- Title
- An experimental investigation of fragmentation occurrence and outcome in the context of rockfall
- Creator
- Guccione, Davide E.
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Rockfall research has made significant progress since the 1980s, with considerable improvements made in terms of design of protection measures, invention of new systems such as self-cleaning meshes, draperies and attenuators, and of rockfall trajectory modelling. The latter is a key step of the design of rockfall protection structures and hazard assessment, as it provides information about impact energy and possible trajectories of blocks. After a rockfall event, breakage of rocks is often observed. Rock fragmentation is one aspect of rockfall that is still poorly understood and usually not modelled because the current state-of-the-art knowledge is not sufficient to predict it or model it adequately. Indeed, it is a complex phenomenon influenced by many factors, such as rock strength, presence and properties of discontinuities in the block, stiffness of the ground, block shape and impact conditions. In this thesis, an innovative experimental fragmentation cell is presented to produce high-quality fragmentation data that will advance fragmentation knowledge. The cell was designed to conduct controlled vertical drop tests and record key impact parameters including impact force, impulse, impact duration, translational and rotational velocities (of the block before impact and its fragments after impact) and all energy components pre-impact and post-impact. The cell is equipped with four high-speed cameras and two mirrors providing six views, used for the accurate reconstruction of 3D trajectories of blocks and fragments, in translation and rotation. An extensive campaign of drop tests using artificial rock spheres of different diameter (50, 75 and 100 mm) and different mortar strength was carried out. More than 360 spheres were dropped with different impact energy in order to investigate the survival probability of spheres at impact, size frequency distribution of fragments, trajectory of fragments, fragmentation patterns, distribution of energy amongst fragments, key energy dissipation mechanisms, and more. The device was first validated using several series of drop tests and its ability to reconstruct 3D trajectories, including rotation was verified. Then, it was found that the survival probability of spheres upon drop tests follows a linear distribution, opposed to a Weibull distribution as often observed for breakage of particles. It is also shown that the survival probability is size dependent: the larger the spheres, the more energy is required to initiate fragmentation. For the testing conditions considered, it was found that the amount of energy dissipated in fragmentation represents about 3% of the kinetic energy at impact and can be considered constant in the range of 5 to 10 m/s. The extensive fragmentation data clearly indicates that the assumption kinetic energy can be distributed to fragments proportionally to their mass (often made in fragmentation models) is not valid. More research is required to understand the process of kinetic energy distribution amongst fragments. A key point of this research was to tackle the fragmentation phenomenon from a stochastic point of view. The natural variability in material properties and block shape (albeit using the same material) implies that the amount of energy required to fragment a rock is not a unique value but a probability distribution. A novel model was proposed to predict the survival probability of brittle spheres upon impact based on the statistical viability of material strength. The model is based on theoretically derived conversion factors used to convert the critical work required to fail mortar cylinders in indirect tension (i.e. by Brazilian test) into the critical kinetic energy at failure in drop tests. The conversion factors account for the shape and size of the specimens tested and the increase of strength under dynamic loading (strain rate effect). The model was satisfactorily validated (relative errors of less than 15%) for three different sphere diameters and two mortar strengths. This model constitutes the first step into the prediction of survival probability of natural blocks. As far as the author knows, this model is the first of its kind.
- Subject
- rockfall; fragmentation; breakage; drop tests; survival probability; impact; energy balance; 3D trajectories
- Identifier
- http://hdl.handle.net/1959.13/1428317
- Identifier
- uon:38617
- Rights
- Copyright 2021 Davide E. Guccione
- Language
- eng
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