Any time you're in doubt about the soil under your foundation, you'd be wise to get professional help.īrent Anderson is a consulting engineer and concrete contractor who serves on the American Concrete Institute Committee 332, Residential Concrete. As you look at the solutions I recommend, however, keep in mind that high-bearing-capacity soil is assumed. For the benefit of builders in the field and at the risk of oversimplifying, I'm going to use non-technical language in this article to briefly explain a little about how footings work and to present some ideas for dealing with special situations. I find that people understand the problems better if they have some background knowledge. I'm a consulting engineer as well as a contractor, and I get called in to a lot of problem situations. In weak or marginal soils, however, it's best to be very cautious some of the solutions contractors think up may not really work. In very strong soils, minor mistakes probably aren't a big deal. In these tough cases, it's helpful to understand the bearing strength of soil and the reasons behind footing design rules. But if the risk is low, you'd like to keep the job moving. If you think there's a problem ahead, you know you should stop and call an engineer. When the footing is laid out off-center so the wall misses its bearing, when you encounter a soft zone on site, or when the footing is undersized, the builder faces a judgment call. It's the unusual situations that cause the most trouble. A frame house with wood siding and drywall interiors can probably handle up to 1/2 an inch of differential foundation movement, but even 1/4 of an inch of uneven settling is enough to cause cracks in masonry, tile, or plaster. If the whole house settles slowly and evenly, some additional settlement is no big deal but if settlement is uneven (differential settlement), there could be damage. We don't often see outright failure, but it's not uncommon to see excessive settlement when soil bearing capacity is low. A footing that performs well in good soil may not do so well in weak bearing conditions. On the other hand, if you build on soft clay soil or if there's a soft zone under part of your foundation, there can be trouble. After the wide part at the bottom of the hole has. Then dump concrete through the tube into the bottom of the hole. Do this by nailing the sides of the tube to the center of a tic-tac-toe grid of 2x4s at the top of the hole. Trench footing Shallow trench filled with concrete When you pour concrete footings, hold the cardboard concrete form tube about 12 in.Stepped footing Stair-like design spreads out load.That’s why all types of foundations have thickened weight-bearing supportscalled footings at their base to help spread point loads over a larger area. Good choice very easy to handle sturdy long-term results unknown. With the exception of solid rock, no soil type can hold a point load for long without some settling. We easily trimmed the block verticals to fit cross-pieces between closely-spaced joist "ladders". As the concrete is poured into the tube, a helper uses a shovel to agitate the mix every 8 in. The concrete should be just slightly on the wet side, about the consistency of thick oatmeal. Note that the spacing on the blocks is different in each direction.number 1 is for 1 1/2 inch lumber. Fill the tubes with concrete until it reaches the nail. Span tables say a 2 x 6 can span up to 7 feet seven inches. We spaced the blocks four feet or less apart. Unknown if any of the bases will settle, however it's a pool deck at ground level. At the far end, we had to add a 2 x 6 post a few inches high. We built the rim joist first and leveled from the retaining wall to the rim joist. After trying the joist, a bit of adjusting was required. Most of the joists attached to the wood retaining wall surrounding the pool and the best practice seemed to be establishing the height of the joist based on the retaining wall and height of the pool (we wanted to be just below the pool rim), then placing the first base and leveling each subsequent base therefrom. We place 1-3 inches of tamped paver base at each location requiring one-half to one bag of gravel each. Great alternative to concrete block! Just finished a 30 x 40 ground-level deck around an above-ground pool.
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