All the best practice building in the world won't make a home durable, comfortable or safe if the soil beneath it is unstable. Poor quality soil is a real issue for builders today. If not recognized and addressed, shifting or expansive soils, or soils rich in clay, can lead to structural compromises within the home. Landscaping and the home's grading can also suffer, resulting in pooling of irrigation waters and poor storm runoff. Before construction begins, the best practice is to research and assess the soil quality for its ability to bear the weight of the building.
In most cases it's a good idea to hire a soil engineer to inspect and evaluate the soil conditions. A soil engineer can determine the depth to ground water, settlement characteristics and what is needed to bring the site up to code. But before calling in the engineer, it can be easy for the builder to inspect the site and make some early determinations on soil quality of their own if they know what they're looking for.
There are two main factors to consider when assessing soil quality: type and substance.
"Virgin" and "fill" are the two main types of soil. Virgin soil has never been disturbed and may be easier to work with than fill areas. In general virgin soil will be more compact and more consistent throughout the building site. Land that has had fill brought in may need to be compacted. Because you can't be sure of the quality of fill used in the area, a soil engineer can be very helpful in areas with extensive fill.
When trying to determine the substance of the soil or what the ground consists of, here are a few tips:
Soil containing peat or expansive clays are the worst kinds to build on, and a soil engineer would probably suggest you remove the soil from the site before building. Areas prone to earthquakes and landslides are especially susceptible to poor soil quality, and special care by the builder needs to be taken to help protect the homeowner for catastrophic building failure. Unlike the sculptor of the statue with clay feet, builders today have the technology and resources readily available to build a home with a solid foundation.
Note key elements affecting your design, like existing structures and plants, views and water issues.
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