Cave-in halts subway tunneling Orleans St. crater blamed on rupture of water mains

November 16, 1990|By Doug Birch

A section of Orleans Street above a subway tunneling project collapsed into a crater about 12 feet deep and 25 feet in diameter early yesterday, severing numerous utility lines, interrupting some telephone service and shutting down the heavily traveled roadway.

Work crews boring a subway tunnel south from Johns Hopkins Hospital caused settling that undermined and cracked two water mains, said George B. Morschauser, a spokesman for DKP, the joint venture managing the subway project.

Water from the cracked mains eroded the soil under the street, he said, eventually causing the pavement to collapse from its own weight.

Water poured into the circular tunnel floor 70 feet under the pavement, and the machine used to bore through the sandy soil now sits in a pool about 5 feet deep, Mr. Morschauser said. But the tunnel walls, which are lined with concrete sleeves, remained intact and the workers were evacuated safely.

The collapse caused no injuries, and there was no property damage other than the gaping hole in the street and the flooding in the tunnel, said Helen L. Dale, a spokesman for the Mass Transit Administration.

Repairs to the street and tunnel are expected to take about a week, she said.

But Mr. Morschauser said work would not resume until an investigation of the incident is completed. "We're not going to continue tunneling until the contractor can convince us that there are safeguards in place to prevent this from happening again," he said.

By 10:20 a.m. yesterday, the crater, located at the southwest corner of Orleans Street and Broadway, was roped off while a mechanical shovel removed pieces of sidewalk and other debris.

More than 30 hard-hatted city, state, utility and construction officials looked on.

The shovel operator worked gingerly around a low-pressure, 6-inch natural gas main, still intact and carrying gas, that hung suspended above the crater floor.

Baltimore Gas and Electric Co. officials said service to gas customers was not interrupted. A Chesapeake & Potomac Telephone Co. spokesman said the collapse wrecked an underground concrete switching box, disrupting service to an undetermined number of phones at Johns Hopkins Hospital and in the surrounding neighborhood.

Mr. Morschauser said the incident began around 11 a.m. Wednesday when crews working for Kiewit-Shea, the tunnel contractor, dug into a section of loosely compacted sand while operating the huge, wormlike excavation machine called a "digger shield."

The ensuing spill was not large enough to threaten to bury the machine, he said, but it caused a "void" or air bubble to form above the concrete sleeving at the face of the tunnel. That void, he said, migrated upward over the next several hours much as a bubble rises through water.

Work crews drilled holes in Orleans Street yesterday evening, he said, and prepared to inject "grout" -- a concrete-like substance -- in an effort to fill the void.

Normally, he said, such a void could be expected to take about 24 hours to move 50 feet to the surface.

But it apparently took less than 11 hours for the void to reach two cast-iron water mains, one 6 inches in diameter and the other 10 inches in diameter, buried about 10 feet below Orleans Street. Without support from below, both water lines cracked.

Tunnel crews first noted leaking water about 9:30 p.m. Wednesday,

Mr. Morschauser said, but thought the water might have come from the drilling equipment.

Over the next few hours the flow increased, and the tunnel was evacuated. Eventually, the water eroded the ground under the street, triggering the collapse.

Excavation of the two 1.5-mile-long tunnels that will connect Hopkins with the Charles Center Metro station near Calvert and Baltimore streets began in July. The five-year, $321 million extension of the subway to Hopkins is expected to be completed in 1994.

The project, which includes construction of two new Metro stations and the twin tunnels 20 feet in diameter, is expected to remove 4,805,638 cubic feet of ice age sand.

Up until Wednesday, at any given time at least a dozen engineers and laborers -- who call themselves "miners" -- were working 24 hours a day on the outbound tunnel, digging south from a 65-foot-deep shaft in front of Hopkins that will one day become the hospital's Metro station.

They were working at a rate of about 40 feet per day and had extended the outbound tunnel to about 450 feet.

Work began in July on the inbound tunnel, also headed south from the Hopkins shaft, and reached 700 feet before it was halted several weeks ago to permit installation of a pressurized air system.

The system, which will eventually be installed in the outbound tunnel as well, is needed once the tunnels penetrate the water table, to prevent groundwater from leaking in and eroding the walls.

Mr. Morschauser said yesterday's collapse had nothing to do with groundwater leakage.

The "digger shield" excavation machine is essentially a backhoe mounted in the hub of a steel rim 20 feet in diameter. Attached to the rear of the rim is "trailing platform," a steel grate used as a work area and gantry for a conveyor belt to remove soil or "muck."

The operator manipulates the hoe, making it reach out and scrape away layers of soil.

Workers on the trailing platform "steer" the shield by adjusting the position of hydraulic feet, attached to the rim, that are propped against the sides of the concrete sleeves lining the sections of completed tunnel.

The workers navigate with the aid of a laser beam mounted on the tunnel wall and aimed at a target hanging from the top of the shield.

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