UNDRAINED STRENGTH OF ULTRA-WEAK COHESIVE SOILS : RELATIONSHIP BETWEEN WATER CONTENT AND EFFECTIVE STRESS

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This work focuses on the mechanical properties of cohesive soils at ultra-low stresses. It consists of an experimental program to establish the water content-effective stress-undrained shear strength parameters of freshly-deposited cohesive soil. The effective stress levels in this program are lower by three orders of magnitude or more than in traditional geotechnical practice. The experimental program was conducted on fully consolidated Boston Blue Clay beds of various ages, 1.7 to 11 cm in thickness deposited from concentrated slurries. It consisted of water content profile measurements at 0.5 cm intervals using a vacuuming technique, and undrained shear strength profile measurements at 1 cm intervals using a specially designed Automated Fall Cone Device. For effective stresses below 400 Pa and void ratios between 2.5 and 8,a non-unique void ratio-effective stress curve dependent on the bed height was found. The water content increased at the same depth for increasing bed thickness. The undrained shear strength, which varied between 10 and 120 Pa, increased with depth for all beds. The strength was mostly water content dependent at early age, then became mostly effective stress dependent as the bed aged. At the same effective stress and water content, the strength at 6 days was higher than at 3 days. This observed behavior is attributed to thixotropy.
1997-09-15