Sinkage Halts MTA Tunneling : Subway: Problems with digging gear cause ground beneath Lankershim to drop five inches. Experts are brought in to correct the situation.

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 in North Hollywood.

The sinkage, barely perceptible on the surface, caused authorities to close one northbound lane of Lankershim Boulevard for 14 hours. A big drilling rig, a crane and a pump were rolled in to dig holes at 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 tunneling halt and a temporary freeze on federal construction aid funds, the subsidence caused no water pipelines to break and 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 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 to work on the problem--one a civil engineering professor from the University of Illinois, the other a San Francisco Bay Area mining engineer.

Tunneling was halted about 2 a.m. Thursday when workers for Obayashi and the MTA’s construction manager, 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 the pavement on Lankershim.

The planned North Hollywood station on the Metro Rail Red Line, scheduled to open in 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.

Times staff writer Michael Arkush contributed to this story.