The San Roque Multipurpose Project is being constructed on the island of Luzon in thePhilippines. The project consists of a 200 m high earth and rockfill dam, a 345 MWpowerhouse and a spillway capable of passing 12,800 cubic meters per second.To construct the dam, a diversion system was required. This consists of a 55 m highearth cofferdam, two large tunnels, each 10 m wide by 15 m high, in combination with athird tunnel 6 m high by 6 m wide. The tunnels are excavated in rock and lined withshotcrete.The design of the diversions and particularly of the tunnel closure is heavily dependenton the seasonal variation in river flows. Current planning for the closure envisionsdifferent closing sequences for each of the tunnels, including a simple earth cofferdam,steel stoplogs and a steel wheel gate.The tunnels will be plugged with cast-in-place concrete in two stages. In order tomaintain river flows during reservoir filling, one of the first stage plugs will be providedwith a minimum release gate.
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