MTA Tunneling Causes 3-Inch Offramp Drop

A 75-foot section of a Hollywood Freeway offramp, undermined by subway tunneling, has sunk 3 inches, the Metropolitan Transportation Authority said Wednesday.

The subsidence is nine times greater than the MTA’s contract with the tunnel construction firm estimated it would be. The offramp and the freeway are still safe for drivers, however, a Caltrans spokeswoman declared.

An MTA official blamed the sinkage on its effort to pump water out of the porous alluvial soil in an ancient riverbed at the base of the Santa Monica Mountains to prepare the ground for tunneling. He predicted that there will be no more sinking when construction crews burrow farther north, into firmer clay and bedrock.

“Sand compacts when it’s not saturated with water,” said John Adams, the transportation agency’s deputy director of construction. “This settlement was not unexpected.”

The subsidence is within the 3.5-inch limit Caltrans set in the permit it granted to the MTA and the Ventura Boulevard offramp near Universal City will remain open to traffic, said Margie Tiritilli of Caltrans.

“Buildings settle--it’s nothing to be alarmed at,” she said. “If anything were to develop that would compromise motorist safety, we would take immediate action.”

But subway construction critics observed that the MTA’s tunneling contractor, Traylor Bros.-Frontier Kemper, has only just begun to dig north from Lankershim Boulevard, across the street from Universal Studios. They predicted the subsidence would continue and grow worse.

“With only half the tunneling done across Lankershim, almost all of the tolerance for subsidence by Caltrans is exhausted,” said Los Angeles County Supervisor Michael Antonovich, an MTA board member who has frequently criticized the subway project.

“If this continues to sink, it has the potential of crippling the Valley’s freeway system.”

Under the Caltrans permit to the MTA, sinking beyond the 3.5-inch limit would require the contractor to suspend tunnelling while taking steps to shore up the subsidence.The MTA’s contract with the tunnel construction firm estimated that water loss would result in a maximum of a third of an inch of settlement under the Hollywood Freeway. A geotechnical design report dated Sept. 29, 1994, and written into the contract forecast that the compression would be spread over several hundred feet and “should therefore be inconsequential and unnoticeable.”

MTA construction executive Charles Stark said it is not unusual for such estimates to be wrong. “Estimates put in specs are engineers’ best guesses based on boring and pump tests performed prior to final design,” he said, but “ground conditions can vary pretty quickly.”

On Wednesday, a hairline crack could be seen on the Ventura Boulevard offramp of the northbound Hollywood Freeway just north of a bridge over Lankershim Boulevard. All morning, half a dozen MTA and Caltrans surveyors were lining up measuring equipment on the shoulder of the freeway in an effort to gauge the extent of the subsidence.

Adams said the MTA’s contractor pumped 700,000 gallons of grout--cement-like industrial epoxy--into the ground below the Hollywood Freeway from December to February in an effort to solidify the mushy alluvial silt. In January, the MTA board awarded Traylor Bros. $500,000 more for grout than the contractor had specified in its contract bid.

When complete early in the next century, the twin subway tunnels will stretch 2.3 miles from Lankershim Boulevard in Studio City under the Hollywood Hills to a Red Line station on Hollywood Boulevard.

The two 21-foot-wide tunnels now being dug with shovels and jackhammers are just a start. Late next month, two giant tunnel-boring machines will chug through these passages on rails to a spot about 200 feet north of the Hollywood Freeway, where hard-rock tunneling will begin in earnest.

One independent safety engineer warned that subsidence could grow worse then.

“Right now they’re just mining with hand tools and light drills. That’s not much vibration,” said Pat Chiodo, a safety inspector for other MTA tunnel contractors on previous subway segments and for the MTA’s insurance company.

“The real test of settlement will come when they run the tunnel-boring machine in there and start rotating the head. It vibrates like hell. If they’ve left any voids behind ungrouted, a 3-inch settlement could quickly become 10 inches.”

Veteran Newport Beach-based mining consultant Khosrow Bakhtar, however, said the settlement is probably nothing to worry about.

“Caltrans could fix a 6-inch settlement by just paving over the asphalt,” he said. “Three inches is no reason to raise an alarm.”

More serious ground settlements have resulted in long tunneling delays for the MTA.

Last March, the agency stopped tunneling along Lankershim Boulevard in North Hollywood after a 5-inch subsidence. A month before, tunneling halted after a half-inch settlement below Lankershim--at what eventually will be the northern end of the tunnel now being dug beneath the freeway. In June 1995, tunneling halted for four months after a sinkhole swallowed up a long stretch of Hollywood Boulevard. The MTA blamed the incident on a burst water main, but the Department of Water and Power blamed the MTA.