Evolution of small strain shear stiffness by fabric changes in granular materials

The initial shear stiffness at small strain is a significant factor to consider for a wide range of geotechnical applications. Determining this initial parameter is critical for describing dynamic soil behavior in the case of wave, wind, machine or traffic vibrations. It is very sensitive to fabric changing processes. Monitoring shear wave velocity, which is directly related to shear stiffness, is an effective tool to detect these fabric changing processes in soil, such as creep and unsaturation. The complex behavior of initial soil parameters, namely initial shear stiffness and shear wave velocity, under consideration of multiphase conditions and time effect, is in the focus of this research. Fabric changing processes were detected experimentally using a signal processing tool called coda wave interferometry and described mathematically. The properties of soils change with time in a phenomenon called creep. This change has a significant impact on engineering practice since it causes long term settlements of foundations, piles and slopes. The evolution of shear stiffness during creep is established in this research based on measuring shear wave velocity, and a model is accordingly described. This model includes, in addition to the time effect, other physical soil properties at the micro scale, such as particle parameters (interparticle friction, angularity and grain properties) and state parameters (coordination number and confining pressure). When soils are subjected to change in water content, knowledge of unsaturated soils is required for many engineering applications such as dams, slopes and stability excavations. In this thesis, semi-empirical and numerical approaches to predict a relationship between initial shear stiffness and degree of saturation are presented. In the semi-empirical approach, analytical models that link shear wave velocity to matric suction or degree of saturation based on experimental data are presented. Additionally, a numerical approach that predicts the soil-water characteristic curve (SWCC) can be used in the absence of experimental data. The analytical models from the semi-empirical approach associated with the numerical approach allow predicting change in shear wave velocity based on easy to obtain soil properties. This gives researchers and engineers a valuable tool to assess the dynamic behavior of unsaturated soils for different degrees of saturation.




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