Tiptoe Through the Trenches

Compaction from foot traffic may restrict the performance of your drainfield and shorten the life of your septic system

Tiptoe Through the Trenches

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Recently we were sent a few pictures of an installation by a reader that showed two workers walking back and forth in an excavated trench between two and three feet into the soil. They were using rakes to rake the bottom and sidewalls of the trench to repair some apparent smearing from the backhoe bucket.

With this, we felt it was time to revisit some of our “principles for good installations.” Those principles are keep it dry (KID), keep it natural (KIN) and keep it shallow (KIS). 

KID means not working the soil when the moisture content is at or near the plastic limit, which is the point where the soil can be manipulated or formed into the 1/8-inch diameter wire. Under this moisture condition, the soil is likely to be compacted or smeared. 

Walk around it

During compaction, weight from a single intense force or small repeated forces pushes soil particles together, causing them to compact. Compacted soils have reduced macro and micro pore space, which results in limited air and water movement, restricted root growth, reduced infiltration rates and decreased biological activity. Therefore, highly compacted soils are greatly limited in their ability to accept sewage effluent.

KIN means maintaining any soil structure at the infiltrative surface in an uncompacted and unsmeared condition. Soil structure should be maintained in as natural condition as possible. Treat this surface like you would if it consisted of eggs and you didn’t want to break them. Generally, the soil located at or near the land surface — KIS — is best for treatment and dispersal due to soil structure and oxygen transfer potential. 

In addition, any benefits from evapotranspiration and natural biological activity are greatest near the surface. If natural structure is destroyed through smearing (spreading and smoothing particles and structure by sliding pressure) or compaction, the result will be reduced acceptance of sewage effluent and transfer of oxygen around the system, limiting biological activity.

The higher the clay content in soils, the more susceptible they are to compaction and smearing. They also hold water more tightly. If it rains, the installer may be able to work on a new system the next day in sandy soils. But for heavier soils, it may take another day or two before the soil is at a suitable moisture content to proceed.

We are not the only professionals working with soils who worry about compaction and smearing problems. Landscapers also see compaction and smearing as huge problems, which can affect the ability to grow plants including grasses, ornamental shrubs and even trees depending on the severity of compaction. The reasons for these problems are the same, reduced air and water movement in their case limiting root development affecting the plants’ ability to grow. We deal with other associated problems as well, including increased runoff and erosion due to lower water infiltration during precipitation events.

Trench tips 

We highlight this because foot traffic creates one of the largest compaction problems. It is an example of small, repeated forces that can cause significant damage.

So, when we see photos of installers walking on unprotected infiltrative surfaces, particularly when they are trying to repair some smearing problems, we feel they are likely doing more damage than good for the ability of the trenches to accept sewage effluent. With smearing and if the moisture content is high enough, there is a risk of soil compaction simply by walking on the trench bottom.

Since we assume the installer has spent time and effort to maintain soil structure at the trench bottom, they could be undoing all of their efforts to maintain a natural soil condition. Another note here is the trench or bed bottom does not need to be raked. It should be left in the rough condition. The bottom line is do not walk on the trench bottom during installation unless it is necessary and then limit any back-and-forth traffic. 

Here are recommendations to avoid compaction and smearing: 

  • Installation should not proceed if the soil is frozen or thawing. 
  • Excavation equipment or other vehicles should not be driven on the infiltrative surface. Eliminating this possibility is just one of many advantages for seepage trenches over seepage beds. 
  • Where possible, use tracked and low-pressure equipment when working around the soil treatment area. 
  • Pay close attention to how materials are delivered and placed in the trench and bed due to the heavy weight of some distribution media (rock) or other materials. 


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