The true costs of the planned Highway 99 tunnel aren't known because the project is only at a conceptual stage of design so far, an engineering expert hired by Seattle Mayor Mike McGinn said Thursday.

The true costs of the planned Highway 99 tunnel can’t be known because the project is still at the conceptual stage, an engineering expert hired by Seattle Mayor Mike McGinn said Thursday.

The report by Thom Neff, a longtime project manager and soil expert in Boston, New York and Portland, Ore., seems unlikely to change many minds about the tunnel. McGinn remains at odds with the state and most Seattle City Council members about the estimated $2 billion project.

However, Neff did point to several reasons why costs could go up, which McGinn said supports his position that the state must take responsibility for any cost overruns.

The City Council has rebuffed McGinn’s attempt to put conditions on tunnel-related agreements so that they wouldn’t be valid until a state law assigning overruns “to Seattle-area property owners who benefit” gets changed.

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McGinn reiterated that if the council will add that language, he will sign the agreements, which deal with utility access, schedules and other planning issues that would help get the work done by 2016.

Councilman Nick Licata said he was glad the mayor hired Neff but didn’t think the consultant bolstered the mayor’s position.

“The only thing he said that is in alignment with the mayor is this is a big project, and it’s risky, but we already know that,” Licata said.

Neff had no answer Thursday as to whether the state’s risk contingency, likely in the realm of $200 million to $400 million depending on bid amounts, is sufficient to cover obstacles in the tunnel work, scheduled to begin next year. He was more emphatic about the tricky soils below downtown, which he called worse than in Boston’s Big Dig. In particular, there is no place in the route where the soil is all of the same type, he said.

“They’re definitely worse than Boston. We didn’t have abrasive soil, we didn’t have boulders eight feet in diameter, we’re not in a seismic zone, and we didn’t have water pressure,” he said, referring to Boston’s extensive, problem-plagued project.

Earlier this week, the state disclosed that it is down to two bidders approaching an October deadline, because a team led by Kiewit Pacific went inactive.

The pro-tunnel City Council’s own consultant, John Newby, mentioned some of the same risks — especially the abrasive soils — in his report earlier this week. Damage done by similar soils caused two machines to stall on King County’s Brightwater sewer project, one of which has restarted and completed its task.

“Simply, what went wrong with Brightwater could happen here,” Neff said of the Highway 99 tunnel.

But he also lauded the state’s project team, saying it was capably addressing the risks.

Ron Paananen, DOT’s program administrator, said Neff is underestimating the amount of work the state and its experts have done understanding the risk and dealing with it in the contract specifications.

City Council member Mike O’Brien, a tunnel opponent, predicted that given the huge amount of work bidders have yet to do on the design, it’s likely the state will assume more risk as negotiations continue.

Neff said the area’s geology and the huge tunneling machine’s size pose risks that are at or beyond previous engineering feats. He likened it to the Boeing 787 Dreamliner effort that is behind schedule and over budget, or the Deepwater Horizon oil rig where BP lacked ability to prevent a leak.

Asked about the successful Great Northern Railroad tunnel dig under downtown Seattle a century ago, he said that route was entirely above the water table, unlike this one. “They knew what they were doing in the old days,” he joked.

He cast doubt on the state’s cost-estimating process, saying the current figures happen to just fit what the Legislature and Gov. Chris Gregoire imposed in early 2009. Even the experts make subjective judgments, he said.

Paananen replied, “We used the process; we didn’t manipulate it.” For instance, the expert team found costs were soaring last fall, so the tunnel’s south end was shifted from First Avenue to west of the old Viaduct — saving huge amounts of money and bypassing most historic Pioneer Square buildings.

On a more reassuring note, Neff said the DOT’s controversial $500 million performance bond, to restart the project if a contractor goes bankrupt, fits the current practice for the surety industry, even though it’s less than the whole value of a construction contract that’s approaching a $1.1 billion.

Licata said Neff convinced him that the council should push for a strong contract, and that the design-build contract — where builders take responsibility for final design — was the best approach.

The consultant put too much emphasis, he said, on the project being “unprecedented.”

“There’s probably a tunnel precedent-setting construction project once a year,” he said. “Each project is unique.”

Licata is a tunnel skeptic but intends to vote for the contracts the council is negotiating with the state. He believes the city should work with the Legislature to ensure that the state will pay cost overruns, but not at the expense of the project timeline.

“I think it’s naive to say there’s not going to be cost overruns, frankly,” he said. “What’s more important is who picks up the cost overruns.”

Meanwhile, a group called Transit + Tunnel Coalition took aim at Neff on Thursday, saying he “spent three weeks evaluating nine years worth of work” and shouldn’t issue opinions.

Neff said analyzing project strategy is his specialty. And while it’s been nine years since the Nisqually earthquake focused attention on replacing the 1953 Alaskan Way Viaduct, this particular bored-tunnel plan has been under serious study for just more than 1 ½ years.

Mike Lindblom: 206-515-5631 or