Do Not Disturb: The Soil, That Is!

Less is more when we’re talking about disturbing the soil in and around an infiltrative area where a mound or at-grade system will be installed.
Do Not Disturb: The Soil, That Is!
An excavator bucket is used to scarify the soil during surface preparation for a mound system.

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Since the 1970s in Minnesota we have been using mound treatment systems to overcome soil limitations and provide separation for treatment of septic tank effluent. Since the 1990s we have been using at-grade systems where the soils permit. With both of these above-ground systems, the installation process begins with the original soil and the vegetation over the soil.

If this part of the installation process is not done correctly – no matter how good a job the installer does on the other components of the system – it is bound to fail. After all this experience and time, we still get numerous questions and hear erroneous statements about what should or should not be done. So here, for another time, we tackle how the original soil should be treated and prepared if an at-grade or mound system will be installed.

We had a county regulator – in a state that will remain nameless – write us a note that said he regularly recommended and designed mounds where the upper 6 inches of topsoil is stripped away and stockpiled to be used as the final cover over the mound. Since he has been to a number of our classes in the past, this is very disconcerting because it violates rule No. 1 of how to treat the soil for mound installation.

A good bond is key

The surface of the soil should remain as intact and undisturbed as possible. The only thing that should be done to the soil surface is that it should be scarified or plowed so the original soil will make a good bond at the infiltrative surface when the clean sand distribution material is applied. Minnesota regulations specify the soil cannot be moved more than 6 inches from the original location. This allows for scarification and plowing to “turn” the upper 6 inches of the soil over to create that infiltrative surface.

Another point to always remember is that the Keep it Dry D___ (KIDD) principle needs to be followed. Soil cannot be worked when the moisture content exceeds its plastic limit. The soil is too wet to work with if you can take a handful from the surface and role it in the palm of your hands into a 1/8-inch-diameter ribbon. Scarifying or plowing the soil will result in smearing and compaction, reducing its ability to accept wastewater.

The other question or comment we received recently was: “The slope that we intend to install a mound on is covered with brush, so should we just bulldoze that out of the way?” From the discussion above, it should be obvious our answer would be no.

Any brush or trees in the area to be covered by the mound or at-grade should be cut off as close to the surface as possible; roots left intact and the soil scarified by use of backhoe teeth to provide the infiltrative surface. This obviously means there is some handwork involved to cut off and remove the brush without driving over or otherwise impacting the surface of the soil. If possible, brush should be hauled off the site in multiple directions to avoid making any potential pathway for water to follow out of the toe of the system.

Cut trees at surface

The story is the same for larger trees. They should be cut off at the ground surface and left intact. The surface area of remaining stumps is not that large, and much more damage will be done to the infiltrative area by trying to remove them. The soil should be scarified and turned over near the tree stumps. It is important they be cut off at the surface. Use caution when removing the logs. Equipment should not be brought in to “skid” or haul out the logs. This probably means some additional sawing so logs can be removed without heavy equipment.

A note here: Some design manuals say stumps should be removed. This is absolutely not to be done. We have a lot of forested area in Minnesota and leaving the stumps intact is a practice born of more than 40 years experience. One way to quickly have problems in a sloping, forested site is to try to remove stumps and roots. Leave it all intact after the trees or brush are cut.

Other things to remember about surface preparation: If the site has native grass vegetation, the vegetation should be clipped to within two inches of the surface of the soil. Rake and remove vegetation from the site. The remaining grass should be turned over or, as we like to say, “green side down,” when the surface is plowed or scarified. Grass should not be left on the surface. It will decay and create a slime that effluent will run across, potentially leading to leakage out the toe of the dike.

Remember in terms of scarifying the surface, the most tried-and-true methods are chisel-plowing or using backhoe teeth to turn the soil over. The backhoe should be moved around the perimeter of the mound or at-grade, staying off the area that will serve as the infiltrative surface. Under no circumstances should a rototiller or similar equipment be used to prepare the infiltrative surface. This equipment pulverizes the soil structure and when several tons of sand are put on top of the area, the soil becomes compacted, leading to reduced acceptance of effluent.


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