Ahmed S. El-Saei

Railway ballast maintenance commonly combines tamping with subsequent dynamic track stabilization (DTS), yet the in-motion dynamic mechanical response of ballast during these operations remains difficult to quantify. This study develops a three-dimensional discrete element method (DEM) framework to simulate a complete tamping and DTS maintenance sequence, characterize the multiscale mechanical response of ballast across spatial zones, and extract strain-dependent shear modulus and damping directly during stabilizer motion. The model is verified against angle-of-repose behavior, lateral resistance before and after tamping, and dynamic excitation responses over a wide range of loading amplitudes. Independent excitation simulations are further used to establish reference modulus-reduction and damping curves for interpreting the DTS-derived response. The results show that tamping enhances support beneath the tie but severely loosens the crib region, producing a mechanically heterogeneous post-tamping ballast state. Subsequent DTS acts as a spatially selective recovery mechanism, with the greatest improvement occurring in the zones most disturbed during tamping. During DTS, all ballast zones enter the nonlinear strain regime, with mobilized cyclic shear strains ranging from approximately to . The DTS-derived responses remain consistent with the excitation-based reference curve, indicating strain-dependent stiffness degradation comparable to that observed under controlled excitation. Stiffness degradation and damping mobilization are strongly zone dependent and governed by local confinement and contact-network evolution. The proposed framework provides a particle-scale computational basis for evaluating ballast disturbance during tamping, interpreting in-motion stiffness degradation and damping mobilization during DTS, and supporting future stabilization parameter optimization.

Large-diameter bored piles can safely transmit loads from structures by skin friction to the surrounding soil strata and end bearing at the bedrock layer, thereby providing a high compressive capacity. High-Strain Dynamic Testing (HSDT) provides a unique alternative technique to traditional Static Load Testing (SLT) for determining the static compressive resistance of the bored piles, considering its quicker performance and significant cost reductions. This article’s main objective is to numerically explore the performance of large-diameter bored piles during the HSDT and to understand their dynamic behavior under an axial compressive impact force. This research is based on testing pile foundations for reinforced concrete mixed-use towers in the coastal zone of New Alamein City, Egypt. The tested pile is a 1.20 m diameter bored pile. Numerical modeling is performed to simulate both the HSDT and the SLT for two piles at the same site. Non-linear axisymmetric finite element modeling is employed to validate both test records and develop some sort of matching between the two tests. As lumped models, the developed numerical models use the signal-matching process, which is conducted by varying and adopting the strength parameters and deformation characteristics of the ground or soil deposit and the soil–pile interface. The predicted load-displacement curves, developed from analyzing dynamic records employing the Modified Unloading Point (MUP) method, are consistent with the field records. The verified non-linear models are utilized to accomplish a comparative parametric analysis to better understand the drop-mass system aspects. The analysis results emphasize the significance of employing adequate impact energy (i.e., dropping height and mass) to move the pile top to a sufficient extent to mobilize its full resistance. However, a longer impact duration, i.e., larger mass, is more effective for achieving a deeper high-strain wave. The impact load should be developed by a larger drop mass with a lower drop height, not a smaller drop mass with a higher drop height. The results also indicate that, for relatively longer piles, the skin friction of the upper layers surrounding the pile shaft is fully mobilized, whereas the skin resistance of the lower layers is not fully mobilized, regarding the stress wave phenomenon effect. Finally, this study’s findings can be employed to develop guidelines and design procedures for the HSDT to be effectively performed on bored piles.