EROSIONAL AND MECHANICAL STRENGTHS OF DEPOSITED COHESIVE SEDIMENTS

Erosion tests were run on soft cohesive sediment beds freshly deposite d from a concentrated slurry in a large rotating annular flume. A tota l of seven tests were carried out with the bed age varying between 1.8 and 18.9 days. A surface type erosion was observed for the applied sh ear stress range (0.1 to 1 Pa). Following the increase in the water fl ow, the erosion rate increases quickly to a peak value and then drops off to about zero. The eroded depth was less than 8 mm for all beds, b ut the resistance to erosion at a given depth and for a given bed stru cture increased with increasing bed age. This phenomenon was attribute d to thixotropy. The erosional strength of a given sediment layer is f ound 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 strengt hs seem to be affected by the same factors, i.e., bed age, structure, and temperature.