Tag Archives: Engineering

Experience vs. Understanding


Posted on October 1, 2014 by

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.

Building Inspector

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).



Posted on February 28, 2014 by

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)



Posted on June 16, 2013 by

Earth Sheltered Timeline, 2013

January: Finalized the design of the Storm Room, which meant we finally had a good idea of what the house would look like.  I also worked out some details of the mechanical room, etc. with 3D models.

February: I shared the architectural plans with the building inspector who then contacted the architect directly to discuss some minor changes that would make things easier to approve.  Got a set of engineering drawings and had a lot of concerns, set up a meeting for March 1.  Finally settled on a set of windows, also got quotes on things ranging from ICFs to a glass floor between the storm room and the rotunda below (it was too much).  Sherri met with some kitchen places to get some quotes. I had a google hangout with the architect to discuss water drainage and other issues.

March:  Met with architect and engineer to try and get things back on the right path and fix some aspects of the design, especially simplifying the structure at the front of the house and fixing some thermal issues.  At the meeting, I mentioned the high cost of the glass floor and the architect suggested a steel grate instead…  At this point the hours of work remaining seem few, but the weeks stretch on…   I also met with the Shotcrete contractor to make sure things still looked build-able to him, he seemed very comfortable with the whole idea and that made me a little nervous, but not as nervous as if he looked nervous ;^).   I also got quotes on the large precast ribs from several precast companies.

April: Perhaps as an April Fools day joke, the architect/engineer sent a bill stating they were 100% done…  and the Engineer was asking for more money beyond the agreed amount for “scope creep”.  Instead of paying, I wrote out an itemized list of the issues along with dates of emails sent on each topic.  Basically, I established that the things I had asked for were in the very first emails and the later emails were just trying to get them to pay attention to the original emails.  Instead of forwarding the letter to the engineer, the architect decided to continue working on everything else so he would only need to go back to the engineer once at the end.   In April, I was sent the “penultimate update” of the drawings, but I still found many issues (many I had already mentioned, but some were new).  The neighborhood where I am building also had a “work day” to trim and remove trees that were too close to the road.  I went out to help out


I will come back and fill in the missing months when I get a chance…

=> Things really get going in 2014