Bertha takes on the complexities of King Street

Posted on Nov 14 2013 2:55 PM

Bertha, the SR 99 tunneling machine, resumed digging at 9:30 p.m. on Wednesday, Nov. 13, following two weeks of scheduled maintenance. Crews spent the machine’s downtime performing inspections, tuning up equipment and installing new cutting teeth

As she emerged from her protected maintenance area, Bertha faced a new challenge – the ground beneath South King Street, which offers conditions she hasn’t yet experienced and won’t see again during her drive beneath the city.

Having dug similar tunnels all over the world, our contractor knew that tunneling beneath King would be tricky. They closed the street before tunneling resumed, and will keep it closed until Bertha reaches South Jackson Street, about 200 feet to the north. They also made plans to tunnel around the clock until they safely reach the other side of King, since continuous forward progress limits ground movement around the machine. 

Sure enough, the morning of Thursday, Nov. 14 brought Bertha an unwelcome, but not unexpected, visitor: a sinkhole.

Located in the construction yard just south of King, the sinkhole was about 15 feet long, 20 feet wide and 7 feet deep. The hole was quickly filled in as Bertha continued on her way, but not before it illustrated why our contractor spent $53 million building a protected area at the start of the tunnel drive. If ever there was a good spot for a sinkhole, it’s here, in a restricted area protected by underground walls.

Why King Street is unique
The conditions near King Street are unique for a number of reasons. For the first time in her journey, Bertha is digging through soil that hasn’t been strengthened with concrete. Some of that soil is glacial till, which is dense and good for tunneling, but a layer near the surface is fill material dumped in Bertha’s path by Seattle’s early settlers. This fill includes loose soil, sawdust and timber piles that, if disturbed during tunneling, can create voids above the machine.

The good news is that Bertha will say goodbye to fill soil by the time she reaches Jackson. Until then, crews are prepared to respond to any voids or other ground movement that may occur.

Removing the roof
Before Thursday, it was impossible for Bertha to create voids at the surface. That’s because she’s been tunneling beneath the 5-foot-thick slab of concrete that extends all the way back to the launch pit. The slab was there to keep the machine from floating toward the surface. How does a 7,000-ton machine float? Lots of groundwater combined with the shallow depth of the tunnel in the early part of the tunnel drive. Now that Bertha has reached a depth of 35 feet, it’s time to take off the roof, which increases the likelihood that any voids created above the machine will travel to the surface.

The path forward
Thankfully, deeper soils are coming. Our understanding of the machine and the best way to operate it is also deepening with each ring we build. As any experienced miner will tell you, there’s always a learning curve at the start of a tunnel drive.

It’s like learning to ride a bicycle. Early on, you use training wheels. Maybe someone holds on to the seat to help keep you steady. You take extra caution, stop frequently. Then, slowly, you start to catch on.

Bertha’s training wheels are still on. But we’re no longer holding the seat. And before we know it, our little machine will be riding on her own beneath the city.