Clear-water scour at single piers and pile groups

Abstract

The major damage to bridges at river crossings occurs during floods. Damage is caused for various reasons, one of the main reasons being the riverbed scour at bridge foundations. Local scour is induced by the flow field generated around piers typically inserted in movable bed rivers. In Portugal, the tragic accident of Entre-os-Rios was mostly due to scour at one of the bridge piers. Physical and economic reasons lead to bridge foundations composed of a pier column founded on a pile cap, supported by an array of piles. Piers of this configuration are known as complex piers. Frequently the pile cap is completely buried, or in opposition, above the water, being the column or the pile group respectively the only structural element interacting with the flow and causing local scour. In modern bridges it is also common to find structural solutions where the deck is supported directly by pile groups composed of only one alignment, without pile cap. For scouring proposes the structural elements may be considered as single piers, pile groups and pier alignments. The construction of new bridges and the maintenance of thousands of bridges built before the main developments in local scour prediction, amount to costs of billions of Euro and justifies a rigorous prediction of the scour depth, both for economic and safety of human lives reasons. The present study develops an extensive research to systematically map equilibrium scour at single cylindrical piers and pile groups and relate the observations with the characteristic variables of the tests. Special attention is given to the effect of time, relative sediment size and relative approach flow depth at single cylindrical piers and spacing, skew-angle, number of columns and time at pile groups. The pertinence of considering the effect of viscosity it is also assessed. Using dimensional analysis, the following major conclusions are achieved. Regarding single cylindrical piers, it is discussed the required duration of the laboratory tests to render reliable equilibrium scour depths and it is confirmed that equilibrium scour depth decreases with the relative sediment size. It is suggested a predictor for the equilibrium scour depth at single cylindrical piers, function of relative sediment size and relative approach flow depth. The parameters of the equation suggested by Franzetti et al. (1982) are fully characterized rendering a predictor of the scour depth time evolution. Regarding pile groups and pier alignments, it is assessed the effect of the test duration on the equilibrium scour depth and it is confirmed that the spacing factor and the factor for the number of aligned rows recommended in the predictors commonly used in engineering practice are reliable since the scour depth at single cylindrical piers and pile groups remain essentially self-similar in time. The prediction of the equilibrium scour depth at pile groups is improved by the suggestion of a aggregate pile group factor, function of the pile spacing, skew-angle and number of parallel pile alignments. Finally it is revealed that the viscosity may affect scouring in laboratory tests.