Let’s Get to the Bottom of This : How safe is the Metro Rail subway tunnel?

At a total cost of more than $5 billion, Los Angeles’ new Metro Rail subway is part of the biggest public-works project in the nation. It is also a key component of the projected 400-mile rail network that will give this sprawling metropolis a modern mass-transit system that is a real alternative to cars and clogged freeways.

So we had better do it right.

That’s why officials of the Metropolitan Transportation Authority--the new super-agency that is overseeing construction of Metro Rail and operating its system of subways, trolleys, buses and commuter trains--must move quickly to reassure riders and taxpayers that tunnels for the first segment of the subway are as safe as they can be.

THIN SKIN: The Times reported Sunday that segments of the 4.4-mile-long Red Line subway route between Union Station and MacArthur Park were built with less concrete than was called for in the design specifications. According to public records and inspections reports, there are spots along the subway tunnels where concrete may be not as thick as the required 12 inches. These thin-concrete areas cover parts of the tunnel between Union Station and the Red Line’s Pershing Square station.

Such thin patches can occur when there is a slight misalignment in the original tunnel bore, according to experts. Engineers can compensate for such problems with reinforcing steel rods embedded in the concrete or steel plates placed outside it. And while this has been done in the Metro Rail tunnels, publicity about thin concrete will surely raise questions about subway safety here in earthquake-conscious Southern California.

SEISMIC SAFETY: There is disagreement between experts over whether this thin concrete is that much of a problem. Some engineering and construction specialists, for example, say the strength of the concrete used to build the tunnels is more than the design specifications called for, which would compensate for thinness. But at least some reputable engineers warn that thin concrete could pose hazards for the subway tunnels when exposed to stress, such as the seismic forces of a major earthquake.

So questions about the Metro Rail’s tunnels are not just a public-relations problem. They are a public-safety and a public-liability problem. That’s why MTA had better get them answered fast--preferably by a panel of construction and engineering experts who do not work for the MTA. Only that is likely to answer authoritatively the complex safety question.