PSI - Issue 84

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

530

1. Introduction Scour at the base of bridge piers is one of the primary causes of structural instability and potential failure. To ensure the safety and durability of foundation systems, it is necessary to consider both general scour, which results in a uniform lowering of the riverbed, and local scour, which manifests through concentrated excavations in the vicinity of structural elements such as piers and foundation footings. To estimate the reduction in effective stresses in the presence of scour, reference is commonly made to the guidelines of the American Petroleum Institute (API), the Federal Highway Administration for driven piles (FHWA DP), and the FHWA for drilled shafts (FHWA-DS). These documents provide methods for estimating the vertical effective stress in soils subjected to scour conditions (Hannigan et al., 2006; API, 2011; Brown and Castelli, 2010). However, according to Lin and Wu (2018), the FHWA-DP and FHWA-DS guidelines tend to overestimate the lateral capacity of piles by 34–47% and 12–22%, respectively, whereas the API guidelines offer more accurate estimates of lateral resistance, although they remain limited to a specific scour geometry. Due to these limitations, several studies have focused on analyzing the effects of general and local scour on the behavior of single piles and pile groups. These investigations primarily examine variations in the lateral response of piles as a function of scour depth, soil friction angle, and the distance between the pile and the onset of the scour slope (Lin and Jiang, 2019; Wang et al., 2023; Chen et al., 2022; Lin and Lin, 2019; Bennett et al., 2009). An aspect often overlooked in the literature concerns the effects of seepage flows that may develop as a consequence of the disturbance of the hydraulic regime induced by the presence of structures within the channel. Seepage can modify the distribution of pore water pressures within the soil, influencing the effective stress state and, consequently, the shear strength of the material. These considerations highlight the need for an integrated approach that simultaneously accounts for scour and seepage processes in order to adequately characterize potential reductions in soil strength and the resulting mechanisms of structural instability. Nomenclature D 90 sediment diameter corresponding to 90% passing v average flow velocity h water depth 2. Definition of the problem This study investigates the effects of seepage flows, induced by the river current, within the foundation soil of bridge piles, considering three conditions: absence of scour, presence of general scour, and presence of local scour. The objective is to assess how these conditions influence the distribution of pore water pressures within the soil and, consequently, the variation of the effective stress state at the base of the foundation. 2.1. Geometry and Characteristics of the Case Study The analyzed bridge crosses the Malone Torrent and is supported by deep foundations consisting of eight drilled shafts with a diameter of 1.5 m and a length of 35 m, arranged beneath a rectangular pile cap measuring 18 m × 7 m × 2.5 m. The simulations were conducted under three distinct conditions: • absence of scour, with the riverbed coinciding with the top surface of the pile cap; • general scour, with a uniform lowering of the riverbed by 1.5 m, 3 m, and 4.5 m; • local scour, with an excavation depth of 1.5 m, 3 m, and 4.5 m. k r equivalent riverbed roughness k s structural surface roughness ΔP difference between seepage-induced and hydrostatic pore pressure σ’ effective stress φ friction angle