Section 490. Characteristics of soils that determine suitability

A. Color. Color is a key indication of the suitability of a soil.

1. Red and yellow mottlings may indicate slow internal drainage and may indicate a seasonal water table.

2. Gray and/or gray mottlings indicate seasonal water tables for at least three weeks duration.

3. Black appearance may be due to organic matter which has accumulated due to poor soil drainage.

B. Texture. The term texture refers to the relative proportion of various size groups of individual soil grains in a mass of soil. Specifically it refers to the proportion of sand, silt, and clay.

1. Soil Classification. For the purpose of this chapter soils have been categorized into four groups based on texture as follows:

a. Texture Group I—sand and loamy sand;

b. Texture Group II—sandy loam, loam, and sandy clay loam. Texture Group II soils are subdivided into Texture Group IIa and IIb soils. Texture Group IIa soils consist of sandy loam soils with percolation rates less than 31 minutes per inch and no structure development. The remainder of soils within this texture group are Texture Group IIb soils;

c. Texture Group III—silt loam, clay loam, silty clay loam; and

d. Texture Group IV—sand clay, silty clay and clay.

2. The soil texture shall be estimated by field testing. The field test that shall be applied is contained in APPENDIX F and is entitled "Field Guide to Soil Texture Classes." Laboratory estimation of texture by sieve and sedimentation analysis may be substituted for the field test at the owner's request and expense. Samples shall be collected by the laboratory under supervision of the district or local health department.

C. Permeability. The term permeability pertains to the characteristics of the soil that enable water or air to move through its pores. The permeability of a soil profile may be limited by the presence of one nearly impermeable horizon, even though the others are permeable.

1. Estimated rates. The soil classifications contained in subdivision B 1 of this section have been assigned the following estimated rates in minutes per inch for the purpose of design. These rates may be modified when experience has shown that because of soil structure the texture group has a demonstrated rate different from that assigned.

a. Texture Group I—up to 16;

b. Texture Group IIa—17 to 30;

c. Texture Group IIb—31 to 45;

d. Texture Group III—46 to 90; and

e. Texture Group IV—equal to or greater than 91.

2. Percolation tests. When the estimated percolation rates are in question, percolation tests may be performed, however, the district or local health department may require percolation tests to determine "measured" percolation rates.

a. Requirements. Percolation tests are to be performed under the supervision of the district or local health department. Test holes shall be located at points and depths selected and/or approved by the district or local health department. A minimum of three holes representative of the absorption area are required. When the results of the individual test holes have a spread of more than 30 minutes/inch, five holes with at least one hole in the center of the proposed absorption area are required. Records of all percolation tests performed shall be attached to the application (See APPENDIX G).

b. Procedure. All percolation tests shall be performed in accordance with the procedure contained in APPENDIX G.

c. Records. Data on swelling, saturation and measurement of the percolation rate shall be recorded on forms by the district or local health department; examples of these forms are contained in APPENDIX G.

d. Interpretation of percolation test results. The absorption area shall be based on the average percolation rate measured in the test holes. The average percolation rate shall be computed by determining the percolation rate (minutes/inch) for each hole and averaging those values. When the percolation rate for an individual hole is in excess of 240 minutes/inch, the area represented may be retested one time and the most favorable rate used to calculate the percolation rate.

D. Soil restrictions. A soil restriction is a feature in the soil that impedes the percolation of water. Restrictions generally consist of a layer of soil horizon within a soil that is firmly compacted or is very rich in clay. Soils containing restrictions may require verification of the percolation rate by percolation tests. Examples of restrictions are listed below.

1. Pans. The term pans include hard pans, fragipans, clay pans, plowpans, traffic pans, iron pans, and plinthic horizons.

2. Stoniness. The term stoniness pertains to the relative proportions of stones present in a soil. Stoniness reduces the soil volume for absorption, and therefore, may require a larger subsurface soil absorption field than would be indicated by soil texture.

E. Soil concretions. Soil concretions as hard grains, pellets, or nodules from concentrations of compounds in the soil that cement the soil grains together. Concretions are indicative of slow percolation rates, restrictions, and/or seasonal water tables.

F. Shrink-swell soils. Shrink-swell soils may exhibit satisfactory percolation rates when dry and therefore must be thoroughly wetted before a percolation test is performed.

Historical Notes

Derived from VR355-34-02 § 3.5, eff. February 5, 1986; amended, eff. May 11, 1988; Volume 16, Issue 16, eff. July 1, 2000.

Statutory Authority

§§ 32.1-12 and 32.1-164 of the Code of Virginia.