EROSIONAL AND MECHANICAL STRENGTHS OF DEPOSITED COHESIVE SEDIMENTS

Erosion tests were run on soft cohesive sediment beds freshly deposited from a concentrated slurry, in a large rotating annular flume. A total of seven tests were carried out with the bed age varying between 1.8 days and 18.9 days. A surface type erosion was observed for the applied shear stress range (0.1-1 Pa). Following the increase in the water flow, the erosion rate increases quickly to a peak value and then drops off to approximately zero. The eroded depth was less than 8 mm for all beds, but the resistance to erosion at a given depth and for a given bed structure increased with increasing bed age. This phenomenon was attributed to thixotropy. The erosional strength of a given sediment layer is found to be one order of magnitude lower than its undrained and drained mechanical shear strengths measured with innovative testing methods. Hypotheses are given to explain that difference. However, at constant density and soil-water chemistry, the erosional and mechanical strengths seem to be affected by the same factors, such as bed age, structure, and temperature.

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