Yielding criteria and limit-state in a Winnipeg clay
Lew, Khew Voon.
This thesis investigates the applicability of the concept of yielding to Lake Agassiz clay in the Winnipeg area. Careful sampling and laboratory testing techniques have been used. The test program was designed to examine the limit-state for the blue clay at 11.5 m depth. The study also investigated the time-dependent aspects of the YLIGHT model of soil behaviour, strain energy as a criterion for identifying limit-state, and the threshold energy concept at yielding. Eighteen 76 mm diameter undisturbed triaxial samples were tested along various stress paths. Drained, stress-controlled tests show that yielding is controlled by the in-situ grain structure of the clay, and by stress-history effects. For the blue clay at 11.5 m depth, a well defined yield envelope has been identified which supports the YLIGHT model concept proposed for Champlain Sea clay. Yield envelopes from different depths in the Winnipeg clay are fairly homothetic, and can be normalized with respect to p'c. Undrained, strain-controlled portions of the triaxial testing program were used to examine several aspects of the clay's behaviour. On the basis of the (o1-o3)/2max failure criterion, the normally consolidated Coulomb-Mohr strength parameters, c' and o' were found to be 4 kPa and 17.5o respectively. The average value of su/p'c was found to be 0.22. Porewater pressures at failure depend strongly on the stress levels and stress ratios during laboratory reconsolidation. Values of Af range between 0.22 to 1.59. The relative stiffness, E50/su, lies between 168 and 361. For a tenfold change in strain rate in Winnipeg clays, the change in undrained strength is approximately 11 to 12 percent.