Subway Tunnel Alignment

* A recent news article ("Misalignments Found in New Subway Tunnels," April 14) reported the tunnel alignment variations on the Vermont and Hollywood Red Line tunnels as major "problems." The impression has been left that the variations were only recently discovered and that they might adversely affect the subway system. Nothing could be further from the truth.

During tunnel excavation, variations in soil and ground water made it difficult to guide the tunneling machines. Just as excavation started, hard ground damaged the 200-ton, locomotive-like machine in one of the Vermont tunnels, 90 feet below the city. The machine, approximately 250 feet long, could not be reversed, nor could it be picked up and placed back on alignment. To dismantle and remove it or drill a shaft to the surface to lift the machine out would have been prohibitively expensive and time-consuming.

The only viable solution to save the taxpayers money, without compromising the final design, was for the contractor to repair the machine in place and guide it back onto alignment. The decision was made to do this after careful consideration by TEAMETRO--the contractor, the construction manager, the engineer and the Metropolitan Transit Authority.

As mining progressed, the variation in soil, recurring damage to the tunnel machine and continued steering problems required the contractor to proceed very cautiously and to correct alignment problems as they occurred.

One example of the engineering and management efforts that solve these complex construction problems is the handling of the continuing mechanical damage to the machine which now prevents it from digging a perfect circle. This has been compensated for by deliberately varying the alignment to slightly elevate the machine from the planned alignment. This limits the variations to the bottom portion of the tunnel, where they can easily be corrected in the next step of tunnel construction.

Given the extremely difficult conditions found underground, moving ahead with excavation in this manner was not a failure of management, but was the correct and prudent course of action. As a result of these management decisions, the taxpayers' dollars have been saved, and a safe and modern subway will open on time.

To set the record straight, the problems associated with tunnel alignment do not reflect a failure of management, but instead are the very occurrences that the MTA and its construction manager are in place to manage successfully.


Construction Manager

L.A. Metro Rail

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