreased with increasing slurry penetration, and the determined travel time agrees well with that calculated by a newly proposed mixture equation. This novel approach enables the determination of penetration depth without visual observation, providing a powerful measuring solution for laboratory studies and slurry shield tunnelling.

The porosity of fluidised beds subjected to a range of inlet flow velocity at low Reynolds numbers was investigated experimentally and numerically. A permeameter was employed for the fluidisation experiments with packings of spherical particles subjected to upward flow, and the fluidisation process was acquired with a charge-coupled device camera. Internal cross-sections of the sphere packings were observed, which was allowed by matching the refractive indices between the liquid and solid phase to attain a transparent particle-fluid combination. The fluidised bed porosity at the steady state was estimated from the thickness of the expanded assemblies from acquired images. Numerical simulations were conducted using coupled Computational Fluid Dynamics and Discrete Element Method with equivalent hydraulic and geometric conditions. The simulation configuration was rigorously tested to optimise numerical stability and capture detailed fluidisation behaviours, and different drag models were used to assess the potential differences. The temporally- and spatially-averaged fluidised bed porosity was obtained from the numerical output, and was compared with the experimental results, as well as analytical solutions. Good agreement was observed in the variation in the fluidised bed porosity with increasing inlet flow velocity at the steady-state condition, which provides confidence in the simulation results of particulate fluidisation, which is associated with internal erosion.

Although many studies have examined reaction zones in groundwater–seawater mixing areas, little attention has been given to how subsurface processes drive changes in iron (Fe) precipitation over time and space. This gap has limited our understanding of the “iron curtain” phenomenon in coastal aquifers. To address this, this study developed a reactive transport model to investigate how porosity evolves during the oxidative precipitation of Fe(II) in porous media. The model incorporates the dynamic effects of tortuosity, diffusivity, and surface area as minerals accumulate. Validation experiments, conducted with syringe tests that simulated Fe precipitation during freshwater–saltwater mixing, showed that precipitates formed mainly near the inlets, reflecting the development of a geochemical barrier at the groundwater–seawater interface. Scanning electron microscopy confirmed that Fe precipitates coated the surfaces of spherical particles. Numerical simulations further revealed that high Fe(II) concentrations drove pore clogging near the inlet, creating a dense precipitation zone akin to the iron curtain in coastal aquifers. At 10 mmol/L Fe(II), local clogging was observed, while at 100 mmol/L Fe(II), outflow rates (i.e., discharge) were substantially reduced. Together, the experiments and simulations highlight how hydrogeochemical processes influence hydraulic properties during the oxidative precipitation of Fe(II) in mixing zones.

Conventional constitutive models encounter challenges in comprehensively capturing the nonlinear behavior and particle breakage effects of rockfill materials subjected to large deformation and multiaxial loading conditions. To address this issue, this study proposes a constitutive model based on machine learning (ML) that considers particle breakage in rockfill materials. By learning the underlying patterns from a large dataset of experimental data, this model can effectively reconstruct the nonlinear and high-dimensional characteristics of the material while also accounting for its loading history and stress path dependence. The model shows outstanding predictive performance on the test set, with relative prediction errors confined to within +/- 5 %. Utilizing this model, the macro-and micro-mechanical responses of rockfill materials are systematically investigated, revealing the mechanisms and quantitative relationships between particle gradation, intermediate principal stress coefficient, and ellipsoidal axis ratio on particle breakage behavior. Additionally, the ML model exhibits robust interpolation within its training range (mean absolute percentage error, MAPE < 3.5 %), yet its extrapolation performance varies outside this scope, maintaining high accuracy for unbreakable particles and cyclic loading (MAPE < 2.7 %) but declining for extreme shapes (e.g., MAPE > 10.0 % for breakage rate), highlighting data dependency as a key limitation. This study brings fresh insights and establishes a novel theoretical framework for understanding the mechanical behavior of rockfill materials while also highlighting potential avenues for future model optimization.

Geotextiles find wide applications in the field for filtration and drainage. When applied on the soil surface they influence soil evaporation. The objective of this work is twofold: (a) to assess the effectiveness of four different geotextiles as cover materials on soil evaporation, (b) to study the combined effect of geotextile and perforated mechanical barriers on soil evaporation. The first set of experimental programs consisted of three soil samples i.e. kaolin, dredged mud from the port of Brisbane and a locally obtained red mud sample from Queensland, Australia tested with four types of non-woven geotextiles under four controlled climatic conditions. All the 4 geotextiles had suppression effects on soil evaporation to degrees that varied with the type of soil, ratio of pore size to thickness of geotextiles ( M *), product of pore size to thickness of the geotextiles ( N *) and climatic conditions. Geotextiles with a higher pore size ( O 95 ) and M * allowed water vapor to move through relatively easily leading to higher evaporation rates. Geotextile with a higher thickness and N * value provided a higher suppression effect on soil evaporation. In a recently introduced dewatering method involving perforated ventilated well method, evaporation from soil take place through geotextiles and the perforated well. Mimicking this, impacts on soil evaporation with geotextiles sandwiched between soil sample and perforated sections were also studied. Maintaining similar number and arrangement of the perforations, soil evaporation was noted to be higher with rectangular shaped perforation compared to circular shaped perforations.

The frequency dependence of dielectric material properties of water saturated and unsaturated porous materials such as soil is not only disturbing in applications with high frequency electromagnetic (HF-EM) techniques but also contains valuable information of the material due to strong contributions by interactions between the aqueous pore solution and mineral phases. Hence, broadband HF-EM sensor techniques enable the estimation of soil physico-chemical parameters such as water content, texture, mineralogy, cation exchange capacity and matric potential. In this context, a multivariate (MV) machine learning approach (principal component regression, partial least squares regression, artificial neural networks) was applied to estimate the Soil Water Characteristic Curve (SWCC) from experimentally determined dielectric relaxation spectra of a silty clay soil. The results of the MV-approach were compared with results obtained from empirical equations and theoretical models as well as a novel hydraulic/electromagnetic coupling approach. The applied MV-approach gives evidence, (i) of a physical relationship between soil dielectric relaxation behavior and soil water characteristics as an important hydraulic material property and (ii) the applicability of appropriate sensor techniques for the estimation of physico-chemical parameters of porous media from broadband measured dielectric spectra.

The magnitude of swelling pressures must be accurately estimated/predicted using appropriate methods for a safe design and performance of pertinent geotechnical infrastructure on expansive soils. This paper reviews various methods used by researchers in the past decades to quantify swelling pressure in different directions. Methods reported in literature for the determination of swelling pressure can be categorized into experimental, analytical, and numerical studies. Laboratory methods are affected by scaling and boundary effects, whereas fields tests are time consuming and expensive; however, numerical models with appropriate validation can predict swelling pressures efficiently. Various factors that affect swelling pressures are clay mineralogy, dry density, initial water content, boundary conditions, and matric suction. In the present critical review, it is observed that matric suction, a significant factor affecting the swelling behavior of unsaturated expansive soil, was considered in only a few studies.

Mud pumping in railways occurs when finer particles in the subgrade soils infiltrate through the overlying coarser ballast layer. The finer particle migration can lead to local settlement and cavities in the subgrade soils and consequently causes poor track performance. The micro-scale mechanisms of finer particles infiltrating into a coarser layer are investigated in this study using coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM). The early stage of filtration was simulated at the micro-scale with a simplified system where assemblies comprising a base layer of finer particles underlying a filter layer of coarser particles, and an upward seepage flow perpendicular to the interface was employed to trigger base infiltration. Various filter-base size ratios along with different flow conditions were considered to investigate the influence of geometric and hydraulic conditions on infiltration. The simulation results of the fluid behaviours were validated by comparing the numerical, empirical and experimental results of hydraulic conductivity. The hydraulic conductivity of the infiltrated zone was also investigated, which highlighted the capability of CFD-DEM in investigating the hydraulic conductivity change induced by filtration.

The geometric properties of the pore space govern the electrical and hydraulic properties of soils. Recognizing this commonality, this paper proposes a new approach to determine permeability from high-frequency electromagnetic (HF-EM) measurements on geomaterials undergoing evaporative dewatering. Two materials have been used; (1) kaolin, a reference geomaterial; and (2) a dredged sediment sample from the Port of Brisbane (PoB) reclamation site (Queensland, Australia). Based on a dielectric mixture equation that integrates Archie's parameters, the evolution of formation factors as a functions of porosity have been derived. Several models that are available in the literature have been used to estimate proxies for geometric length scales for pore radii. With knowledge of the formation factors and proxies for pore radii as functions of porosity, permeabilities of the materials under saturated conditions have been estimated. The results have been compared with experimentally obtained permeabilities. Using the formation factors from HF-EM measurements and two of the proxies for pore radii (hydraulic and characteristic pore radii, as defined in this paper), a good degree of agreement with the experimental data has been obtained within a variation in one-order of magnitude for kaolin. For the PoB material, the use of hydraulic pore radius yielded the best estimation of permeability.

This paper builds on exploring the applications of biomediated pathways to solve geotechnical challenges. First, the state of the art of biological remediation strategies including microbial remediation and phytoremediation have been introduced and critically reviewed in the context of decontaminating the soils. Next, biopolymerisation, biomineralisation and bioneutralisation processes have been depicted with a special emphasis on the applications including but not limited to soil stabilisation, soil erosion prevention, anti-desertification and pH neutralisation. Each of these methods have their own limitations and bottlenecks while scaling up, and these challenges have been summarised and some possible paths to overcome the challenges have also been discussed. The state of the art of electromagnetic (EM) monitoring methods to capture the effects of biomediation on spatio-temporal soil properties are then highlighted as a non-invasive and rapid pathway to track the progress of biomediated soil processes. Finally, each of the technologies discussed have been evaluated for their maturity level using the principles of technology readiness level (TRL). A majority of the technologies amounting to around 77% are still in the TRL 4–7, i.e. in the valley of death. It is thus evident that development of these technologies needs to be supported with appropriate funding for improving their maturity to a level of industrial deployment.

This paper proposes a generalized framework to estimate the seismic active earth thrust on rigid retaining walls with unsaturated backfill. A novel suction stress-dependent approach was developed to compute the horizontal and vertical seismic acceleration profiles within the backfill. The framework applies the limit equilibrium method with a log-spiral failure surface and considers the influence of suction-stress through the effective stress approach. A detailed validation of the proposed framework is conducted by comparing it with well-established solutions reported in previous studies showing a satisfactory agreement. Validation was performed for the cases of static loading with suction and seismic loading without suction because of the unavailability of the data for the combined case. The influence of suction on the seismic active earth thrust for different soil types and environmental conditions is presented together with other parameters including wall height, soil friction angle, frequency of input excitation, and horizontal and vertical seismic acceleration coefficients.