High speed granular debris flows: new paradigms and interactions in geomechanics

Debris flows (often called mudflows) are a type of rapid landslide where soil, rocks and water flow together at high speed downslope. These natural hazards travel for long distances and pose risks to lives and infrastructure that they encounter in their flow paths. During a debris flow event, solid particle sizes and fluid segregate so that large particle are concentrated in the front, leading to high impact forces. This renders their behaviour both dangerous and physically complex so that there are several mathematical and numerical granular flow theories competing to explain their motion.

This research seeks to improve our understanding of debris flows and the impact stresses they can cause to obstacles they encounter, by conducting a series of novel laboratory scale tests in 2D and in 3D on flows confined within model debris flow channels. In the 2D model experiments, the soil and rock will be replaced by acrylic particles, and a photoelastic method used to enable the forces in the individual particles to be determined as the material flows downslope without considering fluid interactions. The results then will be compared against current theories of granular flow in 2D in order to determine which theories best match the results in terms of particle motion and stresses induced in the particles. In the 3D experiments, the soil and rock will be replaced by glass particles and the water by an optically matched fluid which will render the mixture virtually transparent. A laser plane passing through the system then will create a visual slice of the flow at its centre, so that the 2D slice of particles will appear as dark against a bright fluid background during high speed motion. Capturing this behaviour via high speed photography will enable comparison with granular flow theories in 3D in terms of particle motion in the presence of fluid and away from sidewall boundary effects.

In the second series of tests in 3D, an obstacle, representing a structural barrier to the flow or infrastructure, will be placed in the path of a model glass-and-fluid debris flow so that its interaction with the obstacle can be examined. As well as using more conventional techniques, the use of a novel method, holographic interferometry will be trialed to determine how the deformation and stresses in the structure itself are related to particle impacts from the model debris flow. This method can enable the detection of very small deformations over a whole object so is ideally suited to examining the complex interactions of multiple particle impacts with obstacles of differing shape and size.

The overall outcomes will be a several datasets that can be used to better model debris flows numerically, which uniquely includes both the influence of a debris flow on a structure and a structure on a debris flow. It is hoped that the results may lead to better design of barriers to debris flows, enhancing and protecting the natural and built environment and hence leading to greater protection of human life.

This research and its outcomes will have impact on a number of diverse groups and in several ways. The initial (first test series) outcomes will be a 2D and 3D dataset that can be compared with granular mechanics theories of flow in better detail (by generating force-chain behaviour in 2D, and particle interactions and flow velocities via digital image correlation in 3D) than has previously been developed. This dataset will be published in academic journals that are read by members of both the granular physics and engineering communities, thereby fostering a multidisciplinary understanding between geotechnical / hydraulic engineering and physics.

The second test series outcomes will have a much wider impact beyond academia, being of particular value to model developers in the hazards mitigation community. This goal of this group, working in academia, private companies and in government organisations, is better predictive modelling of flow behaviour towards the reduction of risk posed by debris flows, floods and landslides. The soil-structure interaction effects are particularly important as engineered barriers are usually designed by private companies and implemented (in general) according to local government edict to protect vulnerable populations and lifelines. Such barriers must meet the needs of the local community in question and data from these physical model experiments will assist in determining both debris flow behaviour upon meeting a rigid structural barrier and the best design of such barriers. This will have both an economic and societal impact in enhancing protection of our natural and built environment and hence, of quality of life.

The second series of outcomes also will generate impacts directly in geotechnical and multiphase research, being the result of the development of a novel research approach to experimental geotechnical engineering. This novel approach, holographic interferometry, while used in other disciplines such as medical research and in non-destructive testing, will be extended further for use in the multiphase problem posed by fluidized soil. If such an approach should prove successful, it could pave the way for further outcomes in research on geotechnical soil-structures interaction and has the potential to feed into other multiphase problems (e.g. industrial processing of granulated media and biomedical research).

Finally, in terms of public outreach, natural hazards occupy a special fascination with the public as does novel physics. This research proposal combines both within the context of an engineering and societal problem - namely, how to cope with an increasing global population living on more marginal land at the same time as more extreme climatic events lead to greater risks from debris flows and related hazards. The research project and its outcomes would be a unique opportunity to showcase to the public the interconnected reality of science, engineering and the natural world.