Water may prove a costly hazard for tunnel

As construction goes, there are easier things than undertaking a tunnel in Seattle

After all, it involves building a structure under the city's waterfront. Lately, it also is one of two leading alternatives to replace the 53-year-old Alaskan Way Viaduct.

But the area around the proposed site tunnel is layered with water -- thousands of gallons of it.

To keep themselves safe as well as to construct a reliable tunnel, workers will need to handle many conditions. But of all the natural obstacles, water may be the tunnel's most persistent complication.

It flows from the downtown hill above it, then settles between layers of ground under the surface. Then it rises and falls with Elliott Bay's tides. Simply, it's everywhere.

The cost to manage it? Close to $30 million -- and that's barring the remote risk of an earthquake-generated tidal wave.

"It's a pretty difficult engineering problem," said state engineering geologist Jim Struthers, a Department of Transportation leader on the project. "There are a lot of things to be prudent about."

Two water sources drain the steep slopes leading from the downtown hill to the waterfront.

Plus, Struthers said, state studies showed two water sources may come from underground layers of water "throughout the tunnel area." Tests are under way to more precisely map the water's reach.

But the most recent soil samples taken along the viaduct corridor are consistently turning up moisture-laden soil 10 feet to 15 feet below the surface "and if (the soil) is granular, which is to say sand and gravel, it'll have water," said Erik Blumhagen, a hydrologist for the Shannon and Wilson consulting firm in Seattle, which is overseeing the sampling.

This year, University of Washington engineering professor Jeff Parsons along with an expert panel scrutinized the state's designs for the tunnel and a new, elevated highway, the other leading viaduct-replacement option.

"It's just really complicated, and you have a lot of variability along that area, and along any coastal bluff for that matter," said Parsons, who teaches at the UW and consults for a private firm that sometimes works for the state.

Parsons said "water was a big issue in Boston," during construction of the city's Central Artery/Tunnel, or "Big Dig," project, where costs escalated from $2.5 billion to $14.6 billion, partly because of problems encountered during excavation. One Big Dig contractor didn't install waterproofing as required, and a follow-up contractor installed it improperly, adding $1 million to costs.