A lot of specialized experience and understanding are required to properly build an earth sheltered home.
Many contractors have experience, usually learned from mistakes made. Specifically, they learn from mistakes that negatively impact their portion of the build. They often don’t understand how the design or their adjustments and shortcuts will effect other portions of the build or the final performance. Many engineers have understanding of what effects the final performance, but don’t have the experience to understand how their instructions will be interpreted during real construction.
Contractors who do larger portions of the build are more likely to discover negative impacts of their earlier work and, therefore, build more experience. A General Contractor (GC) sticks around for the whole project and hopefully accumulates more experience (via hindsight) that he can apply to the next job. If you are GCing your own home, you need to figure out how to avoid mistakes rather than just learning from them.
The building inspector is often a retired builder or someone with similar experience. They should be on the side of the home owner and help to keep the builders in line with acceptable practices.
However, for practical reasons, their inspections are only at certain key points in the process. The inspectors have a lot of other homes to inspect and can’t spend too much time at each one or keep up on understanding all the design intent. Even with a good building inspector, there is still a lot of room for serious errors that the inspections won’t catch. The GC and home owner still need to be vigilant.
For example… The building inspector checks out the footings before the concrete has been poured. He can see that the rebar and the forms look OK and may approve it, but he is unlikely to check all the rebar or all the forms. Even if they do check the prep carefully, they may not see what is under it and they are not around when the concrete is actually placed and the builders are stomping the rebar into the ground.
Here are some anecdotal examples while I sort this out in my head… I am sure I will see more as I continue my build.
The engineer understands that concrete is strong in compression(20 – 40 MPa or 3000 – 6000 psi), but weak in tension (2 to 5 MPa or 300 to 700 psi). Reinforcement is used to increase the tensile strength and resist cracking. The Engineer understands that when the concrete footing is under building loads, its top portion is in compression (unlikely to fail or crack), but the bottom is in tension and needs reinforcement. at some point between these two, the compression and tension balance out to zero. Therefore, the engineer will often specify that the rebar should be placed in the bottom of the footing.
However, the engineer also understands that the reinforcing is pretty useless if it is not fully encased in concrete so that its tensile strength can be shared. So the engineer may specify something like placing the rebar 2 inches from the bottom of the footing.
Later, the guy installing the footing has none of this understanding. If you are lucky, he knows that he can use rebar chairs that will put the rebar right in the middle and he will probably do that if you let him. When laying out the rebar, they may not overlap corners like they should or worry about rebar being placed in certain critical directions where the tensile load is likely to be greater.
In other situations, the installer may have no concerns at all about pulling the rebar or welded wire reinforcement (W.W.R) up into the concrete to place it at the correct depth. They will just stomp it down and focus much more on the surface finish (that you see when you pay them) and not spend any time worrying about what you can’t see.
Getting this wrong is not something that they notice during the install (no failure as far as they can see), so you can’t trust them to get it right by experience. The guys on my project made all of these mistakes and probably more. I talked to them about these things as they came up and realized that while they have many years of experience installing footings, they had no real understanding of why they were putting in rebar or why it mattered where it went. As a GC on your own home, you will need to keep a careful eye on your installers and inject some understanding (or just tell they what to do) as needed.
Here is a case where experience is the most important thing, probably because failure happens during the install. An engineer can calculate lateral loads in mega pascals, he can understand how those loads translate into forces that will try to tear the form-work apart. Meanwhile, the experienced builder has felt the weight of the forms in his hands and the compactness of the dirt under his feet. He has seen these things fail and really wants to prevent that from ever happening again.
In my case, one of them said “put as many stakes in as you think you need to keep the forms in place, and then add two more.”
I still had some bow outs on my build because they overestimated how well the sand could hold in stakes, but generally speaking, their experience was an asset.
For my garage, the plans were a little unclear on the depth of a grove that was to be formed in the top of the concrete to catch the base of the wall. I understood what it was for, but thought it was based on the depth of a 2×10 board (1.5″ x 9.5″). The first guy to work on it had never done anything like this and did the work on a day when I wasn’t around, so he put the wrong sized board in the wrong place, etc. I contacted the boss the next day and he send out someone else to fix it. The new guy stiffened the board and placed it 3.5″ into the form. While he was working, I noticed it was “too deep” and went to ask him about it. He just looked at me and politely said, “I have done about 20 garage slabs for quonset huts and they always do it like this.” Recognizing experience (and confidence), I backed down and went to check the plans. He was right (and I let him know).