The stage-II diversion tunnel of the Nurek hydroelectric scheme is designed in conjunction with the stage-I and stage-In tunnels to pass flood flows during construction of the first-phase dams and the hydroelectric station building, and to release irrigation water downstream of the hydropower complex during the reservoir filling and lowering period within the range of levels below the sill of the intake portal of the stage-III diversion tunnel.Depending on the hydrologic conditions the tunnel has had two operating periods: The first was an unregulated period . when it came imo operation with the rise in upstream water level at the commencement of a flood; the velocity of the flow in the tunnel during this period did not exceed 15-16 m/sec. The second was the regulated period , when the head on the gates reached 110 m and the velocity below the gates ranged up to 42 m/sec. To ensure free-flow conditions in the tunnel, the chambe~ housing the regulating gates is provided with an air-supply conduit with a cross-sectional area of 9 m z.With regard to the above-mentioned operating conditions, the lining in the section upstream of the gate chamber, where the tunnel is under press=c and velocities do not exceed 20 m/sec, was designed to be of Mark-200 concrete; in the free-flow section downstream of the gate chamber, where the velocities range from 42 m/sec (at the start) to 32 m/sec (at the end). the lining within the limits of the wetted perimeter is constructed of Mark-400 concrete, with a slump of 2-4 cm for the invert, and the upper part is of Mark-300 concrete. In order to prevent cavitation in the tunnel or keep it to a minimum, strictrequirements were set for the quality of the lining surface (see Table 1).

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A meeting of the joint council of Ail-Union Planning, Surveying, and Scientific-Research Institute (Gidroproekt) on underground structures for evaluating the current state of the underground machine hall under construction at the Rogun hydroelectric station was held at the end of 1990. The reason for raising the question was the numerous negative features in the state of the workings of the machine hall (MH) observed both visually and by means of on-site measurements. The main points and results of analysis of the state of the MH and its causes examined by the joint council are given below. Description of the Object -Engineering-Geological Data, Main Designs, Predictive Evaluations. The object was already characterized earlier on pages of the journal [1, 2], and therefore only data about it needed for understanding the essence of the problem are presented below. The machine hall of the underground powerhouse of the Rogun hydrostation, designed by the Tashkent branch of Gidroproekt, is an underground working unique in size: span 21 m, height 70 m, length 220 m. Parallel to it, at a distance of 63 m at the axes, is an also rather large working -the transformer room (TR): span 19 m, height 37 m, length 182 m. In conformity with the design, in addition to functional transverse tunnel workings (busbar ducts, draft tubes), the rock mass enclosing the powerhouse is weakened by a number of auxiliary workings. The most significant with respect to effect on the stress-strain state of the MH are the access adit P-lb driven in the pillar between the workings of the MH and TR, parallel to them (Fig. 1), and the 12 x 20-m assembly chamber (AC) for assembling the shells of the penstocks of units 5 and 6. The set of workings of the powerhouse is located at a depth of more than 400 m in a single tectonic block bounded by active faults No. 1 (Ionakhsh) and No. 35. Within this block is also noted a lower tectonic disturbance, fault No. 70, extending at an angle of 45 ~ to the longitudinal axis of the MH.