Quantifying the effect of failure mechanism on dam breach: A phyiscal modelling approach

Abstract

Tailings dams have received significant attention in the last century due to the catastrophic consequences of historical failures. To assess the associated risk of dam failure, breach studies are conducted to predict the key spatial and temporal concerns. To advance the state of practice, this thesis describes a series of 12 large scale laboratory experiments designed to investigate the characteristics of dam breach during failure. A series of 1 m tall sand dams were tested to explore the effect of failure mechanism on the breach outflow hydrograph. Three failure mechanisms were explored: 1) notch overtopping, 2) geotechnical seepage failure, and 3) a wide-width of overtopping. Results indicate that despite varying widths of initial overtopping, the outflow hydrographs were interchangeable with similar values of peak discharge and timing of peak outflow when compared from a formation time. Of the failure mechanisms explored, the geotechnical seepage failure was investigated further to understand the effect of downstream slope geometry on the evolution of retrogressive slope behaviour. Measurements of transient slope geometry for four physical model dams were used to develop a simplified geometric relationship to define the evolving geometry of the retrogressive failure based on the conservation of mass between the fluvial and sapping zones. This geometric slope model was then applied within a finite element method (FEM) and limit equilibrium (LE) analysis to compare outcomes to the observations of breach in physical model dams. Limitations of LE analysis indicate that the transient slope behaviour could not be investigated inside the static geometry of the model without an understanding of the erosional sediment transport processes altering the downstream geometry. Finally, the development of a new experimental facility to observe the breach and runout behaviour following instantaneous release of a fully characterized liquefiable material is presented. Results from an experimental trial of the facility demonstrated the challenges of conducting research in 1 g stress conditions and suggest that the rate of release impacted the viscous response of the sand.