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Desperately seeking soil: As we expected, Bertha is off to a slow (but good) start

After slogging through 15 feet of concrete, Bertha gets a taste of the good stuff – but why did it take so long?

Engineers depend on math. It is the thing that, more than any building material, gives shape to their designs. Want proof that what you’re building matches the design? Check the plans, do the math.

But when it comes to tracking the progress of Bertha, the SR 99 tunneling machine, you can’t rely on an equation. Yes, we told you that Bertha would average 6 feet of digging per day during the early part of her journey.  Which means that, 18 days in, Bertha has traveled 108 feet, right?

Not exactly. So far, Bertha has traveled 24 feet. So what gives? It’s not a miscalculation. It’s just that understanding Bertha’s methodical start requires a bit more than math.   

Weekend plans

The first thing to remember is that Bertha isn’t digging on weekends. Each weekday consists of two 10-hour shifts of tunneling. Our contractor, Seattle Tunnel Partners, may choose to work weekends at times during the project, but for now weekends are reserved for planned maintenance to Bertha.

A little support, please

Digging through 850,000 cubic yards of soil isn’t easy. Bertha needs help, something strong to push off of as she makes her way forward through the earth. Once she’s fully underground, she’ll push off the curved concrete segments she’s piecing together to make the tunnel walls. Until then, crews are building temporary rings in the launch pit that look just like the permanent rings that will soon take shape beneath downtown.

But without the support of the earth around them, those temporary rings wouldn’t withstand the force of Bertha pushing against them. Crews have to fasten steel supports to each ring – a process that requires Bertha to stop every 2 feet. The result: slowness.

For fun, let’s do the math. Bertha has to build 13 temporary rings before she starts building the real thing. Each ring is 6.5 feet wide. That means Bertha has to tunnel a total of 84.5 feet before she can start installing permanent rings.  Given her progress to date, she has 60.5 feet to go.

Hitting the wall

Bertha was designed to tunnel through the soils beneath Seattle. But before she hit soil, she had to make her way through the 15-foot-thick concrete wall at the north end of the launch pit, a task she completed on Aug. 13. The dig through the headwall is standard for all tunnel projects, and it’s not unusual for it to take a while. Fiberglass-reinforced concrete is tough to dig through and tough to process, particularly while crews are still testing and calibrating the machine.

Bertha is making progress

All of which is to say that Bertha is off to a good, albeit slow, start. Those of you following her journey closely should expect her progress to vary as we head into fall. She might dig 2 feet one day and 20 feet the next. Soon enough, though, we expect her to hit that 6-foot-per-day average.

In fact, our engineers are counting on it.