The effect of changes in pore fluid geochemistry on the elastic-plastic behaviour of Lake Agassiz clay

Abstract

Fresh water retention dykes at Seven Sisters Generating Station have experienced instabilities on an irregular basis since they were heightened in the late 1940's. Previous investigations compared three locations at the site; one section of dyke that had experienced previous instability; a second section that had remained stable; and a third, background location, that had not experienced dyke loading. Laboratory results on clay specimens from the stable and unstable sections showed greater brittleness and anisotropy at the unstable section. Slope stability modelling indicated that the stable and unstable sections both had safety factors greater than unity if post-peak strengths were used. Both were close to unity if residual strengths were used. Differences in stress-strain behaviour and the time dependent nature of the problem suggested that the forebay water was leaching naturally occurring cementation from the foundation clay and subsequently changing its behaviour. This project compared the pore fluid geochemistry of samples from the unstable, stable and background sections. Both sections beneath the dyke had noticeably lower electrolyte concentrations, particularly components of gypsum, than the background section. To examine a hypothesis that changes in pore fluid geochemistry, due to leaching of the forebay water, reduces cementation bonds in the high-plastic foundation clay, a laboratory testing program was undertaken to isolate the effects of pore fluid geochemistry on the elastic-plastic behaviour of Lake Agassiz clay. Yield loci were obtained for the natural clay from the background and unstable locations and a number of clays reconstituted with different pore fluid geochemistry. Geochemical conditions examined included batches of clay reconstituted with deionized (DI) water, gypsum rich water, water acidified with sulphuric acid, sodium chloride brine and two batches that were washed with DI water (one and six pore volumes). The results of the test program showed that yielding was influenced by the presence or absence fo gypsum cementation. The size of the yield locus was reduced when pore fluid concentrations of calcium and sulphate were lowered to conditions that would result in the dissolution of gypsum. Loss of gypsum cementation reduced the amount of strain required for strain-softening from peak to post-peak and the development of a shear plane. Elevated pore fluid electrolyte concentrations increased the normally consolidated peak strength, post-peak strength and residual strength. The yield locus for artifically-weathered clay was smaller in p',q-space than that for the un-weathered soil, but its general shape was similar. Using the laboratory yield locus, stress-deformation modelling indicated that the upper foundation clay yielded during the inital construction and raising of the dykes. Reducing the size of the yield locus by leaching would increase the amount of yielding beneath the dykes. Induced excess pore water pressures due to time-dependent yielding would be sufficient to cause a condition of instability of the Seven Sisters dykes.