OK, so lately I have been trying to convince myself to design one last house and have it built. I have always wanted to build an ICF house, so maybe ICF this time. Toward this goal, I found what appears to be a nice-looking lot near Casper, WY.
I just discovered this area is known for expansive soil and also areas with shallow bedrock. 15-years ago, I personally watched as 3, million $+ Street of Dreams homes slowly slid downhill in Portland OR, because of expansive soil. Eventually, all 3 were condemned. But then again, I live currently on a 7000-ft2 post-tensioned, slab on grade, foundation built on south Houston’s disgraceful clay soil, and it seems to be working.
Casper is a cold environment with a footing depth of 42″ minimum, so originally, I was thinking of a foundation with a standard footing below 42″ with an insulated slab on grade floor and ICF walls. That is now a forgotten option.
However, there are other possibilities. One, is a product called VoidForm, which uses what you might think of as cardboard spacers upon which a concrete slab is poured. Later, the cardboard deteriorates leaving a space under the slab for vertical movement of expansive soil. I don’t think this would help much with lateral movement, but maybe site selection could minimize that.
After thinking about VoidForm, I wondered about poured concrete decks like those of BuildDeck or LiteDeck. And then, what about pre-cast concrete flooring systems. All of these could potentially be elevated above ground level giving significant space for vertical soil expansion/contraction.
Has anyone tried any of these options to successfully survive a similar situation, or am I trying overly hard to fit a square peg into a round hole?
Thanks!
Replies
Have you considered a grade beam foundation?
I am not sure if a straight up beam on grade would work well in this location with a frost depth of 42". Would it be freeze free by being located inside of the regular foundation and walls?
A grade beam is a concrete beam that sits level with grade but does not bear on the soil. In stead it is supported by pylons that extend to whatever depth is made necessary by the soil conditions. In your case it could be the frost depth or penetration past the expansive soil. The beam spans between the pylons. What ever structure goes on top can be designed to span between the beams. This could be a reinforced slab. Since nothing actually bears on the expansive soil it is no longer an issue. My son used to build in the LA area where expansive soil is the norm and grade beams were a common solution. It will require engineering.https://www.google.com/url?sa=i&url=https%3A%2F%2Fciviconcepts.com%2Fblog%2Fgrade-beam&psig=AOvVaw2utiTGuFcjyB5RLgsGpbmn&ust=1687280107886000&source=images&cd=vfe&ved=0CA0QjRxqFwoTCMD2t7Dmz_8CFQAAAAAdAAAAABAI
Thanks for the information. At least there might be some hope for this site. I guess I need to have a geotechnical survey done as part of the offer for the land. This area is hilly, so lateral movement might also be an issue, which I think would be an unsolvable problem.
Those houses in Oregon had no chance since they were slowly, but surely, sliding down the hill 2-3 inches/year.
Grade beam foundation are excellent for hill sides because the pylons can go as deep as needed. I've heard of 30' or more.
I have a fair amount of experience with foundations in the expansive soils of the Woodbine Formation around the DFW Metroplex plus frost-exposed foundations in upstate NY like your Wyoming site. Following is my take and recommendations:
1. You WILL need to engage with a regional knowledgable structural engineer and a geotechnical engineer with site-specific soil borings.
2. VoidForms need to be used under the grade beams. Grade beams need to be supported by either drilled , most likely reinforced, concrete piers that may or may not need to be belled and/or helical piers.
3. If you choose the route to use Void Forms under a concrete slab, that slab will have to be designed and reinforced to be structurally-supporting. This slab will be at least 5" thick to accomodate the multiple layers of rebar and the minimum concrete cover required by Code. This will have to be designed by a structural engineer, AND installed by a knowledgable, probably commercial, rebar installer. Forget about using residential concrete contractors for this work. They have neither the experience, knowledge or equipment to do this work, nor fabricate and install the rebar cages needed for drilled concrete piers or grade beams. Most likely they will give you a cost per square foot to do the work, then immediately quit when they arrive on site and see the rebar they have to install. BTDT.
4. Consider an alternate design for your floor. Consider using a crawl space with wood floor joists supported by grade beams or an engineered subgrade with a concrete slab on-grade. The slab on grade probably would still need at least (not less than 42" below frost line) perimeter grade beams for frost protection around the perimeter. It would require isolation joints between the grade beam and the slab to accomodate the differential movement. Where we used engineered fill for slabs-on-grade in the Metroplex area, the (select) fill typically was at least 12" thick, compacted in 4" loose lifts compacted to not less than 95% of Standard Proctor at optimum moisture content. You'll need a testing lab to monitor and verify the required standards are met. Note that tracked equipment can't be used for compaction- you need specialty equipment to achieve the required results. If you can get to where the Potential Vertical Rise is 1" per year or less, then you should be ready to go.
5. Your plumbing line trench(s) need to be plugged with a clay plug where they pass under the grade beam or you will be injecting moisture into the expansive soil in the middle of your slab on-grade. This does very bad things to the interior of your building.
Thanks Doktor for the very detailed reply. I have some things to consider!