The geomechanical behaviour of peat foundations below rail-track structures

This thesis presents the results of research conducted to define the response of peat foundations underlying railway embankments to heavy axle loads. Three field sites were investigated, two in Northern Alberta on Canadian National Railway’s Edson and Lac-La-Biche subdivisions, and one on the Lévis subdivision in southern Quebec. The scope of this thesis is fourfold: the development and installation of instrumentation; the laboratory testing of peat specimens retrieved from each site; the development of a conceptual model for the behaviour of peat beneath an embankment subjected to heavy axle loads; and finally, the modelling of the strength of the peat foundations relative to the applied loads.;The first component of this research included the development and installation of instrumentation to measure in situ the distribution and magnitude of strain and pore pressure generation. The development included the assembly of instrumentation systems to measure all of the required parameters, and the development of the ShapeAccelArray™ (SAA) from Measurand Inc. to measure horizontal cyclic motion under train loading. It was found the SAA, as provided by the manufacturer, was not able to provide accurate measurements of displacement. A method for determining the magnitude of cyclic displacement from the output of the MEMS accelerometers was developed from the laboratory testing data done as part of this study. This resulted in the ability to obtain a profile of cyclic displacement with depth.;The second component of the research was the laboratory testing of peat specimens retrieved from the sites. Consolidated undrained triaxial tests and direct shear tests were conducted on remoulded peat, remoulded peat fibre and Shelby specimens of peat to investigate the fundamental mechanisms which control the strength of peat. The results were analyzed within the frameworks of elastic behaviour of cross-anisotropic materials and shear strength of fibre-reinforced soil. The test results from samples collected at all three sites were compared and the influence of peat fibres on the undrained strength was explored.;The third component was the development of conceptual models for the undrained behaviour of peat, and peat foundations subjected to moving axle loads. A model for peat was developed from the cross-anisotropic response observed during the laboratory testing and correlations to fibre reinforced soil literature. This material model was then applied to the spatial distribution of stresses and orientation of principal stresses below embankments.;The final component of the project was an analysis of the field data collected from all three sites to provide the magnitude and distribution of strain and pore pressure generation developed within the peat foundations. Further analysis of the measured response was conducted with both with calculations of effective stress paths and finite element modelling to determine the distribution of stress, the locations of potential yielding within the foundations and to determine how close to yielding the peat is under the maximum applied stresses.;The results of this thesis provide new tools for the railway industry to evaluate the response and stability of railway embankments over peat foundations. The development of the SAA allows for the in situ measurement of the magnitude and distribution of displacement, and from this the strain, within soft soils under heavy axle loading. The conceptual models developed and applied to the undrained response of peat foundations provide a framework to evaluate the stability of railway embankments over soft foundations. The results of the application of this framework to the study sites included in this thesis provide context for further investigations.