This research experimentally investigated local scour depth around complex bridge piers in a clear-water condition. The term “complex pier” is used to define a bridge pier composed of three different sections: a column, cap of pile, and group of pile. Eighty-two experiments were carried out using six types of complex pier models to understand the impacts of the pile cap longitudinal extension from column, arrangement (or configuration) of pile group, extension occurring upstream of pile group, and thickness of pile cap. Through the experiments, a mathematical relationship between the upper limit of the pile cap undercut elevation and pile cap thickness was presented. In fact, the proposed formulation aimed to assess the undercutting elevation of pile cap. It was found that the variation of pier-foundation geometry significantly affected the maximum scour depth. The result showed that increasing pile cap thickness thereby decreased the pile cap undercutting elevation. When the piles number in line with flow increased, the maximum scour depth decreased.

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