Predicted and actual risks in construction of the Mercer Street Tunnel

The Denny Way/Lake Union CSO Control Project includes three short tunnels constructed under active railroad tracks and the Mercer Street Tunnel constructed in a densely populated urban setting. Tunneling was conducted in recent glacial and modern fill deposits During planning and design, tunneling risks were evaluated. These included groundwater inflow, ground settlement, running sands, boulders, methane, highly plastic silt, railroad ballast, and buried obstructions such as wooden piles, railroad trestles, shoreline sea walls, concrete, and other construction debris. Risk registers were prepared to evaluate the potential severities, ramifications, frequencies, mitigation measures, and expected costs. A Monte Carlo simulation addressed the probabilistic aspects of these risks. These studies led to key decisions incorporated into the project contract documents. During construction of the tunnels, some of the predicted risks were realized and managed. A comparison between the predicted and actual risks has been assembled and is the subject of this paper.