Workers at the Highway 99 tunnel project have begun drilling 12 vertical wells to pump away massive amounts of groundwater surrounding Bertha, the Highway 99 tunnel machine.
If the operation succeeds, there will be less pressure immediately inside and outside the cutting face, simplfying efforts to clear a blockage in Bertha’s underground path. The machine has been stuck near Pier 48 since Friday night. The leading theory is that a huge boulder is in the path, and the surrounding sand is too loose to hold the rock firmly enough to be cracked by tunnel-machine blades. (A concise description of the soil history, with illustrations, can be found here at geologywriter.com.)
Anyway, the current position of the machine is so wet, from both the uplands and from Elliott Bay, that pressures are similar to being 100 feet underwater, said Matt Preedy, deputy project administrator for the state Department of Transportation. Ballard Marine Construction has already placed two white container-type buildings, with Christmas lights on top, on the job site in case Seattle Tunnel Partners (STP) needs to send in divers for weeks on end, using jackhammers or drills to shatter the obstruction.
But Preedy hopes dewatering would allow engineers and workers to inspect and remove the blockage, without the more difficult and expensive “hyperbaric intervention” using divers. “That is our intention,” he said.
Theoretically, rapid removal of groundwater poses risk of soil settlement beneath Pioneer Square buildings. But the neighborhood and Alaskan Way Viaduct were already shielded from Bertha’s vibrations by rows of buried concrete pillars, which Preedy said ought to also prevent sudden water loss inland.
The wells should be done and the pumping started late next week, Preedy said. “The key to successful dewatering in this scenario is you have to pump it out faster than the [ground]water can recharge,” Preedy said. Water will be pumped into green settling tanks to remove sediment, then into city stormwater pipes.
Earlier this week, STP manager Chris Dixon thought it likely divers would venture into the machine’s cutting chamber next week. But that means Bertha must exert extra air pressure to keep dirt and groundwater from inundating the workers — and that pressure might blow out the soil all the way to the surface, causing a sinkhole. So Preedy said the team wants to try dewatering first, before taking more extreme measures.
Preedy wouldn’t give odds Friday on whether the wells will succeed.