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  Site C : Clean Energy Project - Diversion Tunnels

  The Site C Clean Energy Project is currently under construction in northeast British Columbia, Canada. The 1100 MW hydroelectric project includes two diversion tunnels 700 m and 792 m long, excavated to nominal 12 m diameter. Final, concrete-lined diameter is 10.8 m. Host rock is a horizontally bedded weak clay shale classes as ISRM R1 to R2. Despite the weak rock mass classification, the construction of a fully instrumented 11-m diameter Test Chamber at the location of the diversion tunnels demonstrated the ground behaviour was essentially elastic, recording a few millimetres of convergence. That experience was factored into the layout of the diversion tunnels and the design of the support system. The tunnel support system of tensioned rock bolts and fibre-reinforced shotcrete with steel sets at the portals performed well during excavation. The monitoring of the tunnel performance during construction also indicated that beyond the influence of the valley wall relaxed zone near the inlet and outlet portals, the clay shale response to tunnelling was essentially elastic, with the tunnels performing as if excavated in massive rock. The slaking nature of the weak clay shale was mitigated by the use of road headers to advance the top heading, 4-m long advance rounds and by application of fibre-reinforced shotcrete after rock exposure.

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