Erodibility Testing of Cohesive Soils

Abstract

The Erosion Function Apparatus (EFA) is used to measure erosion rates for cohesive soil samples subjected to water velocities tangent to the sample surface. EFA testing was performed on soil samples taken from three bridge sites in Alabama: Talladega County, Sumter County (Sucarnoochee River), and Dallas County. Results from Talladega County varied greatly. One sample yielded a critical shear stress of 0.49 N/m2 and produced a scour rate of 100 mm/hr at V = 2.0 m/s, while another sample yielded a critical shear stress of 0.049 N/m2 and caused a scour rate of 120 mm/hr at V = 1.44 m/s. No erosion functions were generated for three samples, but testing indicated they were very scour resistant at V = 1.90 m/s, 1.60 m/s, and 1.00 m/s. Samples from Dallas County were tested at V = 6 m/s and were extremely resistant to scour. The internal structure of the sample from Sumter County was very complex, which made surface preparation an issue. Attempts to trim the surface flush with the EFA flume bed greatly disturbed the sample and resulted in mechanical erosion that was not reflective of the soil’s true erodibility. However, one test was successfully performed with minimal disturbance and was highly erosion-resistant at V = 1.00 m/s. The EFA was also used to measure erosion rates for “model” soils composed of bentonite and sand. These soils were studied to allow for controlled variations of bentonite content and wet density. Samples ranged in bentonite content from 3.5% to 25% and were compacted to a wet density of either 1.7 g/cm3 or 1.9 g/cm3 at a water content of 20%. Critical shear stress ranged from 0.62 N/m2 at 5% bentonite to 11.45 N/m2 at 25% bentonite (both at 1.9 g/cm3). An increase in density from 1.7 g/cm3 to 1.9 g/cm3 reduced scour rates from 84 to 34 mm/hr at V = 6 m/s. In addition to the EFA tests, model soils were subjected to impinging jet erosion. This facilitated more rapid testing for soils with relatively smaller bentonite content (less than 8%) that were not amenable to EFA testing. Erosion rates were impossible to measure in samples with less than 8% bentonite in the EFA because of a rapid rate of increase in erosion rate with respect to time. Soil was compacted in PVC caps, and a free jet from a water column and orifice was directed at the horizontal surface of the soil. Tests showed a significant decrease in soil erodibility with increasing initial compaction and increasing bentonite content.