Watch Where You’re Rolling With That Heavy Equipment

Long-term impacts of soil compaction can render a promising building site useless for wastewater treatment options

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Any of you who have read this column over any extended period of time know that we talk about several underlying principles related to proper installation. One of those principles is the acronym KINN, which is Keep it Natural … and you are left to supply the final word, such as Keep it Natural Newt! This has been a way to inject a little humor into a serious discussion.

Recently, we saw an article in a professional journal related to the Soil Science Society of America that, even though it was focused on long-term compaction related to crop production and crop yields, has relevance to our industry as well.

Two major results of the study were that the impacts of compaction from a single pass extended to approximately 20 inches of depth and that the effects on soil structure can last for years or decades.

PATHWAYS FOR DRAINAGE

Deformation or destruction of soil structure increased soil bulk density, slowed fluid transport and increased mechanical impedance. Soil structure — by definition — is the aggregation of individual soil particles (sand, silt and clay) into larger units called peds. These peds have distinct shapes: granular, blocky, prismatic or platy. Granular, blocky and prismatic have vertical cracks and planes that are pathways for water to move through soil. When the structure is destroyed or deformed, water is forced to move only through the pore spaces between individual soil particles.

In our industry, this translates to slower infiltration of wastewater into and through the soil and impedes the movement of oxygen to depth. So, compacting soil provides us with a double whammy so to speak. It reduces the long-term soil ability to accept effluent, resulting in a lower long-term acceptance rate. This can lead to system failure and reduces the amount of oxygen at depth, which is necessary for the aerobic organisms in the soil to help break down the organic material in the effluent. Also, a more resistant biomat is a result, further reducing the long-term acceptance ability of the soil and leading to system failure.

A question we always get when we have discussions about soil compaction is: “How long does it take for the soil to recover and what can I do about it?” The answer is still that it depends, but the study verifies that the effects are long lasting and not easily fixed. Relying on natural processes such as wetting/drying, freeze/thawing, and plant root growth to fix the problem indicates that the range in time to recover is years to decades. When the area in question is the only one or two locations on the lot that are suitable for the soil treatment area, this is not the answer an installer or customer wants to hear, but it is apparently accurate.

One other thought to bear in mind as an installer is that the effects monitored are the result of a single pass. Other studies have indicated that with multiple passes, the damage can go even deeper into the soil profile and be even more long lasting. For an installer, one pass is bad enough. Think of the times systems have been installed in areas where multiple passes have been made over the treatment area by heavy equipment, either by the installer or others working on the site. Once compaction has occurred, the soil is no longer “natural” from our perspective and will respond differently to the application of sewage effluent; those responses will be hard to predict.

AVOID TRAFFIC

This study highlights the need to protect soil treatment areas from construction and other related activities. We always recommend that the site be flagged, staked and otherwise protected from unwanted traffic. All it takes is one errant heavy equipment operator to do significant damage to the site, making it unusable.

One other way a site can be compacted is by the installer excavating and installing the soil treatment unit when the soil is too wet. Hopefully everyone remembers the simple field test to determine if the soil is subject to compaction or smearing: Take a golf ball size amount of soil and roll it in the palms of your hands, and if you can form it into a 1/8-inch ribbon, the soil is at its plastic limit and too wet to work. Installation of this part of the system must be delayed for a day or two until it is safe enough to work without causing compaction or smearing. This is our KIDD principle; “Keep it Dry …”.

For those thinking, “Well, I will just excavate below the compaction level in the soil,” remember: the deeper you excavate, usually the lower the permeability of the soil and the less oxygen available for treatment. So, the size of the soil treatment area will likely need to be increased, and going deeper may not be possible due to other limiting conditions, such as a high water table, seasonally saturated soil, or bedrock.

The bottom line for an installer is not to let the compaction happen in first place; in this case, avoidance is preferable to any type of mitigation effort.



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