Conduct a Soil/Site Review

Buying a house is, for most people, the largest financial investment of their lives. Therefore, it is
important to know about soil characteristics that make a site suitable for home construction or
that may cause problems on an established homesite.

Make Sure Land is Suitable for Your Homesite’s Planned Uses

There are several questions you should answer before you buy land to build on or before you buy an established home.

Potential problems and costly mistakes can be avoided by the homeowner and contractor if a study of the soil and site characteristics at the site is made before construction begins or before a house is purchased.

• Are the soil properties favorable for establishing and maintaining lawns, shrubs, trees, and gardens without extensive and expensive soil modifications?
• Is there a flood hazard? (Is the site on a floodplain?)
• Are there soil factors that prevent or limit the soil’s use for septic tank absorption fields or sewage lagoons (if public sewer system is not available)?
• If you plan to install a basement, will its construction be limited by such factors as:
• a. High water table, either temporary or permanent?
• b. Depth to bedrock?
• c. Drainage – surface ponding or excessive runoff?
• d. Shrink-swell potential of subsoil?
  

• What is the slope surrounding the building site? Will the site be stable? Will there be excessive water runoff?
• What are the erosion conditions and landslide potential? Previous erosion may have caused gullies and/or have limited the depth of topsoil, requiring leveling and filling. Erosion will give a clue about the stability of soil on a slope uphill from the house.

Understanding the nine critical soil and site factors will help determine if there are any limitations for the planned uses of your homesite.

These nine site and soil properties are critical to evaluate for homesite selection: (A) surface texture , the amount of sand silt and clay in the soil; (B) permeability , the rate at which water enters and passes through the soil; (C) depth of soil to bedrock , including both topsoil and subsoil; (D) slope , steepness and length of the slope; (E) erosion hazard , the amount of topsoil currently on the site and the potential for future losses; (F) surface runoff , the rate at which water flows off the site based on slope, drainage and texture; (G), shrink-swell of the soil , which involves changes in volume based on soil wetness; (H) water table , the depth at which water occurs in the soil both seasonally or permanently; and (I) flood hazards , the frequency that water from storm runoff inundates the site.

Typical planned uses for a homesite are gardening and landscaping, foundation and basement construction, on-site wastewater system, and soil stability.

Soils are judged for homesites or building sites by the properties that may limit or prohibit a planned use. A favorable soil property may pose “no or slight limitations” to homesite development, but if it creates unfavorable conditions that require correction or a modification of the building plans, the limitation is categorized as “moderate,” “severe,” or “very severe,” depending on the severity of the condition.

The final evaluation of a building site depends on the limitations of the individual soil properties. The soil property with the most severe limitations automatically classifies the site in the same category. For example, if all soil properties are rated as “slight” but one is “severe,” the site evaluation for that use is also classified as severe. Hence, the building site is judged by its most limiting soil property.

Soil and Site Factors

Surface Texture

A special factor in foundation and gardening. 3 classes.

The texture of a soil is determined by the relative proportion of sand, silt, and clay particles on the site. Texture is most easily determined by rubbing the soil between your fingers and feeling for a slick, floury, or gritty feeling.

Clayey (fine): Severe limitations for all uses. Soil is sticky when wet, hard when dry, and difficult to work when used for flower beds, shrubs, and gardens. They may be droughty and require frequent watering for plant growth. Special planning and design are required for foundations.

Loamy (medium): No to slight limitations for all uses. Loamy soils are floury and provide the best texture for landscaping and gardening. Loamy soil is easy to excavate and absorbs wastewater well. Care should be exercised during construction to be sure the surface soil is not covered by less desirable material.

Sandy (coarse): Moderate limitations for all uses. Soils are gritty and may require stabilization with organic material and/or loamy topsoil to improve moisture- and nutrient-holding capacity for desired plant growth. Erosion from water and wind may be a problem during construction. Sandy soil is easy to excavate and absorbs wastewater well.

Permeability

Affects your on-site wastewater treatment system choices. 4 classes.

Permeability measures the rate at which water moves through soil and is an important factor when deciding between a septic tank system or another type of on-site wastewater treatment
system. Soil percolation tests are required before making further plans.

Rapid: Slight limitations for foundations with basements, moderate limitations for lawns and gardens, and very severe limitations for septic systems and sewage lagoons. Permeability is >2.0 inches per hour. Septic systems may not adequately filter effluent under rapidly permeable conditions, which creates a very severe limitation. Seepage from lagoons would make it difficult to maintain adequate depth of water and could contribute to pollution of ground water.

Moderate: Moderate limitations for sewage lagoons and no to slight limitations for all other uses. Permeability ranges are 0.60 to 2 inches per hour.

Slow: Severe limitations for septic tank systems, moderate for foundations with basements and for lawns and gardens, and no to slight for sewage lagoons. Water movement can range from 0.06 to 0.60 inches per hour. Problems are generally similar to the very slowly permeable soils, but the modifications required for use are not as great.

Very Slow: Very severe limitations for septic systems, no to slight limitations for sewage lagoons, severe limitations for foundations with basements and for lawns and gardens. Water movement is generally <0.06 inches per hour. This requires a prohibitively large field of lateral drains or costly modifications. Septic systems are generally not recommended. Shrink-swell potential is often high.

Soil Depth

An important consideration for foundations and other site uses. 5 classes.

Depth of soil includes topsoil and subsoil. Severity of limitations for depth varies greatly for different uses in homesites; therefore, Table 1 is useful as a guide for evaluating soil depth for alternate uses.

Table 1: Homesite use limitations by soil depth class.

Slope

Impacts site stability, water runoff, and other uses. 6 classes.

Refers to the general slope steepness and length of slope. Slope is important to erosion, water runoff, and site stability. Table 2 will aid in homesite interpretation of the slope condition.

Table 2: Homesite use limitations by slope class.

Erosion Hazard

Current erosion on the site indicates the availability of topsoil and the future potential of soil loss after construction. 4 classes.

If the topsoil is less than 6 inches in thickness, problems with establishment and growth of plants for landscaping and gardens are likely.

No/slight erosion: Topsoil is greater than 6 inches. No to slight limitations for any use.

Moderate erosion: Topsoil is between 4 to 6 inches. No gullies are present. Slight limitations for all uses.

Severe erosion: Topsoil is less than 3 inches thick. Occasional gullies may be present. Severe limitations for lawns and gardens; moderate for all other uses.

Very Severe erosion: Topsoil is less than 3 inches thick and non-existent in some areas. Deep furrows and gullies may be present and actively eroding. This condition will require extensive filling and leveling, extra cost for septic systems, extensive modifications for landscaping, etc. Erosion control measures should be carried out during construction. Severe limitations for all uses.

Surface Runoff

A special factor for foundations and basements. 4 classes.

Water runoff is an important factor in connection with slope, drainage, permeability, and erosion. Special attention needs to be given to water flowing from surrounding areas and from upslope of the homesite. Runoff from adjacent areas onto planned or established homesites can cause ponding and water accumulation around the homesite, wet or flooded basements, or instability
of slopes and soils.

Rapid: Severe limitations for lawns and gardens. No to slight limitations for foundations with basements and for septic systems.

Moderate: No to slight limitations for any use.

Slow: Severe limitations for foundations with basements and for septic systems. No to slight limitations for other uses.

Very Slow: Very severe limitations for foundations with basements and for septic systems. No to slight limitations for other uses.

Shrink-Swell

Many clays swell when they absorb water and shrink when they dry. This is most noticeable in the subsoil where fine-textured soil layers are found. 3 classes.

The red clays of the central and western counties of West Virginia have a high shrink-swell potential, swelling to over twice their dry volume and causing very severe land use limitations. They tend to be prone to erosion and slide downhill on sloping sites. Swelling pressure of such clays may cause damage to foundations and retaining walls, and cause restricted drainage and limited permeability. Keep in mind, however, that not all clays (fine-textured soils) demonstrate equal shrink-swell.

Low: No to slight limitations for all uses. Coarse-textured soils.

Moderate: Moderate limitations for all uses. Medium-textured soils.

High: Severe limitations for foundations with basements, septic systems, and lawns and gardens. No to slight limitations for sewage lagoons. Clayey soils.

Water Table

The presence and depth of a water table can cause limitations that restrict the soil for certain uses. 3 classes.

A water table within the depth of basement construction or septic tank/drain field installation may result in a wet basement or inadequate wastewater treatment unless special precautions are taken. Water table must be evaluated on the basis of both depth and permanence, requiring measurements during different seasons of the year. Soils with no water table have a bright uniform color (brown, yellow, and red), and those with a water table are often pale or have a washed-out grayish color. Mottling of the soil (mixed yellow, orange, red, and gray spots) is an indication of a shallow water table for at least part of the year. Sometimes mottling looks like rust spots.

Deep: No to slight limitations for all uses. The water is present at >72 inches in depth.

Moderately Deep: Moderate limitations for all uses. The water table is present between 48 to 72 inches in depth.

Shallow: Severe limitations for all uses. The water table is present at <48 inches in depth.

Flood Hazards

The occurrence of floods is a frequently overlooked factor. 3 classes.

Flooding may not occur on an area for many years, but then a serious rainstorm can flood homes that were built during drier periods. Urban development in a watershed may increase runoff by up to 70% in streams, greatly increasing flood hazards. Soils can give an indication, but long-term rainfall and flooding records must be studied to determine the true condition. Position on the landscape and proximity to nearby streams are good indicators of frequency of flooding.

None: No to slight limitations for all uses. Flooding frequency is <1 year in 4 years.

Occasional: Severe limitations for foundations with basements, septic system, and sewage lagoons. Moderate limitations for lawns and gardens. Floods 1 or 2 years in 4.

Frequent Flooding: Very severe limitations for all uses. Floods >2 years in 4.

Recommendations for Homeowner/Future Site Buyer

Before construction, topsoil should be salvaged and placed in a pile. Once the home is built, the area around the home should be graded and the topsoil replaced on the surface. This will ensure that growth of grasses and landscaping plants will be successful. Runoff from roofs should be drained to areas away from the house.

When considering purchasing an existing home or a site for home construction, buyers should make on-site examinations of these nine soil and site factors, study the soil map of the site, ask longtime nearby residents about site conditions, and seek assistance from soil scientists of the Natural Resources Conservation Service or specialists of the West Virginia University Extension before making the final decision.

NOTE: If a septic tank is needed.

If a septic tank system or similar on-site wastewater disposal system is needed, you must have the county health department sanitarian conduct a soil percolation test and issue a permit before construction is started.

Where to find information on soil characteristics.

Detailed county information on soil characteristics can be found in the published county soil survey report. Check the Internet for Web Soil Survey or contact the Natural Resources
Conservation Service Office in your area. Many areas have been mapped even though the maps and reports may not have been published. Your WVU Extension county office may also be able to assist you in obtaining information before you build or buy your home.

Final Comments

Careful consideration of the factors discussed above may not answer all of the questions encountered by the prospective homeowner or builder, but many costly mistakes can be prevented by careful advance study and by seeking the advice of soil scientists.

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