Tunnel Fire Still Unsolved : Metro Rail: The blaze consumed much of the physical evidence, officials say. Cave-in has compounded the problem of finding the origin.

As construction workers prepared to begin excavating a collapsed section of Metro Rail subway tunnel, top fire officials told the Los Angeles Board of Fire Commissioners Thursday that the cause of the devastating blaze may never be known.

Fire Chief Donald Manning said that the searing 2,000-degree heat “pretty well consumed all the physical evidence” in the unfinished subway tunnel that runs beneath the Hollywood Freeway.

“We’re going through the process of trying to reconstruct everything that was in there,” Manning said. “It will be a long process.”

The work of arson investigators was made more difficult by the collapse of a 150-foot section of tunnel immediately south of the freeway. Officials said the cave-in was compounding the problem of locating the fire’s point of origin.

Deputy Chief Don Anthony said investigators have concluded that the fire did not begin at either end of the 734-foot-long tunnel, but started somewhere inside. Anthony told reporters there is “a very good chance” the cause will not be determined because of the damage.

The blaze, which broke out just before 2 a.m. last Friday and was not fully extinguished until Sunday morning, was fueled by wood timbers and a plastic liner used to keep methane gas and other toxins out of the tunnel.

Anthony said that the Fire Department will recommend that the contractor use precast concrete, instead of wood and plastic. “Exposed wood and exposed plastic (was) basically the only thing to burn in there,” Anthony said. “We would hope that they (the contractor) wouldn’t use it in the future.”

The tunnel was one of only two short sections of the 4.4-mile Metro Rail subway built with the plastic liner and heavy wood timbers called lagging. The other section of tunnel runs from Union Station to Temple and Hill streets in downtown Los Angeles. The wood and plastic is later encased in concrete.

When the fire broke out, Anthony said, “The tunnel was probably at its most vulnerable state . . . with all the exposed wood and plastic.”

As the tunnel burned Friday morning, Anthony said the Fire Department ordered the posting of guards around the clock in all Metro Rail tunnels built with wood and plastic.

In the remaining 3.75 miles of the subway system, precast concrete sections are being used to shore up the tunnel walls, instead of wood and plastic.

Officials at the Los Angeles County Transportation Commission agreed that the damaged tunnel may have to be reconstructed differently.

“We’re leaning 99% toward a concrete and steel liner,” said John Adams, executive vice president of Rail Construction Corp., a subsidiary of the commission. But a final decision will not be made until an independent investigation of the tunnel blaze is concluded, Adams said.

Edward McSpedon, acting president of Rail Construction Corp., said last weekend the design used in the burned-out tunnel is the same as the rest of the system but “the construction procedures are a little different. That’s the contractor’s option. Both designs are available as options.”

Tutor-Saliba-Perini, the low bidder on the tunnel between Union Station and Metro Rail maintenance yards near the Los Angeles River, chose to use the wood lagging. Adams said the cost of construction varies, depending on what type of material the contractor has on hand.

Officials at Tutor-Saliba-Perini could not be reached immediately for comment.

As arson investigators began to examine the rest of the severely damaged southern part of the tunnel, Anthony said the department had no information to indicate that sabotage or foul play was involved in the fire, the worst incident in the $1.4-billion subway project’s history.

“Foul play is something that we always look at,” he said. “We cannot say it was. We cannot say that it wasn’t at this point. We simply don’t have enough information. Over half of the tunnel has not even been looked at yet.”

While the Fire Department investigation and a separate district attorney’s probe of possible safety violations continued, all but the southbound lane of the Hollywood Freeway closest to the cave-in were open to traffic. Freeway on-ramps at Vignes and Alameda streets remained closed and truck traffic was still banned in the area on the southbound U.S. 101.