This study conducted load-bearing capacity tests to quantitatively analyze the impact of permafrost degradation on the vertical load-bearing capacity of railway bridge pile foundations. Meanwhile, a prediction model vertical load-bearing capacity for pile foundations considering permafrost degradation was developed and validated through these tests. The findings indicate that the permafrost degradation significantly influences both the failure patterns of the pile foundation and the surrounding soil. With the aggravation of permafrost degradation, damage to the pile foundation and the surrounding soil becomes more pronounced. Furthermore, permafrost degradation aggravates, both the vertical ultimate bearing capacity and maximum side friction resistance of pile foundations exhibit a significant downward trend. Under unfrozen soil conditions, the vertical ultimate bearing capacity of pile foundations is reduced to 20.1 % compared to when the permafrost thickness 160 cm, while the maximum side friction resistance drops to 13.2 %. However, permafrost degradation has minimal impact on the maximum end bearing capacity of pile foundations. Nevertheless, as permafrost degradation aggravates, the proportion of the maximum end bearing capacity attributed to pile foundations increases. Moreover, the rebound rate of pile foundations decreases with decreasing permafrost thickness. Finally, the results confirm that the proposed prediction model can demonstrates a satisfactory level of accuracy in forecasting the impact of permafrost degradation on the vertical load-bearing capacity of pile foundations.

Freeze-thaw (FT) cycles significantly affect soil permeability and could cause geological and environmental disasters. This study investigated the influence of FT cycles on the permeability of compacted clay through triaxial permeability tests, considering freezing temperature, cycle number, water content, and confining pressure. Scanning electron microscopy and nuclear magnetic resonance tests were performed to analyze the microstructure and pore characteristics of the clay during FT cycles. The results show that the hydraulic conductivity of the clay decreases significantly at high confining pressures due to soil consolidation. When the confining pressure exceeds 150 kPa, the impact of FT cycles on hydraulic conductivity becomes negligible. The increased number of FT cycles, exposure to lower freezing temperatures, and higher water content lead to more pronounced soil structure damage, resulting in a substantial increase in hydraulic conductivity. FT cycles cause macropores and microcracks to form and increase the average pore radius, creating preferential seepage pathways. Correlation analysis indicates that the increase in macropore content under various FT cycles is the primary reason for the increased hydraulic conductivity. Based on the modified Kozeny-Carman equation, a prediction model is developed to effectively estimate the hydraulic conductivity. These results provide valuable insight into the damage mechanism of clay permeability in seasonally frozen regions from a microscale perspective.

Resourceful utilisation of tailings waste remains a hotspot in global research. While silica-aluminate-rich copper tailings can serve as raw materials for geopolymer preparation, their high Si/Al ratio significantly limits the geopolymerization degree. This study investigates the feasibility of developing copper tailings-based geopolymers for road base applications, using copper tailings as the primary raw material supplemented with 30 % soft soil, 15 % fly ash, and 5 % cement. The effect of NaOH content on the strength characteristics of copper tailings-based geopolymers was explored by the unconfined compressive strength test and triaxial test. The mineral composition and microstructure of copper tailings-based geopolymers specimens were characterised based on the microscopic technique. The results show that: (1) With the increase of NaOH content, the unconfined compressive strength of the copper tailings base polymer increases and then decreases, and reach the maximum value when the NaOH content is 1 %. Compared with the sample without NaOH, the addition of 1 % NaOH increased the unconfined compressive strength by 47 % at the early stage and 69 % at 28d curing age. (2) An optimal NaOH content significantly improves the shear performance of the copper tailings-based polymer, primarily by enhancing its cohesion. Triaxial test results demonstrate that 1 % NaOH addition increases cohesion by 73 % at 28d curing age. (3) The NaOH promotes the formation of geopolymer gel, refines the pore structure, and increases sample density, thereby enhancing strength. Overall, the research results can provide a reference for the application of copper tailings solid waste in roadbed materials.

In recent years, the application of green, low-carbon geopolymer cementitious materials in engineering construction has increased. However, the winter freezes and spring thaws in northwest China often result in structural deterioration. To investigate the freeze-thaw (F-T) resistance and microstructure change of the solidified soil with steel slag-fly ash geopolymer (SF-GP), a series of mechanical and microscopic tests were conducted under the condition of F-T cycle. The objective of these tests was to systematically analyze the F-T resistance and disintegration changes of geopolymer-solidified loess after undergoing F-T cycles. The results indicated that the deterioration degree of the solidified soil would increase as the number of F-T cycles increased; yet, the addition of SF-GP could effectively reduce the deterioration degree of the solidified soil. After seven F-T cycles, the unconfined compressive strength and cohesion value of samples with geopolymer additions of 0%, 10%, and 20% decreased by 23.4%, 16.5%, and 12.0%, respectively, and 51.0%, 42.6%, and 42.1%, respectively. After 15 cycles, the reductions were 34.0%, 21.7%, and 25.3%, respectively, and 83.4%, 67.3%, and 67.1%, respectively. The incorporation of SF-GP effectively reduced both the disintegration rate and the total amount of disintegration, which increased with the number of F-T cycles. The mechanical properties of the solidified soil were analyzed from a microscopic perspective and the change of physical image, allowing a deterioration prediction model between the mechanical properties of the solidified soil, number of F-T cycles, and amount of SF-GP to be established. Overall, the study findings can serve as a foundation and theoretical guideline for studies on the deterioration of geopolymer-solidified soil in cold regions as well as practical engineering applications.

Cementation, even in small amounts, tends to alter the mechanical properties of soil significantly. Ordinary Portland Cement (OPC) is a widely used binding admixture, but there has been an increasing need for replacement owing to its carbon footprint. One such alternative is Calcium Sulfoaluminate cement (CSA), which has higher initial strength gain and lower carbon footprint than OPC. Since existing strength prediction models available from literature were developed for conventional cement types such as OPC and Portland Blast Furnace Cement (PBFC), those are not applicable for predicting the strength evolution of soil treated by other types of cements (e.g., underpredicting the initial strength of CSA treated sand). It is because the prediction models available are generally either soil-specific or cement-specific. This paper proposes a unified strength prediction model that works irrespective of cement and/or soil types by introducing a slope parameter that controls time-dependent strength gain. The proposed model is validated by data collected from literature on various soils and cement types. The three-parameter model demonstrates strong applicability for predicting the strength evolution over a wide range of water-to-cement ratios.

Concrete pavements in saline soil environments of cold regions are not only subjected to vehicle loads but also severely impacted by freeze-thaw cycles (FTC) and composite salts, resulting in durability issues that shorten their designed service life. This paper induced fatigue damage in concrete based on the fatigue cycles derived from the residual strain method. It investigated the variations in the physical and mechanical properties of fatigue-damaged concrete during 100 cycles of FTC and chloride-sulfate attack, revealing the deterioration mechanisms through NMR, XRD, and SEM analysis. Utilizing the GBR algorithm, the prediction model for damage layer thickness were developed. The results showed that, due to physical crystallization, salt freeze-thaw damage, expansion of ionic attack products, and fatigue loading damage, Friedel's salt and ettringite were initially the primary products formed. Subsequently, gypsum emerged, and ultimately Friedel's salt underwent decomposition. After 10 attack cycles, the porosity and the proportion of macropores and capillary pores continued to increase, resulting in a rapid decrease in mass, dynamic elastic modulus, and flexural strength, accompanied by an increase in damage layer thickness. As fatigue damage degree increased, the pore structure degraded, thereby amplifying these changes in macroscopic properties. Incorporating basalt fibers into concrete could enhance its resistance to degradation, with the optimal dosages being 0.15 % and 0.10 %. The GBR-based model of damage layer thickness demonstrated a high degree consistency with experimental data, resulting in a correlation index of R2 = 0.989. This study lays the foundation for assessing the durability of pavement concrete in salt-freezing environments.

Pisha sandstone (PS) rapidly collapses in water and its performance deteriorates seriously, and its special engineering properties have always been the focus of researchers. In areas where fillers are scarce, it is of great significance to use PS as roadbed fillers to slow down soil erosion and green environmental protection. However, the long-term deformation characteristics after construction need further study. To reveal the long-term dynamic characteristics of Pisha sandstone fillers (PSF) under vehicle load, this study conducted the cyclic loading test of PSF by using the GDS triaxial test system. The deformation characteristics of PSF under different cyclic stress ratios (zeta) and load frequencies (f) were studied. The grey correlation analysis method was used to obtain the correlation degree of each influencing factor to the cumulative plastic strain (CPS) of the PSF. Finally, the grey GM (1,1) model is used to predict the CPS data of PSF. Based on this, the classical semi-logarithmic strain model is modified, and the CPS prediction model of PSF is established. The results reveal that the zeta and f will promote the development of axial deformation of PSF. The axial elastic deformation (epsilon(e)) and CPS of PSF increase with the increase of zeta, and the zeta has a great influence on the CPS. The influence of f on epsilon(e) is more significant at high stress levels and less significant at low stress levels. The influence of f on CPS is opposite to that of epsilon(e), that is, the influence of high stress level is small, and the influence of low stress level is large. According to the degree of correlation, the factors are sorted according to the degree of influence: static strength (sigma(f)) > confining pressure (sigma(3)) > dynamic static stress ratio (eta) > load frequency (f) > cyclic stress ratio (zeta). The GM (1,1) model has high accuracy and reliability for the quantitative description and prediction of the CPS of PSF. At the same time, according to the test and GM (1,1) model prediction results, the CPS prediction model of PSF was established. The research can provide insights and references for the establishment of cumulative deformation and prediction model of PSF under cyclic loading.

This paper presents experimental and theoretical research aimed at deepening the understanding of the lateral response of monopiles in sand subjected to cyclic loading. A series of 1-g model tests were performed for varying cyclic load and magnitude ratios, as well as for different pile stiffnesses. The broadly phenomenological behaviors of the monopile including accumulated displacement, cyclic secant stiffness, bending moment and reloading responses were captured. The results reveal the effects of cyclic load ratio, amplitude ratio and pile stiffness on the development of accumulated displacement and secant stiffness, and point out the action mechanism that the cyclic bending moment of rigid piles tends to increase while that of flexible piles tends to decrease. The elastic threshold of the reloading curve gradually increases with cycling, and increases with the increment of cyclic magnitude ratio. Crucially, a generalized model capable of describing the hysteretic characteristics of loading curves of monopiles was established, and the computational formulas for predicting the peak accumulated and residual displacements were derived. The reasonableness of the proposed method was verified under different loading parameters and pile-soil systems, which could be used for the preliminary design of offshore monopiles.

In this study, the size effect on the tensile properties of compacted clay was investigated by using deep beam specimens. The equation for calculating tensile strength considering the effect of specimen thickness was established based on the results of finite element analyses. By using deep beams, Brazilian discs, and three-point bending beams, the tensile strength of compacted clay was tested to verify the rationality of deep beam specimens. Furthermore, differences in the tensile properties of deep beams of different sizes (widths of 50, 75, 100, and 125 mm) were explored. The results showed a significant size dependence of the peak load and peak displacement. As the specimen size increased, the tensile strength of the soil exhibited a linearly decreasing trend, whereas the energy required for tensile damage gradually increased. The Ba & zcaron;ant size effect model was used to predict the strengths of compacted clays, and a peak load prediction model that considers the structural parameters of the specimens was developed.

To address the long-term settlement of embankments over structured soft soil during the in-service stage, artificial structured soils with different interparticle bonding strengths and initial void ratios were prepared, and repeated triaxial loading tests were conducted to investigate the effects of bonding strength, initial void ratio, stress amplitude and cycle number on the accumulative deformation characteristics. The results show that the relationship between the accumulative plastic strain and cycle number can be classified into stable, critical and destructive types, and an empirical relationship between the stress sensitivity and dynamic stress ratio is established. Furthermore, two different empirical models for accumulative plastic strain are presented that incorporate soil structure. Reasonable agreement between the model predictions and the experimental results for different natural soft soils demonstrate that the proposed models can accurately capture the accumulative deformation behaviour of structured soils. In addition, considering the accumulated plastic deformation of soil subjected to cyclic loading as static creep, a simplified method for calculating three-dimensional cyclic accumulative deformation is proposed by implementing the proposed model in a finite-element simulation utilizing an implicit stress integration algorithm. Finally, the effects of the dynamic stress level and structural strength on the accumulative deformation are analyzed. This has important implications in controlling the long-term settlement of embankment in soft soil area.