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Hey all you soil engineers & foundation experts!

AAC_NC | Posted in GBA Pro Help on

So. I’ve read up a lot on piers, especially the helical kind, they seem nice. After growing up on hard slabs, never thought I’d build on one. But my soil is poor (sand) and my water table high (so low shear strength). Both a floating slab or drilling down to solid WILL work. I estimate piers, in my application, are about $4k more, which isn’t a lot in the grand scheme, but I’m building with CMU so I don’t want any flex… I’d need really solid grade beams. Or a floater.

Structurally, which is the overall BETTER option, and why?

PS I’m south so no worry about frost heaves (code is 24″ footer), but I am a little worried after hitting water 42″ down

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Replies

  1. alexqc | | #1

    Helical piles are really popular here in Quebec. I think the technology was modernized here so that may explain it.

    Anyway, I'm a big fan of helical piles. What I like about them is that you can measure the load a pile is going to support just by monitoring the torque required to install it. This is huge in my opinion. You don't need to test the soil and you don't need to guess how much it's going to support. You get a plan signed by an engineer and you have the peace of mind that your foundation is never going to sink or fail. Foundations aren't cheap to fix so that's why I would go with piles. It's cheaper to put them before then after the foundation is built :-)

    Yes they are a little bit more expensive than a traditional foundation but with a traditional foundation it's always recommended to have a soil analysis performed by a geo engineer anyway so the costs are pretty much the same in the end.

    1. Romn322 | | #3

      Have you built with piers? Whenever I hear someone say something is more expensive, I wonder if they've looked at all the secondary costs. Ease of installing radon mitigation or repairing waste pipe in a pier / slab system compared to monolithic are examples. If you've built with both & have a ballpark for % difference, I would love to learn!

      1. alexqc | | #4

        Not yet but I'm going to next spring. My dad built a house addition on piles ten years ago though. I was impressed by the system. Here, the frost line is 4 feet deep so he saved lot of money by not excavating a basement that he didn't need.

        Before comparing the cost, let's establish a few things first because people often compare slab to piles and it my opinion this is not a good comparaison. Piles don't replace a slab (unless your building on a mat slab). Piles replaces traditional concrete footing or the thickened edges of a slab if you are pouring it in one shot. They transfer the weight of the building to the soil just like a spread footing does. You can build a slab on piles if you want or just grade beams and float your slab or build an entire wood subfloor just like you could do the same with a concrete footing. It's up to you. Piles don't dictate your choice of subfloor.

        The main difference between piles and spread footing is that the piles are a deep foundation while the spread footing are a shallow foundation. A spread footing works by spreading the weight of the building on a large area to support it. Therefore, the soil need to have the proper bearing capacity otherwise your foundation will sink or crack. Hence why if you have any doubt on your soil bearing capacity and you are going with a spread footing, you need to have it tested. You also need to make sure your footing is under the frost line which means some excavation is usually required.

        A pile works differently. Instead of spreading the weight of the building over a bigger area, it works by transfering it deeper to a soil with a better bearing capacity. What is really nice about helical piles compare to concrete piles is that you can guarantee the soil bearing capacity just by monitoring the torque applied to the screw piles. When the torque reaches the desired value, you know you have the bearing capacity you need and that the pile will be able to support your house. No need to test anything and that's why I really like this technology. It's more expensive than a concrete footing (3 to 4 thousand more from the quotes I got) but if you compare it to the footings and the soil test, I think the cost difference is pretty small. Plus, installing screw piles takes only one day while with a concrete footing, you have to wait for it to cure. It's up to you to decide if the peace of mind is worth it.

        Once you decide what support the weights of your house then you can choose the subfloor you want. Personally, I prefer building a wood subfloor on top of a conditioned crawlspace because it makes repair so much easier and I don't like concrete floors. They are hard on knees. And if you are afraid of flooding, it's better to have your house a little bit up in the air so it doesn't get damaged by raising water. The benefits of a concrete slab though is that you don't need stairs to access your house, it's cheaper and you can use it directly as your floor if you want. But upgrading and repairing your plumbing can be more difficult.

        You should check Matt Risinger and Jordan Smith on youtube. They made videos on piles and slab and are both based in Texas.
        Hope this help.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Doug,

    If there are no technical limitations that force you to choose one over the other, use the architecture as a guide to which one is more appropriate. What relationship do you want between the house and the site? Do you want it to be seamless, where you can walk out onto patios or gardens without stairs? Would you prefer the the house was set up a bit yielding better views with raised decks?

    It's sometimes easy to get into a mindset where you let technical considerations dictate the form of your house, but try and remember that your primary goal is to create a place you enjoy living in that both supports your lifestyle and delights you. Whenever possible those considerations are what should drive the form. If you look through Architectural Periodicals or Home Magazines, there aren't many photos of houses where the architect let building assembly choices dictate designs. If you have choices that don't involve compromises, use the one the makes the house better to live in.

  3. Romn322 | | #5

    I think you have me figured out. This is what I really needed to hear. I tend to get lost in the technical perfection & lose sight of the aesthetic.

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