For the last several weeks, workers have flown by helicopter into the rugged Santa Ana Mountains, where they have dug deep into the earth to help determine whether a highway tunnel can be built between Riverside and Orange counties.
If tests prove successful, in a decade or so trucks and cars coming from near Corona could be traveling through an 11.5-mile tunnel under the Cleveland National Forest, avoiding the congested Riverside Freeway, transportation planners say.
Engineers first need to determine the groundwater level and the type of soil and rock they must dig through, said H. Tony Rahimian, a consultant coordinating the geologic tests.
"We really don't know the depths of the tunnel yet, which will change when we know where the groundwater is," Rahimian said, adding that the groundwater level would help determine both the tunnel's alignment and elevation.
The tests will continue through October. Holes are being bored from 1,200 to 2,200 feet deep at five sites, the majority accessible only by helicopter. A final report is due next year.
Transportation planners once had a very skeptical view of whether a tunnel could be built beneath the forest. But the political muscle -- and potential funding -- gathering behind the idea has changed some minds.
Chris Norby, Orange County Transportation Authority Board chairman, said getting construction money may be difficult but not impossible. The tunnel is not on the county's transportation master plan, but Norby and planners in both counties have not excluded financing the construction costs, estimated at $8.5 billion, by selling bonds, charging tolls or by allowing a private developer to build it.
The entire OCTA board "is very interested in the idea, but it's a matter of being technologically possible. There are a lot of people who would pay a toll to get from the Inland Empire to south Orange County," Norby said.
The Metropolitan Water District of Southern California is exploring the same route as a way to get additional water to Orange County. Engineers with the agency bored two deep holes last fall to explore groundwater levels along the same route. Officials recently concluded that building a 10- to 12-foot water conduit can be done.
"Based on the information, we will design a special tunnel-boring machine so it will fracture rock into little chunks by shaking and vibrating," said Gordon Johnson, the MWD's chief engineer. "Those chunks will then be removed by a conveyor belt."
The MWD is aware of earthquake faults in the area, such as the Elsinore fault, but the proposed route is outside the affected zone, Johnson said.
It is still too early to fully assess the earthquake risk for the highway tunnel, Rahimian said.
"Neither our tunnel nor MWD's goes through an active fault," he said. "The Elsinore fault runs parallel to the I-15 and to the best of our abilities we've tried to avoid any active faults."
The MWD wants a conduit to carry water from Lake Mathews near Riverside to lines within Irvine, where it would be distributed to growing southern Orange County.
Initially, water and transportation officials wanted to build the project together.
They have shared information, but some planners believe a joint project would be difficult.
The MWD wants to build deeper into the mountain and rely on gravity to help move the water inside the pipe rather than expensive pumps. By contrast, highway tunnel engineers want a higher elevation to avoid the water table.
"MWD can go deeper than we can," Rahimian said. "They only have a 10-foot-wide tunnel to bore, where ours is 50 feet wide. . . . Like them, we need a good understanding of the groundwater in the forest so we don't adversely affect the environment."
The tunnel is one of several options under study to relieve traffic on the heavily traveled Riverside Freeway, the main connector between Riverside and Orange counties.
A tunnel would connect Interstate 15 near Corona to the Foothill Eastern tollway in Irvine.
The OCTA and Caltrans plan to widen the freeway by a lane in each direction. The Riverside County Transportation Commission has already added lanes to the freeway and plans to extend the toll lanes along the route farther into the Inland Empire.
Improvements must be made to alleviate congestion on the Riverside Freeway, said Cathy Bechtel, project director for the tunnel corridor, which is a joint venture of the two counties' transportation agencies.
"If nothing is done," she said, "engineers would have to blow out the [freeway] to 22 lanes to handle the projected traffic demand by 2030."
There is no single fix to the traffic problem, Bechtel said.
"That is why we need to do a variety of improvements, including studying the feasibility of building a tunnel," she said.