During a build I’ll inevitably get hung up on some detail that bogs me down. I just can’t wrap my head around it and I’ll avoid it working on some other part of the build until I can’t proceed unless I figure out the problem. For the T-Brick shed it was how to attach the roof to the wall. For the cabin, it was how to integrate the standing seam metal panels with polycarbonate panels for the porch roof. There was also a compound angle for the roof rafter for the hipped sections of the roof that resulted in a lot of head scratching.
For this build it’s been the berm retaining wall, how to build it and how to tie it into the cottage. I’d considered using railroad ties, treated timbers, concrete blocks, dry stack stone and earthbags. Finally, I decided to return to my wheel house and use cob stabilized with hydrated lime. I’ve seen stabilized cob used in unprotected applications but never for a retaining wall so be forewarned this could fail horribly.
What is Stabilized Cob?
Cob is sand, clay and straw. Hydrated Lime is calcium carbonate or limestone that has been fired in a kiln. The high heat causes the limestone to release carbon dioxide turning it into Calcium Oxide or quicklime. Quicklime is mixed with water (H2O) to become Calcium Hydroxide or Hydrated Lime which is available in 50 lbs bags at most home improvement stores.
Mixing hydrated lime with cob gives the cob resistance to water penetration while still remaining vapor permeable. Over time hydrated lime will also absorb the carbon dioxide it lost during the firing process, a process called carbonation. In effect, it’s turning back into limestone.
Building a Cob Retaining Wall
We built the forms for the retaining wall grade beam with 2x8s and staked them down with outriggers. The grade beams are 1 foot wide and 5-6 foot long.
For the grade beam I put down a couple of inches of gravel and then instead of portland cement based concrete I used a mix of sand, small gravel, straw and hydrated lime. It took about a week for it to set firm enough to build on. Even then if you really pressed on it you could leave a small impression. However, now, several month later it’s rock hard.
Before pouring the grade beam I installed four inch perforated pipe wrapped in landscape cloth around the perimeter of the cottage. One end is tied into the french drain running underneath the cottage foundation that drains to a dry well. The other exits through the bottom of the berm wall. So, any moisture that makes it to the exterior wall will have an escape path.
With the cob wall I could only go up six to eight inches at a time. If I went up too much it bulge out. Fortunately, the weather was sunny and dry so sometimes, I could get away with putting on a course in the morning and then another in the late afternoon. For a couple of weeks I would cob for about an hour a day and then do other things like work on the earth tube system. More on that below.
To tie the wall in with the cottage I drilled large screws into the frame of the cottage and worked the cob around them.
As I went up I gradually sloped the wall toward the berm. At 2-3 feet up I installed a deadman tieback in the wall. This consisted of a 2×4 anchored to the wall with several pieces of rebar. I notched two more boards so one end fit around the 2×4. One of end of each board is fastened to the 2×4 and the other end extends into the berm so that the weight of the berm will reinforce the wall. I also put deadmen in the end of the walls to provide an attachment points for a possible future fence.
I transitioned the upper section of the retaining wall(that will not be in ground contact into a fence. I set the bottom horizontal portion of the fence into wall and drove in short pieces of rebar to tie it into the retaining wall. The 4×4 posts are attached to the horizontal boards with box joints and metal fasteners
After assembling the structural components of the fence I buried the bottom six inches of the fence to really anchor it to the wall. After backfilling is competed I’ll attach vertical cedar fence planks.
Laying the ground work for Earth Tube system
An earth tube is a buried ventilation duct. Below six feet ground temperatures remains fairly consistent throughout the year. In our area that temperature is around 68° F. I’ve heard 50-55° but I think it depends on your areas average annual temperature. Moving air through an earth tube should cool or warm to the ground temperature.
This earth tube system consists of two runs of 4 inch diameter HDPE pipe. Each one runs into a 55 gallon barrel with the intent being to add a little more volume to the system. From the barrels the pipe runs along the perimeter of the building and then up to a little insulated air exchanger attached to the side of the cottage. An exhaust fan mounted on the interior of the cottage will pull air through the tubes and into the cottage (theoretically).
There are a couple of issues that may prevent the system from working very well, namely the length of the pipe (each run is only about 30 feet) and the depth (only about five-six feet). I’ll post more details about the system once it’s in operation and I have the various temperature and humidity sensors in place.
How does the insulated air exchanger work. Does it mean that the outside air does not go into the cottage, only cooling the inside air that circulates through it?
No, I should have called it a combiner box. The fan pulls fresh air from outside through the pipes that converge in the exchanger/combiner box.