My parents sent me this link a few weeks ago, knowing I would appreciate it. At first glance, it was interesting. A guy built a pretty cool dome home (in Thailand) from cement blocks. He only spent ~$6000 for the basic structure and got it done in about 6 weeks… The home looks nice and organic, but it is not earth sheltered and it didn’t have to deal with our cold weather. Not a lot of lessons that I could apply in North America…
But then I saw this… And I realized that someone on the team, maybe Steve or one of the guys he was working with, was quite brilliant.
This pivot arm is key to the whole thing…
Instead of working with complicated geodesic shapes, he is just using regular concrete blocks. Before starting the construction, he placed 3 of these posts, each with a pivot arm. Then he could just swing the arms to mark the footings.
Then he placed each row of blocks using the end of the pivot arm to set them exactly to form a sphere.
You can see more pics on his site, but eventually, he had several intersecting concrete domes.
Then he just coated them, inside and out, with stucco for a nice smooth finish.
Brilliantly simple. I probably still can’t apply it directly to my build, but I sure do find it interesting. Maybe I will experiment with building something else this way… When I have time ;^)
And here is a video tour of the completed project… (and some practice embedding youtube video)
Basement under the home in the Earth?
Most earth sheltered homes do not have a basement… This is mostly due to concerns about natural lighting, depth to the water table, etc. Structurally speaking, two buried floors would experience a lot more lateral forces from the earth. If you don’t want to rely on a sump pump, you also need to dig deeper drains, etc.
However, in my case, with a nice big hill of sandy loam soil that would have relatively low lateral forces and great drainage. I also liked the idea of using QuadDeck ICF (Concrete) flooring that would act like a “shear floor” against lateral loading. After not having the benefits of a basement in my current home, adding one seemed like a good idea to me. I just had to work out the egress exit/natural lighting and figure out where to give up space for the stairs and I was ready to go.
For the stairs, I tried a number of locations before I figured that out. I explained that process in a post a while back.
For the basement egress, since the basement was only on the North side, I had to put a window well on the north side. That was the side I originally planned to bury, but since I had such nice views there, I had already relaxed on that and put in a few windows. Now I would need to put the basement egress directly under one of those windows. Rather than a small “vertical shaft” window well, I thought it may be more interesting to put in a larger conversation pit. I could use the pit to get closer to the side of the hill and perhaps actually end up with an egress with a view, as well as a cool sheltered place to hang out.
Count the cost
While it is true that a basement is a relatively inexpensive way to gain square footage, mostly because it doesn’t need an additional roof, it does still need its own walls and floor and electrical and plumbing and that all adds up. also, the suspended floor over the basement costs considerably more than the slab on grade floor that would be needed without the basement.
Eliminating the basement would also simplify the construction process starting with a much simpler excavation, shallow drainage pipes, etc.
My specific design only called for a partial basement. I hoped to limit the complexity, but because I have a sandy site, the engineer specified a slope of 1/2. Meaning that my 10 ft deep basement will affect the construction for 20 ft around. The design with the basement required more expensive step footings, taller stem walls, two levels of french drains, etc.
On the north side, the egress window was a challenge for earth sheltering the house because I needed to be a lot more careful about retaining the earth around it. The plan looked good in 2D, but my 3D model revealed some concerns about the scale and cost of the retaining walls that will be required to keep earth from spilling into the basement.
In my original gantt chart (building schedule), I planed to spend 1/5th of my costs and a month of my schedule on the basement… Knowing that I wouldn’t have the option to come back and decide to add a basement later, and generally adding space to an earth shelteted home is difficult and because it would make a lot of my passive HVAC stuff work better, I decided to go for it.
Reality update
The basement is in now and I can say that the costs were well estimated. Shotcrete went considerably over estimates, but I saved money in other areas and ended up with a fairly affordable basement in the thirty-something-dollars per square ft range for a rough basement. If I decide to plaster the walls or finish the floor, that will raise costs, but I don’t need to spend that money until I need the space. At that point, it will seem like a bargain compared to adding space from scratch.
However, I didn’t factor in what a disruption it would be on the building site, and therefore to the building schedule. The excavators and footings contractors didn’t like the step footings. I checked with the building inspector and he didn’t like them either. This meant that I was not able to do the footings all at once. Keep in mind that the basement is only under a portion of the house. I would need to backfill the basement before I could do the footings for anything else. Before I could backfill, I needed the basement shotcrete, waterproofing, plumbing, septic field (and the trench to get there), etc. Each of these things had delays, especially the septic field which slowed us down by a month due to a gravel shortage and trouble getting the septic tanks ordered. By the time we got the footings in for the rest of the house, it was pretty much the end of the summer construction season.
The silver lining is that the basement was a bit of a trial run. We got to see shotcrete applied in a less critical area of the home. The resulting mess has lead to some adjustments in the plan for the main level. If we had started with the main level, where the walls are higher and the shotcrete also needs to be applied overhead, it could have been much worse.
Structural considerations
With lateral loading on either side of a shear wall or shear floor, the connection across that support dramatically effects the deflection in the walls. It is important that the basement wall acts like a single element from footing to the roof. If there is a joint between the basement floor and the main floor, the shear floor between them will not be nearly as effective.
(I will come back another time with some illustrations)
As you may recall, my design features pre-cast concrete arches to support the heavy earth loads and let me have open spaces without requiring large spans. I had some old posts about how these will be built, and even my own experiment to build quarter scale models.
I did get quotes on having these ribs done by professional concrete pre-casting companies. One even sent me nice faux stone concrete samples, but when the cost estimates came back, they ranged from $40K+shipping to $80K (with shipping) for the 10 ribs… I thought that was ridiculous considering that each rib only used about 130$ worth of concrete and less than 200$ worth of rebar. I asked the companies how many forms they would make and what they would make the forms out of… All three said that 10 was a small order, so they would just make 1 form out of wood…
Obviously, they were charging way too much and I was going to have to take this into my own hands… I had designed the ribs to be cast easily in a 1ft deep form. I could do this.
But first, I wanted to make a computer model to figure it all out.
My model revealed that the cost of reusable parts, assuming I went with a rather expensive Melamine base and 2 layers of 1/4 inch smooth plywood for the side walls, would be about $575. I would probably make 2 in order to cut down on crane visits (the crane will have to come at least 5 times with 2 forms). Then each rib would require about $350 worth of rebar, concrete, etc. I also decided that I would need to buy a concrete polisher (wet) ($200) and a sawsall concrete vibrator attachment ($50).
All told, that would mean about $1150 for the 2 forms, plus $3600 for the rib materials, plus $250 for tools, which gives less than $5000. If I add 20% to cover misc, it comes to $6000. I plan to work out a deal with the concrete company to rent their crane for a reasonable cost. They currently use it to place pre-cast septic tanks which are about the same weight (35 cuft at 130 lbs each is 4550 lbs, plus the weight of the rebar).
Along the way, I thought about things like layout, materials, form removal, etc. For instance, I plan to build these on the front half of the garage slab. I will build the back half of the quonset for use as a shop, and then cover the front half with a large tarp to keep the rain out. I can then remove the tarp so the crane can pick up the ribs more easily. I plan to use some #4 rebar to create hooks on the top of the casting. In order to remove the form later, I will need a slot in the form that I will plug with pieces of scrap insulation during the pour. For a base, I plan to use melamine sheets that will provide a non-stick surface. I will then need to polish both sides of the form to get a similar finish on the trowel finished side of the concrete. I plan to build two forms, one left handed and one right handed… There are a few different configurations to lay these out next to each other to minimize the space needed, I think I will go with a 24×24 layout that will require 5 sheets in the top layer, 6 in the middle layer and 1 in the bottom.
Here are some pics with a few more details…
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Rib assembly. The brackets are screwed into the 2x# forms, which are bridged together with plywood scraps and all of that is screwed down to the melamine base… but the sidewalls are not actually screwed to the brackets…
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A detail of the hanger. The PVC pipes along the spine will make it easier to set the vertical rebar and feature ridged insulation wedges. The PVC is held to the form by a screw which is removed before the forms.
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Rebar skeleton, mostly #5 rebar tied together with #3 rebar loops.
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The loops are mostly spaced by 8 inches, but the engineer specified that the spacing be reduced to ~4″ near the high end of the lower spandrel.
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Closeup on the high end spandrel. You can see the PVC holes that will help tie the rib into the central tower.
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Low end of the form. The white PVC pipe will allow the rib to slide over a 1″ steel post in the foundation. The green “hangers” are 2×6 boards that will keep the form from bowing and help me suspend the rebar skeleton.
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Table to calculate costs of the forms and rib material
