During the past year, we have taught soils classes in several states. Some states or counties require installers to take a soils identification course in order to get a license or permit to install systems.
Needless to say, at least some installers ask why they need to go through the same soils information as the site evaluator and designer. Our typical response is that as the installer, you are the person the homeowner relates to most closely, and so you will be the first person called or sued if there is a problem.
You need to be able to recognize if there has been a mistake in the site evaluation that could result in a mistake: the wrong type of system, the wrong location or the wrong size.
Aside from landscape and site characteristics, the two most important soil properties to know are soil texture and color. Soil texture is used to determine system size. Soil color helps you ensure that you maintain the proper separation distance for treatment and determine whether the area has been disturbed. Here is a quick primer on soil texture and its relationship to soil absorption system sizing.
How much of each?
Soil texture is the relative proportion of soil particles, by weight, finer than two millimeters in diameter (sand, silt and clay-sized particles). Materials larger than two millimeters are called rock fragments and are removed to determine the texture of the remaining fraction (the material that passes through a number-10 sieve).
Rock fragments are important because they influence water storage, and they dilute the volume of soil that can provide treatment. In some states, when rock fragments exceed 15 percent, the soil-sizing factor is adjusted to reflect this condition.
While most of us recognize sand particles because we can see and feel them, silt and clay-sized particles are different. It is difficult to imagine a particle just 0.002 mm in diameter. If a sand particle were magnified to a size of 10 inches, a silt particle would be about 1 inch in diameter, and a clay particle would be the size of a grain of sugar.
Soil texture classi-fications are defined according to the distribution of the sand, silt and clay-sized par-ticles (soil separates). The 12 textural classes are often displayed on a textural triangle. The classes in order of increasing proportions of fine particles are:
• Sand
• Loamy sand
• Sandy loam
• Loam
• Silt loam
• Silt
• Sandy clay loam
• Clay loam
• Silty clay loam
• Sandy clay
• Silty clay
• Clay
The sand, loamy sand and sandy loam classes may be further subdivided into coarse, fine and very fine. This all sounds complicated and some people make it that way. But for sizing soil treatment systems, a number of these classes get lumped together into four or five that are easier to distinguish with a little practice.
The relationship between soil texture, percolation rate and soil-sizing factor used in Minnesota, where we are based, is provided in the table as an example.
In the field
Texture can be measured in the laboratory by determining the proportion of the various sized particles in a soil sample. This is called mechanical analysis, and the laboratory uses either a hydrometer or pipette procedure. For installers, the issue is how to estimate the texture in the field as a check against the system design.
You make the determination in the field by feeling the soil with the fingers, and sometimes by examining it with a hand lens. This requires a little familiarity and a little practice, but with modest effort you can become competent at estimating texture.
The procedure is to moisten a sample of soil 1- to 2-inches in diameter. There should be just enough moisture so that the consistency is like putty. Too much moisture results in a sticky material that is hard to work.
Press and squeeze the sample between your thumb and forefinger. Press the thumb forward to try and form a ribbon from the soil. Sandy soils will often be loose and feel gritty. Silty soils will feel smooth like flour, and clay soils will be sticky.
The way a wet soil develops a long, continuous ribbon (or doesn’t) gives a good idea of the amount of clay present. If the soil sample forms a ribbon (silt loam, loam, clay loam or clay), texture is determined by the length of the ribbon and whether the sample feels smooth or sticky.
Smooth with less than an inch-long ribbon would be silt loam or loam. Sticky with an inch-long ribbon would be clay loam. Sticky with a ribbon longer than 2 inches would be clay.
Case in point
Using the Minnesota sizing example, let’s say you are starting to excavate for a system of trenches for a three-bedroom house with an estimated daily sewage flow of 450 gpd. The site evaluator and designer used the sizing factor for a clay loam.
This would mean the size of the system in terms of the amount of trench needed would be about 450 gpd x 2.20 sq. ft./gpd = 990 square feet. With a 3-foot-wide trench, that results in 330 lineal feet of trench.
However during excavation, suppose you notice that the soil seems a lot looser and with a lot less clay than would be present with a clay loam. You conduct the ribbon test and get a ribbon no longer than one-half inch before it breaks apart. This indicates a loam texture.
The amount of trench needed then would be 450 gpd x 1.67 sq. ft./gpd = 751.50, or 750 square feet. That means you need just 250 lineal feet of 3-foot-wide trench. These figures in the Minnesota example would be adjusted for depth of rock under the pipe. But as you can see, they still could make quite a difference.
You would then need to discuss this information with the designer and site evaluator so that everyone would be evaluating the site the same way. Obviously, this example would be more critical if it went the other way and showed the need for a larger system. It is in your best interest to be able to do this analysis in the field. It will make for better systems and fewer call backs in the future.
