What is the probability of encountering unexploded bombs on a U.K. construction site?

Quantitative risk data is essential for the efficient and consistent assessment of risk and allows for the effective use of cost-benefit analysis for assessing mitigating measures. The quantitative assessment of the risk posed by unexploded bombs (UXBs) on construction sites within the U.K. involves the identification of a set of contributory risk factors and modelling the interaction between them. The interaction of the risk factors has been modelled using Monte Carlo Simulation built into a new risk assessment tool (JG-MCS). Some of the output results from the tool are presented, including the limited validation that is possible against published work. The tool is used to assess the sensitivity of risk to some of the problem variables and several of the underlying assumptions are investigated. The model has also been extended to explore the influence of investigative mitigation measures. In this paper a set of fundamental questions are raised regarding the risks being assessed, how they are presented and the efficacy of some mitigation measures. For the scenario used, it was found that the overall risk of UXB encounter was low before mitigation and only slightly lower after mitigation. It was also found that the cost of implementing measures to reduce the risk posed by UXBs might be considered to be below the bracket of what is described by the HSE as ‘reasonably practicable’.