The use of weathered phyllite waste slags generated from the excavation of cuttings and tunnels as roadbed filler material can effectively address issues related to filler scarcity, environmental protection, and cost. This study focused on weathered phyllite obtained from a highway expansion project in the Longnan Area of Gansu Province, China. Various experiments were conducted in a laboratory setting, including compaction, unconfined compressive strength (UCS), California bearing ratio (CBR), permeability, and disintegration tests, to investigate the response of mixtures with different gravel contents (GCs), ranging between 30 %-70 % by weight of weathered phyllite filler (WPF). The test results indicate the presence of a critical GC threshold. At 55 % GC, the WPF exhibits optimal compaction, the highest UCS and CBR values, and the lowest permeability and disintegration rates. Upon reaching this critical GC threshold, the phyllite gravels contact each other to form a skeletal structure, while fine grains fill the gaps within this structure to create a denser skeleton configuration. Coarse phyllite gravels are more prone to fragmentation into finer grains, which can effectively occupy large, medium, and small voids between particles. Consequently, the WPF exhibits enhanced structural density and improved mechanical and hydraulic properties. These findings provide a theoretical reference for the engineering application of phyllite in mountainous projects.

Expansive soil, a commonly distributed clay, is unsuitable for direct engineering applications. This study proposes a method to produce foam lightweight soil from expansive soil, effectively mitigating its expansive properties. The physical and mechanical properties of expansive soil-based lightweight soil (E-LS) were systematically investigated under varying water-solid ratios, wet densities, and expansive soil contents, using tests for flow value test, drying shrinkage test, pH test, and compressive strength test. An orthogonal experiment was conducted to quantify the influence of these factors on unconfined compressive strength (q(u)), leading to the development of a strength determination method. The results show that the preparation of E-LS modifies the expansive soil structure, completely eliminating its expansiveness. Compressive strength of E-LS increases with both wet density and curing age. For expansive soil contents ranging from 30% to 60%, the unconfined compressive strength at 28 days (q(u-28 d)) varied from 0.21 MPa to 1.58 MPa. Specifically, for E-LS with 50% expansive soil content, a water-to-solid ratio of 0.8, and a wet density of 900 kg/m(3), the q(u-28 d) reached 0.92 MPa, meeting the requirements for embankment construction. The factors affecting compressive strength are ranked as expansive soil content wet density water-solid ratio, and a predictive model for E-LS strength was developed. E-LS exhibits the capability to fulfill diverse embankment filling requirements in engineering applications, while demonstrating distinct advantages including expansive property mitigation, compaction-free implementation, and construction efficiency, thereby presenting significant potential for practical engineering deployment.

The accumulation and discharge amount of coal gangue are substantial, occupying significant land resources over time. Utilizing coal gangue as subgrade filler can generate notable economic and social benefits. Coal gangue coarse-grained soil (CGSF) was used to conduct a series of large-scale vibration compaction tests and large-scale triaxial tests. The results indicate that the maximum dry density of CGSF initially increases and then decreases with the increase in fractal dimension. The stress-strain curves of the samples exhibit a distinct nonlinear growth pattern. Analysis of the compaction effect suggests that the compaction degree of CGSF should not be lower than 93%. As the confining pressure increases, the extent of failure strength improvement due to increased compaction decreases. Additionally, the failure strength of samples initially increases and then decreases with the increase in coarse particle content. A modified quadratic polynomial fractal model gradation equation was proposed to describe the gradation of samples after particle breakage. Based on this, a new quantitative index for particle breakage was established. Analysis of particle breakage in samples revealed that higher confining pressure and greater coarse particle content lead to increased particle breakage. The breakage exhibited a significant size effect, and the impact of particle gradation on sample breakage was greater than that of confining pressure. The stress-strain relationship of CGSF was analyzed by using a logarithmic constitutive model, and the correlation between model parameters and the newly derived particle breakage index was generated. A constitutive model incorporating particle breakage for CGSF was established, and its accuracy was validated.

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.

Using coal gangue as a subgrade filler will produce good benefits, and its application prospects are very broad. It is of great engineering and scientific value to study the improvement method and dynamic characteristics of coal gangue subgrade filler under traffic load. Combining the properties of coal gangue material, fly ash and lime and soil were added to improve the bearing behavior of coal gangue subgrade filler. Then, a compaction test was carried out using the principle of orthogonal experimental design. By analyzing the compaction test results, the optimal proportion of each additive was obtained. A large-scale dynamic triaxial test was carried out with the proportion of each admixture in the maximum dry density group in the compaction test. Based on the dynamic triaxial test results, the effect of confining pressure on the permanent strain was analyzed, the analysis model of permanent deformation and cycle number of traffic loading was proposed, and the correctness of the model was verified. In addition, a modified Hardin-Drnevich model was established, which can describe the dynamic stress-dynamic strain curve of coal gangue subgrade filler under traffic load, and then, the dynamic modulus and damping ratio were analyzed.