It has been a while since I’ve written about the walipini, but it has been getting steady work over the last year. A walipini is an in-ground greenhouse. It uses the thermal mass of the earth to help keep its internal temperature from fluctuating too quickly in the winter. It is essentially a pit with a retaining wall of earthbags (which I call “engineered sand bags” full of clay, hydrated lime, and straw and/or woodchips, at a 7:1:2 ratio, held together by barbed wire between the layers) with a post-supported roof over it.
For those who want a quick refresher, check out this post describing the planning of the structure, the site and bags, and the foundation and earthbag fill.
This spring I made a big push to finish the interior walls. Each bag is filled with about 50 lb of the mixed fill. Each morning, I’d mix up a batch of clay, straw, woodchips, and lime and start filling the bags. In total it took about 5 minutes per bag to mix, fill, and place it. As each row was finished, two lines of 4-point barbed wire was laid down and held in place by weights (logs in my case, visible on top of the uppermost row in the picture below). The bags were stacked so that the joints between bags didn’t line up with a joint in the row below. Every three rows I hammered in a 2-ft length of 1/2-in rebar. This bound about five rows together. The last inch of rebar was left exposed to stab into the next row. I tied 2 ft of baler twine onto each piece of rebar and pulled it into the structure interior; it will be used to tie the chicken wire to the wall to help hold up the earth plaster in a later step.
As the wall grew, I backfilled behind it, on the outside of the structure. I had dug out deep trenches to build the wall in (see this post). The fill outside the trenches was moved back against the walls as they rose. Between the earthbags and soil was a 7-mil plastic sheet. Each bag also has a 3-mil plastic bag inside of the woven poly bag, so the fill should not be getting much of a moisture fluctuation, which could cause the wall to heave, especially with a clay base material. Luckily, my clay does not expand or contract much with water (just a percent or two).
The wall is built in an oval shape when viewed from above. We avoided straight walls because they do not stand up to the external forces of the backfill pushing against them. The wall also slopes out about 1 in for every 1 ft of rise. To further reduce the likelihood of the walls pushing in, I installed about a dozen cable-tied braces, pictured below. The cables were pushed through the wall towards the fill. On the outside, they were secured to 4-ft-long T-post stakes driven into the ground at the outer edge of the fill. Inside the walipini, the cables were secured around wooden battens. Then the cable was tightened by twisting the two strands.
And just in time for winter, I was able to get the last bag in place, pictured below. You can also see the chicken wire on the left side of the wall. As I noted, this will help hold the clay plaster that will be used to cover the earth bags. The woven poly material is not great against UV light.
This spring we hosted the Sustainability Skill Share. One of the classes built a masonry mass heater in the walipini, but I’ll cover that in an upcoming post.