Test prospective sites for soil compaction potential, and protect the septic system site during and after installation.

Humans, along with all our activities, cause widespread soil compaction. An ideal soil has 50 percent pore space: some air-filled pores and some filled with water. In addition, 45 percent of a typical soil is composed of mineral materials, with 5 percent composed of living and dead organic materials. The term compaction includes soil compression, soil compaction and soil consolidation.

  1. Compression is the loss of soil volume. Soil compression leads to a loss of total pore space and aeration pore space, and an increase in capillary pore space. In other words, large air-filled pore spaces are crushed, creating more small, water-filled pores. Compression is most prevalent in soils under wet conditions.
  2. Compaction is the translocation and resorting of textural components in the soil (sand, silt and clay particles), destruction of soil aggregates and collapse of aeration pores. Compaction is facilitated by high moisture contents.
  3. The third primary component of soil compaction is consolidation. Consolidation is the deformation of the soil, destroying any pore space and structure, and water is squeezed from the soil matrix. This process leads to increased internal bonding and soil strength as more particle-to-particle contacts are made and pore space is eliminated.

Compaction constrains oxygen (O2) movement in the soil and shifts soil toward anaerobic conditions. Less O2 diffusion into the soil leads to a chemically reducing soil environment. Compaction leads to smaller pore spaces and slower infiltration rates.

Common compaction causes

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  1. Wet soils – For every soil type there is a soil moisture content at which the soil can be severely compacted with minimal effort. The higher the clay content, the higher the likelihood that the soil will hold water. Soils that are saturated or nearly saturated have lower soil strength and compact, smear and move more than the same soil under dry conditions.
  2. Vehicles, pedestrian and animals – The pounds per square inch of force exerted on the soil surface by walking, grazing, standing, and concentrated humans and other animals can be great. Traffic with tracks and wheels puts a force on the soil surface. Narrow rubber tires can transfer many pounds of compaction force to the soil.
  3. Soil handling – The movement, transport, handling and stockpiling of soil destroys aeration pore spaces and disrupts soil aggregates. Soil cuts, fills and leveling compacts the soil. Soil-handling equipment can be large and heavy, allowing compaction many inches deep.

Compaction can occur on any soil texture. In order to assess the potential of a soil to compact, the plastic limit should be tested. Plastic limit means a soil moisture content above which manipulation will cause compaction or smearing. The plastic limit can be measured by the American Society for Testing and Materials Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils, ASTM D4318 (2005). Below is an abbreviated version of the test:

  1. Select a handful of soil for testing (any non-soil material – rocks, roots, etc. – should be removed). Do not add moisture or let it dry out. Sample should be taken at the depth of excavation (absorption area).
  2. Roll the sample between the palms into a pencil or worm shape.
  3. Continue rolling the thread until it reaches a uniform diameter of 1/8 inch if possible.
  4. If the sample does not reach a diameter of 1/8 inch, the soil is above the plastic limit and construction can proceed.
  5. If the sample is rolled into a diameter equal to 1/8 inch before breaking, the soil is too wet and construction should not occur.

The septic system site must be protected. During construction, the proposed soil treatment and dispersal area site should be protected from disturbance, compaction or other damage by staking, fencing, posting or other effective method.

The best way to minimize compaction is to not allow equipment on site. However, this is seldom possible, so minimizing impact is the next best option. Distributing the weight of vehicles by tracks is beneficial. 

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After the system is built, traffic on the soil treatment system by both humans and animals should be avoided. Instruct homeowners to keep pets off the system. Warn them never to drive a car or other vehicle across the mound or mow when the soil is wet. Compacted soil can lead to soil erosion and impedes the flow of air around the system. In winter, activity on a mound can cause frost to penetrate, resulting in freezing problems.

Soil compaction is permanent. Studies demonstrate that after half a century, compaction still affects soils under natural conditions. Recovery time for significant compaction is at least two human generations. Soils do not “come back” from compaction. With surface compaction, time will help. Normal freeze/thaw cycles, root activity and weathering will help to loosen up the compaction.

Any modification is temporary and is less than ideal. This does not mean that modifications cannot work, but that it simply may be more complicated and costly, and may increase maintenance and monitoring requirements. Percolation tests or other hydraulic soil tests are also useful in providing a better understanding of site impacts (e.g. compaction, fill, etc.). 

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When a compacted site must be used, you may consider pretreating the wastewater prior to it reaching the soil and using reduced loading rates. In both instances, the reduced organic and hydraulic loading rate will maximize the ability of the soil to treat and accept the wastewater. If the compaction is very severe, experimental methods include deep plowing and ripping and mechanical soil fracturing. In these applications, removal of the material may be needed.  

About the Author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

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