Reading the Soil

Installers who know and practice the basics of soil and texture study can help ensure the onsite system design is correct and construction is done under optimal conditions.
Reading the Soil
Opening a soil pit gives the best look at depth to limiting layers, and the ability to analyze soil color, structure and texture.

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As discussed last month, the two major soil characteristics determining the required size of the soil treatment area are soil texture and structure. They impact the size of the system and are important in providing treatment. It also bears repeating that other system sizing factors are daily sewage flow and composition of the wastewater. So, recognize in this discussion that the size of the system is impacted by other factors.

Soil texture is simply based on the size of the individual soil particles. The USDA classification system designates 12 texture classes of sand, silt and clay particle sizes. It is important to note that soil particles are 2 millimeters or less; anything larger than that size is considered rock.

Surface area

There have been a lot of examples to describe the impact of texture on permeability and treatment capability of soils. One measure of the potential for treatment is how much surface area is in a given amount of soil that sewage contacts as effluent moves through the soil. If you take a pound of sandy soil versus a pound of loamy soil, the sand has 3 acres of surface area for treatment and the loam 15 acres. So, from a treatment standpoint, the loam will do a much better job. On the other hand, sand will accept water faster than loam soil. So we like to talk about a balancing act where we want the soil to accept the water we put in while providing the treatment needed. This is the reason we sometimes need to provide some additional treatment for effluent applied to sandy soils versus soils with higher clay content.

There are two ways to determine particle size and soil texture class. One is through a laboratory test that involves a settling cylinder and hydrometer. The other is through the standardized texture-by-feel method conducted in the field as a part of the site evaluation. This is particularly useful since trying to remember the size ranges by their particle diameter range is not really helpful in the field. One of the best ways to determine texture is by recalling how each of the separates feels. Sand – gritty; silt – smooth, velvety like baking flour; clay – slick, sticky.

Having a working knowledge of separating textures into general classes such as sandy, loamy and clayey can help the installer field-check that system sizing is in the right ballpark. For instance, a medium sand would have a loading rate of 1.2 gallons per day per square foot while a clay loam has a loading rate of 0.2 gallons per day per square foot, which obviously makes for a much larger system with the same amount of effluent. Of course these loading rates also take into account soil structure and development of the biomat at the infiltrative surface. This does demonstrate the difference in acceptance rates between soils.

Shapes and structure

What about soil structure? There are four major structure shapes to recognize: granular, platy, blocky and prismatic. How the soil is glued together can improve the amount of pore space (porosity), increase the pore size, connect the soil pores, and increase the percolation rate. When a soil lacks any structure and it is not single-grain, it is termed massive. From a water acceptance perspective, granular, blocky and prismatic improve water acceptance for a given texture material. Platy and massive reduce the ability of water to move through soils. This is reflected in loading rates with a sandy loam and massive structure of 0.2 gallons per day per square foot versus 0.6 gallons per day per square foot with a moderate structure. The size and grade of the structure, along with the consistence – or how well the soil is held together – will also adjust the loading rates.

If the installer keeps general summaries in mind of the impact soil texture and structure have on movement of water into and through the soil, they can anticipate potential problems as they excavate the system. The finer textures, such as loams and clay loams have fine pores, high porosity and good treatment capability, but low permeability. Coarse textured sands and loamy sands have large pores, low porosity, low treatment capability and high permeability.

Fine structure will have high porosity and good treatment, coarse structure low porosity and low treatment. Strong structure will have high porosity and high treatment, weak structure low porosity and low treatment.

Sound a warning

One last comment on structure: An installer can affect soil structure and reduce the capacity of soils to accept septic tank effluent. If structure is damaged or destroyed by smearing or compaction during installation, permeability is drastically reduced. Remember if a handful of soil can be worked and rolled into a ribbon 1/8-inch thick, the soil is too wet for the installation to take place. This test can easily be done in the field.


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