(10)Probabilistic Dam Break Assessment and Flow Slide Analysis for Tailings Storage Facilities

摘要

Dam breaks leading to a flow slide on the scale of Mt Polly, Los Frailes, Merriespruit and Stavahave been shown to have multiple root causes that combined as a chain of faults and events that ledto failure. These failures all experienced issues with regard to operation, management, constructionquality, unrecognised foundation or materials behaviour as well as structural failure. Whencombined these issues had the effect of removing support to a mass of poorly consolidated tailingswhich underwent static liquefaction and flowed with disastrous consequences. While forensicinvestigation of the root causes of a dam break provides invaluable insight to failure developmentthe key issue is how we can use the results of the investigation to analyse, track and ultimately stop,the progression towards a dam break and flow slide on a current operation. A fault-event analysisprocess provides a useful semi-quantitative probabilistic method for understanding the relativecontributions of the root causes and the way in which they can combine to precipitate a failure. Butit is also vital, when assessing the risks associated with a tailings storage facility, to work throughthe consequences of a dam break and flows slide. This requires simulation of the outflow rate ofthe tailings through the dam break over time and prediction of the lateral spread and distance ofthe outflow. The simulation also needs to bring into consideration the rheological properties of theflowing tailings and the uncertainty of these properties along with the uncertainty of other keyparameters including the width of the breach and the volume of the material that liquefies andflows. This paper sets out through a worked example a probabilistic model for flow slidemodelling that incorporates probability density functions for the main variables. When combinedwith the fault-event analysis the flow slide model enables the risk of flooding specificinfrastructure, as well as the depth of flow, to be determined on a semi-quantitative probabilisticbasis. It is demonstrated how, together with the fault-event analysis, the flow slide analysis enablesthe development of management protocols and infrastructure protection works that, ifimplemented properly, will ensure that the chain of faults that could lead to a dam break can bebroken, and a flow slide prevented.