dam-hydropower Engineering

At part load the swirl leaving the runner intensifies and the flow winds into a helical vortex rope inside the draft tube; the rope's eccentric precession makes wall pressure oscillate periodically, feeding power swings into the unit and vibration into the powerhouse. Admitting a metered amount of air through the head cover or the shaft center hole surrounds the rope's low-pressure core with an elastic gas volume, flattening pressure peaks and disrupting its rigid rotation. The correct admission rate is calibrated by model tests, since excessive air drags down turbine efficiency.

The pure-friction formula counts only normal pressure times a friction coefficient and ignores the cohesion of the bonded rock-concrete interface, making it overly conservative and blind to the benefit of grout curtains and drainage. The shear-friction formula includes both cohesion c′ and friction coefficient f′, consistent with SL 319 and US Bureau of Reclamation criteria, matching the shear-slip failure mechanism far better. Designs additionally verify deep-seated critical blocks, adding shear keys or prestressed anchors where margin is insufficient.

A gravity dam resists horizontal thrusts such as water and silt pressure through friction mobilized by its own weight on the foundation contact plane, and the sliding safety factor must satisfy code limits. Reservoir water percolating along foundation fissures builds a vertical uplift pressure beneath the base that cancels a large share of the effective dam weight, making it the governing adverse action for stability; a grout curtain backed by drain holes typically cuts uplift by more than thirty percent, noticeably slimming the section and saving concrete.