Transportation official Cathy Bechtel is asked the question all the time: "When is that tunnel going to be built? When can we drive in it?"

Commuters are understandably eager to try out the highway that one day might be built under the Cleveland National Forest, connecting Riverside County bedroom communities with Orange County jobs, said Bechtel, director of project development for the Riverside County Transportation Commission.

There's just one problem, she said: "We don't know if it's even possible yet."

Riverside County transportation officials hope to determine whether or not it is possible to build the $6 billion road by sometime late next year. And officials say the answer lies deep beneath the chaparral- and oak-covered peaks of the Santa Ana Mountains that straddle the Riverside-Orange county boundary.

In 2005, The Riverside County Transportation Commission, in a bid to deliver relief to commuters who fight congestion on Highways 91 and 74, backed the concept of a highway under the mountains, following a study of a half-dozen options for improving travel between the counties. The new road would run 12 miles from Cajalco Road and Interstate 15 in south Corona to the junction of Highways 241 and 133 in Irvine.

The four- to six-lane highway would run in twin parallel shafts approximately 54 feet in diameter, with eastbound lanes in one bore and westbound lanes in the other, said Tony Rahimian, a Santa Ana consultant helping the agency study the concept.

A possible third tunnel could house trains, trucks and utility lines.

First things first

Because of the huge financial mountain officials will have to climb to pay for the expensive venture, Rahimian said consultants have recommended the highway be financed as a toll road.

The tunnel toll-road concept emerged in late 2005 from a joint Riverside County-Orange County study that examined options for providing congestion relief. That study, in an admittedly very rough guess, estimated that construction could begin in 2017 and take six years to complete.

And, so, the answer to the question everyone is asking is that it will be 2023 -- at the earliest -- before anyone gets to drive through a tunnel to Orange County.

Before commuters even begin to think about using the road, though, transportation officials will have to answer the fundamental question of whether the project is technically feasible.

In their quest to find that answer, Riverside County officials recently applied for permission from the U.S. Forest Service to drill nearly a half-mile down into the mountains. Ranger Keith Fletcher of the Cleveland National Forest's Trabuco district said it is "highly likely" the request will be granted, and that permits will be issued early next year.

The commission is hoping to obtain rock and soil cores at five different locations and has suggested seven potential sites for the borings, Fletcher said.

Water and rock

Eric Haley, the transportation commission's executive director, said "the borings are absolutely central to determining the viability of the project. We need to know what kind of rock we're dealing with and how viable the soils are for a tunnel. And we need to know a lot about water tables."

Rahimian said it is important to find out where the ground water lies because that will help planners and consultants gauge the water pressure under the mountain.

"We want to make sure that the tunnel is leakproof, so that water doesn't leak into the tunnel," he said. "It won't be acceptable to anyone if we change the ecology of the mountains."

Rahimian said planners also want to know whether the rock is solid or loose. The more solid the rock, the easier --and less expensive -- it will be to construct tunnels through the mountains.

Bechtel said the commission has hired Kleinfelder of Irvine to drill for the answers. She said the firm's $6 million survey will get under way as soon as the forest permits arrive.

Rahimian said it should take between three and six months to drill. He added that the drilling would present a challenge because most of the potential sites cannot be reached by truck or car, and a helicopter would have to be flown in.

Each core will be drilled approximately a mile apart, he said.

The money for the undertaking would come out of a $15.8 million federal grant approved a few years back for the highway study.

No drumbeat of support

The drilling would follow a similar exercise undertaken several months earlier, in the same area, by Metropolitan Water District. The district drilled two holes.

Metropolitan spokesman Bob Muir in Los Angeles said the giant water agency is analyzing the cores to see if it is feasible to punch a pipeline through the mountains to connect a planned treatment plant near Lake Mathews with its water delivery system in south Orange County. Muir said results are anticipated in December.

Regional officials have raised the possibility of pooling their resources -- and saving taxpayers money -- by putting the road and pipeline in the same set of tunnels.

However, he said that may not be practical.

Because Metropolitan intends to use gravity rather than costly pumps to move water from Riverside County to Orange County, the water tunnel likely will have to be built well below the mountaintops, Rahimian said. And, in order to make the highway leakproof, transportation officials may have to build the toll-road tunnel higher.

And even if the technical, environmental and financial hurdles are overcome, there is still a political obstacle threatening to kill the dream of a tunnel linking Riverside and Orange counties. That obstacle is the significant opposition from south Orange County residents and elected officials who fear their lifestyle could be compromised.

"I would characterize the view of Orange County as cautious at this point, early in the game," the commission's Haley said. "It is fair to say that there is no drumbeat of support coming from Orange County, so there is still some selling to do -- if this turns out to be a viable project."

Contact staff writer Dave Downey at (951) 676-4315, Ext. 2623, or ddowney@californian.com.