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)

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