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Thread: Finally starting to plan

  1. #1

    Finally starting to plan

    Hi all, first post in awhile.

    I'm in Pittsburgh, looking for cheap land. Bad credit, no money as of now.

    I'm wondering about the foundation and the earth that it will sit on top of.

    Assuming I can find an acre that isn't on top of a current or former longwall mining site (that is a problem in some parts around here),
    how can i be sure that my concrete piers will stay put once I put them in place?

    I've thought that maybe if I can find a spot with a think layer of rock a few feet or yards below the surface, that would be ideal.

  2. #2
    LHBA Member rreidnauer's Avatar
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    I'm about an hour East of Pittsburgh. It's all room and pillar method approximately 300 feet below the surface out my way, as best as I can determine. The area is known as the Laural Highlands, a large upheaved plateau formed from plate tectonic forces, and has hydraulically eroded away to form numerous stream and river valleys.
    I'm building atop on one part of this plateau, as the reason for it not eroding in all areas, is rock that was not as easily weathered away as it was where there are now those streams. Solid rock can be found directly on the surface, or several feet below. For my area, it is a type of sedimentary shale, but can change drastically over the area. I've dug up large sandstone rocks despite not really having sandstone as a primary strata on my property. (there is a primarily exclusive sandstone mine about 7 miles away, so I must be on the fringe of that strata)
    For my build site, I found the solid rock layer not to be flat, but rising up at a shallow slope. It was about three feet thick. Material found above and below this seam is a sort of fractured shale that I'm assuming is clay that has been partially metamorphosized into shale. It makes for easy digging and a good foundation base. The solid seam was a bit more work for me, not that the sedimentary layers didn't come apart easily when dug out, but because of the slope, trying to maintain a flat and level bottom to my excavation for a continuous footer to be placed. In the end, it came out fine, just took additional time and care.
    Another area of excavation on my property found the solid rock layer about a foot beneath the surface and at least four feet thick. Because of the nature of the excavation, (a 3x6 ft vertical wall hole, 5 ft deep) the backhoe couldn't be used to break it out. I removed the rock by hand using an electric demo-hammer, pick, and muscles. Never broke through the bottom, so no idea how thick it was there. Other excavation areas, I never found any ledge-rock. I've dug many trenches for electric, water, and sewer without running into ledge very often. Meanwhile, part of the drive in to the property is literally atop this rock ledge.
    So the point of this very long-winded answer is, that it will be nearly impossible to guarantee you'll find solid rock to build some or all your piers upon. Adaptation is required. Personally, I'm not a fan of pier foundations in cold climate areas, but if I were to go that route, and had mixed strata to build upon, i would strongly consider first pouring a continuous footer to then pour the piers upon.

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  3. #3
    Thank you. I'm still looking at possible plots and locations.

  4. #4
    LHBA Member mudflap's Avatar
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    I agree with Rod- go with the stem wall. I have the opposite soil here in alabama- no rock, just loan, and the threat of clay in some areas.

    The reason I comment is because of an interesting finding with my pier foundation- according to rocklock- that a continuous footer was not necessary.

    I went with the pier foundation for 2 reasons: a high water table and cost. Since code says "frostline" in our area is 12", piers will use less concrete.

    As far as strength- and this is the interesting part - since the logs are continuously connected by rebar to the foundation and each other, the logs themselves provide the function of a continuous footer for the structure. This satisfied my local engineer and the building inspector, and I was given the green light on my plans.

    I suspect a very low risk of earthquakes in my area contributed to the approval as well.

    I imagine that a continuous footer, plus the lhba butt and pass method is substantially stronger than just the piers alone, if there is an enhanced risk of earthquakes in the build area.

    Also, if the frostline is deep- like 3', then the cost of piers can begin to exceed the cost of a stem wall.

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