Emergency repairs are underway to shore up a narrow mountain road east of Geyserville where a landslide threatens a vital wastewater pipeline.

Crews are busy installing a sturdy retaining wall on a section of Pine Flat Road that washed out during a rain storm last Christmas Eve.

They're trying to finish the job before the rainy season returns and runoff from the steep hillsides in the area further undermines the road under which millions of gallons of treated wastewater is forced uphill daily toward the Geysers geothermal fields.

"A rupture in a pipeline of that diameter and that pressure could be catastrophic," said Mike Prinz, deputy director of subregional operations at the Llano Road wastewater treatment plant.

County crews using large boulders did a passable patch of the road following the slide, which sent tons of mud and debris hurtling down a steep embankment into Sausal Creek.

But with such a key piece of regional infrastructure embedded in the road at risk, city officials decided that a more lasting fix was needed.

"We don't take any chances with a pipeline of this importance," Prinz said.

They hired local engineering firm GHD, which, when it was Winzler & Kelley, did much of the design work on the 41-mile pipeline that went live 10 years ago in November.

The 3-foot-wide pipe carries up to 17 million gallons of treated wastewater daily from the cities of Santa Rosa, Rohnert Park, Cotati, Sebastopol, and Windsor 3,200 feet up the Mayacmas Mountains where it is injected into the geothermal steam fields and turned into enough energy for 100,000 homes.

Pushing that much water up such a steep hill requires a high-pressure pipe and three powerful pumping stations. The slide occurred just above the lowest of the three, the Bear Canyon Pump Station. At that location, the pipe pressure is one of the highest in the system, about 440 pounds per square inch, Prinz said.

It's not just the pipeline that was at risk from the slide. A fiber-optic cable that lets the pumping stations and other sensors communicate and a high-voltage electrical wire are also embedded in the one-lane roadway.

With so much on the line, the city was able to get the $250,000 project fast-tracked and approved by various regulatory agencies in short order.

Nine 40-foot steel beams were embedded deep into the hillside last week. Next week, sheets of concrete and steel will be installed between the beams to hold the hillside in place.

Drain pipes that were full of debris will also be cleared as part of the project, said Matthew Kennedy, a project manager at GHD. Drainage ditches and culverts in the area around the slide have become clogged over time, which likely contributed to the landslide, Kennedy said.

"Maintenance of the drainage system that is up there is important," Kennedy said. "If that doesn't happen, you end up with things like this."

The project should be finished by mid-October, Kennedy said.

You can reach Staff Writer Kevin McCallum at 521-5207 or kevin.mccallum@pressdemocrat.com. On Twitter @citybeater.