Avoid Digging Deep When Designing Drainfields

Keeping the trenches shallow allows the trench bottom to be located above the required separation distance from a limiting soil layer

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A few months ago an installer we have worked with sent soil pictures to interpret with the comments that when he dug up an existing failing system “the best I could dig was about 8.5 feet deep in about 4 different spots. Soil was tight and when it broke out it was almost like rock, and hitting it with a hammer and it would break easily.”

His description would fit perfectly with describing soil layers massive in structure; very dense and compact in nature and very slowly permeable. Given his location, this describes natural soil layers and not soil that has been moved, compacted or manipulated. He asked what kind of replacement system should be installed?

The answer depends on more than just the soil conditions at the site, including conditions such as topography and slope; the elevation of the house sewer; and location of sewage tanks providing effluent pretreatment. In addition, while soil pictures are useful, it is often difficult to determine soil characteristics and depth to limiting soil area from photos due to light conditions, camera resolution and other factors. This may be a topic for a future column — how to take good soil photographs.

Avoid ponding

Our initial recommendation was to go with either an at-grade or mound system to utilize the natural soil surface. It was no surprise when he responded back that the homeowner would not go along with adding a pump tank, pump and pressure distribution to the system. Fortunately, the topography allowed the installer to extend the supply piping to an undisturbed soil area with enough elevation difference to install shallow trenches.

Keeping the trenches shallow allows the trench bottom to be located above the required separation distance from a limiting soil layer. In this case, the limiting layer begins at the top of the dense massive structure parent material. The dense layer will impede the downward movement of effluent and tend to pond the effluent where the soil layers change, leading to reduced treatment and lower rates of water movement downward. 

By installing the trenches shallow and maintaining a minimum separation of 3 feet from the bottom of the trench to the beginning of the layer, treatment will be provided, and the effluent will be allowed to spread out laterally from the trenches using more area for the water to infiltrate. Other types of limiting layers that can be avoided using shallow trenches are impermeable bedrock, creviced limestone bedrock, seasonally saturated zones and permanent water tables.

The 3-foot separation distance we use in Minnesota was based on early research in virus and bacteria movement, which indicated using proper loading rates and resulting biomat formation movement of pathogens was about 2 feet beneath an operating system. Your state or local permitting authority may have a different set of separation criteria, so you always need to check your local requirements. Providing adequate separation distance is not the only advantage from installing shallow trenches. 

In a natural soil profile, the horizon or area of the most biological activity, most well-developed structure and the best permeability for both oxygen and water is at the surface. When sizing the area of soil treatment trenches to accept the amount of water being generated in the house, using the most permeable area of the soil will generally result in the smallest system area-wise, while still providing adequate treatment. Shortening trenches can save the homeowner addition expense. It may also save on your equipment by not needing to dig deeper into or through dense layers.

Being in the most biologically active zone for naturally existing aerobic soil organisms enhances treatment of the effluent and controls formation and growth of the biomat. To break down the organic constituents of wastewater (BOD, suspended organic solids), aerobic soil organisms need adequate oxygen and water to be most effective. Since there are many more organisms in the soil surface, there will be more efficient breakdown of the waste.

Easier maintenance

In terms of treatment of pathogens and bacteria, the surface horizon with more organic matter and better structure provides more surface area for them to be captured on soil particles. This prevents their downward or lateral movement away from the system and makes them vulnerable to attack by the aerobic soil organisms or simply using their ability over time to be infectious.

Keeping excavation depths shallow also helps during rainy periods because the surface dries out first allowing installation to take place sooner. During system operation there is an advantage because there will be more evapotranspiration from a shallow trench versus a deeper trench. While in humid areas this may be an advantage for only a month or two in the summer, it can be significant. In drier or arid areas, evapotranspiration can account for up to 40% of delivered wastewater. 

Finally, with trenches closer to the surface, inspection and maintenance are easier for service providers. It should not be hard to figure out why one of the overriding principles for good trench installations is to keep it shallow. 


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