Characteristics of force jumps and energy release during shearing of granular material - acoustic emissions measurements and fiber-bundle models

The thesis presents experimental and theoretical studies of acoustic emissions reflecting individual events of abrupt elastic energy release during shearing of granular media. Modeling of these events is based on the fiber-bundle model (FBM) formalism. The FBM is capable of characterizing rupture and interaction of disordered material constutuents and can be used to predict progressive failure in complex systems such as granular assemblies and Earth materials. Fast geologic mass movements, such as shallow landslides, rock falls, or snow avalanches, often occur without obvious warning signs. Heterogeneity of host materials, complexity of the underpinning mechanical processes and limitations to measurement techniques often hamper the prediction of such mass movements. Triggering of geologic mass movements is typically associated with the accumulation of internal failure and localized deformation events culminating in the formation of a slip plane, which eventually leads to an abrupt and often catastrophic mass release event. Local internal failure events may include rearrangement of the grain contact network, particle-to-particle friction, rapid liquid-air interface displacement, breakage of cementing agents or rupture of biological components occurring in natural geo-materials. Presenting an abrupt perturbation of the internal force network deformation-induced failure processes typically result in emmision of elastic waves. For their characteristic frequency spectra in the kHz range such elastic waves are often termed acoustic emissions (AE). Based on investigations of failure-induced AE and measurable granular material stress-strain behavior we present a framework for the observation and prediction of shear zone formation in granular materials. To study characteristic AE signature of micro-failure events in granular shear zones we extended and applied a conceptual fiber-bundle model (FBM). Individual building-blocks of the FBM, called fibers, represent either single grain contacts, elements of the granular force network or other load-bearing entities occurring in granular geo-materials. Using the FBM we can provide links between AE-generating grain-scale failure processes and shear zone formation. In preparation for the theoretical and experimental studies, we initiated an exhaustive literature review to assess the current state-of-the-art of AE application for geologic material testing, and to pin down potential source mechanisms. We review important issues of acoustic signal propagation and attenuation, and outline potential strategies to apply the method on the field-scale for the assessment of slope stability. In acknowledgment of the great potential of FBMs for the modeling of grain-scale failure events associated with granular shear zone formation we adapted the FBM framework considering the peculiarities of granular materials. Analytical expressions of fiber rupture-induced energy release form the base for the simulation of acoustic events. Shear-frame experiments with glass beads as prototypical representation of granular matter were performed, including high resolution shear force and AE measurements. Experimental tests were in good qualitative agreement with predictions from the FBM and revealed a close connection between low-frequency acoustic emissions and small fluctuations of the shear stress. The simultaneous observation of continuously released high-frequency AE events suggests a relation of those signals to other source mechanisms, such as friction or rolling at individual grain contacts. We amended the FBM with an AE generation model that is rooted in the principle of failure-induced excitation of the granular force network and bridges the gap between macroscopic shear zone mechanics and failure-induced AE signals.