At CD + C, we recently began a residential job on a site that required a 40-foot foundation. While this number looks like we added an extra zero since the average foundation for a home is four feet, this is accurate. Why did it require such a high foundation? Well, because of poor soil with a very minimum load bearing capacity.
Soils are rated according to their load bearing capacities, meaning how much load the existing soil can handle without risking the chance of a building settling. If the bearing capacity is exceeded by the weight of the building, the soil will compact, resulting in cracked foundations and walls, which compromises the safety of the structure. Some of the more common types of soil that have insufficient bearing pressure include peat, sand and expansive clay.
All buildings must be engineered or designed to anticipate some settlement. However, taking measures to have soil analyzed and addressing underlying issues will prevent costly and time-consuming repairs in the future.
In general, softer soil and soil that expands or contracts with changing moisture levels are more unstable and will be able to bear less weight. Excess moisture can saturate foundation soils, which often leads to softening or weakening of clays and silts. Increased moisture within foundation soils is often a consequence of poor surface drainage around the structure, leaks in water lines or plumbing or a raised groundwater table. Houses built over soil that have uneven bearing strengths – such as part soil and part rock – will settle unevenly, creating cracks in walls and foundations.
The best soils to build on show no signs of minimal settlement. Some of the things we look for are no evidence of unstable land, such as nearby landslides in the vicinity, and no organic soil, peat or soft clay. Stiffer or denser clay is more ideal because the less porous a substance is, the less it will be affected by water. The site we are currently building on is comprised of a soft, silty soil with a high moisture content. As soils dry out, they shrink or contract, resulting in a general decrease in soil volume.
Measures to fix problem soil aren’t cheap but will cost less in time, money and aggravation in the long run. There are several websites property owners can visit to obtain free information prior to construction:
- Visit www.usda.gov for prepared soil maps.
- For US Geological surveys and topographical maps, visit www.usgs.gov.
Fixes include increasing the depth of poured foundations, fills and bracing. Your contractor and/or structural engineer can make the appropriate recommendations.
Costs vary based on geographic region or depth of boring, which basically means drilling to determine the physical properties of the soil on the site; analyzing the results and making recommendations based on the proposed building design and structural needs. The number of borings needed varies with the lot and building size.
However, it is important that a professional tests the site for you new Chicago home. Your home builder will likely hire geotechnical engineers to test the soil to detect problems prior to construction. Individual cities, such as Chicago, require that home builders take precautions; this is built into the city’s building code.
Different problems require different solutions. For example: if peat is detected, it can be removed if the layer is thin enough. Alternatively, a specially designed raft foundation and matt floor slab may be needed. For sand, which varies in size and compaction, piles will need to be driven down to a good bearing layer in conjunction with a concrete slab. Building on a clay soil will affect the ground moisture content and result in a different pattern of expansion and contraction. Moisture content will also be reduced by large paved areas, tree planting and subsoil drainage.
Regardless of the method, it is important to determine the load bearing capacity of soil to ensure the most solid and stable foundation is built for your new home in Chicago. While this takes extra time, effort and money up front, the end result will be a building that will be permanent and less costly in the long run.
For more information on the services provided by Chicago Design+Construction, visit our website at www.ChiDesign.net.