Pettey, Angus
(2023)
A micromechanical investigation of soil-structure interface behaviour.
PhD thesis, University of Nottingham.
Abstract
A limiting factor in the drive to deliver performance-based design is the lack of knowledge regarding the constitutive behaviour of soil-structure interfaces, particularly in the case of cyclic loading. Attempts to model the behaviour of these interfaces have failed to consider the long-term effects of cyclic loading, in particular how both the soil and structure may degrade over the course of thousands of cycles. To address this shortfall in understanding, consideration must be given to not only the geotechnical aspects of the interface, but also the structural aspects, and how these two components interact over the course of a structure's lifetime. This thesis presents a micromechanical investigation of the behaviour of soil-structure interfaces, with a particular focus on these cyclic effects by carrying out novel experimental testing at the macro-scale and single-particle scale.
At the macro-scale, a series of direct shear tests were carried out on a smooth stainless steel interface under a constant normal load. After continued shearing, the interface experiences a rapid elevation in the shear force transferred, accompanied by an increase in roughness of the surface and crushing of the Leighton Buzzard Sand grains. These observations are found to corroborate similar behaviour witnessed in literature. However, the initial trigger of this rapid increase in shear force cannot be explained by existing models, or verified by macro-scale observations. Therefore it was necessary to investigate the behaviour of the interface at the single-particle scale. A novel testing apparatus was developed to carry out single-particle direct shear tests on a smooth stainless steel interface. Testing revealed that the steep elevation in shear force also occurs at the single-particle scale and is caused by abrasive wear at the interface.
To investigate the abrasive wear at the single-particle scale further, a method was developed to accurately model the contact geometry of the particle. A particle virtualisation methodology was implemented to capture high resolution 3D meshes of the \SI{1.5}{\milli\metre} particles, with a provision to directly compare the grain meshes prior to and after testing. Using this methodology, it was found that the particle undergoes a significant change in shape during testing, with the particle becoming flattened and the nominal contact area increasing. This insight, of abrasive wear to the equivalently harder abrasive particle, has not been readily considered by tribological studies due to the difficulty of modelling and monitoring the contact geometry of irregular particles.
The frictional response of irregular particles during abrasive shearing therefore required further investigation, to establish a method for characterising local 3D angularity. Using the particle virtualisation methodology, a novel method was developed to characterise the local 3D angularity of the particle, and the evolution of this angularity during shearing. A new parameter, 3D attack angle, has been established, which characterises the angle an irregular abrasive grain makes with a planar surface. This new parameter is found to have a strong correlation with the rapid increase in shear force transmitted at the interface, whereby at the point of sudden shear load increase, there is a corresponding sudden change in 3D attack angle. It is therefore concluded that the rise in shear force is caused by an initial decrease in the 3D attack angle of the particle, which causes cutting abrasive wear to the surface.
With a better understanding of the micromechanical processes occurring at the interface, the macroscopic mechanisms that govern global response can be viewed in a new light. As such, more informed engineering decisions can be made regarding the design of soil-structure interfaces, which will ultimately lead to more efficient and sustainable infrastructure.
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