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Coupled mechanical and ultrasonic investigation of physical processes in granular gouge layers
Gheibi, Amin
Gheibi, Amin
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2020
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2021-01-03
Abstract
Granular materials are abundant in nature, and depending on the external and inter-particle loading conditions, they exhibit complex features and states that are significantly different than the properties of the individual grains. Understanding the particle-scale mechanisms in granular materials that dictate their mechanical behavior is a critical issue in the evaluation of deformation in geotechnical structures and geosystems. In order to monitor and model the behavior of granular materials, particularly the non-accessible layers (e.g., fault gouges), it is important to develop practical methodologies through which particle-scale mechanisms can be accurately portrayed. Hence, the main goal of this thesis is to investigate the underlying physical processes during compression and shear in granular gouge layers utilizing ultrasonic measurements. The main hypothesis is that monitoring the variation of ultrasonic wave attributes can provide valuable information to evaluate the evolution in the inter-particle true contact area, pore volume changes, and time-, slip-, and velocity-dependent particle-scale mechanisms. Accordingly, an experimental setup equipped with a fast wave propagation system was developed for conducting one-dimensional compression experiments as well as single and double direct shear experiments with simultaneous ultrasonic measurements. The experiments were conducted on thin layers composed of either granular quartz sand or glass beads with different properties to simulate the gouges or rock joint filling materials under different stress conditions. During compression, the variations in compressional wave peak to peak amplitude, velocity, and dominant frequency, as well as the evolution in particles crushing, were documented through successive loading and unloading cycles. The experimental results showed that the peak to peak amplitude exhibit a clear sensitivity to the stress history, which resulted in finding the seismic yield stress that corresponded to the mechanical gouge layer yield stress. In addition, the ultrasonic transmissivity of the granular gouge layers were found to be dependent on the particles size, which led to finding a close link between particle comminution and the amount of evolution in the dominant frequency. The shear behavior of the granular gouge layers also was studied through various slip modes (stable sliding, slow stick-slip, and dynamic stick-slips) using single and double direct shear experiments. The experimental results obtained during stable sliding and slow stick-slips showed that the signatures of the geometry- and time-dependent variations of the inter-particle contact quality and pore volume changes with sliding velocity and slip accumulation were clearly captured from the variations in the transmitted wave amplitude and the dominant frequency, respectively. In addition, the variations in dominant frequency were found to correspond to dilation and compaction of the granular gouge layers during compression, stable and unstable shearing. Further experiments on glass bead gouge layers during the dynamic stick-slip sliding mode in conjunction with continuous wavelet analysis, demonstrated the frequency dependency of ultrasonic wave propagation in granular gouge layers. Moreover, the amplitude that corresponded to the different frequencies was found to follow different trends depending on the inter-particle contact stiffness, normal stress, and pore volume sizes. The inconsistent amplitude variations thereby were filtered out and the integrated ultrasonic energy was obtained for certain frequencies of the transmitted and reflected shear and compressional waves. The integrated ultrasonic energy exhibited close sensitivity to the different phases during stick-slips and also the point of slip initiation, which was identified as the precursory signature to dynamic shear failure during stick-slips. The findings in this thesis have several practical applications and can pave the way for possible new approaches in the application of ultrasonic waves to better monitor time- and stress-dependent particle-scale mechanisms in granular gouge layers.
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