Yielding and rupture in a lacustrine clay

Abstract

Recent tests have examined the properties of block samples and tube samples from depths to 12 m in Winnipeg clay. Careful trimming and a wide variety of testing procedures have permitted new insights into the behaviour of the clay that is markedly anisotropic and non-homogeneous. The effective strength envelope for "blue clay" from 6 to 12 m depth can be simplified into three straight-line sections. At low stresses, a section of the envelope has been identified having a low cohesion intercept and a high "friction" angle. This is thought to be caused by close Assuring in the clay, and controls the field behaviour in many small embankment, riverbank, and excavation problems. At in situ stress levels, the clay dilates markedly as failure is approached. Porewater pressures depend strongly on stress levels during laboratory reconsolidation. Residual strengths are low, and are influenced by the methods used during testing. Yielding has been clearly identified for shear failure of the grain structure, but is more poorly defined for increasing normal octahedral stresses. The work has emphasized the importance of selecting appropriate testing procedures and stresses for field applications.