Changes planned to prevent cave-ins


November 16, 1990|By Mark Bomster | Mark Bomster,Evening Sun Staff

George Morschauser wants to make sure the streets along the Metro subway extension don't cave in anymore.

Morschauser is the construction manager on the $321 million Metro project, where tunneling came to a halt yesterday after a big patch of Orleans Street collapsed just above the tunnel.

Construction officials say it could be at least a week before the contractor, Kiewit/Shea JV, is ready to resume work on the tunnel.

"We would like to review his plan of approach to prevent . . . this from happening again before we let him go ahead," said Morschauser, who works with the construction management firm of DKP Joint Venture.

Among changes being considered:

* Making sure that the contractor has equipment and material immediately available to deal with any future erosion, which apparently was not the case yesterday.

* The possibility of chemically treating the loose soil with a substance that would make it more stable and less likely to collapse during tunneling.

* Tunneling within an area filled with compressed air as a way of keeping material from seeping into the tunnel and making the process more stable.

"It's not a normal incident," Morschauser said of the collapse, though he added that "it's something that could be expected," given the geology of the soil in that area.

No one was injured and the tunnel was not damaged. The street collapsed after tunnel construction work caused underground water lines to break. The result: a giant sinkhole that has forced the closure of Orleans Street between Broadway and Central Avenue. Traffic is being routed through the area along a parallel service road while the hole is repaired. That arrangement is expected to last until early next week.

"We've got a lot of wet, sloppy material inside the tunnel at this time," Morschauser said. He said that material is "basically an inconvenience at this time" but it will take about a week to clear it away.

The subway tunnel is part of a 1.5-mile extension to the existing 14-mile Metro line. When completed, it will link Johns Hopkins Hospital and Charles Center, adding two new subway stops.

The new line actually consists of two parallel subway tubes 22 feet in diameter and more than 60 feet deep. They are being dug by specially built, $2 million tunneling machines known as "moles," which have been working since July. The furthermost machine had proceeded about 700 feet from Broadway.

Morschauser said the collapse took place when the contractor was unable to keep up with the erosion of soft earth above the tunnel, a situation compounded by the broken water lines.

It happened like this:

The tunneling machine was digging away at the dry, loose sand that makes up much of the soil beneath the surface in the area of Johns Hopkins Hospital.

When the machine digs forward, some of the loose sand just above the tunnel tends to fall or "run" into the tunnel itself. That leaves a gap just above the tunnel.

The sandy soil above the gap then tends to flow into the gap, a process not unlike sand running through an hourglass. That continues right up to the surface, unless checked.

Ideally, the contractor checks that process by injecting cement "grout" into the air bubble, drilling down from the surface or up from the tunnel itself.

In this case, however, by the time the contractor had set up its drills, the air bubble already had risen to the level of the buried water lines about 8 to 10 feet down, according to Morschauser.

With no soil to support them, the 6-inch and 10-inch water lines broke, flushing soil and water into the tunnel and creating a sinkhole in the street about 30 feet long, 25 feet wide and 15 feet deep.

A gas line that ran through the area did not break, and a sewer line that broke at the same time apparently did not contribute to the collapse, a Mass Transit Administration spokeswoman said.

A number of steps can be taken to make sure the same kind of collapse does not happen again, Morschauser said.

For example, "we will insist that the contractor has the proper material to drill immediately, to prevent this from happening."

Project officials also will consider treating the loose earth with an inert, non-toxic substance, sodium silicate, that chemically binds and solidifies the earth, making it less likely to erode during tunneling.

The contractor also may speed up the process of tunneling in an area filled with compressed air, which was expected to take place farther south as the work dipped below the level of ground water.

Morschauser said work would not get under way again until construction officials are satisfied with the precautions.

He added, however, that "this area is probably the worst-case situation" for the type of loose material that can cause this type of collapse.

Morschauser said workers in the tunnel had a warning that something was wrong when water began flowing into the tunnel from the wall that was being dug away. Those workers were evacuated.

"I don't feel there was any danger to the workers in the tunnel or to the public safety," he said. "The workers were out of the tunnel at the first sign of water."

He also said there was no danger to the public, since Orleans Street had been closed off for several hours before the collapse.

The water was quickly diverted, resulting in little loss of service to the hospital or to residents, according to an MTA spokeswoman.

The cave-in interrupted phone service to Hopkins hospital and repairs were delayed because Chesapeake & Potomac telephone workers could not get into the hole to make repairs.

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