The aim of this study was to investigate the effect of roughness and relative density on the mechanical properties of sand–concrete pile interface. A series of direct shear tests were carried out on the interface using a large-scale direct shear apparatus with various relative densities of sand (73%, 47%, and 23%) and concrete blocks with four roughness values (I = 0, 10, 20, and 30 mm). Various mechanical properties (such as shear stress, volume change, peak shear strength, secant friction angle, and normalized friction coefficient) from the interface tests were compared with trends obtained from the pure sand direct shear test. For the smooth interface, the shear stress–horizontal displacement curves of the dense sand specimen exhibited a slight softening response, which became more apparent as the roughness increased. The curves of the loose sand specimen demonstrated a hardening response. The volumetric response was influenced by the combination of normal stress, relative density, and roughness. The peak shear strength demonstrated a nonlinear increasing trend as the normal stress increased. With an increase in the normal stress, the secant friction angle and peak friction coefficient decreased as exponential and power functions, respectively. Additionally, a critical roughness value I_cr resulted from the tests, which halted the upward trend of the peak friction coefficient and normalized the secant friction angle when I exceeded I_cr.

To address the track irregularity at transition zones between subgrade and rigid structures (bridge, tunnel, etc.), some common transition approaches, such as trapezoid subgrade, were adopted in many engineering areas. However, in regard to a mountainous area, the common transition approaches may not be practicable anymore due to the limitation of the length between subgrade and rigid structures. In this paper, a new type of bridge‐tunnel transition section with a deeply buried pile‐plank structure (DBPPS) for short‐distance transition is introduced. A three‐dimensional finite element model that considers vehicle‐track‐subgrade coupling vibration is proposed to study the dynamic performances of a DBPPS transition section in the Shanghai–Kunming high‐speed railway. With this model that has been validated with measured responses from field tests, the dynamic responses and the smoothness in track stiffness along the transition zone are analyzed. In addition, the influences of train speed, axle load, and train direction on dynamic responses are investigated, and the influences of two optimization strategies, including varying‐length piles and constant‐length piles, on the stiffness smoothness of the DBPPS transition section are discussed. Results show that the vibration level of the DBPPS transition section is lower than that of the abutment and the tunnel, and the additional load caused by vertical track stiffness difference aggravates the vibration at the connections between the DBPPS transition section and the abutment (or tunnel). Furthermore, the smoothness in stiffness along the transition zone can be significantly improved by the improvement strategy with varying‐length piles.

Due to the complexity of pile–soil interaction, there is little research on active–passive piles that bear the pile-top load transmitted from the superstructure and the pile shaft load caused by the lateral soil movement around the pile simultaneously. The purpose of this study is to analyze the displacement and internal force of active–passive piles. Most of the pile design codes in China use the elastic resistance method to describe the relationship between the lateral soil resistance and the horizontal displacement of the pile, but this is not accurate enough to analyze the internal force and deformation of the pile when the pile displacement is large. For this case, the passive load on the pile shaft caused by the adjacent surcharge load can be described in stages, and the p–y curve method can be used to express the relationship between the lateral soil resistance and the horizontal displacement of the pile. Additionally, taking both the active load (vertical force, horizontal force, and bending moment on the pile top) and the passive load into account, the deflection differential equation of the pile shaft is herein established, and a corresponding finite difference method program is implemented to obtain the calculations pursuant to the equation. The correctness of the analysis method and program was verified by two test cases. The results show that our calculation method can effectively judge the flow state of the soil around piles and accurately reflect the nonlinear characteristics of pile-soil interaction. Moreover, the influence depth of the pile displacement under the passive pile condition caused by the adjacent load is significantly greater than that under active pile condition, and the maximum pile-bending moment appears near the interface of soft and hard soil layer.

Modifying weathered argillaceous slate (WAS) to fill the subgrades of high-speed railways not only leads to obvious environmental benefits but also significantly saves natural resources. Previous studies have used several additives together with cement/quicklime. Using cement/quicklime as an additive would also improve the strength of weathered soft rock. Therefore, cement/quicklime can be used effectively as a sustainable solution. To illustrate the feasibility of cement/quicklime in WAS improvement, the mineral components and physical, mechanical and hydraulic characters for cement/quicklime-modified WAS were investigated via laboratory test. The dynamic behaviors were then analyzed by establishing a 3D vertical coupling dynamic model and performing dynamic triaxial tests. After mixing cement/quicklime, the following was proved: (1) The particles were coarsened, the content of kaolinite and amorphous phase increased, and the content of muscovite decreased, resulting in weakened hydrophilicity, reduced plasticity and enhanced water stability; (2) Mechanical and deformation characters were enhanced, reflecting an unconfined compression strength of quicklime/cement-modified soil greater than 0.5 MPa and elastic deformation for the subgrade of less than 0.1 mm; (3) For the quicklime-modified WAS, the dynamic strength was below the allowable value, and the sample disintegrated in water in 2 min, indicating it to be unsuitable for subgrade filling in HSR; (4) For cement-modified WAS, the water stability and dynamic strength were improved greatly, as well as the static strength. The sum of results highlights that 5% cement-modified WAS is recommended for filling subgrades of HSR for comprehensive factors.