Gas Explosions in Tunnels Easily Avoided, Experts Say

Despite the presence of extensive natural gas deposits, constructing a subway tunnel beneath Wilshire Boulevard is no more dangerous than tunneling along the alternative route favored by federal officials, according to construction experts.

The technology for preventing natural gas explosions during tunneling is highly developed and relatively easy to implement, say experienced tunnelers, and much of it has long been used in the coal mining industry.

The keys are adequate ventilation to dilute the natural gas and the use of sophisticated sensors to detect any potential buildup of the explosive fumes. “That’s basically it,” said George Fox, president of Grow Tunneling in New York City.

Construction companies routinely, and safely, tunnel through areas even more gassy than the Wilshire route, and several small tunnels have already been built under Wilshire Boulevard for other purposes, said Byron Ishkanian, an engineer with the California Division of Occupational Safety and Health.

“You’ve got to be very careful, but it can be done safely,” Ishkanian said.

Ed McSpedon, president and chief executive officer of the Rail Construction Corp., a subsidiary of the Los Angeles County Transportation Commission, said the Metro Rail subway could be safely built anywhere in the city.

That is not to say that tunneling through areas laden with natural gas presents no risks. “Nothing is without risk,” McSpedon said. “But we have the means and methods to manage that risk effectively.”

The original Metro Orange Line plans called for a subway along the densely developed Wilshire Boulevard route. But those plans met a roadblock when natural gas, commonly called methane, seeped into the basement of the Ross Dress-for-Less store at 3rd Street and Fairfax Avenue, causing an explosion in 1985 that injured 24 people.

Prompted by the explosion, the city ordered a survey of underground methane concentrations in the area and ultimately declared a 400-square-block area a “potential risk zone.” The gas is produced naturally by underground oil fields and rises through wells or cracks in the ground. Spurred by Rep. Henry Waxman (D-Los Angeles), Congress blocked the use of federal funds for subway construction in any potential risk zone.

Transportation officials then settled on an alternative route that bypassed Wilshire Boulevard and went beneath Pico and San Vicente boulevards. The commission hired Pasadena-based Engineering-Science Inc. to survey methane levels along the new route for the required environmental impact report.

Measuring underground methane is a straightforward process. Researchers simply drill a small hole into the soil and insert a methane probe connected to an automatic recorder. According to McSpedon, the holes are drilled at regular intervals and extra holes are drilled at sites where geologists have reason to suspect high concentrations.

According to the original environmental impact report and a re-evaluation conducted in 1991, the soil under the proposed Pico-San Vicente route contains roughly the same amount of methane as under the Wilshire route. On a risk scale of 0 to 4, with 4 being the highest risk, both rate a 3.

But the Pico-San Vicente area has not been formally declared a “potential risk zone” by the city, so federal funds can be used to construct along that route.

Given that the two potential routes have equal methane risks, the question then becomes whether either can be constructed safely. The answer to that question appears to be a virtually unqualified “yes,” according to experts throughout the country.

“There is very definitely a consensus that it can be constructed safely,” said Joe Sperry, a tunnel construction consultant in Boulder City, Nev., who is widely considered one of the leaders in the field. “That consensus has grown in the last five years.”

The precipitating event in forming that consensus was a 1988 methane explosion in a sewer tunnel under construction in Milwaukee, in which three workers were killed. That tunnel had inadequate ventilation, and the explosion was triggered during an inspection while tunneling operations had been shut down.

As an outgrowth of that explosion, the U. S. Bureau of Mines became involved in tunneling safety because of the coal industry’s extensive experience in tunneling through gassy areas.

Mining engineer Fred Kissell of the Bureau of Mines in Pittsburgh investigated the Milwaukee explosion and others that had occurred earlier and drew up guidelines on how to prevent explosions.

“We didn’t have something before that told the (tunneling) operator how to ventilate and how to monitor methane, etc.,” Sperry said. “Kissell’s book is very practical.”

Confidence about the Los Angeles tunnel is also based in large part on policies adopted by Cal/OSHA and that are now being used in Metro Rail construction. Since those policies were adopted in 1973, “We haven’t had a gas ignition underground in all of California,” Ishkanian said.

In fact, using the Cal/OSHA standards, a contractor had safely tunneled directly underneath the Ross store only a week before the March, 1985, explosion.

Contractors are also using the guidelines to build a water supply tunnel in Kansas City through an extremely gassy area. That tunnel is parallel to another that was built in 1927 with few controls. There were eight ignitions in that earlier tunnel and a total of seven people killed. So far, there have been no incidents with the new tunnel.

The most important safety factor, everyone agrees, is adequate ventilation. In earlier times, ventilation was carried out at a rate of about 30 cubic feet per minute. Modern guidelines call for 100 cfpm “and it’s a rule of thumb to double that if we think there might be methane.

To achieve this ventilation, large fans are set up at the exit of the tunnel and about every 2,000 to 3,000 feet inside. Fresh air from the surface is carried to the tunnel by a steel pipe three to five feet in diameter.

The guidelines also call for methane sensors to be installed every 3,000 feet along the tunnel and two to be installed on the front of the tunneling machine where an encounter with methane is most likely. The Rail Construction Corp. uses seven sensors on the front of its tunneling machine to check for methane pockets.

All electrical equipment used in the tunnel is “spark-proof,” or what engineers call “intrinsically safe.” In practice, that means that motors or anything else that might form a spark are enclosed so gases cannot reach them.

The final safety precaution is drilling a small exploratory hole, about three inches in diameter, along the tunnel path well ahead of the tunneling machine. Probes extended through this hole contain methane sensors to detect unusual concentrations of the gas and a magnetometer to detect metal objects, such as a buried pipeline or a capped wellhead.

“What is suggested (by Waxman) is that there will be a sudden concentration of methane,” said Fox of Grow Tunneling. “I don’t believe, generally speaking, that that cannot be dealt with. . . . When you’ve got plenty of ventilation up front, you very quickly lower the concentration.”

With these precautions, experts agreed, the Wilshire route for the Red Line could be safely constructed. And McSpedon emphasized that tunneling under Wilshire is not really that much different from working anywhere else in the area.

“What you have to realize is that methane is part and parcel of any underground construction in L. A.,” he said. “It has to be taken into account if you want to be safe.”