Introduction
Laboratory Tests
Large-Scale Direct Shear Tests
Large-Scale Impact Tests
Modeling of a Geogrid and Ballast in DEM
Determination of Micromechanical Parameters
Parameters | Geogrid | Ballast |
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Particle density (kg/m3) Coefficient of friction Contact normal stiffness, kn (N/m) Contact shear stiffness, ks (N/m) Contact normal stiffness of wall-particle, kn-wall (N/m) Shear stiffness of wall of wall-particle, ks-wall (N/m) Parallel bond radius multiplier, rp Parallel bond normal stiffness, knp (kPa/m) Parallel bond shear stiffness, ksp (kPa/m) Parallel bond normal strength, σnp (MPa) Parallel bond shear strength, σsp (MPa) | 850 0.45 4.68 × 106 2.34 × 106 1 × 107 1 × 107 0.5 6.42 × 107 3.21 × 107 386 343 | 2500 0.85 6.84 × 107 3.42 × 107 1 × 108 1 × 108 |
Results and Discussion
Shear Stress–Strain Responses
Measured Stress Distributions
Contact Force Distribution
Coordination Number
Orientation of Contacts
Conclusions
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The inclusion of geogrids significantly increased the shear strength and decreased the dilation of ballast under direct shear loading conditions. This was due to improved interlocking between the ballast and geogrid, resulting in a higher peak shear stress and a reduced particle displacement.
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DEM simulations proved that the geogrid could impart a higher value of Ncontact as compared to the unreinforced one. This results in a reduced maximum contact force, Fmax (with geogrid: Fmax = 542, 562, and 522 N with inclusion of BG1, BG2, and TG3). In contrast, without the inclusion of a geogrid, the maximum contact force was up to Fmax = 754 N. This finding implies that the applied forces are not solely transmitted through the large aggregate skeleton; they also transmitted to the geogrid and subsequently mitigate the impact of large contact forces, and thus effectively reducing track deformation.
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The inclusion of geogrid resulted in increased coordination number (Cn). Indeed, the unreinforced ballast has the lowest values of Cn varying from 3.5 to 6.2; the TG3-reinforced ballast assembly shows the highest values of coordination number (Cn = 5.6–9.4). The increase in Cn is reflected by the interlocking, facilitating the transmission of forces throughout the assembly.
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The analysis of contact orientations in both unreinforced and geogrid-reinforced ballast assemblies during shearing revealed that particles were re-arranged and rotated to support induced loads. With the inclusion of a geogrid, contact forces were redistributed and reoriented, aligning more toward the horizontal shearing direction to support the applied shear loads.