Soil Maps help to identify critical issues that may affect Land Development. A careful review of soils in advance of planning a layout on a development site can help avoid costly and sometimes ineffective approaches to drainage and stormwater management, as well as possible legal issues arising from impacts to wetlands and water bodies.
Hydric soils, hydrophytic vegetation and wetland hydrology are the three criteria for identifying jurisdictional wetlands. Soil maps provide information about hydric soils thus helping to predict the presence of federally regulated wetlands. The interpretive information associated with the soil map indicates which map units are composed of hydric soils as well as soil map units that may contain small areas (inclusions) of hydric soils. If the soil map for a land development project has map units that are hydric soils or have hydric soil inclusions, then the County will require a jurisdictional determination before that project is approved. In a jurisdictional determination, a private consultant is hired to check the site for wetlands. If any are found, the wetland areas are marked in the field and then mapped by a surveyor. The mapping is verified by the United States Army Corps of Engineers (USACE) in conjunction with the Virginia Department of Environment Quality (DEQ), the two agencies that regulate wetlands in Virginia. If the project proposes any disturbance to the wetlands, then an impact permit must be obtained from USACE and DEQ. Any disturbance or destruction of wetlands without a permit may result in enforcement action by the regulating agencies.
Expansive soils, also known as shrink – swell soils, are those that contain specific clay minerals that absorb water molecules between their layers, causing the minerals to expand when wetted and then shrink when the water molecules are lost as the soil dries. While the expansion is not huge, generally less than an inch, it can exert up to 30,000 lb per square foot, thus damaging foundations, basement walls, roads, sidewalks, pipes and other structures. As the soil shrinks, it can remove the support for the foundations, roads and pipes, resulting in additional damage. Geotechnical and civil engineers are some of the professionals involved in testing the soils and designing appropriate structures.
The Interpretive information associates with a soil map also addresses shrink – swell. The map units are rated for their shrink – swell potential, generally low, moderate, high or very high. Current building code required the building plan to address the problem if the soil map for the project area shows a rating of “moderate” or higher. Contact the Building and Permit Center (540-422-8230) for additional information on building permit requirements or the County Soil Scientist for more information on soil properties.
Bedrock shallower than 5 feet from the soil surface is part of the information provided by the soil map. Shallow bedrock can increase cost of development due to difficulty in excavation for basements, roads and pipe lines. Shallow bedrock can also limit septic system options. It affects the type of plants that can grow and thrive.
Seasonal or Permanent High Water Table
Zones within the soil can be saturated with water during all of or portions of the year. The movement of water through the soil may be prevented by restrictive layers such as heavy clay or bedrock. In some cases, the soils are saturated due to their landscape positions. They are in low-lying positions where surface water collects. The water leaves only though the slow percolation through the soil or evaporation. The high water table can make excavation difficult, limits septic system options and cause wet basement and crawl space issues. The water table can also limit the plants that will grow and thrive on the site.