Septic Systems: Site Evaluation and Testing - Soil Depth

Soils must have enough depth so that wastewater is properly treated. Soil depth from the soil surface to the saprolite, rock, or parent material is a major factor in determining the suitability of the site for on-site systems. See Figure 4.5.3 for a drawing showing soil depth requirements, as stated in the rules.

Soil Depths and Site Suitability for Septic Systems
  • Twelve inches or more of aerated soil is generally required beneath the trenches to adequately treat the wastewater. However, a total of 48 inches of acceptable soil is necessary for conventional on-site system installation. If the total soil depth is between 36 inches and 48 inches, a modified on-site system can be installed, requiring 12-24 inches of soil above the trench, a 12-inch trench, and 12 inches below the trench.

  • The only exception to the soil depth requirement is for sites with less than 36 inches of soil where the site evaluation has determined that a modified or alternative system can be installed. For instance, site suitability for low-pressure pipe systems must be based on the first 24 inches of soil beneath the naturally occurring soil and surface. See 15A NCAC lSA.1956 or .1957 for the rules governing installation of modified or alternative septic systems.

Soil depth and slope.

On steep slopes, the depth of soil required for installation of trenches may be greater than the depth called for in the rules. This extra soil depth is needed to keep the bottom of the trench level and at the proper depth on the sloping site. For example, if a treatment and disposal field bas a slope of 60% and a modified conventional system with a trench depth of 24 inches is to be installed, then the minimum soil depth at the lowest elevation must be 57 .6 inches. This depth is required because on a 60% slope with a 36-inch wide trench, there is a difference of 21.6 inches between the uphill and downhill sides of the trench. To keep the downhill side of the trench 24 inches deep and to have 12 inches of soil under the trench bottom, there must be 57 .6 inches (21.6 + 24 + 12) of total soil depth.

See Figure 4.5.4 for another explanation of the extra depth of soil required to install trenches on slopes. Table 4.5. 7 lists the differences in the uphill and downhill sides of a trench for trenches of various widths on various slopes.

Difference in depth in inches between uphill and downhill side of trench.

Soil depth and the use of saprolite.

If soils are not deep enough to install an on-site system but are underlain by saprolite, the site may be reclassified as PROVISIONALLY SUIT ABLE under certain conditions. A trench or pit investigation of the saprolite must be conducted in order to determine if the following physical properties and characteristics are met:

1. Saprolite must have weathered from igneous or metamorphic rocks.

2. Saprolite texture must be sand, loamy sand, sandy loam, loam , or silt loam.

3. Clay mineralogy must be SUITABLE (non or slightly expansive).

4. Moist saprolite consistence must be loose, friable to very friable or firm for more than 2/3 of the material.

5. Wet saprolite consistence must be nonsticky or slightly sticky and nonplastic or slightly plastic.

6. The saprolite must have no open and continuous joints, quartz veins, or fractures relic of parent material to a depth of 2 feet below the proposed trench bottom.

Saprollte depth. When saprolite is used rather than soil to treat wastewater, a separation distance of 24 inches is necessary between the bottom of the trench and any weathered rock or bedrock (Figure 4.5 .5). If the trench is placed partially in soil and partially in saprolite, then the separation distance is 24 inches - x, where xis the depth of the soil in inches (Figure 4.5.5). For example, if a 12-inch trench was composed of 9 inches of soil and 3 inches of saprolite, then the total depth of saprolite necessary to treat the wastewater would be 15 inches (24 inches - 9 inches= 15 inches).

Septic Trench Placement in Saprolite Soil

From the North Carolina Onsite Guidance Manual