Bertha has much to be thankful for as she nears South Jackson Street

Posted on Nov 25 2013 12:20 PM

Sometime in the next few days, Bertha, the SR 99 tunneling machine, will find herself surrounded by soil that spent thousands of years under the weight of mile-thick glaciers. Fortunately, there’s no place she’d rather be.

Bertha will encounter eight different types of soil as she digs her way beneath Seattle, each with different characteristics. In general, the looser the soil, the more likely it is to move as you tunnel through it. Sand, for example, is harder to control than soil compacted by glaciers.

Until recently, Bertha had been digging through ground that crews strengthened with concrete. She then pushed her way into untouched glacial soils topped by a thin layer of unstable fill soil dumped in her path by Seattle’s early settlers. By the time she reaches South Jackson Street, Bertha will be completely buried in glacial till, which is the kind of soil she'll tunnel through for the remainder of her journey. 

Maybe she owes those glaciers – which made most of the soil she’ll encounter dense and stable – a debt of gratitude as she digs her way toward the Thanksgiving holiday.

Other things Bertha is thankful for

Barges
On Nov. 23, crews started using barges to remove excavated soil from the dig site. The barges will take the soil to Mats Mats, a quarry near Port Ludlow. Previously, crews used trucks to remove soil and dispose of it at facilities in the Puget Sound region. Moving forward, they’ll use a combination of barges and trucks, which will increase the speed and efficiency of the tunneling operation.

Good friends
@BerthaDigsSR99 recently welcomed her 8,000th Twitter follower. Not bad for a tunneling machine.

A favorite milepost
Milepost 31, the SR 99 tunnel project information center, has seen more than 20,000 visitors since it opened in December 2011. It's a great way for the public to learn more about Bertha, WSDOT's efforts to replace the Alaskan Way Viaduct and Seattle's historic Pioneer Square neighborhood. Milepost 31 will celebrate its second anniversary with a public event on Dec. 5 

Firm ground
The ground above Bertha has remained steady since crews discovered (and quickly filled) a sinkhole above the machine on Nov. 14. Given the soil conditions, crews may see some additional ground movement before the machine reaches Jackson.  

Great progress
Bertha has tunneled nearly 350 feet in the two weeks following her last scheduled maintenance stop. She’s traveled a total of 774 feet and installed 111 concrete rings since the start of tunneling.  

Crews hope to reach Jackson before they break for the holiday. What’s on Bertha’s Thanksgiving menu? You should know by now.