The dynamic responses of subgrade bed layers are the key factors affecting the service performance of a heavy‐haul railway. A 3D train‐track‐subgrade interaction finite element (FE) model was constructed using the ABAQUS code, where different vertical irregular track spectra were simulated by modifying the vertical node coordinates of the FE mesh of the rail. Then, the dynamic stresses in the subgrade bed layers subjected to heavy‐haul trains were studied in detail. The results showed the following: (1) the transverse distribution of the dynamic stress transformed from a bimodal pattern to a unimodal pattern with increasing depth; (2) the pass of adjacent bogies of adjacent carriages can be simplified once loaded on the subgrade since the dynamic stresses are maintained around the peak value during the pass of the adjacent bogies; (3) the dynamic stress at the bottom of the subgrade bed surface layer was more sensitive to the train axle load compared with that at the subgrade surface because the dynamic stresses induced by the two rails were gradually overlaid with increasing depth; (4) the maximum dynamic stress at the subgrade bed bottom was reduced by approximately 70% compared with that at the subgrade surface; (5) the vertical track irregularities intensified the vertical excitation between the train vehicle wheels and rails, and the maximum dynamic stress at the subgrade surface under the action of the irregular heavy‐haul track spectrum increased by 23% compared with the smooth rail condition; and (6) the possible maximum dynamic stress (σ_dm) at the subgrade surface under the action of irregular track spectra can be predicted using the triple standard deviation principle of a normally distributed random variable, i.e., σ_dm = μ + 3σ (where μ and σ are the expectation and standard deviation of σ_dm, respectively).

Subgrade is a critical component of railway systems since it provides a stable platform for the track substructure. The implementation of quality assurance methods is very important during the construction of a new railway subgrade and in monitoring the working performance of operating railways. There are several indicators available to measure the compaction and mechanical performance of a completed subgrade system, including compaction degree ( K), porosity ( n), modulus of subgrade reaction ( K₃₀), basic bearing capacity ([ ]), strain moduli ( E_v), dynamic modulus of deformation ( E_vd), and light dynamic penetration ( N₁₀). The objective of this research was to compare and evaluate the frequently used engineering methods in order to select the important and reliable indicators of field parameters. Four soil models with different degree of compaction were considered in the common laboratory tests and detection methods in China to determine the construction quality. The implementation and test results of these methods were analyzed and compared. The study results indicate that K correlates with n linearly. There is also a good correlation among K₃₀, E_v, and E_vd. As a recommendation, K is considered as the controlling indicator for compaction performance, and E_v2 and E_vd are considered as the controlling indicators for railway subgrade stiffness evaluation. The bearing capacity or strength indicators may be added to the quality detection work in heavy-haul railway construction.

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.

Critical dynamic stress (σ_cri) and accumulative plastic strain (ε_p) are primary indicators regarding the dynamic stability of unbound granular materials (UGMs). This study aims to seek an effective method to evaluate the dynamic stability of UGMs used in railway subgrades. First, the dynamic characteristics of an UGM used in railway subgrade bed construction were investigated by performing a series of large-scale cyclic triaxial tests, with the results showing that ε_p versus cycle number (N) curves can be categorized into stable, failure, and critical patterns. Grey relational analyses were then established, where the analyzed results demonstrated that the ε_p–N curve pattern and final accumulative plastic strain (ε_s) of the stable curves are strongly correlated with the moisture content (w), confining pressure (σ₃), and dynamic deviator stress (σ_d). The analyzed grey relational grades distributed in a narrow range of 0.72 to 0.81, indicating that w, σ₃, and σ_d have similar degrees of importance on determining the ε_p–N curve patterns and the values of ε_s of the UGM. Finally, a data processing method using a back-propagation (BP) neural network is introduced to analyze the test data, and an empirical approach is developed to evaluate the σ_cri (considering the effects of σ₃ and w) and ε_s (considering the effects of σ₃, w, and σ_d) of the UGM. The analyzed results illustrated that the developed method can effectively reflect the linear/non-linear relationships of σ_cri and ε_s with respect to σ₃ and/or σ_d. The σ_cri approximately increases linearly with increasing σ₃, and a simple empirical formula is proposed for the σ_cri. In addition, ε_s and its variation rate increase non-linearly with increasing σ_d but decrease non-linearly as σ₃ increases.

Theoretical solutions were derived to calculate the additional stress/prestress in a newly-developed prestressed embankment (PE), and the diffusion characteristics of the prestress in a PE with a lateral pressure plate (LPP) having width of 0.9 m were clarified using the theoretical solutions and a 3D finite element analysis. The results show that (1) the application of the theoretical solutions requires the net spacing between the LPP and the embankment shoulder is greater than the LPP width; (2) the maximum prestress appears in the upper part of the loading area of a LPP, and the maximum and minimum prestresses present an order of magnitude difference at the shallow depth, but the difference attenuates and the prestress gradually tends to be uniform with increasing depth; (3) the prestress propagates to the core zones that mainly bear the train loads with certain peak stress diffusion angles, and the values for the analyzed case are 50° and 58° in the external regions of the LPP along the slope and longitudinal directions, respectively; and (4) a continuous, effective and relatively uniform prestressing protective layer with a prestress coefficient greater than 0.1 can be formed above the core zones when the LPP spacing is properly designed.