ABSTRACT. High pressure ground water influx into the Alfalfal Main Tunnel: characteristics, remedial action and interpretation. During excavations of the Alfalfal Hydroelectrical Project Main Tunnel a case of large ground water inflow occurred, with 150 l/s influx into the tunnel, with an unusually high measured pressure of 100 bars. There are no previous records of similar pressure under the same conditions in Chile.The Alfalfal Consulting Team designed a special method of excavation and support by sections, as well as a systematic cement-bentonite slurry injection system in order to solve the problem and continue with the normal tunnel excavation. The hydrogeological interpretation considers the existence of a water zone that coincides with one of the main fracture systems, with orientation NS-5°W and dipping ca 35°E. Its opening is favored by the rock stress situation, thus allowing the 1,000 m connection with the surface.