The thermo-mechanical (TM) behaviour of the energy pile (EP) group becomes more complicated in the presence of seepage, and the mechanism by which seepage impacts the EP group remains unclear.In the current work, a 2 x 2 scale model test bench of EP group was set up to investigate the TM behaviour of EP group with seepage. The test results indicate that the heat exchange performance of EP group with seepage can be significantly enhanced, but also leads to obvious differences in the temperature distribution of pile and surrounding soil along the seepage direction, and thus causes evident differences in the mechanical properties between the front pile and the back pile in pile group. Compared with the parallel connection form, the thermal performance of EP group with the series connection form is slightly attenuated. However, the mechanical properties of various piles in the EP group differ significantly. Under the action of seepage, the mechanical balance properties of various piles in the forward series form are optimal, followed by the parallel form, and the reverse series form is the least optimal. A 3-D CFD model was established to further obtain the influence of seepage and arrangement forms on EP group. The findings indicate that seepage can not only mitigate thermal interference between distinct piles but also expedite the process of heat transfer from pile-soil to reach a state of stability. Concurrently, the thermal migration effect induced by seepage will be superimposed along the seepage direction, resulting in the elevation of thermal interference of each pile along the seepage direction, and the superposition of thermal migration effect increases with the time. Under the same seepage condition, the cross arrangement can enhance the thermal performance of EP group, optimize the temperature distribution of pile and soil, and thus the imbalance of mechanical properties among pile groups can be reduced. In addition, the concepts of thermal interference coefficient and heat exchange rate per unit soil volume are introduced to facilitate a more precise evaluation of the thermal interference degree of each pile in the pile group and the heat exchange performance under different pile arrangement forms.The standard deviation and mean value in the statistical method are used to evaluate the equilibrium of mechanical properties of pile group, which is more intuitive to compare the differences in mechanical properties of pile groups under different working conditions.

The operational performance of energy pile (EP) group with seepage is strongly influenced by seepage parameters. In this paper, a model test system of 2 x 2 EP group with seepage is built to study the influences of seepage water level and seepage velocity on thermo-mechanical behaviour of EP group. Also, a numerical model of EP group considering seepage is developed to obtain the variations of thermo-mechanical behaviour of EP group under different seepage parameters. The findings demonstrate that an augmentation in seepage water level can enhance the heat exchange performance of EP group, but it also exacerbate the imbalance of mechanical properties between piles in the short term, in which the seepage only have a significant effect on the temperature of piles and soil below the seepage water level. Increasing seepage velocity and circulating flow rate can strengthen thermal performance of EP group and improve the equilibrium of pile axial force and displacement between the pile groups, but increasing seepage velocity also increases the imbalance of mechanical properties between the front and back rows of pile group. At the same time, compared to the circulating flow rate, the change in seepage velocity has a dominant impact on the thermo-mechanical characteristics of EP group. Moreover, when the seepage angle is within 0-45 degrees, increasing the seepage angle can effectively improve the heat transfer performance of EP group, and the temperature distribution of pile and soil is obviously different for different seepage angles, in which the mechanical properties of EP group have the best equilibrium when the seepage angle is 30 degrees for current simulation conditions.

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.

The influence of mechanical loading paths on the characteristics of gap-graded granular assemblies was investigated using the discrete element method (DEM). Dense and loose gap-graded assemblies with finer fraction content, f(c), ranging from 0-100% were prepared and subjected to drained triaxial compression and extension loading paths. After examining key macroscale quantities, micromechanical analyses were conducted to elicit the particle-scale characteristics including the evolution of the fabric of the assemblies under the different loading paths. The results of the DEM analysis confirm the validity of the Mohr-Coulomb failure criteria at the critical state. While the mobilised friction angle at the peak is higher under extension than in compression, no significant difference was obtained in the critical state friction angle for both loading paths. Despite the higher mean stress transmitted by the gap-graded assemblies under compression in comparison with extension, the contribution of the finer particles to the total mean stress is not significantly influenced by the loading paths. Our data show that the variation in the fabric of granular assemblies under different loading paths does not always stem from an initial inherent anisotropy. Fabric anisotropy is marginally higher under extension than in compression despite having an initial isotropic fabric.

This study investigated the influence of sample preparation methods, moist tamping and wet pluviation, on the erodibility and mechanical behaviour of gap-graded soils with three gradations: fully stable, unstable, and on the borderline of stability. Drained triaxial tests were performed using a modified erosion-triaxial apparatus, followed by micro-CT scanning to assess pore network properties. The results indicated that for fully stable and fully unstable samples, the preparation method had minimal impact on both erosion and mechanical behaviour. However, for the samples on the borderline of stability, wet pluviation method resulted in fine particle segregation, creating a heterogeneous structure with reduced pore connectivity. This led to lower erosion rates (0.4 gr/min reduction compared to the moist tamping technique), but mechanical properties remained largely unaffected, as confirmed by similar intergranular void ratios and stress-strain responses. Micro-CT scanning quantified differences in pore structure, showing that wet pluviation samples exhibit lower connected porosity compared to those prepared by moist tamping. These findings highlight the critical role of specimen preparation in assessing suffusion susceptibility and erosion behaviour, particularly for soils near the threshold of instability.

Waste generation has been a source of environmental concern in case of inadequate management. However, the potential for resource recovery from waste has been highlighted, and circular economy strategies have been greatly promoted to achieve sustainability goals. Municipal solid waste incineration bottom ash (IBA) and mine tailings represent two relevant waste streams under study for geotechnical applications. The present work aims at investigating the physical, mechanical, chemical, and ecotoxicological characteristics of two mixtures of 90 % bottom ash and 10 % of two different mine tailings (one of iron and another of tungsten, tin, and copper) to evaluate their safe utilization. The results indicated that mixtures of IBA and mine tailings have good compressibility, permeability, and shear strength properties, comparable to granular soils. Additionally, adding 10 % mine tailings in the mixtures had minimal effect on the mechanical behaviour of IBA alone. No substantial concentration of potentially toxic metals or relevant ecotoxic effects were found in any of the analysed materials and their eluates. These results suggest that mixing IBA with mine tailings for geotechnical use, e.g., in embankments or road base/subbase may be a safe option. This represents a promising alternative for valorising both waste streams while promoting sustainable and circular solutions.

The evolution of microstructure induced by loading and unloading has a significant impact on the hydro- mechanical behaviour of soils, including volume change, shear strength, water retention and permeability. In this paper, a constitutive model based on the evolution of microstructure is established building on the approach of an existing mechanistic model. In this model, the evolution of microstructure is represented via changes in the pore size distribution (PSD) and assumed to be related solely to the change of void ratio induced by loading and unloading. A PSD-dependent Bishop's effective stress coefficient chi*, which represents the coupled impact of PSD evolution on hydro-mechanical behaviour of soils, is used to replace the Bishop's effective stress coefficient chi. The model can reproduce and predict the hydro-mechanical behaviour and evolution of microstructure and their interaction within a unified framework. It also has potential in studying the soil-water characteristic curve and multi-field-coupling of soils. Model response and sensitivity analysis are reported based on idealized parameters to give a primary evaluation on the model's performance and feasibility of using PSDs from mercury intrusion porosimetry. It is found that whilst the model is sensitive to parameters representing inter-aggregate pore size distributions it can be satisfactorily applied to represent the hydro-mechanical behaviour and microstructural evolution of unsaturated soils.

To accurately predict soil thermal effects is of great importance for simulations of complex boundary value problems such as the energy foundations and nuclear waste disposal. Existing thermo-mechanical constitutive models only account for clay, and cannot simulate the sand's behaviour under thermo-mechanical conditions. In this study, a unified thermo-mechanical bounding surface (UTMBS) model is proposed for saturated clay and sand. Based on the thermal effects on the isotropic compression line, the model proposes a new unified thermal softening relationship and a plastic modulus for clay and sand, with the thermal cyclic behaviour replicated by a memory surface. The unified model considers the thermal effects on the critical state line and the shape of the bounding surface, accounting for both drained and undrained shearing of clay and sand at different temperatures. In addition, the non-linear elasticity relationship represents the hysteresis loops of the stress-strain relationship in the mechanical cycles. The performance of the proposed model is evaluated against existing experimental results for clay and sand in terms of their thermal cyclic behaviour, drained/undrained triaxial compression, and mechanical cyclic behaviour at different temperatures. It is evident that the UTMBS model is able to simulate various thermo-mechanical behaviours of clay and sand.

Polyurethane solidified ballasted track (PSBT) offers a novel solution to the frequent maintenance requirements of ballasted track, delivering a high-quality track infrastructure. In this study, laboratory tests were carried out on polyurethane solidified ballast (PSB) specimens under compression, tensile and shear conditions to obtain stress-strain curves and deformation characteristics. The numerical model of the PSB specimens was developed based on the discrete element method (DEM). The effects of the parameters related to the bond elongation on the tensile and compressive properties of the PSB specimens were analysed, and the parameters were calibrated by the test results. The results showed that the compressive, tensile and shear strengths of the specimens increased with increasing polyurethane foam density. During uniaxial and split loading, the stress-strain curves of the PSB specimens gradually entered the stress softening stage after an elastic phase. The compressive-tensile strength ratio of the specimens was around 1.55. From the perspective of deformation, the PSB specimen is primarily strained by the highly compressible polyurethane materials, and the specimen generates a considerable residual strain at the initial stage of cyclic loading. Thus, it is necessary to pre-compress the PSB to achieve an optimal load-bearing condition. DEM simulations show that there is a strong correlation between the mesoscale bond elongation of particles and the macroscopic tensile and compressive strengths of the specimens. It is therefore possible to utilise a high value of bond elongation to simulate bonded granular materials with low compressivetensile strength ratios. The results obtained from the simulation of the numerical method used in this study are in high agreement with the test, which provides a new idea for revealing the meso- and macro- mechanical properties of PSB and its application in PSBT.

The structure of unsaturated loess has a significant impact on its hydraulic and mechanical properties. An elastoplastic model considering the structured evolution of unsaturated loess is presented in this paper using double stress variables consisting of average skeleton stress and suction. The model divides the structure of unsaturated loess into inherent structure and suction-induced structure from the structured composition of loess and gives isotropic compression equation for unsaturated loess that considers the structure evolution. At the same time, a soil-water characteristic curve considering the influence of the void ratio is introduced to reflect the coupling between the hydro-mechanical behaviours of unsaturated loess. The proposed isotropic compression equation is extended to axisymmetric stress space with the aid of the yield surface and flow law in the modified Cam-clay model. The proposed model can not only reflect the structured evolution of loess, but also predict reasonably the mechanical and hydraulic behaviour of loess under different stress paths. The reasonableness and availability of the proposed model are initially verified by comparison with the results of the unsaturated loess isotropic compression tests at constant suctions, the wetting test at constant net stresses, the complex stress path tests and the triaxial shear tests as well.