Angat Project—The Leak in Tunnel Two

A month after diversion closure, a severe leak developed at the head of one of the diversion tunnels of the Angat Project in the Philippines. The leak was found to be in the intake concrete at the top of one of the two closure gates. Initial leakage flow was 7 cumecs under a head of 80 m, increasing to 32 cumecs at a head of 90 m. The leak opening could be reached with objects lowered from the surface into the gate slot of the intake structure. Attempts to close the leak with oakum masses rolled in canvas and with blocks of marine fender rubber failed. A later attempt with pervious rolls and mats of fencing succeeded in plugging the leak opening. Large wooden blocks were placed next, followed by successively finer materials down to sand and clay to seal the mass. Finally, the intake was completely covered with barge-dumped fill reducing leakage to 0.6 cumecs. The permanent concrete plug in the tunnel was built using a two-stage bypass system to carry leakage flow past the plug.