Maximum Scour Depth around a Bridge Pier in Sand and in Clay: Are They Equal?

The maximum scour depth around bridge piers in sand is calculated using well established formulas based on experimental model calibrations. There are no such formulas for the maximum scour depth around bridge piers in clay. In practice and by conservatism the maximum scour depth in clay is taken to be equal to the maximum scour depth in sand. However no such evidence exists and common sense tells us that clays scour much more slowly than sand. This paper presents flume test results of pier scour in clay. The piers are cylinders with diameters varying from 25 mm to 210 mm. The soils were a low plasticity porcelain clay, a medium plasticity armstone clay, a high plasticity bentonite clay and two uniform sands. The results show that the maximum depth of scour is approximately the same in the sands and in the clays. However the rate of scour is drastically different. This shows that in clays a site specific scour rate analysis is necessary while it may not be necessary in sands.