PSI - Issue 84

Luca Vené et al. / Procedia Structural Integrity 84 (2026) 529–535

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3.3. Numerical Modeling of Seepage Flow in the Soil In the geotechnical model developed with FLAC3D, the soil was idealized as a homogeneous, isotropic medium with drained behavior. The assumptions of homogeneity and isotropy allow for a simplified treatment of the seepage problem, enabling the application of the Laplace equation. Furthermore, the assumption of drained behavior implies the absence of persistent excess pore pressures, which are assumed to dissipate instantaneously. This assumption permits separating the hydraulic analysis from the mechanical analysis. To reduce computational effort, only half of the foundation and the surrounding soil volume was modeled. In this study, only the hydraulic analysis of seepage flow was carried out, while the mechanical modeling was omitted. This choice is justified by the presence of water above the ground surface, which requires applying not only hydraulic pressures at the riverbed but also the water weight as a distributed load in terms of total stress to correctly evaluate the effective stresses. In the present case, the free surface is inclined due to the disturbance induced by the pier, making it impossible to apply a uniform loading scheme. For this reason, only the seepage flow within the soil was simulated, with the objective of evaluating the deviation of pore pressure distribution relative to the reference hydrostatic condition. 4. Results Regarding the analysis of the results, the pressure difference ( ΔP ) was defined as the difference between the values computed using FLAC3D, which accounts for seepage flow, and those obtained assuming a purely hydrostatic pressure distribution. Fig. 1 shows the plan view and the cross-section of the foundation to identify the areas from which the pressure distributions were extracted in the FLAC3D model and to contextualize the scour conditions. The pressure values were extracted along three types of sections: • a horizontal line located at the center of the footing base, extending longitudinally from upstream to downstream; • three vertical lines positioned around the footing—upstream, downstream, and lateral—spanning from the ground surface to the base of the foundation; • four vertical lines located upstream, on the sides, and downstream of the piles, extending from the pile head to the tip.

Fig. 2 presents two graphs illustrating the pressure distributions for three different conditions: no scour (solid line), generalized scour (dashed line), and localized scour (dotted line). The left-hand graph shows the horizontal distribution of ΔP along the base of the footing. The right-hand graph shows the vertical distributions of ΔP corresponding to the three vertical sections around the footing, as identified in Fig. 1 (left side). Fig. 1. On the left: plan view of the foundation of the bridge crossing the Malone Torrent, with the flow direction indicated by the red arrow. The highlighted contour and piles represent the portion of the foundation modeled in FLAC3D. The three points along the perimeter of the footing indicate the locations of the vertical lines from which the pressure distributions around the footing were extracted. On the right: longitudinal section of the foundation showing the scour depths considered in the analysis.