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Foundations for cob house in subduction zone.  RSS feed

 
Perry Tart
Posts: 15
Location: Pacific Northwest
bike goat woodworking
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Hello, long time lurker, first time poster. I've been doing a lot of reading in preparation of building a home here in the Puget Sound Region (focusing at the moment on the Green River Valley and surrounding area because of family and other important considerations like doctors, etc.) and have a lot of preliminary plans, but I am struggling to figure out a good way to build a foundation for a home. I know that "a good hat and boots" is important to prevent cob from crumbling or molding, and I certainly see the importance. My concern comes from the fact that there are three major earthquake zones in this region, and I am worried about building a house that will fall as if it were made of cards.

In particular, here in the Valley there is a lot of alluvial soil, so in an earthquake, liquefaction will happen. The last major earthquake to occur in the region was the 2001 Nisqually earthquake, which measured at a 6.8. I remember that earthquake pretty damn well. For a bit more background, I am also doing CERT training, and am acutely aware that it is not a matter of "if" a superquake will occur, but "when", and want to plan accordingly. (Other major concerns are easier to plan for, like thunderstorms, winter storms, interface/wildfires, drought, and lahars; I have plans for those already). I am also concerned about using a lot of cement, and dislike buying poly bags for earthbagging a foundation because of how serious the microplastics problem is.

What this leaves me is with a complete loss for what to do for the foundation. I'm already planning on using wood found at the site to build post and beam above ground, and could consider it for below ground with appropriate cautions (it looks like birch wood, which is not rare around here, would be a good support in case of an earthquake) I'm just... not sure what to do. Would a rubble trench be good enough? Would I have to pound down to bedrock like the developers who are building new apartments in downtown Auburn did? Is cement even an option? (I have reservations, but if it's the safest way to make sure my house doesn't collapse, I will deal with those reservations.)
 
Mike Feddersen
Posts: 357
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Perry,

Your question roused my curiosity, so I went looking around the web for some answers.
What I found may have some answers, but also may raise some new building ideas.

I saw this site earlier in the eve while searching Pinterest for 'cob building' pictures for
future design use Ideas. Rubble Trench Foundation

This site drew me in with just the beauty of plasters applied on walls. Earthen Acres

From above, Rubble Trench

This site has all kinds of information from all over on Natural Homes

Natural Homes Facebook Page read comments under the pictures for more ideas.

You were mentioning using post and timber construction, I remember from awhile ago a guy talking about
the strawbale home he built, saying if he built another it would be a framed home with strawbales as insulation.
Evidently the strawbales did some settling that he needed to follow behind and do reinforcements and rebuild.

In the 1st website above, the person mentioned "daub and weave" construction of walls. This pinterest photo
shows an exterior wall being weaved. Woven Wall

Here is a video on compressed earth block building, they use a bit of cement but the end result is very strong.



I include some videos on earthquake testing of cob structures, the second one is long winded but informative.



 
Terry Ruth
Posts: 698
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I'm a little over half way through the first video. Alot of what is stated lines up with my research other than it is not quantified into a design guide or international code. For example, how thick to make walls to get them to perform (breath, etc) per a climate zone. My research and lab test I have seen says only an inch or two in from the surfaces is needed and a thermal brake.

At 32 min the shaker table test were interesting. The way that test is set up is not really replica of how earthquakes hammer earth construction from below. It sounds like Berkley, CA did the better test and there are some on Strawbale they did that did well too @ richter 7 ish reinforce with steel mesh and bar I do not personally like since it corrodes. I think if you pay a little more for s-glass or bassalt rebar it will sustain longer. New Mexico has seismic earth code to follow, check with Albuquerque building and safety office(BSO). Of course PS sound has it's own building code you will want to follow if in a non-permit rural zone anyway.

Earth construction in PS is going to take a knowledge base from an Architect and PE, and BSO office with decades of proven builds in the area, otherwise, it is a real risk for the average homeowner/DIY earthen blocks included. It is not just the foundation, it is the roof to the foundation as a seismic system that needs to be designed properly. The loads from below react at the top of the walls where all the weight is that cause shear failure the steel grid resisted and the earth stand alone fails miserably. . Dangerous!

Thanks for the vids Mike, I may have some more comments. The guys trying to push earth blocks there are other forms such as CMU's/ ICF's, that do just as well in seismic and high wind, flying debris, bullets, all over the internet he fails to mention. Wood does not do that great especially light framing...Yes the Texas A$M test of 150 mph studs were interesting solid concrete @ 4" thick did well, roofs usually come off tho unless pored.

I tried to push earth codes here with seasonal tornados. The city PE wanted to put rebar all through it since that is all he understands and we are in the lowest seismic zone in the country...he was concerned about national average 90 wind 3 sec gust most of the country should design to and we use 2x4's and OSB for. I argued earth walls do not need it unless in seismic since they are 2-3 times thicker than 4" concrete or ICFs. Trying to explain to people that only understand concrete and wood is like pulling teeth. Next time someone says earth construction does not work in seismic show the video. I wish I could find Berkleys.
 
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