Building Cob with no local clay?

I am in San Diego, and everything is decomposed granite here! East County specifically. There is the local river and lake, but you're not allowed to dig up clay at either (it's all either a park or protected wildlife area). There are drainage streams everywhere for when it DOES rain, of course, but those are all on private property and tiny. However, we are absolutely sold on cob, and determined to find a way. We are starting with a small shed to practice.

So, with no local alternative available without being sneaky (and I don't prefer that at all), what are our options? Are there places that sell clay by the truckload? Is clay-based dirt used for anything else - as in, would dirt available at rock/block landscaping supplies possibly be high in clay content? Or some other source? I know local pottery supplies sell clay by the barrel but it's VERY expensive, not a possibility on our budget. And I can't afford to get a handful of dirt from every source and send it in to be tested.

I understand the attitude of 'use what you have', but decomposed granite is useful for almost nothing, with no rocks large enough to build anything with. And, besides, we just love cob. So cob it is.

Thanks!

Howdy Sahara, welcome to permies!
I would check out any rock or landscape places for sure. Just ask them for clay and see what they have.

It's all decomposed granite?? Where are you in east county that you can't find clay? It's pretty hard for me to think of a place in San Diego county that doesn't have clay right under your feet. That's kind of why they made the missions out of adobe; it was right under your feet everywhere you look.

Have you dug down 2 feet, screened the rocks out of the soil, and done a soil test? Failing that, have you talked to excavation companies that need a place to dump fill dirt? If you go around to different excavation sites, you may find someone who is digging up just what you need and would be glad to sell it to you for cheap.

Hi Sahara,

Big welcome to Permies...

....but it's VERY expensive, not a possibility on our budget. And I can't afford to get a handful of dirt from every source and send it in to be tested...

I would be remiss, even though I want to be encouraging, to not suggest to you that the above quote from you kinda states the reality of working in an earth based material. As a traditionalist, environmentalist, permaculturist, etc. I can not in good faith, propose using materials (or recommend materials) for the bulk of construction that do not have a local source. You are also inhibited by your pecuniary limitations, as you shared them, and the fact that you do face tectonic activities that often counter indicate "pure cob" construction. (Traditional forms are fine and stand well to earthquakes.)

I am glad you love the idea of building with earth, yet it would seem that many have greatly over romanticized this medium, and thereby lose sight of some realities. To conduct "good practice," in architecture the construction process of "means, methods and materials," should (especially with environmentalism in mind) be of the vernacular, natural, and as easy to facilitate as possible. We all ignore these rules (or some of them) at different times. I admit to ignoring them very often as I ship timber frames (especially old ones...) all over the place, but I own the faux pas, and try to make up for it in other ways whenever possible.

So...

I think you will probably find a local source, as already suggested from many different sources. You may even find a contractor close by digging a foundation that has appropriate clays (decompose granite does make up many clay forms) but none of these are going to be either easy or inexpensive. Perhaps you should consider a hybrid? Or better look to the local indigenous cultures and see what they have built with historically. This method will most likely be your prime option, and it may even include some clay...

Some positives for your plight:

http://www.sandiegohistory.org/journal/v49-3/adobe.pdf

http://www.sandiegostrawbale.com/index.php?ticrat-adobe-conservation-article,45

http://www.utsandiego.com/uniontrib/20080425/news_1mc25cob.html



Regards,

jay

Dig 2' down and you should find plenty of clay there. I think we found clay only 1-1.5' deep when I helped on a friend's project in SD. Another place to look, is along the side's of roads where the earth has been cut into. I really don't think you'll need to buy any clay. Dig deeper, do a jar test, and practice making a few blocks to see how they hold up and get the mixture right. If you find dry clay, just combine the clay with the sand first, before water. As long as it holds together, it should be sufficient. A cob garden wall I built was only about 15% clay and 85% sand. It's held up just fine, although it wasn't structural or in a seismic zone. Clay's job is really to just hold everything together, so as long as it's not too wet or too dry you should be fine. A few considerations, though. Make sure your walls are thick enough for the earthquakes you WILL get. Though the lack of mortar joints in cob makes it great for zones with seismic activity, no structure is earthquake proof, so I'd still suggest making your structural walls 1.5-2ft thick, curved or with buttresses, and maybe with strategic arches on inner walls or future courtyards. I'm always concerned with earthquakes, so I personally would have support beams, even if some pure cob structures in major seismic zones have held up for centuries. But I am paranoid and tend to overdue things. Anyway, good luck!

I know I'm late to the party on this thread, but were you able to find Cob on your property after all, or were you able to find a local source?

If so, how did the building go? Were there permitting issues with using Cob as structural load-bearing material?

I ask because I am also in San Diego and would like to build with Cob one day.

Crazy question from a total novice.  I also live in the Desert in Arizona and would like to build some small out buildings with cob or Adobe style bricks in the fall. I truly font understand how to know you've got clay underneath you on your land. Sorry if its a stupid question can anyone spell it out for a beginner?

Hi,

First check the USDA soil map for you location: https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx
If you want to make adobe or compressed earth blocks you need  between 15-30% clay contents in your soil. 25% is theoretically optimal, but it has to be determined empirically.
If you want to make your own mix from scratch then it's convenient to have access to pure clay.
Few years ago when I started building my CEB house in California and I faced the same problem, but within 1 day of internet search and few phone calls I have found a place that sells rocks, gravel, sand and also clay. I have checked it first, found the lab analysis of this particular clay and purchased it afterwards.

Digging around as well is the only way. Maybe use a hand operated hole drill, or ask around the district.

If you have no clay, think about strawbale, but you will need clay to finish it.
Have you researched alternative materials?

I also googled "finding clay in the Arizona desert" and was surprised at the information presented.
I hope it helps you.

If you want to get clay out of soil with not much clay in it there is any easy way that I learned of by accident. Find a gully or a big ditch that has really muddy water when it rains. Dig 2 holes in the bottom the first hole is to catch the sand. The second hole is for the clay. Throw all the dirt down hill of the 2nd hole to make a dam. If the runoff escapes form the pit it will carry the clay with it. You have to catch all the water. Once you have done that you will have a free source of clay that is replenished every time it rains. The clay you capture will probably be way to pure for Adobe and will need sand added.

I also live in Arizona but don't know where to find Clay/ soil

I am curious, can you use clay that is sold at ceramic art suppliers for cob? I don't know about cost... Just an idea?

You certainly can - it would be purer than anything you are likely to dig up - but it would be unreasonably expensive for anything larger than an art experiment.

This is.just comical.  I have, one foot below the topsoil, a vast amount of clay.  It holds the rain from draining off  for weeks. The jar test  had two bands.  Clay and water.  Not enough of anything else to show in the test.

I might do another test with the topsoil included just to see if i can wring some sand out of it.  Buying for things like this just seems wrong.

The catch is that I have no sand.  The second most common substance in the world and I have none.

I wish we could make a material exchange, but we would all need dump trucks to get it around.

To use that sample in the bottle you will need to add sand and gravel to get a range of particle sizes from miniscule to say 2mm.

Fortunately, sand is cheap and pretty local most everywhere. If you can get a dump truck in to your location, you can get enough to build a cob cottage for a few hundred dollars. If all you have to do is mix sand and clay (and straw), you will have excellent cob for much less work than many places. My native clay is glacial till, with lots of sharp stones and gravel, and while it is perfectly proportioned for cob as is, the texture makes it dangerous to mix by foot.

sand is "cheap".  it still costs something.  something that is in severely short supply for us.  But, like I said the stuff is everywhere, I just need to find a free source that isn't illegal in some way.  I haven't spent alot of time working on sourcing things as I am still contemplating all that I can do with Rocket Mass Heaters and cob building.  our area in particular is horrible for water retention.  now I know that that is due to the clay the entire region appears to sit on.  the water problem will prevent me from doing alot with cob that I might want until such time as I can also find a free source of rocks or "urbanite" to make a good foundational wall.  i've got a good contatct through work (a concrete contractor) that might be able to get as much free concrete chunks as I would like.  I can't see what they would use them for or how they would make money from them.
the problem remains of transportation, and getting the wife on board for anything of a sizeable nature.  

speaking of clay type, how does one positively identify what type a particular clay is?
I  ran across some geologic information that mentioned a particular formation that my area falls in, and that some 18% of that a type of clay that is used often in even making fireclay or firebrick.  so that would be a plus if that were my type.