Andrea Maria Deganutti, 2008, The hypermobility of rock avalanches, 2008,
URL: http://www.cnr.it/prodotto/i/271106

This study focuses on rock avalanches which are large and very fast landslides characterized by volumes higher (on average) than two to ten million cubic metres with velocities of the order of tens of meter per second and are among the most destructive natural phenomena. The aim of the research is to increase the knowledge on rock avalanches, putting at scholars community's disposal a new contribution on some long- debated questions related to some aspects of their behaviour that have not been completely understood yet. In particular the subject of this work is directed to the transport stage of these phenomena, being the motion characteristics of rock avalanches one of the most puzzling questions in present days geological debate and none of the related theories advanced so far has been widely accepted by the scientific community. The strangest aspect of the behaviour of fast landslides with a volume of at least 107 m3 is that they travel much longer than one would expect by normal Coulomb friction mechanics, which, on the other hand, work rather well for rockslides of small volumes; this behaviour can be called hypermobility of rock avalanches. In the first chapter of this thesis a general introduction to the rock avalanche theme and related problems is given, followed by the general aims of the present study and the scheme of the adopted methodology. The second chapter is dedicated to the review of the international literature on the long debate on causes, mechanics and characteristics of rock avalanches, proposed theories for low friction behaviour, stated relations for maximum runout distance forecast and so on. The aim of this chapter is to give a general picture of the knowledge state-of-the-art on the matter. Then an introductory sub-chapter is given on the novel theory of dynamic fragmentation advanced as an explanation for some geological phenomena characterized by abnormally low friction, among which the hypermobility of rock avalanches. As an example of how dynamic fragmentation can act on rock avalanches and to give an illustration of the evidence of its effects, three real cases are presented at the end of this chapter. Two of them regard cases of rock avalanches, the third refers to a block slide, all three events happened in New Zealand. Chapter 3 is on the laboratory part of the present research_ a new concept rheometer, capable of high pressure and high shear rate, has been designed and built, in order to obtain an experimental evidence of the effect of the fragmentation in a shearing sample of rock grains. The main constructive difficulties, together with the apparatus capabilities and limitations are 9 described. The results of the fragmentation rheologic tests are reported and commented with special reference to the effects of fragmentation on rock avalanche behaviour. The fourth chapter is dedicated to the development of a distinct element method (DEM) numerical model of the fragmentation rheometer; this model has been conceived for the purpose of getting a versatile and trustworthy tool capable to simulate the effects of dynamic fragmentation with reference to both laboratory rheometric tests (the numerical model does not have the structural limitations of the real one) and real low friction phenomena. The numerical model results are presented and commented; pros and cons are reported as well. The model performances have proved good and worthy of further developments. The theses is concluded by a chapter (5) with the final remarks and conclusions of the whole study, with some future research perspectives opened by this research. Two Appendices (A, B) are attached at the end of the Reference listing_ Appendix A contains some more photographs and construction sketches of the high fragmentation rheometer with an extended set of resulted graphs. Appendix B contains more information and results of the numerical model of the rheometer with some examples of PFC (Particle Flow Code) and FISH (programming language embedded within the code) as it has been used to build the rheometer model.

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