Metro Doha Green Line: Fibre reinforced tunnel linings - Design and construction experience

The Design and Build project “Metro Doha Green Line Underground” has been awarded to a joint venture composed of Porr, the Saudi Bin Ladin Group and HBK in summer 2013. It contains the “ready to use” construction of the underground part of the Green Line with 2 × 17 km tunnels, six stations, switching areas as well as cross passages and emergency exit shafts. The 34 km of tunnelling, realized with six EPB machines simultaneously, have been completed in only 18 months. The main impact on the determination of the design and build specification in terms of the segmental ring design is based on the extraordinary geological environment properties of the project area of Doha City. The geology is characterized by a dominating 3-rocklayers system comprising massive upper and lower limestone deposits, which are commonly separated by a thin shale layer. Additionally, and in regard to the nearby located sea-side the type of identified aggressive exposure conditions relevant to the concrete segment lining in terms of significant sulphate and chloride impact requires a durable reinforcement solution fit for required 120-year service life. Especially due to chloride attack an ordinary steel bar reinforcement system including a projected 90 mm nominal concrete cover at lining's intrados and extrados turns out statically difficult and constructionally non-practicable. Therefore, steel fibres are chosen to meet the strict design constraints.