The contact of the upstream faces of concrete dams with the rock foundation arouses particular interest as the most probable place of occurrence of the limit state. The presently accumulated materials of on-site observations and theoretical investigations enable revealing a number of important regularities for their consideration in standard documents.We will present briefly the results of investigating the contact at operating structures. The Bratsk dam was the first object of such investigations begun by S. Ya. I~idel'man in 1965 [1]. During filling of the reservoir to the level of 84-90% of the design head, the instruments installed at a distance of 1 and 4 m from the upstream face showed a decrease of compressive strains and then an increase of tensile strains. At the normal pool level (NPL) disruption of the contact was recorded on a length of 10 m with a maximum open width of 0.3 ram. Control drilling revealed a zone of intense fracturing of diabases at a depth to 3 m under the base of the structure. Opening of the contact increases in the winter and decreases in the summer under the effect of seasonal variations of the hydrostatic pressure and outside air temperature. It has a tendency toward an increase with time and according to the readings of the piezometers behind the grout curtain presently reaches 15 m. This process is accompanied by a decrease of compressive stresses in the concrete at the base of the upstream face and increase of the seepage uplift pressure, During the past 15 years the discharge of the piezometers under the first column has tripled [2]. The Ust'llim dam is equipped with contact remote strain gauges installed at a distance of 0.5 m from the upstream face. In six sections there are longitudinal gauging sites within the first column [3]. Disruption of the contact was noted in the majority of sections, with a greater degree of opening in the powerhouse sections, with the exception of the three start-up sections that had a high construction readiness of the powerhouse by the time of filling the reservoir. The length of the fracture exceeded 12 m, the open width was more than 0.4 ram, owing to which some of the instruments failed. Formation of the fracture was accompanied by a decrease of compressive stresses in the concrete of the lower part of the upstream face.

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