Soil Concerns an Installer Can Control

It’s important to protect the installation site and prevent compaction of the soil treatment area
Soil Concerns an Installer Can Control

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When soil characteristics are not interpreted correctly, they can negatively impact long-term performance. Assuming the installer is not the one that made the soil interpretations and design (although I recognize a lot of you do soil and site evaluations and design), what are some of the potential soil problems an installer can control?

One of the biggest factors an installer can control is not excavating or working with soil when it is too wet. In my area, the construction season can sometimes last only a few months and rainy periods during the summer can make a short season even shorter. This puts pressure on installers to get a lot of systems installed in relatively short windows. Working the soil or having traffic over the area that will be used for the soil treatment unit can cause significant compaction and smearing. This compaction reduces soil permeability to both air and water.

The site evaluator and designer may have technically made the right calls regarding the soil, but because it was compacted during installation, the soil will not accept the amount of effluent intended. As with some of the other conditions discussed previously, once this compaction occurs and it’s not mitigated, no amount of management short of soil treatment unit replacement is going to fix the problem.

There is an easy field test to determine if the moisture content of the soil is at or above the plastic limit, which is where compaction and smearing will occur. Take a handful of soil and try to roll it between the palms of your hand. If you can roll the soil into a ribbon 1/8-inch in diameter the soil is too wet to work. The soil should be tested at the depth the system will be installed. So if a trench is 24 inches deep, that depth and above is where the test soil should come from. If it is a mound or at-grade, then test at the surface. Compaction, particularly at depth, is very hard to fix. So, if it looks like you are compacting and smearing the soil, work should be stopped on that phase until the soil is dry enough.

Another situation I have seen very often is that it’s not the installer who does the damage to the site, it’s the other contractors working on the residence. This happens a lot during new construction or remodeling. The shortest path to bring in the concrete blocks or cement truck is across the drainfield site. This heavy equipment can seriously compact the soil in the area to depths of 2 to 3 feet or more given the right (or wrong, in this case) soil conditions. This has led regulatory agencies to require protecting the drainfield area by fencing and signage. Even if this guideline isn’t a part of your local regulations, as an installer you should protect the site from other disturbances. Remember, you are the one the homeowners will call first when the system is not working.

About the author 
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program and is an emeritus professor in the university’s Department of Soil Water and Climate. Send him questions about septic system maintenance and operation by email to

This article is part of a series about designing and installing onsite septic systems with future management in mind:


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