Bachelor Thesis. The effect of material properties (sand vs. glass beads) on the structural development of analogue Coulomb wedges

Transcription

1 Bachelor Thesis The effect of material properties (sand vs. glass beads) on the structural development of analogue Coulomb wedges Bachelorarbeit zur Erlangung des Grades Bachelor of Science (BSc.) am Department für Geo- und Umweltwissenschaften der Ludwig-Maximilians-Universität München betreut von Prof. Dr. Anke Friedrich Abgabetermin: Florian Hofmann Blombergstr München florian.hofmann42@googl .com Matrikelnr.: München, den

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3 CONTENTS 1 Contents 1 Abstract 4 2 Introduction Critical Taper Wedge Theory Mechanics Application to accretionary wedges and fold-and-thrust belts Non-cohesive, dry critical Coulomb wedges Analogue experiments of accretionary wedges Classic experiments GeoMod2008 Benchmark experiments Aim of this study Materials and Methods Materials and Material properties Nomenclature for analogue materials Analogue materials Schulze Ring-Shear-Tester Ring-Shear-Cycles Internal and basal friction and cohesion Angle of repose Experiment setup Push vs. pull setup GFZ setup LMU setup Image analysis Image acquisiton Image preprocessing Determining conversion factors and velocities Different methods of determining surface slope Perl program Input values and basic constants Filtering of reflections Slope detection for the whole wedge Progressive linear regression Wedge width and wedge height Particle Image Velocimetry (PIV) LaVision DaVis Strainmaster Software PIV analysis of sidewall pictures PIV profile plots PIV plot analysis: thrust spacing, activation time, reactivation Area of shear relative to sidewall, basal shear and frontal thrust length Sieve experiments

4 CONTENTS 2 4 Results Material properties Internal friction and basal friction Theoretical prediction of the critical taper Angle of repose Sieve experiments Volume throughput and mass throughput Minimum filling time for sandbox models Poured bulk density Bulk density as a function of sieve height Individual sieving styles Wedge geometry Surface slope Wedge height and wedge length Thrust spacing and activation time Sidewall friction Discussion Optimizing the sieving process Common problems Measurement of surface slope α Temporal evolution of surface slope and the kinematics of pushed wedges Wedge width and wedge height over time Effect of material properties on the structural development of pushed and pulled wedges Applicability of analogue materials Results in context with similar experiments Key differences between push and pull experiments Conclusions 73 7 Further work Experiment iterations and different equipment D analysis of pushed and pulled wedges Comparison of force predictions and dynamometer measurements Acknowledgements 76 9 References 77 A List of figures 81 B List of tables 86 C List of abbreviations, variables and constants 88 D Analogue materials and material properties 89 D.1 Internal friction and cohesion D.2 Basal friction and cohesion of materials on Alkor foil D.3 Angle of repose

5 CONTENTS 3 E Experiments 92 E.1 Scale factors for all GFZ experiments E.2 Velocities for all GFZ experiments E.3 PIV profile plots F Results from other studies 96

6 1 ABSTRACT 4 1 Abstract This study aims at investigating the effect of material properties of granular materials on the structural development of convergent Coulomb wedges in analogue experiments. Analogue materials used in this study include natural quartz sand, glass beads of different grain size ( µm) and corundum sand. Internal friction and cohesion of analogue materials, as well as basal friction and cohesion of material on Alkor foil are both measured with the Schulze Ring-Shear-Tester. The influence of sieving style on material properties and experiment outcome is investigated by performing experiments in which material is sieved from heights of 10 cm to 90 cm by different people and the resulting bulk density of the granular material is measured. A series of systematic analogue experiments in both the push and pull setup are performed for different analogue materials. The experiments are documented with digital sidewall pictures and 3D laserscanning. The digital images are evaluated with Particle Image Velocimetry (PIV) to visualize the strain evolution over time. Thrust spacing, activation time and reactivation of in-sequence thrusts are measured for each experiment. A new spatially and temporally high-resolution approach to measuring surface slope is introduced and applied to image analysis and to the evaluation of profiles obtained with laserscanning. This allows the visualization of the temporal development of surface slope, wedge width and wedge height. This procedure is compared to other methods of determining surface slope. The effect of sidewall friction on experiments is quantified by measuring lateral changes in surface slope. This dataset is also used to identify the main differences between pushed and pulled wedge experiments. Surface slope was found to be highly transient for pushed wedge experiments, whereas it reached and attained a stable value in pulled experiments. Pushed wedges are supercritical and they typically exceed the critical surface slope by Wedge width and wedge height grow as square root functions of convergence. The density of granular material is highly dependent on sieve height. Sieving from a height of less than 50 cm produced a bulk density that was up to 10% less than the maximum bulk density. Glass beads were found to produce a more regular structure of in-sequence-thrusts in both, space and time, than sands, while displaying less variability. Pushed wedges are not described by Critical Taper Wedge Theory, because they exceed predicted surface slope by more than 5-15.