Local Geology Kicks Project’s Arse


Confession: this post is mostly an excuse to post my super-awesome front loader and dump truck photo:

Check out the dirt-dumping action!

How awesome is that?  I’ve never had so much fun photographing a dump truck before.  Comes to that, I don’t think I’ve ever photographed a dump truck before.  But when Cujo and I were out walkies, looking for nice cherry blossoms, we passed by the site of this mysterious building project that’s been going on for half of forever.  Usually, it’s hidden behind walls, but the wall has come down, and the whole thing is revealed!  Also, there’s a sign we never noticed before:

Sooper-seekrit projeckt revealed!  Image credit Cujo.

Ah-ha!  ‘Tis a wastewater treatment facility.  And if you’ll direct your attention to the lower left of the photo, you’ll see there’s this tunnel they’re excavating that goes out to the Sound.  This tunnel is where the problems begin.

Cujo sent me this article in the Seattle Times that shows what happens when you drive a tunnel through gobs and oodles of glacial sediments: sinkholes.  And how.  Check this out:

Kenmore Sinkhole, image credit and copyright TunnelTalk

Allow me to direct your attention to a paragraph in the article describing that incident, from which the above photo was filched:

Neither the owner nor the contractor would discuss the focus of their investigations, but these will likely look at several possible causes, including the experience of the slurry machine operator with the closed slurry system making it difficult to judge the amount of material being excavated during a shove. Another possible cause might be the presence of a large boulder in the face that stalled penetration without slowing extraction of material and caused over-excavation. A third possibility is the meeting of high artesian water pressure and its influence on the excavation cycle. [emphasis added]

All of you geotypes are probably shouting, “Glacial erratic!” about now.  Seattle’s got lots, random boulders dropped by the Cordilleran Ice Sheet during its stay.  According to the articles I found, the tunnel-boring machine’s been encountering quite a bit of sandy soil, which it sometimes proceeds to remove too much of.  Not to mention running in to boulders.  Tunneling through all of that glacial outwash, till, and random erractics has got to be an absolute nightmare, and goes a long way toward explaining why the project’s run over on both time and money. 

TunnelTalk has a nice, simplified geologic cross-section showing what the excavators are dealing with here:

Image courtesy and copyright TunnelTalk

You’ll notice there’s not much clay it gets a chance to run through.  That means it’s grinding itself up against sand and gravel.  According to TunnelTalk, this means more frequent cutter replacements – only trying to get down there to replace a cutter when you’re not in a nice, stable bit of clay is difficult.  And then there’s the propensity for sinkholes.

This is something ordinary folk don’t usually think about when contemplating infrastructure, when they contemplate it at all.  But geology’s critical when it comes to deciding where and how you’re going to dig your tunnels things like wastewater lines.  We don’t have a lot of good choices here.  The bedrock’s down too deep in most places, the water table’s high, and glacial deposits are difficult to deal with.  Planners need to understand and deal with those issues so that the needs of the metropolis can be served.  And this is a good dry run for the gargantuan tunnel they want to replace the Alaskan Way Viaduct with: without this, they may not have been alerted to the true scope of the problems they’re going to face in sending a highway underground.

Oh, Seattle!  You are beautiful, but when it comes to infrastructure, you’re a right pain in the arse.

Comments

  1. says

    and this is a good dry run for the gargantuan tunnel they want to replace the Alaskan Way Viaduct with:Or, a harbinger of what a disaster that could turn out to be. My guess is that they're more familiar with what's under downtown Seattle, since people have been building large structures there since the beginning of the 20th Century. Still, it would be nice to know they were paying attention.

  2. Karen says

    What a mess! Tunneling projects always seem to come in horribly late and massively over-budget; I suspect it's because there's always lots of pressure in the bidding phase to downplay the most serious problems the geology can cause.

  3. Anonymous says

    Laying pipe in earthquake country below the water table in glacial sand and gravel deposits brings to mind two interesting terms:inflow collection system inundation-and-old lace and arsenicThe first term refers to piping leaks that could cause headaches for the public wastewater treatment works (POTW) in coming years…and the second refers to the unfortunate chemistry of wastewater nitrate leakage from damaged piping when the water table dips (seasonally) and results in arsenic release from sediment interstitial waters, which then enters said piping when the water table rises later in the year. Bangladeshi-style.