Walk Away When the Soil Is Wet

Don’t even start up your excavation equipment if the work site has been doused with water. Wait for drier weather.

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We still receive numerous questions about “good” installation practices and their importance in the soil dispersal and treatment area. We will attempt to highlight a few thoughts on installation techniques and importance for different site and soil conditions.

If the system is being installed in areas with high water tables or in soils high in clay content, greater care must be taken to maintain the soil in as natural a condition as possible. Keeping and using natural soil to maintain the ability to accept and treat septic tank effluent is always important.

However, on wet sites where water is close to the surface or in high clay-content soils that maintain water for longer periods after rainfall, keeping it natural and avoiding problems is more difficult. Therefore, it is very important to protect the area from other construction activities. Way too often we have seen instances where the soil treatment area has been damaged by traffic over the site from delivery trucks or other contractors not realizing the site can be damaged.

Excavating or scarifying soil when it is too wet can cause compaction or smearing, reducing the ability of the soil to accept wastewater and making it difficult to predict the long-term acceptance rate. Compaction is the compression of soil particles, which closes or reduces pore space available for water movement. Smearing shuts off the soil pores from acceptance by spreading and smoothing soil particles by sliding pressure.

The result is reduced infiltration and acceptance and usually system failure if installation proceeds without fixing or mitigating the impacts. The effects of compaction and smearing in the soil can last for years depending on the severity. This is why it is so critical not to excavate or scarify soil when it is too wet.


Excavation or working the soil should only be done if the soil moisture content is below its plastic limit and is dry enough to be worked. Obviously if the soil is too wet following precipitation events, it may take a day or more for the soil to dry sufficiently to avoid damage during installation.

There is a quick method to evaluate soil moisture content in the field. The soil tested should be taken from the depth of the infiltrative surface — where effluent will leave the media to enter the soil. At-grade or mound system soil should be tested at the surface to the depth of the scarification.

The test is conducted by taking a clump of soil and working it in the palm of your hands. If the sample can be rolled into a wire 1/8-inch in diameter, the moisture content is above the plastic limit and excavation should not occur. If the sample crumbles, the soil is below the limit and installation can proceed.

Once installation begins, the soil still needs to be protected from activities that could cause compaction or smearing. This means all traffic, including foot traffic, should be kept off the infiltrative surface. We often see installers walking back and forth along the bottom of trenches before the media is placed. Human foot traffic can cause significant compaction by multiple trips across the area. You have spent time and effort to protect the area from disturbance and problems; don’t blow it by not being careful during the install.

It’s important to keep an eye on the weather during installation. Any exposed areas should be covered to protect the infiltrative surface from direct rainfall. Rain itself can damage the soil structure at the infiltrative surface. If this happens, installation should not occur until it dries out. The surface may need to be re-prepared depending on the extent of the damage.

For good system installation the job should be finished before leaving the site. This means a final grade on the system is established so surface water will run off the system and the system area is protected from runoff over the system area. This may involve things such as redirecting runoff from downspouts or berms to direct water away from the system.


Leave a minimum of 6 inches of suitable soil to establish a good vegetative cover over the system. The vegetation should be close-growing and vigorous. This keeps the soil in place during rainfall and in the winter, vegetation provides an insulating layer that will catch and hold snow in place. Avoid trees, shrubs and other and other vegetation with extensive deep root systems. Roots can cause problems with all system components.

Turf grasses, wildflowers and native grasses should be used to establish cover. Turf grasses have fibrous root systems that hold soil in place. They will need to be maintained by regular mowing. Wildflowers and native grasses with fibrous root systems may be preferred because they have the same root system benefits but require less maintenance.

Low maintenance does not mean no maintenance. It is important to keep native vegetation under control and any trees or shrubs that get started in the area should be removed before they become established. The area should continually be evaluated for the presence of burrowing animals such as gophers. These pests should be removed before they cause damage to the system.

Having “good installations” is not difficult; but it does require vigilance from start to finish on the installer’s part. And remember, you are the first person someone will call if there is a problem, so doing it right can eliminate a lot of issues in the future.


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