Local Pressure Response and Load Redistribution Characteristics of Bridge Pier Groups Under Debris-Flow Impact

Bridge piers in debris-flow channels are commonly assessed as isolated members, although an upstream pier may modify the loading applied to downstream piers. This study investigated local pressure responses in tandem twin-pier and equilateral triangular three-pier arrangements using flume tests, surface velocimetry, and coupled SPH-DEM-FEM simulations. Streamwise-location-matched single-pier cases were used to distinguish the downstream-location effect from interference caused by the front pier. Each pier configuration was tested once; consequently, the reported attenuation and left–right difference indices describe the individual experimental realizations rather than statistically established coefficients. Under a debris-flow bulk density of 1600 kg/m3, a flume inclination of 23°, and a nominal release volume of 0.052 m3, detectable pressure was concentrated at the lowest sensor level. In the two tandem-pier realizations, the rear-pier peak pressures were 30.7% and 26.8% of those measured on streamwise-location-matched single piers, and the corresponding pressure impulses were 9.9% and 16.4%. The triangular configurations exhibited weaker rear-pier attenuation and case-specific differences between the two rear-pier records. The simulations used the full nominal source mass of 83.2 kg and provided exploratory comparisons of local SPH contact pressure and cumulative whole-pier loading. Because the numerical pressure ratios, energy balance, and sensitivity results did not support a fully converged quantitative prediction, the experimental attenuation ratios are not proposed as design coefficients. Within these limits, the results show that peak pressure, pressure impulse, and action duration can respond differently to pier-group interference and should therefore be considered separately.