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Expert:Tunnel on house of cards

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Expert:Tunnel on house of cards

Post  mermaid on Wed Aug 24, 2011 6:07 am

Soft clay ‘valid theory’ behind sinkhole
By Richard Weir

Wednesday, August 24, 2011 - Builders of a problem-plagued Big Dig tunnel may have underestimated the sensitive nature of deep underground marine clay — considered as fragile as a house of cards — when undertaking one of the world’s largest “ground freezing” projects, according to an engineering expert.

The Interstate 90 connector tunnel is now undermined by a massive sinkhole, and the Boston blue clay underneath it may be the root cause of the latest fiasco, said Thomas C. Sheahan, a Northeastern University geotechnical engineering professor.

“At a micro level, the soil is like a card house, and when it’s frozen it’s extremely stable,” Sheahan said. “But when it thaws, the water contracts, and now those bonds have been broken, causing that soil to collapse like a collapsing card house.”

State transportation officials are at a loss to explain why the ground under train tracks near South Station has dropped by as much as 8 feet and a water-filled gap — possibly as long as 190 feet — has formed under the I-90 tunnel.

Frank DePaola, the state’s highway administrator, said yesterday that Sheahan may be right: “It could be a valid theory to explain what is going on.”

Yet, according to a series of reports obtained by the Herald, engineers who carried out a series of laboratory strength tests of frozen soil samples didn’t see any reason to halt the piping of supercooled saline 130 feet into the ground, stiffening it so that train service to South Station wouldn’t be interrupted during tunnel construction.

“We estimate about 2.0 inches of thaw settlement will occur at the edge of the influence zone,” engineers from Mueser Rutledge, a Manhattan consulting firm, advised the Big Dig’s contractor in a February 2000 analysis, referring to the footprint of the frozen ground. “Area B will also experience an additional 5 inches of thaw settlement due to melting of shallow frozen marine clay.”

Peter Deming, a partner with the firm, did not return messages yesterday, and one of his engineers, Dong Chang, who worked on the project, declined to be interviewed. Other estimates provided by the firm suggested that settling could exceed 20 inches, documents state.

U.S. Rep. Stephen Lynch (D-South Boston), who is worried about the safety of the tunnel and has called on the Federal Highway Administration to investigate the latest Big Dig failure, expressed skepticism over statements by state officials that greater than expected settling of the frozen ground is the cause of the sinkhole.

“I’m not buying it,” said Lynch, a former ironworker who has an engineering degree. “As things thaw out (over) the years, you will get some settlement, but usually it’s measured in inches. You don’t get 8 feet of settlement from thawing.”

So far, the state has spent approximately $12 million on making repairs to infrastructure damaged by the sinking soil. DePaola said plans are being made to drill below the tunnel to get a clearer picture of the void and, once the thawing ends by 2014, to pump cement into it stabilize the tunnel.
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