Dixon says the project team hopes there is no damage to the machine or cutter head but won’t know until the divers do a visual inspection.
As Bertha progressed on its downgrade, it went through 30 to 40 ft of fill, mixed with a variety of contaminated soils and timber debris. Below 40 ft, crews expected—as per extensive geotechnical data obtained, pre-bore, by Shannon & Wilson—native soils, a complex glacial till mix of silts, sands, clays, gravel, cobbles and boulders. Data shows Bertha will encounter all sorts of mixes throughout the path as it moves, at points, all the way to 200 ft below surface level.
"We know we have a lot of ground variations, big thick layers of clay, big thick layers of sand and gravel and silts and always a potential for boulders or cobble,” Preedy says.
“We don’t know if we are in soils we were not anticipating until we get inside and see what we’ve encountered,” Dixon says. “We know we will encounter boulders along the way—the technical documents are pretty clear [on that].”
If a boulder proves to be the issue, Bertha’s rotating head is designed to break it up. If Bertha can crunch it down to pieces 3 ft dia or less, those pieces will fit in the plenum and move through the system. But boulders don’t always cooperate. Some boulders are so large that the cutter head has difficulty breaking them up; also, if the rock sits in loose soil, the boulder can potentially spin right along with the cutter head, making progress “problematic.”
In either of these situations, divers will move into the 18 in. of space between Bertha and the boulder and use hand tools to break up the boulder.
Bertha is now “roughly on schedule,” Dixon says. Even though labor issues, hardened grout and a conveyor screw clog has delayed boring, Bertha was making better-than-expected progress during what was planned as a “slow and deliberate” drive toward Safe Haven 3, at 1,500 ft—also where Ballard Diving was scheduled for a plenum inspection.
Plans were to push Bertha into “full production mode” after Safe Haven 3 in early 2014, moving her under the existing Alaskan Way Viaduct and under downtown Seattle. First, though, Bertha needs to get to her safe haven, and it will take the intervention of divers to move past this obstruction.
