Deformation and micromechanics of granular materials in shear zones - investigated with the Discrete Element Method

Abstract

This thesis was inspired by the lack of detailed (i.e. particle scale) knowledge concerning deformation processes of mechanically weak sediments, especially intrinsically weak layers on the basal shear plane of submarine landslides. It has been known for some time that many different parameters influence shear strength and localization features. This is true not only for sediments, but also for other kinds of weak layers, such as fault gouge. These parameters include for example mineralogy (e.g. smectite, illite, quartz), sediment composition (clay, silt), sediment structure and texture (microfabric), grain size distribution, excess pore pressure, magnitude of effective stress, and deformation history. However, to date it has not been possible to rank or to quantify the influence of each of these parameters. The main goal of this study is to analyze the influence of some of these parameters and, if possible, rank and quantify them.Standard methods to examine shear strength of sediments and fault gouge are various geotechnical shear experiments. In these, a sample is sheared under defined conditions and resulting coefficient of friction, void ratio change, and other meaningful parameters are analyzed. Unfortunately, it is not possible to 'look' inside a shear box during a test and to analyze grain deformation behaviour on a microscopic scale. Therefore, this study employs a different approach to specifically address the problem of microscopic deformation processes. Here, a numerical modelling technique, the Discrete Element Method (DEM), is used.The DEM is a numerical tool based on the behaviour of granular materials. Within some limitations, soils and fault gouge can be considered as granular. Thus, the DEM allows simulating deformation behaviour of weak layers. The DEM principle is based upon simple physical contact and motion laws and can reproduce a wide range of grain features and behaviour. The technique has already been used to model other kinds of granular deformation processes such as large and small scale deformation processes.Utilizing the DEM a numerical shear box, very similar to geotechnical ring shear tests, was designed. Inside this box, a variety of different numerical 'samples' were generated. These 'sediments', or 'fault gouges', were designed with close specifications in each study (manuscript), respectively. During the numerical experiments a multitude of micromechanical parameters (particle displacement and rotation, microfabric evolution, coordination number, sliding fraction, contact force distribution and orientation) were measured. These also encompass classical geotechnical measurements, such as coefficient of friction, void ratio or volume change.At the end of this thesis a ranking order of tested parameters is presented. In this ranking, it is important to distinguish between purely numerically derived conclusions and their implication for natural materials. Hence, within model limitations, grain roughness and clay size fraction exhibit maximum influence on frictional strength and localization of sediments and fault gouge. The impact of boundary surface roughness is smaller; followed by grain sphericity. The influence of stratigraphic layering is difficult to place into this ranking as it has a different impact in different settings.