Subway Project Faces New Sinkage Danger : Red Line: Tunneling work halts in North Hollywood as engineers study how to keep the soil above from collapsing.

A thin fiberglass rod driven 45 feet into the ground in North Hollywood signaled the warning. The rod had dropped a half-inch, marking trouble below, where a giant tunneling machine had forged 40 feet ahead into the fickle soil.

Engineers knew they faced a considerable challenge when construction of the Metro Red Line subway moved into the San Fernando Valley on Feb. 13. But it wasn’t until the rod’s slight drop April 7 that they realized the troubles were greater than they had foreseen--that the ground was almost continuously collapsing as they excavated the subway tunnel, potentially jeopardizing not only their construction project but buildings and streets above.

Work on the initial two miles of the Valley subway, costing $65.4 million, has been at a standstill since. Meanwhile, some of the world’s leading mining experts are trying to decide how to tackle the geological phenomenon, and officials are bickering over who will pay for the extra work.

“There are thousands of ways of doing the job,” said Al Wattson, construction manager of the 11.6-mile segment of the Red Line that will stretch from North Hollywood to Hollywood. “It’s partly science and partly art.”

As officials engage in the art of paying bills and negotiating, geologists have been immersed in the science of determining exactly how far the ground has sunk. Thanks to a complex measuring system in which sensitive fiberglass rods detect the slightest ground movement, they are certain of their calculations and can now use them to prevent buildings and infrastructure from shifting.

“The numbers are very precise,” said Anthony Stirbys, geotechical coordinator for the Metro Rail construction manager, Parsons-Dillingham.

In a way, building a subway in Los Angeles is a bit like a child’s game at the beach. You cut the top and bottom off an empty tin can and shove it horizontally into the side of a sand dune. What happens? As the can fills with sand at one end and is scooped clean at the other, gravity pulls more sand inside. The result: The surface of the dune drops.

That’s what happened last August in Hollywood, when a portion of Hollywood Boulevard above the new subway tunnel sank almost a foot, potentially compromising the safety of buildings, streets, sewers and utility lines above.

The engineers faced with the delicate task of tunneling from North Hollywood to Universal City experienced the same geological problem as they did in Hollywood, though to a far lesser degree.

In the Valley, too, much of the subsoil consists of sand and gravel washed down over thousands of years from the surrounding mountains.

Thus, building a subway in Los Angeles is different from tunneling in other American cities such as New York and Chicago, where the hard subterranean rock was comparatively easy to drill into and resisted collapse, said Khosrow Bakhtar, a Newport Beach tunneling expert. However, he said, similar troubles with subsidence plagued the subway project in London, a city built on clay, and various other mining projects throughout the world.

The soft soils and fractured rocks in both Hollywood and North Hollywood, Bakhtar said, are “characteristic of the whole Los Angeles Basin from here to China Lake.”

As they began work in the Valley, engineers were initially optimistic that their giant excavating machine would advance eight feet an hour--or as much as 200 feet per day--beneath Lankershim Boulevard. But two months later, after a series of starts and stops, the tunnel had progressed a total of only 216 feet.

“We’re dealing with young alluvial soil, and there’s no way to define the geology ahead,” Wattson said. Although hundreds of test holes were bored along the route before construction began, he added, “you just can’t drill enough test holes to have a continuous profile.” The soil along most of the subway path through North Hollywood is an unpredictably diverse conglomerate of deposits carved and layered over the centuries by the rampant abandon of the Los Angeles River, which often altered its course by miles until it was harnessed in concrete channels only 50 years ago.

Thus the texture of the soil 50 to 70 feet below the surface, where the subway is being tunneled, rapidly changes from coarse to fine sand and from cobblestones to boulders, Wattson said. The contrast in soil types is so dramatic that it can be noticed even within the relatively narrow hole being dug for the subway tunnel, only 20 feet in diameter.

While the sinkage April 7 was relatively small and within the MTA’s guidelines for construction, officials say they don’t want to repeat the lesson learned in Hollywood, where the sinkage caused authorities to briefly seal off nine blocks of Hollywood Boulevard and evacuate residents of an apartment building.

This time, sensing devices like the one that detected the half-inch sag are being placed at closer intervals--100 or even 50 feet apart in sensitive areas rather than the 200 feet originally thought adequate.

Called borehole extensometers, or E-wells, the devices consist of a steel shaft drilled vertically into the ground along the route of the tunnel. Inside the shaft are three fiberglass rods of various lengths, the longest stretching to within five feet of the tunnel ceiling.

The rods can detect movement or subsidence in the soil that is less than the thickness of a dime.

As an army of surveyors, geotechnicians and engineers monitors equipment at the surface around the clock, other crews of operators and miners tend to the digging below.

The subway tunneling machine acts much like the child’s tin can at the beach--or a voracious mole greedily pushing its way through the earth. It ranges in length from 260 to 300 feet. Workers even refer to its ends as the “face” and “tail.” At its face, the machine has a circular nose that is pushed into the soil like a cookie cutter.

Then the dirt is scooped out by a mechanical arm, placed on a conveyor belt, and dumped into a cargo car pulled by a mining train. It is hauled out of the tunnel and trucked away.

When sagging occurs--which one MTA worker said could be caused by no more than a helmetful of sand trickling down--grout can be quickly pumped in to fill the hole and stabilize the soil. That was the remedy used in Hollywood last summer.

But prevention is always preferable, and several techniques are being used--or tested--in North Hollywood in addition to the E-wells. In one proposed method, the top of the tunnel shield--the giant ring of steel that serves as the “tin can” portion of the tunneling machine--may be lengthened to reduce the chance of soil collapsing at the face.

Horizontal steel plates have already been added to the front to stop the dirt from caving in. Wood boards are also being kept ready for workers to quickly swing into place and hold up the soil. But those procedures are time-consuming, impede the operation of the digging arm and do not completely resolve the problem.

Teams of experts are considering still more methods, such as pumping grout into the ground to change the nature of the soil before excavation begins. Work on the North Hollywood project will not resume until solutions are found, officials said.

“There’s always a chance for something to happen, said Wattson, the engineering expert. “We just have to accept that.”