The Behavioural Framework of a Lightly Cemented High Plasticity Clay Under Low Effective Stresses

Abstract

This paper presents the findings of an experimental study on the primary yielding and failure mechanisms of an artificially prepared lightly cemented high plasticity clay, using fly ash as the cementation binder and on the influence of its presence in the soil matrix. The study was conducted for a series of triaxial isotropically consolidated compression and extension tests in the low stress range. Studies on clays with low effective stresses at yield and failure are limited, mostly because of the experimental difficulties. The results showed that the difference between the clay samples, with or without the addition of fly ash, was the shape of the yield locus, resulting in a flattening of the primary yield locus of the fly ash treated samples compared to the wide arch shape of primary yield locus of the untreated sample. Experimental results showed that the yield locus expanded with the presence or increase of fly ash content and this was due to kinematic softening induced by the breakage of inter-aggregate bonds or destructuration. To corroborate this interpretation, the microstructure of untreated and treated mixtures was investigated by polarised light microscopy analysis. Testing also revealed that the mode of failure for the fly ash treated clay is different in compression (shear failure) and extension (detachment of bonds). Shear resistance in extension was found to be less dependent on the normal stress and was mostly due to the cementation bonds.