Strain-time-strength relationships in a marine clay

The main purpose of the testing program was to determine if the strength of a particular clay deposit decreases with time when subjected to a constant shear stress. Strains were examined to ascertain if a critical strain existed. Distinguishing features of clay examined (Leda clay), are its high sensitivity, high liquidity index and tendency toward decreasing rather than increasing shear strength with depth. The consolidated-undrained triaxial compression test was used. A constant consolidated pressure was selected for all tests, and a back pressure of 1 tsf was applied, with a confirming pressure of the sample value. Three series of tests were carried out; first at controlled rates of strain 20%, 2% and 0.2% per hour; next at constant stress, with time to failure being measured; and lastly uniform increments of stress were applied for uniform increments of time, to determine Housel's "yield stress". The loss of shear strength with time up to 25% seems to be a real time effect. Safety factors should be selected to allow for this. The presence of a plastic element of strain ( irreversible strain not dependant on time) and the decreasing modulus of deformation with increasing deviator stress makes a simple application of a rheological model difficult. Normal stress-strain plots of either constant stress rate or strain rate must be curved. The controlled strain rate test may or may not give information equivalent to the constant stress test. Finally some of the test data indicated that the strain at failure might be constant with respect to time for a given clay.Le principal objet du programme d'essai consistait \ue0 d\ue9 terminer si la r\ue9sistance d'un gisement d'argile donn\ue9 pouvait dimineuer avec le temps lorsque soumise \ue0 une contrainte de cisaillement constante. Certaines des donn\ue9es d'essai ont indiqu\ue9 que la d\ue9formation \ue0 la rupture pouvait \uea tre constante dans le temps pour une argile donn\ue9e.Peer reviewed: NoNRC publication: Ye