Highway tunnel tests are planned

Geological tests to determine if a water conduit and a highway tunnel can be built between Riverside and Orange counties are scheduled to begin this week in the rugged Santa Ana Mountains .

The Metropolitan Water District of Southern California plans to drill two deep holes to explore groundwater levels as well as rock and soil conditions along the routes of the proposed projects.

MWD is considering a conduit to carry water from Lake Mathews near Riverside to a location near Irvine, where it would be distributed to growing southern Orange County.

The giant water district has joined with transportation agencies in Orange and Riverside counties, which are studying a proposal for an 11.5-mile highway tunnel through the Cleveland National Forest in the same general area as the water line.

The highway tunnel -- one of several options under study to relieve traffic on the Riverside Freeway -- would connect Interstate 15 in Corona to the Foothill-Eastern tollway in Irvine.

Its most expensive version could cost at least $8.5 billion.

“This is the drilling we must do to determine if we want to proceed with a tunnel,” said Denis Wolcott, a water district spokesman. He said the information the district got would be shared with the Orange County Transportation Authority and the Riverside County Transportation Commission.

Officials at the water district and transportation agencies say the feasibility studies are critical for evaluating routes as well as estimating costs and construction time.

If there are serious complications, such as abundant groundwater and bedrock, officials say construction would be difficult, if not impossible.

“Our biggest concern is to build a tunnel that is leakproof,” said Tony Rahimian, an Irvine-based engineering consultant who is heading the highway tunnel study. “If the water table extends more than 750 feet above the tunnel, there will be too much water pressure and it will be difficult to seal.”

If there is much dense bedrock along the proposed routes, it would increase costs and add to construction time, Rahimian said.

Water district officials say preliminary results from the two drillings should be available in August.

The cost of the MWD study, including permits and agreements with the U.S. Forest Service, is about $5 million.

MWD officials say they will try to seek reimbursement from $15.8 million in federal grant money awarded to study the highway tunnel.

Later this year, OCTA and the Riverside County transit agency plan five other borings along the proposed route of the highway tunnel. The areas are so remote that drilling equipment must be airlifted to the sites.

“The information that MWD is going to obtain will be very helpful to us, but we are not sure it will be enough,” Rahimian said.

The agencies want to complete their analysis by the end of 2008, he said.

dan.weikel@latimes.com