Anticavitation measures used in stage-II diversion tunnel of nurek hydroelectric scheme

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The 4-yr service period of the stage-II tunnel of the Nurek hydroelectric scheme, with velocities in the range of 35–42 m/sec and discharges up to 2000 m3/sec, permits the following preliminary conclusions to be made. 1. The steel lining provides adequately reliable protection of the gate-chamber structure against cavitation, but supplementary protection against its corrosion is required. 2. The cavitationally dangerous zone in the tunnel is limited to a segment 70–100 m long, located downstream of the reducer, convergent, portion of the gate chamber (7–10 tunnel diameters long). The lining of this segment should be constructed of concrete with an increased cavitation resistance and protective coatings. 3. Downstream of the cavitationally dangerous segment the erosive action of the flow on the lining diminishes considerably. The dense monolithic Mark-400 concrete in this zone under actual conditions provided an adequately reliable resistance to the action of the high-velocity flow. 4. Polymeric protective coatings on the concrete are effective, having regard to the increased total service period and the intervals between repairs of the concrete lining. Polymeric coatings should be applied onto dry, well-prepared concrete surfaces, strictly fulfilling the technological requirements. 5. Protective gunite coatings in the cavitationally dangerous zone are not sufficiently stable unless the necessary bond with the concrete is ensured, special additives are introduced, etc. If a good bond is effected, the gunite erodes uniformly. 6. For patching individual cavities and steps in the lining, polymer-solution and polymer-mastic epoxy compounds and water glass proved themselves effective. 7. The termination boundary of the steel lining with the concrete surface of the tunnel lining should, for greater reliability, be constructed in the form of an outward step with an air supply to this area, even if the concrete surface is protected by polymeric coatings.