The paper describes the engineering-geological conditions and the structural decisions of the underground machine hall of Rogun HPP under construction in Tadjikistan on Vaksh river as well as the results of the experimental (model) and theoretical studies and in-situ observations of the variation of the stress-strain state of the surrounding rock mass and the supports of the openings, and the assessment of stability of the opening during and after the construction. The analysis of the results of long-term in-situ observations and previous model geomechanical studies permitted to make the assessment of the present-day state of the machine hall and give recommendations of its further use. The authors of the report highly acknowledge the contribution of the late A. Nikitin for the fulfillment of the present analysis.
The Rogun hydropower project constructed on Vaksh river in Tadjikistan is composed of the 335 m high embankment dam and the underground power plant located at the downstream portion of the dam at a depth of 350–400 m. The site of the hydropower scheme is composed of the rock material- sandstones and aleurolites whose layers are inclined at an angle of 70–75°. The rock is medium and weakly fissures. The strength of the sandstone Rcom= 100 MPa, the strain modulus of the mass composed of sandstones E=9000 MPa. The corresponding parameters of alurolites are 70 MPa and 5500 MPa. The density of the rock material Δ=26,5 kH/m3. The seismicity of the territory is assessed as IX according to MSK intensity scale. In the bounds of the site there are tectonic disturbances along which the present displacements measured as several mm/year are recorded. The determination of the structural layout of the power plant was made with comprehensive account of the site geology: the machine hall and the transformer bay are arranged in one tectonic block characterized by a uniform stress field. The long axes of the openings are oriented transverse to the layers, in the direction of the maximum stress vector. The machine hall is located in the sandstones to the maximum possible extent and the transformer bays are completely in the sandstones (Figure 1). The machine hall has the following design dimensions: the length 220 m, the width 22 m, the maximum height 78 m. The transformer bay is at a distance of 40 m from the machine hall, the relevant dimensions of the transformer bay are: 200; 20 and 40 m. The floor elevation of the transformer bay is close to the elevation of the crane rails of the machine hall At a 20- 40 m distance from the machine hall on the upstream side there is a mounting chamber of the temporary water supply route which is 50 m long, 12 m wide and 20 m high. Besides, the site of the machine hall and the transformer bay is already equipped with various purpose tunnels. The layout of the underground excavations is shown in Figure la.