Building a Biodome Part 3: Foundation

The first thing I did was mark the central point for the dome structure, which is roughly in the middle of my backyard, which I had observed as being the sunniest spot in the garden.  Once I'd decided on the center of the circle, I pounded in a metal peg and using an 8 ft string marked out the edges of where the foundation trench was going to go with rocks.  I removed all the turf within this circle.

Then I started the laborious job of digging a 2 foot deep ditch that would be slightly wider than my bales all the way around.  I had some help with this from a couple of friends, thankfully.  One of them remarked that digging ditches had to be "the second oldest profession in the world" and it had a ring of truth to it.  No way around it, but this is slow, backbreaking work.  It made me think of my grandfather in the first world war and how hard it was to dig 6-10 foot trenches while battle was raging all around, must have been hellish.

Once the trench was dug I placed 9 concrete blocks at equal distances around the trench.  My 'B' struts that are all along the base of the dome are just shy of 5 ft long, so I made sure the blocks were spaced at that distance.   The base of the dome has ten sides, I didn't place a block where the door was going to be, since I planned to cut that strut out of the structure to make the entrance.  I attached around 6 feet of galvanized wire to each block - the idea being I would use this to tie the wooden dome frame into the foundation and prevent it from blowing off the top of the straw bale walls. Once the concrete blocks and wire were in place we started to move 6 tons of 3/4 inch crushed gravel into the trench.

 This was another laborious job and I was grateful for the help of several friends before it was completed.  The foundation needed this much gravel because the land sloped and the foundation had to be around a foot or so above ground level on one side.  I wanted to get the bales well above ground level to prevent moisture coming in from the sides. We leveled the gravel with two 2x4x10 planks screwed together and a level on top.

Work Effort and Cost

Digging the trench took approximately 2 person days.  Moving the gravel into the trench, around another 3 person days, so 5 person days in all.

6 tons of 3/4 crushed gravel cost $130 delivered.  The galvanized wire cost $20.  I had the concrete blocks lying around my yard, not sure what these would set you back, probably $5 apiece.  I spent a total of $155 on the foundation.

Lessons Learned

While my foundation was cheaper than pouring concrete, I paid for it in work effort.  I planned a biodome construction one day workshop and had intended to get the bale walls and the wooden dome built that day, but we spent the whole day moving gravel after realizing that the foundation as built wasn't high enough.  So I felt bad having friends hauling and leveling gravel that day, which didn't seem very inspiring or interesting.  Construction is kind of humbling that way and will tend to take at least twice to three times as long as an inexperienced person is expecting.  Having said that, spending effort or money on the foundation is never a waste, because the rest of your structure will be more 'true' if it is deep and level.

Talking to straw bale construction experts later in the build, they were very concerned about placing bales directly on the gravel foundation.  They thought that the bale walls would wick water up from the dew point and eventually rot as a result.  Their recommendation would be to put a layer of flat rocks, then a layer of earth bags all around to raise the bale wall higher off the ground and create a moisture barrier.  I would go with this recommendation if I were doing this again.

Acknowledgments

Thanks to James, Malte, Julie, Sunni, Jim, Randi and John for your help with the foundation!