Risk management in Outfall tunnelling, the MPSO tunnel, Doha, Qatar

The Musaimeer Pumping Station and Outfall (MPSO) is a successfully completed 10km-long and 3.7m-ID outfall tunnel. MPSO is a part of a bigger system comprising a pumping station, an offshore riser shaft and an 84-port diffuser field. This system will drain storm and groundwater from southern Doha, Qatar, at a peak flow rate of 19.7m3/s. The authors present how the geotechnical risks during tunnelling were identified, managed, and mitigated based on cross-correlation of the results from the client's extensive offshore geotechnical investigations in weak/soft rock mass, the contractor's Seismic Reflection (SR) survey and Electrical Resistivity Tomography (ERT) survey as well as the Bore-tunnelling Electrical Ahead Monitoring (BEAM) System used during tunnelling. Good and continuous correlation was observed between the different types of geotechnical investigations, which significantly reduced the risks during outfall tunnelling and allowed for consistent, safe, and uninterrupted operations along the whole alignment. The authors have also compared the laboratory test results obtained from the borings performed during the first ground investigation phase of MPSO with the ones collected for the upstream connecting tunnel completed in 2017 to expand a database for weak rocks common to the Doha area.