A comparison has been made of the natural stresses measured in the rock damsite of the arch dam being built for the Houdoni hydropower station using the geological, geophysical and geomechanical methods, on one hand, and the gravitational stresses found by the solution of the plane and spatial problem within the scope of the elastic theory, on the other hand. It is noteworthy that the measured natural stresses are generally higher than the calculated gravitational stresses, especially in the horizontal directions.

The damsite section of a 200 m high arch dam of the Houdoni hydropower station consists in a V-shaped canyon with a floor 15 to 20 m wide. The slopes of the canyon are mainly composed of volcano-tufogene rocks of the Bajocian stage formation interlaid with tuff and lava breccia, porphyrites, tuff sandstone and shale. Shale and sandstone of the upper Lias are exposed on the northern periphery of the section. The Bajocian stage rock depth at the damsite section is characterized by a non-uniform petrographical composition and structure. It is divided into four benches in the descending stratigraphical sequence. The first bench being only exposed at upper and medium elevations of the left slope represents mass green and grey porphyrites found as dense, tough and slightly fractured. The second bench consists in tuff sandstone with rare fine-fractioned interlayers of tuff breccia and brown and grey tuff shale. The depth of the left slope bench is 40 to 50 m, and that of the right is 100 to 110 m. These rocks are almost completely exposed on the right slope, and overlapped by drift beds and alluvion on the left. The third bench, the thickest, 120 m deep is lava breccia comprising fine porphyrites based on the lava cement. These rocks are well exposed on both slopes of the valley. The fourth bench is made up of alternating tuff sandstones and shales, and rather thin tuff breccia interlayers. The depth is 40 to 50 m. The aforesaid types of rock with the exception of lithology differ by the degree of schistosity and jointing and possess various indexes of physical-mechanical properties. All rock types in the massif are extremely changed resulting in a large variety of rock properties in the natural bedding. Surface relaxation and weathering are other factors strongly affecting the rock properties and condition in their natural bedding. This results in pronounced heterogeneity of rock physical-mechanical properties of the surfacial part of the profile. A whole complex of investigations was carried out in the determination of the natural stress field at damsite using the geological, geophysical and geomechanical methods. The geomechanical studies using hydrofracture were carried out in two boreholes at depth interval from 50 to 150 m (ref. to Prof.F. Rummel, W. Germany). For the determination of the natural stresses in the massif the following values found in the hydrofracture experiments will be considered as reference: the fracture pressure P e ' the refracture pressure Pr and the static pressure Psl with the closed estuary.

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