Valley Subway Tunneling Delayed as Ground Sinks

In a development that alarmed the Metropolitan Transportation Authority, trouble with a giant tunneling machine Thursday halted work on the San Fernando Valley’s first subway project for the second time as the ground sank five inches below a heavily traveled thoroughfare.

The sinkage, barely perceptible on the surface, caused authorities to close one northbound lane of Lankershim Boulevard for 12 hours. A big drilling rig, a crane and a pump were rolled in to dig holes at the intersection of Lankershim and Weddington Street through which gravel and grout were injected to fill the depression.

Unlike a similar incident in Hollywood last summer that resulted in a four-month halt to tunneling and a temporary freeze on federal construction aid funds, the subsidence caused no water pipelines to break, and local businesses and homes were unaffected.

Still, it was the second such subsidence in three weeks along the North Hollywood subway segment. Tunneling, suspended Feb. 27 because of a half-inch drop in the sandy soil, had resumed only Monday.

“It’s an unexpected problem, and I am worried about it,” said John J. Adams, the MTA’s acting executive officer in charge of construction. “We will have to reassess what we’re doing with the machine and possibly change our method of mining.”

Adams said the tunnel contractor, Obayashi Corp. of San Francisco, planned to fly in two consultants--one a civil engineering professor from the University of Illinois, the other a Bay Area mining engineer--to work on the problem.

Tunneling was halted at 2 a.m. Thursday when graveyard-shift workers for Obayashi and the MTA’s construction management firm, Parsons Dillingham, noticed the subsidence above the face of the tunnel during a break.

Adams said the workers were cleaning the lower part of Obayashi’s digging shield of muck and silt at the time. To do that, they had to adjust a flat support device called a “breasting table,” which acts like a roof to support loose dirt just behind the tunnel’s dead end.

The adjustment caused loose dirt to fall from the upper part of the tunnel’s face. That slippage continued 45 feet to the surface, causing a 20-foot-long trough to form beneath Lankershim’s asphalt.

Adams stressed that the transportation authority’s contract with Obayashi allows for no subsidence at the tunnel face.

Khosrow Bakhtar, a private tunneling consultant in Orange County who works for the U.S. Department of Defense, the Swiss government and others, said the settling could have been minimized but cannot be avoided in some soils.

“If they had maintained the breasting table in place or provided some other means of support while cleaning the lower quadrant, settlement would not have happened,” said Bakhtar, who also teaches a course on hard-rock tunneling at UCLA Extension for miners drawn to Southern California for the MTA project.

The planned North Hollywood station on the Metro Rail Red Line, scheduled to open in the year 2000, is crucial to an ambitious plan to revitalize the community as a major entertainment center of theaters and shops.

Tunneling began Feb. 13 amid fanfare and estimates that work would proceed at a rate of 50 to 200 feet a day. To date, however, the tunnel has progressed only 70 feet in more than a month.

“It’s not unusual to have a lot of starts and stops when you begin one of these tunneling operations,” said Bill Heard, an MTA spokesman. “These machines aren’t like a pickup truck that just starts. You need to get your team working together and customize the machine underground.”

The sinking occurred, however, as the MTA seeks to remove problem-plagued Parsons Dillingham from the $65.4-million North Hollywood project. The agency has advertised a contract to replace the construction management firm, which was in charge when the debacle occurred beneath Hollywood Boulevard.

MTA staff members had selected a Pasadena-based firm to replace Parsons Dillingham, but the appointment was held up by the MTA board after questions over potential conflicts of interest arose.

Dick Robbins, founder of one of the world’s largest tunneling machine companies, was sympathetic to Obayashi, the firm carrying out the actual digging.

“It’s never obvious what you have to do to avoid subsidence,” he said from his office in Seattle.

Indeed, Kevin McCarney of the Universal City-North Hollywood Chamber of Commerce, a backer of the subway extension, said the subsidence beneath Lankershim does not frighten him.

“We were informed that there would probably be a little sinking here and there but that unless it’s over a foot, not to worry,” said McCarney, president of the chamber.

The sinkage across from the El Portal Theater occurred in soil similar to that beneath Hollywood Boulevard, earth described as “collapse prone” in a federally commissioned study. The problem with the soft dirt raises questions over the planned extension of the Red Line westward from North Hollywood.

Critics of a subterranean route through the San Fernando Valley have pointed to problems like those beneath Hollywood Boulevard as proof that tunneling is too risky and expensive.

Times staff writer Michael Arkush contributed to this article.

Tunneling Setback

Tunneling on a section of the Metro Red Line project, which will connect Downtown Los Angeles with the San Fernando Valley, was halted Thursday when the ground subsided 5 inches.