Twin tunnels carrying trains to and from Minneapolis-St. Paul airport were major undertaking

The twin tunnels that carry Hiawatha Line trains to and from Minneapolis-St. Paul International Airport are an engineering wonder, and the awards prove it.

In 2005 Bloomington-based HNTB Corp., the main consultant on the tunnel project, and geology specialists CNA Consulting Engineers received the Seven Wonders of Engineering Award from the Minnesota Society of Professional Engineers.

The scale of the $117 million project alone warrants applause: the nearly two-mile-long tunnels, bored through sandstone 65 feet underground, pass through a subterranean light rail station. Together they are the largest mined space in the state.

What's more impressive is that the work, completed in 2004, took place beneath what was then the eighth largest airport in the country. Even though the tunnels run under two operational runways and four operational taxiways, airport operations continued as normal during the 14-month project.

"It had everything and anything an engineer could want," said Pat Mosites, project manager for the Metropolitan Airports Commission.

The most dramatic feature was the 500-ton Herrenknecht tunnel boring machine, or TBM. The machine was shipped from overseas to Duluth and then driven, in three loads, on two tractor-trailers driving side-by-side down the freeway in the dead of night. When assembled, the TBM was as long as a football field.

Engineers with CNA found that a layer of limestone could act as a natural roof for the tunnels and station without disrupting the areas above. Once underground, the TBM wormed its way through the sandstone underneath, carving out each tunnel in five-foot segments and expelling excavated soil, which was later removed by mining cars and conveyors.

"In underground construction, the potential for mishaps is high," said Rich Johnson, associate vice president at HNTB. "The ground owns the project when you're underground. "

There was one tunneling mishap, which Mosites called "a little exciting. " The TBM ran into an unexpected cavern, into which moisture had seeped in over time and created an underground pond. As the TBM bored into the area it collapsed, sending water rushing into the machine.

"The best thing to do is keep going to get out of it," Mosites said. Crews filled the void with concrete to reinforce the area and keep moisture from getting in later, and the TBM moved on.

All things considered the project came off very well given the "extremely accelerated" schedule, Johnson said.

"We had a unique combination of highly qualified people and we were able to make rational decisions on a timely basis, rather than hashing them over for months on end," he said.

There were two other major construction aspects, aside from boring the tunnels. Where the trains head down into or up out of the tunnels, approaches were built using "cut-and-cover" construction, extending 900 feet south and 500 feet north from where the TBM did its work.

Another challenge was mining the Lindbergh Station, the largest underground public space in Minnesota. The station - 40 feet high and 63 feet long - was dug out of sandstone and lined with precast concrete.

"The precast concrete liners were enormous," Mosites said. To install the 60-ton pieces, cranes as tall as 60 feet were brought down into the cavern.

Of all the engineering elements that went into the project, the limestone bedrock was a fortuitous element - basic geology that became integral to the design. "We were very fortunate," said Mosites. "It made it a lot easier to do this. "

Jeremy Stratton is a Minneapolis-based writer.