The high-speed railway (HSR) subgrade has a strict settlement-control standard at the mm level, but its deformation stability is significantly threatened on permafrost with poor thermal stability and in susceptible-to-thawing settlements. Therefore, the filler suitable for permafrost regions needs to be explored and determined. In this study, the frost heaves, permeabilities and static strength characteristics of three coarse fillers were experimentally investigated, and the optimal subgrade filler was determined for the certain HSR, the first HSR in permafrost regions around the world. The test fillers include pure fillers, 5% cement improved fillers and 5% cement + 3% modifier improved fillers, and the effects of curing time, modifier content and freeze–thaw cycles were analyzed. The test results show that: (1) the frost heave rate and permeability coefficient decrease with the increase of curing time and modifier content, while increasing with the freeze-thaw cycles; (2) After six freeze–thaw cycles, the cement + modifier improved filler’s frost heave rate and permeability coefficient are 0.51 and 0.00331 cm/s, a larger decrease in the frost heave rate (more than 50%) and the permeability coefficient (about one order of magnitude) than that of pure filler; (3) The cement + modifier improved filler shares the highest compressive strength under different curing times and freeze-thaw cycles. In summary, the modifier has a more significant influence on the engineering characteristics than the curing time or freeze-thaw cycles, and the cement + modifier improved filler has the best comprehensive performance. This study will provide a technical reference for the foundation-treatment and disease-prevention of HSRs in island permafrost regions.

A non-proportional reduction in strength parameters is widely used in slope stability assessment, but the current asynchronous reduction in strength parameters only considers the cohesion c and internal friction angle φ, which is suitable for slope stability assessment under static loads. Under seismic loads, however, tension at the rear edge of the slope often accompanies the appearance of ground cracks. In order to consider the relationship between tensile strength, cohesion, and the internal friction angle reduction coefficient, starting with the linear softening attenuation law of soil material strength parameters, a functional relationship between cohesion and internal friction angle is obtained. Then, considering that the failure of microelements in the tensile and shear zones conforms to the tension and shear of joint failure, the relationship between tensile strength, cohesion, and the internal friction angle reduction coefficient is derived. By establishing a homogeneous slope model and comparing and analyzing the progressive instability failure modes of slopes under static and seismic conditions, the stability and potential slip surface differences of slopes under two different working conditions are explored. The research results indicate that slope instability is a gradual, cumulative failure process under both static and dynamic conditions. The instability mode of the slope under static conditions is shear failure. In contrast, under dynamic loads, the instability failure of the slope is manifested as shear failure upward at the foot of the slope and tensile failure downward at the top of the slope. The stability coefficient of slopes under earthquake conditions is reduced by 17.3% compared to that under static conditions. Under earthquake conditions, the potential sliding surface under an asynchronous reduction in strength parameters is shallower than that under static conditions and deeper than that without an asynchronous reduction in strength parameters. Overall, the research results provide a reference for slope stability analysis and support design optimization under earthquake loads.

With the gradual densification of China’s railway network, more and more railways are inevitably crossing active fault zones. Creep dislocation and stick–slip dislocation of the fault zones can lead to uneven deformation and cracks in the railway subgrade, threatening the sustainability of the railway. Therefore, a model test and numerical simulation were used to analyze the deformation mechanism and sustainability of the subgrade and foundation induced by a strike–slip fault dislocation in a flat site under different angles between the subgrade and fault strikes. The test and simulation results show that (1) under the influence of strike–slip fault dislocation, the horizontal direction of the subgrade appeared to have an S–shaped displacement. The top surface of the subgrade appeared to have a vertical settlement. Further, tension–shear cracks and compression crush zones appeared on the subgrade’s surface. (2) When the angle between the fault and subgrade strikes increased, the subgrade’s uneven deformations induced by fault dislocation first decreased and then increased. The angle between the main tension–shear crack on the top surface of the subgrade and the fault strike showed a trend of increasing and then decreasing. In addition, the compressive stress along the subgrade strike decreased, while the tensile stress along the subgrade strike gradually increased. (3) The tension–shear cracks in the subgrade and foundation were spiral and converged to the basement strike–slip fault along the depth direction. This study provides a reference for the safe operation and sustainability of railways near faults.

Part of the Hang-Tai High-speed Railway (Hangzhou via Shaoxing to Taizhou in Zhejiang Provence) in China passes through the diatomaceous earth area, which is the first time in the history of Chinese high-speed railway construction. This type of soil has significant compressibility, swelling and disintegration. Diatomaceous earth also shows a sharp reduction in strength when exposed to water, which severely impacts the safety of the project. However, no studies have been carried out on the engineering practice of building a high-speed railway in the diatomaceous earth area in China. Moreover, there is limited experience in the construction of ballastless track through the diatomaceous earth area. In order to ensure the stability of the high-speed railway subgrade in diatomaceous earth area, and considering the high level of precipitation in the location of this railway, a kind of waterproof and drainage subgrade (WDS) is proposed to reduce the influence of precipitation on the strength of the diatomaceous earth foundation. The subgrade has a flexible waterproof and drainage layer (WDL) inside, which consists of capillary waterproof and drainage plates and medium-coarse sand. In the present study, field tests including immersion tests and excitation test are carried out on a subgrade test section to verify the subgrade structure. The tests mainly focus on construction technology, waterproof performance and dynamic characteristics. The studies show that the subgrade bed with the WDL can effectively avoid the diatomaceous earth foundation from rainfall interference and maintain the long-term stability of the subgrade. The flexible WDL in WDS has a significant energy dissipation effect in comparison with the traditional subgrade (TS) filler and can play a key role in vibration damping, promoting the attenuation of dynamic response in the downward and cross-sectional directions within the subgrade. The dynamic response of the WDS attenuates along the depth. In comparison with the existing high-speed railway subgrade measured data, its dynamic response attenuation coefficient is within acceptable limits. The laying of the WDL does not change the subgrade dynamic characteristics transfer law. The proposed structure meets the requirements of ballastless track construction for high-speed railways, and the WDL can be used in the design of high-speed railways for enhanced drainage protection in diatomaceous soil areas or other special soil areas.

The smoothness of high-speed railway is extremely strict. As an important part of the high-speed railway foundation, the subgrade structure has strict deformation requirements. It is necessary to ensure that the subgrade structure does not have plastic cumulative deformation under the long-term cyclic load of the train, which is the most critical geodynamic problem in high-speed railway subgrade. This paper studied the cumulative deformation characteristics of subgrade filling under cyclic train loading through the cyclic triaxial test, three types of accumulative plastic deformation trends, and the value of critical volume shear strain was discussed, which would provide the theoretical support for the dynamic design of high-speed railway.