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Monitoring seepage in the foundation and body of the high gravity-arch dam of the Sayano-Shushenskoe hydrostation has been being carried out since the start of the construction and operating period (since 1978) and as the dam was constructed the range of monitoring tasks broadened. At the first stages the main tasks were to determine the parameters of the seepage flow and to assess the effectiveness of engineering antiseepage elements. During stageby-stage construction of the dam and filling the reservoir, seepage inhomogeneity of the foundation and the changes in the seeping medium occurring were evaluated. On the basis of the accumulated information the maximum allowable values of the seepage parameters for normal operating conditions were determined and recommendations were worked out for improving the seepage regime and extending monitoring. These tasks were accomplished on the basis of the results of observations of seepage, deformations, stress-strain state of the foundation and banks with consideration of geology, hydrogeology, hydrochemistry, grouting works, and special model and prototype investigations.The main objects of monitoring are the dam foundation and bank abutments composed of strong crystalline schists: paraschists under the right-bank part and spillway sections (36-66) and orthoschists under the powerhouse part of the dam and left bank (1-35). The paraschists have greater strength and toughness; the orthoschists are more brittle, fractured, more subjected to tectonic deformations. The rock foundation is cut by steeply and gently dipping joint systems filled with quartz-carbonate rocks and dense clay and grus filling.The waters of the rocks form an artesian aquifer with a decrease of water permeability deep into the mass. In the upper zone of the foundation, at a depth to 30 m (elevation 280 m), are the most decompressed and permeable rocks with a permeability coefficient Kp = 0.1-0.3 m/day. The middle zone with a thickness of 40 m (to elevation 240 m) has Kp = 0.05-0.1 m/day, below which the values of the permeability coefficient are less than 0.05 m/day.To strengthen the foundation and increase its impermeability, the design called for blanket grouting from 15 m under column II to 30 m under the upstream shoulder of columns III-IV; three-row contact grouting from the upper pool to the deep curtain (rows A, B, and C) with a depth of 35 m; deep two-row 100-m curtain, the downstream row of which is brought practically to the aquiclude; one-row grouting with a depth of 16 m is carried out along the upstream face beneath the upstream apron. During the grouting works, maximum absorption of grout was noted in the foundation of sections 26-31, 34, 35, and 39, and in the powerhouse section the weakened zone was confined to elevation 240 m and in the spillway sections to the upper 30 m (elevation 280 m). Contact, deep, and blanket grouting was carried out also in the canyon-wall abutments. In accordance with the design, drainage of the foundation was arranged from a gallery at elevation 307-316 m by ...