The consolidation and strength behavior of mechanically compressed fine-grained sediments

Abstract

This thesis investigates the consolidation and shear strength behavior of saturated fine-grained sediments over the effective stress range of 0.1 to 100 MPa. The research makes use of samples which are resedimented in the laboratory from natural soils. In addition to practical benefits, resedimentation allows for isolation and quantification of individual factors influencing behavior such as composition, consolidation stress and overconsolidation ratio (OCR). Ko-consolidated triaxial compression tests were performed on eight resedimented soils at room temperature. The results demonstrate conclusively that the conventional assumption of these soils exhibiting constant normalized properties is not valid when behavior is evaluated over a significant stress range. The direction and rate at which a soil's strength properties vary depend on its composition, with high plasticity soils showing a much more rapid reduction in both normalized undrained strength and critical state friction angle with increasing stress compared to low plasticity soils. For all soils, increasing consolidation stress results in a more ductile stress-strain response during undrained shearing as strain to failure increases and the amount of post-peak strain softening reduces at each OCR. Variations in strength properties as a function of stress level and soil type are closely linked to Ko, with higher values of Ko associated with both lower friction angles and lower undrained strengths. During virgin compression, high plasticity soils display a rapid increase in Ko and values in excess of 0.80 have been measured at high stresses. The permeability behavior of a large number of resedimented soils has been investigated over a permeability range of 10-¹⁴ m² to 10-²⁰ m² and a porosity range of about 0.75 to 0.20. The permeability-porosity relationship for a soil can be correlated to its liquid limit, which provides a robust indicator of the combined effects of pore size distribution and clay minealogy on behavior. Virgin compression behavior is strongly influenced by composition at low stresses, although at high stresses all fine-grained soils display a similar compression behavior regardless of their composition. The conventional 'Terzaghi' definition of effective stress is shown to be applicable to fine-grained sediments at pore pressures up to at least 10 MPa.