Construction and performance of the Karkheh dam complementary cut-off wall: an innovative engineering solution

Construction of a dam cut-off wall is one of the most challenging tasks in dam engineering given the deep excavations involved and the complex interactions between stiff cut-off walls and soft surrounding soils. Here, we present innovative solutions for the development of the Karkheh dam’s complementary cut-off wall in southwest Iran which is among the largest structures of this type worldwide with a maximum depth of 115 m. Due to excessive water seepage and high hydraulic gradient following the reservoir impoundment, additional measures were considered among which was the extension of the existing cut-off wall. The main goal was to decrease the hydraulic gradient of the seepage through the dam foundation. The construction of this new wall, which is called as the complementary wall here, was associated with a number of technical challenges among which were: the connection between the new and old walls; trenching and placing of plastic concrete wall through different dam body zones; and slurry loss during trenching through the dam body zones. The complementary wall was constructed successfully producing invaluable engineering experiences including: design of a U-shaped panel as the connecting panel; design of a new method for grouting through uniformly distributed filter/drain materials; and adding cement-based grouts to the cut-off wall panels to prevent slurry loss. The complementary wall helped to decrease both total seepage and the hydraulic gradient; for instance, in the right abutment, total seepage was cut for 25% and the hydraulic gradient was reduced from 0.2 to 0.095.