More Basic Principles

Good practice calls for the soil treatment unit of an onsite system to be kept natural, level and shallow
More Basic Principles
No matter which type of trench media is used, trenches should be be installed according to the same basic principles: they should be shallow, on the level, and on the contour, and the soil must be kept natural.

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Before we move on to more specific discussions of installing soil treatment units in the coming months, we want to finish our review of basic installation principles.

Last month we explored the principle: Keep It Dry (KID). Our next principle is: Keep It Natural (KIN). All excavation of trenches or surface preparation of at-grade or aboveground systems must be done so that the original soil structure is exposed without smearing and without compaction. Then, as the installation continues with addition of media and backfilling, keeping the soil in this natural condition should be a major concern.


Keep vehicles away

If you have taken care not to work the soil when it is too wet, the protection of the soil needs to carry over to keeping excavation equipment and other vehicles off the excavated area. One reason to use trenches instead of beds for the soil treatment area is that trenches discourage others from driving on the exposed area. You should also minimize foot traffic after excavation. In particular, you should resist the temptation to walk on the excavated surface during placement of the trench media.

You will often read in articles or manuals that you should rake the sidewalls of trenches to help expose the natural soil structure. We have found this to be generally ineffective in reducing smearing from the backhoe bucket and reducing compaction from foot traffic on the trench bottoms.

It is important when placing the trench media to avoid compaction. When using rock as the trench media, this means placing the media working upslope and, if possible, using equipment with low ground pressure to distribute the weight over as large an area as possible.

In cold climates, the soil infiltrative surface should not be frozen when the trenches or beds are excavated. The frozen soil generally comes out in large clumps or clods, and this can result in significant compaction. More important, if that material is used as part of the backfill, there will be uneven settling and the presence of large voids. This can lead to freezing during winter and can provide space where excess water can enter the trench in spring, causing hydraulic failure.


Level installation

The third basic principle is: Keep It Level (KIL). In the soil treatment area, this means the bottoms of the trenches or bed excavations must be level, and the top of the distribution media should be level along the slope contour.

One of the most common mistakes with trench installations on slopes is the trench not being level and on the contour. With today’s laser levels, there is no excuse for not having the infiltrative surface and the top of the media level. It is also important in gravity distribution for the distribution box or drop box to be installed level. Obviously, this principle also applies for other parts of the system, such as septic tanks and media filters.

In pressure distribution systems such as at-grade or mound systems, the bottom of the distribution media and the piping must be installed level to achieve equal distribution, unless the difference in elevation has been factored into the design. We recommend that no more than 1/2-inch elevation difference be allowed across the entire distribution network. Of course, as always, check on your local or state codes.


Shallow is better

The fourth and final basic principle is: Keep It Shallow (KIS). Generally, the best soil for installation of a soil treatment unit is at or near the surface. This part of the soil usually has the best infiltration rate and the best structure for oxygen exchange. In addition, evapotranspiration and natural biotic activity on the part of soil bacteria is higher. There is also less likelihood of encountering seasonally saturated soil or other limiting layers.

On sloping sites, you must identify the elevations of the bottoms of the trenches in relation to any limiting condition, including bedrock, before construction begins. You then use these elevations during the excavation to ensure that the required separation is maintained. To maintain level and keep the separation distance, wheeled backhoes can self-level through use of stabilizers.

If the slope is too steep, or if you use tracked equipment, you may need to make a bench for the equipment by cutting out soil on the upslope of the first trench and placing the excavated material downslope to create a bench for the second trench.

Next month we will look at gravity distribution and installing drop and distribution boxes. We will also look at trench media choices and media installation.


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