The erosion of foundation soil at the Zagorskaya 2 pumped-storage hydropower project was caused by the "inefficient performance" of the plant's impervious system, according to a recent statement by Russian utility JSC RusHydro.
The company said workers first noticed water seeping into the 840 MW plant's turbine room on September 13, at which time 15 people working in the powerhouse were evacuated.
The turbine room and other areas were flooded during the next several hours via broken expansion joints and intakes of unfinished water pipes, caused by the erosion of soil that made the building to sag.
JSC RusHydro (LSE: HYDR) began restoration efforts immediately following the accident. An earthfill link was built on the side of the tailrace to isolate the plant building from the lower reservoir, while "practically all" of the water has been pumped out and is now below the floor of the powerhouse.
Some of the equipment has already been examined, though the company said it will wait until the powerhouse is fully dried up by the end of February 2014 before conducting a full report.
RusHydro is currently in the process of preparing documents for insurance companies, though a release said it will not know the total cost of the damages until a restoration plan is developed.
Zagorskaya 2 is unique, RusHydro said, in that the plant is built on soft soil instead of hard rock as is preferable. Such foundations are not typical in Russia's central region, however, and the company said its designers did not properly account for the soil around the plant's base.
The utility had wanted to put the plant's first pair of turbines into operation earlier this year, though the delayed completion of a transmission line by the Federal Grid Company (FCG) have moved Zagorskaya 2's commissioning date back.
Work on the pumped-storage plant began in July 2007.