This study presents an investigation into the mechanical behavior of geogrid-reinforced ballast subjected to cyclic loading focusing on the macro- and micromechanical features of the geogrid-ballast interaction mechanism. Key areas of interest include the effects of geogrid placement depth, aperture size, and stiffness on the motion of ballast particles, formation of contact force chains, and energy dissipation. A three-dimensional discrete element model, calibrated with experimental data, simulates ballast box tests performed on 300-mm-thick ballast layers reinforced by geogrids placed at depths ranging from 50 to 250 mm below the tie. The findings reveal that geogrids located within the upper 150 mm of the ballast layer significantly reduce tie settlement by minimizing particle movement, creating well-connected soil structures, and decreasing energy dissipation. Upon identifying 150 mm as the optimal geogrid placement depth, a parametric study evaluates the impact of the geogrid aperture size (A) and stiffness on the behavior of geogrid-reinforced ballast. The geogrid aperture size (A) is varied to give aperture size to ballast diameter (D) ratios ranging from 1.09 to 2.91, while the geogrid's stiffness ranges from 9.54 to 18.00 kN/m. Results indicate that A/D ratios greater than or equal to 1.45 are required for geogrids to perform satisfactorily, while stiffness appears to wield a negligible influence on the response of geogrid-reinforced ballast.

The construction of airport runways or other infrastructures on soft soil might risk damage and create potential hazards if inappropriate foundation treatments have been conducted. Preloading on soft soil is a commonly used ground improvement method in airport runway construction, owing to the cost efficiency and simplicity of this method. However, the theoretical basis for the design parameter estimation of this method has not been fully understood, for example, preloading height calculation and preloading time determination. In this paper, calculation models of settlement characteristics for soft soil preloading treatment and influence factors of preloading for soft soil are proposed, in accordance with Terzaghi one-dimensional consolidation theory. Preloading is counted as a dynamic process in this theory, and the settlement calculation model is expressed as an integration formula. In addition, a simplified calculation model is proposed in this paper for a specific condition of preloading treatment. Reliability of the models was verified with in situ data from an airport runway construction site in China. Results from multiple analysis reveal that preloading treatment can significantly increase the total settlement and consolidation, and accelerate the settlement rate of soft soil. Sensitivity analysis of the theoretical model found that preloading height is the key parameter affecting the preloading efficiency when ignoring the material type of soft soil. Moreover, the thickness of soft soil is the critical input parameter, as analyzing the factors affects the preloading height, and preloading treatment is a continues process rather than an instantaneous process, as can be noted from analyzing the settlement characteristics of the two loading conditions. The proposed settlement characteristic model provides valuable information for the design of preloading parameters of similar projects. Preloading is a simple and widely used method for improving the strength of soft soil; the design of preloading parameters mainly relies on the construction of a trial site or engineering judgment. The theoretical basis of this method is still weak. Thus, this paper proposes a settlement characteristic model under preloading conditions, which was verified by utilizing in situ data from an airport runway construction site. In addition, the influences of design parameters on such settlement characteristics as preloading height and thickness of backfill soil were analyzed. It was found that preloading height is the key parameter affecting the settlement characteristics of preloading treatment. In addition, sensitivity analysis was conducted on the preloading height design using an analytical model, and the results show that the thickness of soft soil is the critical factor affecting the preloading height. In that case, accurate site investigation is significant for the preloading design of soft soil.