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January 22, 2014 at 11:58 AM

Rock was just the right size to get stuck in Bertha

A rock or concrete piece that workers found lodged in the Highway 99 tunnel machine cutter head is about 3.5 feet wide, state transportation spokeswoman Laura Newborn said today.

That measurement is meaningful for a few reasons.

Tunnel machine Bertha is designed to digest boulders of up to 3 feet in diameter, by pulling them through the conveyor system.  Anything larger should have been cracked open by disc-shaped cutters on the rotating face of the giant drill.

However, that kind of force can only be exerted if the rock is firmly held in place by surrounding soil. And in this part of the tunnel route  – along Elliott Bay near South Main Street — studies show that groundwater content is high, ranging from 15 percent to 35 percent. So the dirt may be too wet to hold a boulder steady. It should be possible for workers to break it soon, using power drills or hammers.

Soil studies in 2010 predicted the tunnel-boring machine would encounter 500 boulders, ranging from two to eight feet wide along the 1.7-mile route, mostly in firmer soil.

Here’s more of what we reported about the other objects — including plastic and steel-pipe fragments — workers found inside Bertha.

“None of these things in and of themselves is enough to slow the machine down,” Newborn told The Associated Press this morning.

Comments | More in Traffic & Transit | Topics: bertha, boulder, highway 99


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