Geotechnical investigation of the abandoned mine workings under Newcastle

There are often differences in ‘expert opinions’ of what constitutes appropriate design parameters for determining the level of stability of the existing pillars of old coal mine workings. For example, there is seldom 100% certainty about the value of any design parameter. Instead there are varying degrees of confidence (or belief) for each possible design value. This uncertainty in design parameters can be represented by probability distributions. The inclusion of such probabilistic information into a probabilistic risk analysis will enable the probability of failure to be estimated. To illustrate the utility of risk analysis for decisions taken with uncertainty, a probabilistic risk analysis has been conducted to assess the uncertainty of design parameters on the level of stability of existing pillars within a disused coal seam beneath a proposed surface development in the Newcastle area. The case study considers dimensional and level of inundation uncertainties. This case study provides a preliminary framework for a risk-based approach to decisionmaking for a geotechnical system subject to high uncertainties. The outcomes of the risk analysis are probability of failure and annual economic risks (expected losses per year). The paper will describe the steps taken in the risk assessment, risk acceptance criteria and how results from a risk analysis may be interpreted by a decision-making development consent authority.