Bridge Scour in Cohesive Soils: A Case Study

The project site is located in Gordon County, Georgia, which is in the northwest part of the state. State Route 156 crosses Salacoa Creek and it's ~4000 ft wide floodplain. The existing 400 ft long bridge is to be replaced by a 590 ft long main bridge along with the addition of a 400 ft long overflow bridge to meet The Georgia Department of Transportation's (GDOT) hydraulic design criteria. GDOT's procedures do not allow for preliminary borings prior to the approval of the bridge layout. Therefore, the grain size used for scour calculations must be estimated using other means. A soil survey sample was obtained by GDOT. However, the depths of the sample were not deep enough to be useful in the scour analysis. The Soil Conservation Service Soil Survey for Gordon County has 90%–100% of the soil at this site passing the No.200 sieve. This combined with information obtained from the Sediment Grade Scale pointed to a D₅₀ of as low as 0.00002 ft/0.006 mm (very fine silt). This small grain size produced extremely high clear water scour values on the order of 50 ft in the area of the overflow bridge. Due to the excessive values, an alternative approach was researched.