Spatio-temporal characterisation of local scour around a circular bridge pier in cohesive soil with different clay/silt ratio

Abstract

This paper presents new data on scour around a vertical pier in cohesive soils. It specifically focuses on the temporal evolution of local scour hole around the pier with increasing the clay/silt ratio content. Series of flume tests were performed using soil mixtures made with 85% fine sand and 15% fines. The fines fraction used in the soil mixtures includes silt, kaolinite clay, and various combinations of both with different proportions. The water velocity during the tests was kept constant and slightly above the erosion critical velocity of the fine sand used in the mixtures. A 3D laser scanner was employed to continuously record the geometry of the scour hole. Results show that increasing the Kaolinite clay in the fine fraction reduces significantly the scour. The mixture with a range around 7.5–10% of kaolinite clay fines content provides the threshold composition for cohesive soil behaviour effect in scouring process, recognized through scour pattern change. As the clay content increases, notable reductions are observed in the dimensions of the scour hole. Furthermore, the temporal evolution of maximum scour depth measured in flume was in agreement with the results from SRICOS-EFA (Scour Rate in COhesive Soil - Erosion Function Apparatus) method.