TUNNEL’S VISION BESET BY BOULDERS
Large rocks have added more headaches to already delayed CBBT expansion
Construction of a new tube at the Chesapeake Bay Bridge-Tunnel is now two years behind, in large part because of the latest headache: The granite boulders armoring the manmade islands are proving exceptionally difficult to deal with.
The boulders, some so big only two could fit on a rail car when they were hauled in 60 years ago, provide form and protection for the four artificial islands anchoring the portals of the facility’s two original tubes.
Expansion plans call for adding two additional tunnels to parallel the old ones, putting an end to the two-way traffic flow inside in the existing tubes.
To do the job, a fresh technology — at least in Hampton Roads — was chosen. A tunnel boring machine will be used instead of the traditional immersed tube method. Problems have now pushed the wrap-up date to 2024.
Work on the first parallel tunnel started in 2017. Diving under the shipping channel closest to Virginia Beach, running more than a mile between the first and second islands, it was scheduled to be completed for $756 million in 2022.
Permitting caused delays. Another holdup occurred last year when a primary subcontractor
was removed for nonperformance.
And now, the boulder obstacle.
Everyone knew the rocks were there, of course. But according to Mike Crist, the bridge-tunnel’s deputy executive director of infrastructure, the project’s main contractors — Dragados USA and Schiavone Construction — didn’t grasp just how formidable the boulders would be.
“They’re saying they’re thicker and deeper than they thought,” Crist said. Here’s the picture:
The islands were created by placing boulders, chiseled from the mountains of central Virginia, on the bay bottom in an oblong ring, more than a football field wide and at least four times as long. Sand was pumped in to fill the interior, then more boulders placed on top, like a turtle’s shell.
Tunnel boring machines come in different models, with some designed to eat through such hard rock, but the machine ordered for the CBBT can only handle soft soil, which makes up the vast bulk of the tunnel’s route.
But to reach that soft soil, it must be launched from a pit on the island and mine its way out.
Boulders can be removed to create a hole, but it’s not simple. Removing too many could endanger the integrity of the entire island and the existing tube, especially if a nor’easter or hurricane rolls in.
Removing just enough requires building a structure to shore up the walls of such a narrow opening. That’s what contractors have been attempting to do, pounding steel pilings through the outer shell to create a sort of coffer dam.
“Imagine trying to drive a nail through granite rock,” Crist said. “Progress going through there is much, much slower than anticipated.”
Crist said the contractors are searching for answers and plan to meet with bridge-tunnel authorities soon to discuss any options they come up with. No tax money has gone into the project, and Crist said any additional costs are expected to fall on the contractors.
“Or at least that’s our position,” he said.
More boulders await at the far end, where the boring machine will need to surface on the second island, so the challenge must be solved, Crist said.
Construction on the next parallel tunnel isn’t expected before 2037, so who knows what technology will be available then.
A tunnel boring machine will be used to build new tubes at the Hampton Roads Bridge-Tunnel, a project that recently received the green light to start construction.
But the HRBT won’t encounter the same trouble, Crist said.
The HRBT’s islands are similar to the CBBT’s but larger, offering more options for portal placement, allowing a boring machine to angle under the perimeter rock instead of through it.