Virginia Power is drilling holes into the Bath County mountain where that nation's largest pumped storage hydroelectric plant is located to relieve high water pressure that could cause landslides and millions of dollars in additional repairs.

The water pressure is caused by leaks in tunnels that carry water from the lake on top of the mountain at the Bath County site to a lower reservoir.

Unless relieved by the holes, the water pressure could cause landslides on the uninhabited mountain in the Allegheny mountain range, but no substantial threat to lives or property exists, officials said. Another solution would force the company to shut down the plant until the leaks in the tunnels could be stopped, said Virginia State Corporation Commission officials and sources close to the project.

"At this point, we're deeply concerned," said one SCC official. "Our concern is over extended shutdowns and the cost of increased major repairs."

Virginia Power said yesterday instruments installed in the mountainside had detected a sharp rise in water pressure on Jan. 22. The company said the pressure was stabilizing after drilling 36 drainage wells in a mountain gully called Johnston City Hollow.

The wells allow water that has seeped out of the project's tunnels to drain into the plant's lower water reservoir instead of building up inside the mountain.

The pumped storage facility, the largest in the United States, has two giant reservoirs, one 1,000 feet above the other. The plant generates electricity when water from the upper reservoir flows through vertical tunnels to the lower reservoir, making power as it flows through turbines. At night, when other Virginia Power plants are turning out more power than the company's customers can use, the excess electricity is used to pump the water back into the upper reservoir -- where it can be released the next day to generate more power.

"There has already been a measurable decrease in pressure," said James E. McDonald Jr., spokesman for Virginia Power. An upswing in internal mountain pressures "increases the chances of surface mudslides and groundslides, and we don't want it to happen," he said.

More drainage wells may have to be drilled to further relieve pressure, said Teed Wafle, director of engineering for Virginia Power. But so far, "we put in relief wells to relieve the high pressures, and the drain holes are working," he said.

Internal pressures could only cause slides if they were building near the mountain's outside slopes, but they have built up "well back in the mountain," he said. Even if slides occurred, they would not threaten the 22-story powerhouse where about 100 employes work, he said.

The drilling is expected to add $250,000 to $300,000 to the cost of the $1.7 billion project that Virginia Power said is on budget.

Last June, the Virginia State Corporation Commission, the state's utility regulating commission, hired Shannon and Wilson of Seattle to investigate the engineering, design and construction work on the project after leaks were detected in March in one of three mountainside tunnels.

Virginia Power then spent $60 million to inject grout into the mountainside to stop the leakage, and recently Shannon and Wilson gave the utility a clean bill of health. Virginia Power's McDonald said the grouting work cut the initial leakage in half.

The Virginia SCC official said yesterday the high water build up of pressure was not a good sign. The SCC said Virginia Power "did admit that essentially they weren't sure the situation had stablized, and they could in the future have high readings" of internal pressures.

If pressures do not stabilize, the SCC official said, "The next major thing to do would be to strengthen the tunnels with steel liners . . . the plant would be out of service for three months." He could give no cost estimate for the liners.

One source close to Virginia Power's project said pressures that had built up were about 1,600 feet away from the powerhouse and that wells were being drilled because "we want to keep the high pressures back there, we don't want them to migrate towards the powerhouse."

The source said there was no evidence of any landslides, but that it was too early to tell whether pressures would stablize. Even if landslides occured, "you might see small clumps, but no large mass fall on the powerhouse," he said.