Soil suitability is a measure of how well the qualities of the soil support the requirements of the proposed land use.
Examples of suitability for a building include the following:
- Support: The soil must be capable of adequately supporting its foundation without settling or cracking.
- Drainage: The soil should drain well so that basements remain dry and owners can install septic systems in localities where sewers are not available.
- Depth: Soil depth must be adequate for the excavation of basements, sewers, and underground utility trenches.
- Fertility: Surface soils need to be capable of supporting plantings.
How well a soil can support development is a function of several factors, including its:
- Moisture content
Surface and bedrock geological conditions also affect site suitability for development
Evidence of appropriate soil suitability includes if the soil:
- Drains well
- Is sufficiently permeable
- Has low shrink-swell potential
- Has low frost hazard potential
- Has low side seepage
If these characteristics are missing, it may cause problems for development and the project might need to take mitigation measures.
The type of soil is not the only development issue. When the soil issues combine with other features of the site, including the height of the water table, the slope stability, and the potential of subsidence or settling of soils due to the extraction of mineral and geological deposits beneath the surface, additional problems can arise.
Nonetheless, most soils are suited for development, and adverse soil conditions can be overcome by installing drainage, replacing soil with structural fill, or using special foundations. However, these measures can significantly add to project costs or conflict with resource management goals such as the preservation of floodplains or farmlands. In certain urban areas, the high cost of available land may justify the high cost and potential resource impacts associated with these measures. In suburban and rural localities these factors justify the selection of an alternative development site.
- Is there evidence of ground subsidence, seismic activity, a high-water table, erosion, or other unusual conditions on the site?
- Is there any visible evidence of soil problems (such as foundation cracking, heaving, settling, or basement flooding) in the neighborhood of the project site?
- Were structural borings or a dynamic soil analysis/geotechnical study needed and conducted? If so, please discuss the findings of the report.
- Are there visual indications of filled ground?
- Will the project site significantly affect or be affected by unsuitable soil conditions? Is climate change expected to exacerbate unsuitable soil conditions due to rainfall variability and warming temperatures?
- Will the project significantly affect soils that may be better suited for natural resource management activities such as farming, forestry, unique natural area preservation, etc.?
Perform an initial screening test to determine if the foundation soils are compressible or unstable. Also consider using Soil Survey Maps prepared by the National Resources Conservation Service (NRCS) or state natural resources department.
If the potential exists for any of the following problems at the project site, a soil engineer, or geologist should examine the site:
- Underground Hazards: Looking back at the Contamination and Toxic Substances section of the site’s environmental assessment can assist in determining if hazardous substances might be present. If indicated, a test boring can be done to determine soil stability and the possible presence of hazardous substances. A field observation can help determine if the site was a former dump or landfill. Trash, random vegetative growth, odors, and/or rodents can be evidence of filled ground. Sometimes landfills contain toxic chemicals (consult the section on “Hazards and Nuisances Including Site Safety and Noise”). Follow appropriate engineering principles and techniques if the project involves placing buildings on a landfill or dump.
- Bearing Capacity: The bearing capacity of site soils and surficial geology determines the foundational support capacity of a project site. In general, well-drained coarse-textured soils provide the best structural support. Poorly drained clay and organic soils provide the least support. Two frequently used rating scales for soil engineering performance are the American Association of State Highway Officials scale and the NRCS Unified Soil Classification system. Both ratings are provided in NRCS Soil Surveys. State and local building codes may also establish standards for soil bearing capacity.
- For mid-rise or high-rise structures, or in those areas where bedrock is close to the surface, the bearing capacity of the geological substrate is important. Use geotechnical engineering standards to interpret the potential structural loadings for various categories of surface/bedrock geology configurations; however, site-specific analyses may be required for major structures.
- Frost Susceptibility or Liquefaction: If frost susceptibility is a problem, contact the city or county engineer or a geologist. Look at the frost line, foundation depth, soil types, and water table. In general, poorly drained soils are more susceptible to frost action than well-drained soils. Sandy soils or filled areas in which high water table conditions exist are subject to liquefaction in the event of ground tremors or in the presence of large vibrating machinery. Under these circumstances, soils lose nearly all structural bearing capacity. In such locations, consult with an engineer or geologist.
- Shrink-Swell Potential: Consider how much the soil volume changes with ambient moisture. Soils with high clay content, that are subject to changes in moisture due to groundwater withdrawal, drainage, and increase in paved areas, are the least suitable for development. If the site has soils with a high shrink-swell potential, consult a soil engineer to determine if settling might occur due to changes in moisture content of the soil.
- Subsidence: Ground sinking can lead to the collapse of existing structures, changes in drainage and vegetation and safety hazards. Conditions that may indicate subsidence include:
- Extensive underground mining (shaft/tunnel)
- Presence of limestone or other soluble bedrock in areas of moderate to high precipitation
- Large withdrawals of groundwater from aquifers
- Excessive wetting of low-density soils subject to hydro-compaction.
Contact the city or county engineer or a geologist to determine if subsidence is a potential problem in the area.
- Water Table: A high water table might produce damp or flooded basements or foundation damage. High water table conditions may also limit the use of septic systems for onsite wastewater disposal. Check the soils survey to determine if the seasonal water table is higher than the lowest elevation of the structure; future projections of season water tables are preferable, as they may be altered by climate change. A soil boring test or soil percolation test can provide a more in-depth analysis.
In addition to the federal requirements under NEPA, legal requirements for soil suitability are found primarily in state and local building codes, zoning requirements, and subdivision regulations. EPA National Pollutant Discharge Elimination System (NPDES) and Stormwater Pollution Prevention Plan (SWPPP) requirements address issues related to subsidence. Additionally, many communities have local building codes or zoning ordinances that address soil suitability.
Mitigation measures call for both soil and foundation engineering solutions. Solutions might include:
- Replacing problem soil with more satisfactory fill to reduce or eliminate problems
- Injecting additives
- Improving drainage
- Altering foundation design by embedding the foundation, using pilings, or increasing the bearing areas of spread footings
Problems with subsidence or lack of suitable soils for onsite wastewater disposal may require considerations of alternative locations. In addition, an SWPPP (which is required as part of the NPDES process) considers erosion, slippage, settlement, subsidence, or other related problems and incorporates measures to reduce issues related to soil suitability.