This study investigated the dynamic properties of red mud (RM)-reinforced volcanic ash (VA) by dynamic triaxial tests. The effects of stress state (dynamic stress sigma d, confining stress sigma 3), dynamic frequency (f) and load waveform (F) on the accumulative plastic strain (epsilon p) have been investigated. The findings indicate a significant influence of the stress state on epsilon p. When sigma d reaches 120 kPa, the specimens exhibit insufficient strength, leading to shear failure. As sigma 3 increases, the dynamic stresses that lead to specimen destabilization also exhibit an upward trend. The effect of f on epsilon p is limited. The epsilon p does not exhibit a clear or consistent developing pattern with increasing f. As for the F, the epsilon p exhibited by the specimens subjected to sinusoidal wave loads is less than that observed under trapezoidal wave loads. Shakedown theory classifies deformation responses into plastic shakedown, plastic creep and incremental collapse. The epsilon p curve patterns of RM-reinforced VA exhibit plastic shakedown and incremental collapse without significant plastic creep characteristics under cyclic loading. A predictive model for epsilon p under cyclic loading is established, which has good predictability. This study presents a novel application of VA and RM, offering substantial research insights into waste recycling.

Salinization of road base aggregates poses a critical challenge to the performance of coastal roads, as the intrusion of chlorine salts adversely affects the stability and durability of pavement structures. To investigate the cyclic behavior of salinized road base aggregates under controlled solution concentration, c, and crystallization degree, omega, a series of unsaturated cyclic tests were conducted with a large-scale triaxial apparatus. The results showed that variations in solution concentration had a negligible influence on the resilient modulus of road base aggregates, and no significant differences were observed in their shakedown behavior. However, the long-term deformational response of the aggregates was affected by the precipitation of crystalline salt. At low crystallization degrees, a significant increase in accumulated axial strain and a decrease in resilient modulus were observed with increasing omega. Once the crystallization degree exceeded a critical threshold (omega(c)), there was a reduction in accumulated strain and an increase in resilient modulus. The precipitation of crystalline salt also disrupted the shakedown behavior of road base aggregates. During the nascent stages of crystallization (omega < 0.33), the presence of fine crystalline powders and clusters in the saltwater mixture destabilized the soil skeleton, resulting in a transition from the plastic shakedown stage to the plastic creep stage. This poses potential risks to the long-term characteristics and durability of the road base courses.

In previous train operations, traffic loads were typically considered continuous, disregarding the intermittent effects of successive trains on subgrade loess. To investigate the cumulative plastic strain behavior and critical dynamic stress of subgrade loess under intermittent train loads, a series of dynamic triaxial tests were conducted considering factors such as cyclic stress ratio, confining pressure, and frequency. The deformation characteristics of subgrade soil under different stress levels were analyzed, and the dynamic behavior of specimens was categorized based on the development trends of strain rate and cumulative plastic strain. Then the critical dynamic stress levels for plastic shakedown and plastic creep states were determined. The results indicate that intermittent effects suppress the development of cumulative plastic strain and excess pore water pressure in the soil. The more cycles of the unloading-drainage stage the soil undergoes, the stronger its resistance to failure. Under intermittent loads, cumulative plastic strain increases with higher cyclic stress ratios and frequencies. When the cyclic stress ratio is constant, the increase in confining pressure enhances soil stiffness, but this increase is insufficient to counteract the strain induced by greater dynamic stress amplitude, resulting in increased cumulative strain. Combining cumulative plastic strain and plastic strain rate, a classification standard for the deformation behavior of subgrade loess under intermittent loading conditions was established, and the critical dynamic stress was identified. The critical dynamic stress increases with higher confining pressure but decreases with frequency. Accordingly, empirical formulas for critical dynamic stress concerning confining pressure and frequency were proposed. These findings are crucial for understanding the mechanism of intermittent train load effects and analyzing subgrade settlement.

Well-graded granular materials are extensively used in the foundation layers of roads and railways. Excessive deformation developed in these layers under traffic-induced cyclic loading represents a major contributor to structural deterioration, while no viable methods are currently available for rehabilitating these layers without causing substantial disruption. In this context, biocementation holds promise as a non-disruptive solution, yet dedicated investigations have been lacking. This study, through a series of multi-stage cyclic triaxial tests, explores the feasibility and effectiveness of biocementation in improving the deformation and shakedown behaviour of a well-graded aggregate representative of typical granular materials used in road and railway foundations. The results show that both uncemented and biocemented aggregates exhibit distinct stable and unstable responses with increasing cyclic stress. Biocementation effectively enhances deformation resistance and elevates the shakedown limit in the stable regime, while it aggravates brittleness in the unstable regime. Further interpretation using the normalised stress ratio (NSR) reveals the existence of a unique critical NSR zone that separates stable and unstable regimes, independent of both cementation level and confining pressure. Microstructural characterisation elucidates multiple precipitation patterns. A cementation mechanism where small aggregate particles and CaCO3 crystals merge to form cementing bonds between large particles is postulated.

Soil modification is an effective method for enhancing the mechanical properties, including its strength, deformation capacity, and dynamic mechanical stability. Nanomaterials have broad prospects in soil modification due to their small particle size, large specific surface area, and non-toxic and harmless properties. Using the laboratory dynamic triaxial test method, this paper presents a scientific evaluation on the dynamic stability and freeze-thaw resistance of loess modified with nano-silica. This study has investigated the effects of nano-silica content, dynamic stress amplitude, confining pressures, and freeze-thaw cycles on the cumulative deformation behavior of nano-silica modified loess subjected to cyclic loading. Based on the shakedown theory, the shakedown state of 60 samples was evaluated, and an equation for the critical dynamic stress of modified loess was established under the shakedown limit state. The experimental results show that nano-silica can effectively fill the micropores in soil and form a cohesive gel that enhances the bonding between soil particles, significantly increasing the cohesion of the loess due to its nanoscale (10- 9) small size. The 2.5 % content of nano-silica is the optimal dosage for reinforcing loess. Under the same confining pressure condition, the failure strength of the 2.5 % nano-silica modified loess is about 1.4-2.1 times that of the loess, and the residual strength is about 1.2-1.5 times that of the loess. The incorporation of nano-silica significantly improves the dynamic stiffness and freeze-thaw resistance of loess, increasing the reinforcement factor by 51 %-69 % under unfrozen conditions and still increasing it by 43 %-64 % after experiencing one freeze-thaw cycle. Similarly, nano-silica significantly enhanced the dynamic strength and strength parameters of loess. Nano-silica exerts an influence on the shakedown state of the soil, wherein the impact becomes more significant with increasing dynamic stress amplitude.

The subgrade structure of high-speed railways is an important foundation for the safe and smooth operation of high-speed trains, and the scientific design of the subgrade structure provides a fundamental guarantee of its durability and technical economy. As, in the development of high-speed railways in China, higher speeds are being pursued, more requirements have been put forward for the dynamic stability of subgrade structures. To address this issue, this article focuses on the control requirements for the long-term stability of subgrade deformation, and various design methods for high-speed railway subgrade structures are presented. Considering the energy dissipation and dynamic stability characteristics of subgrade filling materials, the dynamic performance of coarse-grained soil filling materials in the bottom layer and graded crushed stones in the surface layer are revealed. The methods for determining the values of dynamic parameters such as the dynamic modulus and damping ratio are provided. Based on the dynamic shakedown theory, the stress-strain hysteresis characteristics of fillers and the variation law of dissipated energy are revealed. The correlation between unit volume dissipated energy and shakedown state under cyclic loading conditions is identified. A criterion for determining the critical shakedown state of high-speed railway subgrade structures based on equivalent unit volume dissipated energy is proposed, and a method for determining the design threshold of dynamic stress and dynamic strain is also proposed. The results show that the shakedown design critical values of equivalent unit volume dissipated energy in the bottom and surface layers of the foundation were between 0.0103 similar to 0.0133 kJ/m(3) and 0.0121 similar to 0.0149 kJ/m(3) , respectively. The critical dynamic strain range was 0.8 x 10(-3)similar to 1.3 x 10(-3). On this basis, a high-speed railway subgrade design method based on energy dissipation and dynamic shakedown characteristics was developed. The results can provide theoretical support for the design of high-speed railway subgrade structures with different filling material alternatives and control standards.

Long-term traffic loadings will induce strong vibrations in the saturated ground, and it probably produces excessive settlements of saturated ground and even various distresses (such as cracks and leakage) of the tunnel structure. To better understand the long-term cyclic deformation behaviors of saturated clay subjected to cyclic traffic loading, a series of cyclic undrained hollow cylinder apparatus tests were performed on Shanghai saturated clay. The secondary cyclic compression stage of permanent axial strain, energy dissipation, and damping ratio are employed to identify the distinct shakedown ranges of saturated clay. Moreover, attempts are made to establish a link between the permanent deformation behavior invoked by different levels of dynamic stress and a kinematic yielding framework. The cyclic test results of Shanghai clay can be classified as plastic shakedown, plastic creep, and incremental collapse, and Y-2 and Y-3 yield limits are interpreted as threshold cyclic dynamic stress to divide the shakedown ranges. Additionally, the effective cyclic dynamic stress ratio can better identify the shakedown ranges of saturated clay. Eventually, a criterion is recommended to identify distinct shakedown ranges of saturated clay. The findings will contribute to the safe design of the transport infrastructure in saturated ground.

Structured soft clay is characterised by high sensitivity and compressibility and accumulates excessive deformation under long-term dynamic loads, e.g., traffic loads, which likely threatens the service performance of overlying structures. In this work, to model the long-term mechanical behaviour of structured soft clay and efficiently capture its structural degradation, a new constitutive model was developed. The structural properties of soft clay, i.e., high yield strength and cohesive strength, were considered by a proposed yield surface, with their evolutions related to the combined plastic volumetric and deviatoric strains. The cyclic response of clay to undrained conditions was described through bounding surface theory. Moreover, the influence of the loading frequency on the dynamic response of clay was incorporated into the plastic modulus, and the softening effect caused by the generated excess pore water pressure (EPWP) was described by the shrinkable yield surface. Model validation was then carried out by reproducing both the accumulated strains and EPWPs of five types of reconstituted and structured soft clay. The acceptable consistency between the simulated results and experimental data and the independent and physical meaning of the featured model parameters confirmed the efficiency of the proposed model. More importantly, the evolution of the structural internal variables S-i and p(t)' with the development of plastic strains effectively represented the structural destruction process of soft clay under long-term cyclic loading conditions.

This study aims to explore the accumulated behavior of reinforced coarse-grained soils through cyclic triaxial tests and to develop a prediction model for the plastic shakedown limit. Cyclic triaxial test results illustrate that the reinforced specimens, especially those incorporating geocells, demonstrate the lowest accumulated axial strain and the highest plastic shakedown limit when compared to unreinforced ones under identical cyclic loading. Additionally, the accumulated axial strain at the plastic shakedown limit for reinforced specimens is determined. This strain is then used to determine the additional confining pressure exerted by geogrid or geocell, employing a function proposed by Yang and Han. By integrating the additional confining pressure into the plastic shakedown criterion for unreinforced specimens, a prediction model for the plastic shakedown limit in reinforced specimens is ultimately established. The model's applicability and the accuracy of computed additional confining pressure values are validated using experimental data.

In this paper, a study of the cyclic plastic creep phenomenon during cyclic loading was performed. The plastic creep phenomena in the field of soil cyclic loading are defined as a process that occurs under a medium stress state and leads to excessive accumulation of plastic strains. Recent literature shows the existence of plastic creep, which is a part of the shakedown theory for cohesive soils. Nevertheless, when the soil is subjected to a relatively small stress state, the axial plastic strain may occur as a phenomenon called abation, where plastic strain increment occurs in a constant decreasing manner. Cyclic triaxial tests were performed to understand the abation. The results indicate a specific phase in soil response, such as pre-failure stages marked by pore pressure increase and subsequent effective stress reduction, indicating a steady state during repeating loading. In this paper, the proposition of a cyclic plastic creep mechanism based on test results is presented. The soil response was divided into three phases based on the pore pressure, stress paths, resilient modulus, and plastic strain change analysis.