Measuring the Erodibility of Cohesive Soils

The scour depth around a bridge pier founded on soil or soft rock varies with time. There is no method available currently to predict the scour depth versus time curve for a bridge pier subjected to a real velocity hydrograph. Existing methods give one predicted scour depth independent of time and based on the design velocity. These methods tend to be conservative because they must consider the worst time factor condition. This is particularly true for scour in fine grained soils and soft rocks. The SRICOS method was developed to fill that gap. A computer program called SRICOS and a new apparatus called the EFA have been developed. The input to the program consists of the pier diameter, the velocity hydrograph, the erosion function from the EFA. The erosion function is obtained by testing samples from the site. It is the relationship between the erosion rate and the hydraulic shear stress applied. The erosion rate is measured by timing how long it takes for one millimeter of the soil or soft rock sample to erode when this one millimeter protrudes into the flow of the EFA pipe. In order to simplify this method to the point where the calculations can be done by hand on the back of an envelope, the simple SRICOS method was developed. The velocity hydrograph is replaced by the maximum flood velocity and an equivalent time factor which quantifies the velocity content of the hydrograph. This paper describes the simple SRICOS method and gives an example.