Bridge failure and pier scouring: laboratory experiments on maximum scour depth around wide piers

The presence of bridge piers within the river causes troubles like potential clogging of limited spans, scour around each single pier, and costs related to provisional work. All that suggests the reduction of their number increasing the dimension of each single pier. This leads to the realization of wide piers, that according to the literature are piers characterized by width less than 2-3 time the water depth. Scouring around piers is widely studied in the literature, but the complexities of the overall phenomena still leave grey areas in the prediction of the maximum scour depth around wide piers. An accurate prediction of the scour depth is fundamental in the design of bridge foundations. The underestimate of this value may cause the pier foundations exposure leading to serious damage or failure of bridge, while its overestimation causes unnecessary extra-construction costs. In the present work, laboratory experiments have been developed to minimize the causes of uncertainty appearing from literature analysis. The experiments were run in a rectangular flume 1 m wide, using uniform sand characterized by median grain size d50 = 0.35 mm. Different pier widths (D) were tested with different water depths (y), covering the range y/D = 0.25-3.20 and limiting the ratio between pier width and the grain size (D/d50) in the range 140-470. All tests were run in steady state clear water conditions adopting a constant ratio between flow and sediment critical velocity. To get a deep understand on the scour process, the flow field in the scour hole is experimentally investigated through ADV measurements and compared with Large-Eddy Simulation (LES). Moving from narrow to wide piers, the maximum scour depth as well as the shape of the erosion change, revealing a reduction of the maximum relative scour depth but an increase of erosion footprint.