Geotechnical Aspects of Sub-Sea Tunnelling on the Musaimeer Pumping Station and Outfall Tunnel Project
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Musaimeer outfall tunnel is one of the longest storm water tunnels in the world with a total length of 10.2 km. The tunnel is connected via a drop shaft to the main pump station. This system will accommodate surface and storm water received from the drainage networks of 270 km2 of urban areas in southern Doha. Though the geological conditions remain similar to those in Qatar Metro projects, in this project, the tunneling faced new challenges because the outfall tunnel alignment is 40 m deep (25m below the water of Gulf plus 15m under the seabed). This project involves the construction of the outfall tunnel with an internal diameter of 3700 mm sloped at 0.05% upward to the riser shaft. The riser shaft, which is located at the end of the outfall tunnel, is connected to a diffuser field located on the seabed. The tunnel will be excavated by Tunnel Boring Machine (TBM) and is expected to encounter possible water inflows at high pressure, complex mixed ground, weaker ground strata prone to cavities or voids and the presence of vertical and lateral fractures connected to the seabed. The geotechnical parameters in sub-sea tunnelling are being assessed through all main project stages a) Evaluation b) Verification and c) Application during the execution of this project which is currently in progress. The tunnel alignment traverses through the Rus formation, Midra shale and Simsima Limestone. The TBM will require periodic atmospheric or hyperbaric interventions at the cutter head for cutter tool maintenance. During this process, the face can be mapped and compared to off-shore borehole logs. Geophysical seismic reflection/refraction and resistivity surveys have been carried out along the tunnel alignment. A non-intrusive radar system facility, installed on the TBM cutter-head, able to probe ahead in real time the ground conditions is also presented. By looking at TBM excavation parameters such as, thrust force, torque, penetration, cutter head rotation speed, correlations can be made to the above surveys and a look ahead plan can be developed to aid TBM operation.