Erosion Function Apparatus for Scour Rate Predictions

Scour is the number one cause of bridge failures. Scour in coarse grained soils (sand, gravel) is relatively well known, but scour in fine grained soils (silt, clay) and weak rock is not. In coarse grained soils, scour takes place very rapidly and the scour rate is rarely an issue because one flood is likely to create the maximum scour depth. In fine grained soils, the scour process is much slower; as a result, even after a hundred years, a bridge may not experience the maximum depth of scour. Therefore, in fine grained soils it becomes necessary to predict the rate at which scour takes place. A new apparatus called the EFA (Erosion Function Apparatus; 〈http://tti.tamu.edu/geotech/scour〉) has been built and tested to measure the erosion rate of fine grained soils; the EFA can also be used to measure the erosion rate of coarse grained soils if necessary. The end of a Shelby tube sample from the bridge site is fitted through a tight opening at the bottom of a pipe with a rectangular cross section. Water flows through the pipe and erodes the soil sample, which protrudes 1 mm above the bottom of the pipe. The rate at which the sample erodes is measured, and the shear stress imposed by the water on the soil is calculated. The plot of erosion rate versus shear stress is the result of an EFA test. It indicates the critical shear stress at which erosion starts and the rate of erosion beyond that shear stress. This function can be used to predict the rate of scour at a bridge.