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  Design and repair works after collapse within the Estí headrace tunnel – Panamá

  strength and deformability of volcanic and sedimentary rocks may be underestimated in underground hydropower works particularly regarding their medium to long-term behaviour under high pore pressure conditions. In a similar geological scenario in some stretches of the Estí headrace tunnel (D=9m, L=4.8km) the characteristics of the rock mass have been affected by water in the tunnel and its sudden pressure changes. The provisional and final supports, implemented according to the initial design, appeared to be appropriate, but effective only for a few years, exhibiting some rock and shotcrete falls as well as a huge collapse 7 years after its commissioning. The reengineering of the final lining and the collapse repair works are presented in this paper. The design provides a fibre reinforced cast in place concrete lining on the whole tunnel length supporting the remaining potentially unstable tunnel stretches and reducing the risk of flushing of the degradable lithotypes. For the geologically sensitive zones, specific support solutions have been studied and implemented, including overcoming and repairing works of the main collapse area. The investigation, design and repair works have been carried out in just 11 months, therefore demanding considerable efforts from all the involved actors.

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