Low (8 m high) dykes on soft plastic clay in SE Manitoba failed intermittently over many years. The clay is smectitic and cemented with gypsum. Tests showed that clay at an unstable section was more brittle and anisotropic than at a nearby stable section. Both sections had noticeably less gypsum than nearby unloaded clay. Tests examined the impact of pore fluid chemistry on mechanical properties of reconstituted clay from the site. Yielding appeared to be controlled by cementation bonding and by environmental effects. Stress-strain behaviour, and softening from peak to post-peak and then to residual strength, was influenced by ionic strength and the Na/Ca ratio.