BOSTON - The Central Artery/Third Harbor Tunnel project in downtown Boston will cost about $76 million more than planned because of soil problems and an unexpectedly severe winter.

The 1% increase in the total cost of the project was necessary to assure that the Third Harbor Tunnel will open on time, according to a report from Peter M. Zuk, the project's director.

If all goes as planned, commercial traffic will be able to use the tunnel and the artery by late 1995. The tunnel will be open to all other traffic by about 2002.

Last winter was particularly severe in Boston, with a snowfall of more than 96 inches, and icy conditions slowed work on the tunnel. The weather forced project management to add a third and fourth shifts and weekend shifts.

Additionally, the report said, unstable soil has slowed the progress of the tunnel and forced increased blasting and more geotechnical surveys than were expected.

Since the project's inception, its cost has grown from about $2.3 billion to an estimated $7.7 billion. About 85% of the cost is scheduled to be paid by the federal government. Despite the huge amount of federal aid, the state has been slow to develop a plan to pay for its share of the project's costs.

A report released this March by the U.S. Highway Administration was highly critical of the cost increases of the Third Harbor Tunnel project and said the federal government should consider placing a cap on its spending at the $7.7 billion level.

If spending goes beyond $7.7 billion, the report calls on the federal government to lower its share of the cost to 50% from 85%.

Highway Administration officials say Massachusetts should refine its cost estimates and provide more information to the federal government about where its share of the project's costs is coming from.

State leaders say they believe President Clinton would most likely not approve the cap, pointing to Massachusetts' influential representation both in the House of Representatives and the Senate.

The concerns voiced by the federal government, though, have increased the urgency to finish a plan to pay the state's share. Many Statehouse observer's say they expect a portion of the state's costs will be paid through the sale of municipal bonds.

Legislation proposed earlier this year by Massachusetts Gov. William F. Weld lays the groundwork for a unified Boston Metropolitan Highway System. The system would consist of the Sumner and Callahan tunnels, the Third Harbor Tunnel, the Mystic-Tobin Bridge, the Central Artery, and the Boston extension of the Massachusetts Turnpike that runs from Boston to Route 128.

Under the proposal, the Massachusetts Turnpike Authority would administrate the unified system. The authority would provide a portion of the new Third Harbor Tunnel's toll revenues to the state to offset Massachusetts' share of the costs for the Weld proposal.

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