Repairing Tunnel Will Be Costly, Time-Consuming

Like everything else associated with Metro Rail--the costliest transportation project per mile ever built--repairing the tunnel that was damaged by Friday’s fire promises to be an expensive and lengthy process.

But engineers said they do not expect to postpone the opening of the $1.4-billion subway, which already has been pushed back because of construction delays from 1992 to September, 1993.

Meanwhile, engineers patted themselves on the back for having squirted a cement-like gel into the ground under an elevated section of the Hollywood Freeway that passes over the burned tunnel. The gel, sprayed in advance of the tunneling, was meant to stabilize the ground in the event tunneling weakened it.

“If it wasn’t there,” said Al Vardanian, project engineer in charge of the tunnel segment for the Southern California Rapid Transit District, “it’s possible that the freeway wouldn’t be there now.”

Insurance is expected to cover the cost of repairs. A “wrap-up” insurance policy covers Metro Rail contractors, managers, designers and the RTD, said Barbara Anderson, in charge of risk management for the RTD. The builder has a $25,000 “risk deductible,” Anderson said. The balance of repair costs would be paid under insurance policies underwritten by Argonaut Insurance Co. and Lloyd’s of London. Anderson said the policies--one of which is for more than $75 million--are big enough to cover the fire damage.

The builder, Ronald Tutor, president of Tutor-Saliba Corp., termed the damage “mind-boggling.” He said the tunnel, which was three weeks behind schedule, would have been finished by Sept. 15. Now, he said, it probably will not be finished until next year.

On Friday, firefighters used mounds of dirt to choke off the fire at both ends of the tunnel, a process that an engineer likened to “closing an oven door with the oven on.”

Nonetheless, engineers and firefighters managed to enter one end of the tunnel in late afternoon. They estimated that temperatures were between 120 and 150 degrees Fahrenheit.

Engineers were attempting to assess how much shoring up would be needed before the freeway could be safely reopened, and which of at least three technical methods to use to rebuild all or part of the affected 729-foot tunnel segment--about 150 feet of which apparently collapsed.

Ground over the collapsed section had not been fortified with the gel, called chemical grout, because it supported only a vacant lot, engineers said.

Los Angeles Fire Marshal Dave Parsons said that it could be days before it is reopened. Late Friday, officials confirmed that the freeway would be closed at least through the weekend.

“We made a hard decision” not to reopen until the shoring is in place, he said.

Engineers said they expect to be able to use a parallel tunnel--eight feet away from the one that burned and apparently undamaged--to keep construction of other parts of the subway on track.

The next major step in building the 4.4-mile rail line is laying permanent track. The rail line extends from the fire site, in a service yard just east of Union Station, to stations at the Civic Center, Pershing Square and the Financial District before ending at Wilshire Boulevard and Alvarado Street.

Engineers said they need to be able to lay the track, which comes in pre-welded, 1,400-foot segments, from one end of the system to another before they can lay cables that will connect the system electronically and eventually feed information on train locations to a computerized control center.

They said they expect to be able to use the undamaged service-yard tunnel to get track-laying access, through a crossover tunnel, to the rest of the two-tunnel system.

The fire and partial cave-in came in a tunnel segment that is only about 17 feet below the surface. West of Union Station, Metro Rail tunnels dive gradually to a depth of about 50 feet.

In addition to the freeway, part of the final segment of the El Monte Busway passes over the damaged tunnel.

Gel-like chemical grout was also used to strengthen the ground under the busway, engineers said. In addition, the busway’s concrete piers were specially extended well below the level of the tunnel.

Preliminary engineering estimates Friday were that the busway would be all right, but Parsons said the busway also would not be reopened until shoring was in place.

The tunnel that collapsed was dug by a backhoe-type shovel housed at the front of a steel tube.

The tube, slightly larger than the diameter of the finished tunnel, is called a shield because it protects workers from cave-ins.

Workers assembled segments of this tunnel’s circular lining inside the shield. The shield’s operator, who sat closest to the tunnel’s face, then moved the machine forward by pressing hydraulic jacks against the end of each newly assembled liner segment.

The damaged tunnel, built as part of a $38-million contract for the entire service yard won by Tutor-Saliba, used a lining of steel ribs and timbers--the most popular method among U.S. contractors.

The steel rib--a circular steel I-beam--is put into place first. The shield pushes off against the steel rib.

The four-foot space between a set of ribs is then filled with lagging--in this case, timbers that are 4 inches by 4 inches.

The inside of the tunnel looks like the inside of a barrel.

Progress is slow--typically 40 to 50 feet a day.

“They do it in four-foot segments,” James Monsees, Metro Rail’s chief tunnel engineer, said in an earlier interview. “They erect a steel ring, set in four feet worth of boards . . . erect another steel ring. Shove forward and just . . . keep going.”

This tunnel segment “holed through” about a week ago, Vardanian said.

At that point the shield was removed.

Depending on the extent of damage, the contractor could elect to have workers dig out the tunnel by hand.

He could attempt to insert the same shield he used the first time. Theoretically, there still would be room for it to fit. But it would a close call, engineers said. Thus, a new shield might have to be built.

Or he could use a method called “open cut” construction in which he would dig a hole down to the tunnel from the surface and repair it from there.

Once the lagging is in place, and covered by the plastic sheeting that prevents gas from entering the tunnel, steel bars to reinforce concrete are placed around the circumference of the tunnel.

Special retractable steel forms are then employed and a foot-thick layer of concrete is pumped into the tunnel, filling the space between the plastic sheeting and the forms. The concrete is the tunnel’s permanent lining.