Experimental modeling of local scour in cohesive soils

Safe and economic design for bridge foundation requires an accurate prediction of the scour depth and pattern around the piers. Scour depends on the flow field and the type and properties of the erodible bed. No general formula is presently available to estimate pier scour in cohesive soils due to complex erosion process, lack of a general erosion model, and difficulty of scaling the properties of cohesive soil in scour experiments. Most of the previous experimental studies have yielded inconsistent results since the scale effects on erosion resistance were not properly taken into account. The present study focuses on the erosion resistance of cohesive soils and how it is modeled in scour experiments. Based on surface erodibility tests on natural cohesive soil samples from different locations, an erosion resistance model is presented to describe the variation of erosion resistance with soil properties (moisture bulk density and water content). Cohesive soil can be modeled in pier scour experiments by scaling down its erosion resistance to be appropriate to the scaled down geometry and flow. A model soil (mixture of silica powder and kaolin clay) with low erosion resistance is used as erodible media. Scour around two prototype circular piers are modeled with laboratory experiments of different scales and satisfactory agreement is found for scour pattern and bottom shear stress from comparison of scaled-up experimental data at the prototype scale. Satisfactory agreement is found between the scaled-up experimental results and the field scour data for prototype piers with different shapes (rectangular and irregular). Erosion resistance is found to influence scour depth and pattern. Cohesive soil is successfully modeled in pier scour experiments by modeling the erosion resistance instead of modeling I the individual properties of the soil, which is difficult and perhaps impossible.