![]() 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. ![]() ![]() Trench footing Shallow trench filled with concrete.Stepped footing Stair-like design spreads out load. ![]()
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