On structure and rate dependence of Perniö clay

Abstract

Thesis evaluates effects of anisotropy, bonding and viscosity of soft sensitive Perniö clay. These key features were examined in relation to sample disturbance, strain-rate, and stress ratio conditions in consolidation. Experimental evaluations form the basis of the study. The comprehensive sampling and laboratory testing programmes on soft sensitive Perniö clay comprised of three sampling techniques and numerous advanced oedometer and triaxial tests on natural and reconstituted specimens, resulting in a unique experimental database. Experimental results were interpreted in terms of physical parameters enabling detailed evaluation of the key features of soft soil behavior. Firstly, the study explained effects of structure and sample disturbance on compressibility and consolidation characteristics in incrementally loaded oedometer tests (ILOT). Commonly accepted paradigm of cα / cc being constant under one dimensional consolidation conditions was yet again questioned. Secondly, the study explained effects of sample disturbance and strain-rate on compressibility characteristics in constant rate of strain oedometer tests (CRS). Utilizing advanced interpretation approaches the influence of strain-rate on slope in compression was clarified. With regards of triaxial testing, the stress ratio in anisotropically consolidated triaxial tests (CAD) was proven decisive for compressibility and destructuration response. Creep coefficient and cα / cc under triaxial test conditions were shown to be related to consolidation stress ratio and load increment ratio. Furthermore, the study explained the effects of sample disturbance, consolidation history and strain-rate on the clay response in undrained triaxial compression tests (CAUC). Undrained shear resistance and strength were found to be influenced by the strain-rate and destructuration. On the basis of characteristic points, advanced approach for defining strain-rate influence on undrained shear response was introduced. Based on experimental results on Perniö clay, creep characteristics in one dimensional and triaxial test conditions are strongly related to sample disturbance and consolidation stress history. Furthermore, study clarified effects of sensitivity and destructuration. Approaches in interpretation of strain-rate effects on compressibility and undrained shear are extended. This is important to account for in any future sampling and testing programmes, in particular if the testing is aimed at improving advanced constitutive modelling of soft natural clays. Grounded on thesis findings, guidelines for enhancement of constitutive models for soft structured clays are set with emphasis on effects of structure and rate dependency on compressibility and strength.