EFFECT OF AGING AND STRESS HISTORY ON THE UNDRAINED STRENGTH OF ULTRA-WEAK COHESIVE SOILS

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This work is concerned with the mechanical properties of freshly-deposited cohesive sediments. It was previously established by the authors that the undrained shear strength in the ultra-low stress range is effective stress dependent and increases with bed age. This paper consists of an experimental program to quantify the strength gain with time, and to study the effect of stress history on the strength of the sediment bed. The experimental program was conducted on fully consolidated Boston Blue Clay beds of various ages, 3.5 to 11 cm in thickness deposited from concentrated slurries. The bed age varied between 2 and 23 days, and the undrained shear strength measured with the Automated Fall Cone Device varied between 10 and 120 Pa for effective stress values below 400 Pa. For a given water content, the strength exhibited a two-fold increase over the 21 day period attributed to thixotropy. This thixotropic strength gain was mainly a function of the existing effective stress with some dependence on water content. Although no measurable swelling occurred upon unloading (unloaded effective stress up to 190 Pa), the shear strength decreased substantially depending mainly on the magnitude of the unloaded and the remaining effective stresses, but also on the existing water content.
1998-12-15