Urban forest cob experiment part II: trenching

This week, my focus was on digging a trench for our cob cottage.   Once complete, it will be filled with drain-grade gravel (smooth, not sharp pieces) and a drainage pipe.  It’s called a rubble trench foundation, an ancient technique popularized here by architect Frank Lloyd Wright.

I calculated that I could dig about one side per day, every other day.  I found, however, that I was not as tired/sore as anticipated, and was able to dig one trench side each day, working only 2-3 hours each day.

Along the way, I checked the size of my trench with Rob’s leftover drainage pipe that I’ll most likely be using in my project (though it’s larger than the 4″ recommended, so I’ll do a bit more research).

Drain pipe

Cozy drain pipe!


After reading more about foundations that double as drainage systems, I discovered that I’ll need to dig the trench about twice as deep and another half as wide.  Our frost line is about one foot, so I should be safe at about 1.5 feet deep, maybe a foot wide.

Then, stem walls — the above-ground portion of the foundation (in this case, urbanite*, our old brick, and stone is what I’ll be using) — will be built to keep the cob off the ground to protect it from water exposure.

I also read more about the need to level the ground, and began the process by laying a piece of wood across the plot to see just how much higher the South side of the house was.

Scientific measuring stick

Scientific measuring stick, top center


The stick told me what I already knew, and that was that I needed to level the plot.  I began digging on the South side and came across a very hard bit of what I guessed to be concrete.


It wouldn’t budge.  I kept digging.


The Rock of Gibraltar? Rock of Ages?

Clothesline: a foundation

Clothesline: a foundation

Naw.  But once it supported a clothesline.  Rob said, “there’s probably another one over there,” pointing to another part in my tiny plot.  Let’s hope it was a long, long line more than 12′ away.

By the way, heavy.

All this digging and leveling did allow me to find the best part of the soil.  And it’s not silly putty.

But — as an aside, and for the record, silly putty?  Love it.  It’s not allowed around Rob (just cracking the plastic egg initiates eye-rolls and wise-cracks) but I used to carry the “original” version in my purse.  One never can anticipate when squeeze hand exercises might be needed (or newspaper print copies, or moldings of Rob’s whiskers, etc).

Similarly, clay, play-doh, and other squishy mold-y things have always been of tactile interest to me. There is something satisfying about creating shapes, destroying said creations, then starting again fresh on will.

So I was happy toward the end of the week, after our torrential rainstorms and leveling work, that I finally unearthed solid Carolina red clay from beneath my foundation trench.


Or Carolina orange? I really hope this is clay.

Or Carolina orange? I really hope this is clay.


I also note, as part of last week’s soil test, that my jar had changed:

Still a big question mark in a jar

Still a big question mark in a jar


What does this mean?  That’s rhetorical.  I have no idea.

Next week, on to widening/deepening the trench, hopefully grading it down toward a temporary pond, and leveling the floor of the site.

Thanks everyone for your interest in learning along with me, hanging out to smoosh some cob when the time comes, and for your future presence in the cob mansion itself.

Total cost to date

$2 (Unchanged from last week)

* Urbanite: leftover chunks of concrete and other non-geological (human-made) materials suitable for cob foundations. My urbanite is (are?) chunks of our former concrete walkway.

And PS – Rob’s pallet deck and pallet couches (seen in my crooked picture angle from East of my future cob house, looking South) – still needs to be finished, but it’s useable and I love it (never mind the propped up post being stored there temporarily):


It’s not crooked. But my photo-taking was.



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