Scour at complex bridge pier

Abstract

Bridge pier is a key component of bridge that vertically supports the bridge deck and other superstructures and is usually located in a river’s main channel or on its floodplain. A bridge pier is commonly built with complex pier form (typically with a wall-like column, a pile-cap, and a group of piles underneath) to meet the structural, geotechnical, and hydraulic requirements. The existence of a bridge pier obstructs the flowing water and thus leads to sediment entrainment and erosion at the base of the pier, with a scour hole forming gradually. The existence of the scour hole may undermine the stability of the scoured pier and lead to more severe loss once the bridge fails. Therefore, study on scour at complex bridge is of great importance for both researchers and practitioners. The thesis presents a series of experimental studies on scour at complex bridge pier to investigate unknown scour phenomena and deepen our understanding of scour mechanism. The main content consists of six independent but also closely related research topics: clear-water scour at skew complex bridge pier, temporal evolution of clear-water scour, live-bed scour in column-only situations, live-bed scour at complex bridge pier, scour at complex bridge piers in close proximity, and scour at complex bridge pier caused by combined waves and current. The six topics cover a wide range of scour scenarios that usually occur in field situations but have not been well investigated and understood. Under clear-water conditions, the equilibrium scour depth and pattern at complex bridge piers are significantly affected by pier skew angle, relative pile-cap elevation, and pier form, which do not apply to circular piers with uniform cross-sectional shape. Similarly, the temporal evolution rate of clear-water scour at complex piers are also influenced by the geometric complexity and tend to be non-uniform. Four evolution stages are observed, including initiation, stagnation, development, and equilibrium stage. Under live-bed conditions, the scour pattern at a complex pier resembles that at singles piers if the pile-cap is deeply buried and only the column contributes to the scouring process. Under such circumstances, the mean scour depth with small ratio of flow depth-to-pier width may be significantly overestimated by the existing predictors. If all the components of a complex pier contribute to scour, the mechanism will be much complicated. The mean scour fluctuation, which is caused by migrating bed-forms, at the pier will be significantly enlarged when the pile-cap is fully buried by the original undisturbed bed. The enlargement is because that the increase (or decrease) of flow’s scouring capability and the decrease (or increase) of the sediment supply into the scour hole may happen concurrently. If two complex piers stay in close proximity, both the clear-water and live-bed scour depth at the piers tend to be affected significantly. Generally, the mean scour depth and scour fluctuation at the downstream pier will be attenuated due to the protection of the upstream pier. In contrast, two tandem complex piers have insignificant effect on each other. Under combines waves and current, three types of scour temporal evolution (ascend-descend, asymptotic, and transferred type) are identified, and the scouring processes (structure exposure, cyclic sediment motion) at the scour initiation and development stages are also discussed. Most of the scouring effect can be observed in the first half duration of a complete wave period when the streamwise wave orbital velocity dominates. The dependence of time scale and equilibriums scour depth on various parameters are also investigated. Finally, a structured scour prediction method is proposed according to the existing methods and covers various scour scenarios included in this thesis. The new method significantly improves the prediction accuracy and can be easily modified when more data become available in the future.