Look Before You Leap Into a Trench Excavation

Before that first bucket of soil is removed on a sloped lot, be sure you understand site conditions and the lay of the land so your system will fit and work properly.

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During one of our workshops, an installer asked about the layout of trenches loaded by gravity and placed on a sloping lot. It was a good question, and reminded us to emphasize basic aspects of installation along with some of the more complicated issues. So here’s a refresher course on the trench installation that may get lost among the more complex topics we address when class is in session.

Drainfield trenches should be used because they provide the most adequate treatment of septic tank effluent when installed in good soil conditions. Those conditions include a permeable soil that will accept a daily loading rate of greater than 0.2 gallons per square foot and less than 1.2 gallons per square foot as determined by soil analysis. Some areas still use a percolation rate to set this threshold, so the numbers are typically between five minutes per inch and 60 minutes per inch. In addition, there needs to be no limiting soil condition such as bedrock, high water table or dense, slowly permeable soil within the required separation distance from the bottom of the trench to ensure treatment occurs.

These requirements will often restrict the depth trenches can be installed. Our Minnesota code specifies a maximum excavation depth of 48 inches to ensure the trenches are kept shallow to avoid potential limiting conditions and to utilize what is usually the most permeable and biologically active part of the soil for treatment. Regardless of whether your code has such a limit, keeping trenches shallow is in general better than deeper.

An excavation is considered a trench instead of a bed when it is less than 4 feet wide and should not be wider than 36 inches. Actual trench width is determined by the width of the backhoe bucket used. Bucket widths vary from 18 inches to 36 inches with the majority of trenches 3 feet wide. Narrow 18-inch buckets can be used but will usually increase the length of trench and take longer to excavate. There may be cases where a narrower, deeper trench will help fit the system on a small lot. Part of the reason not to go narrower is to allow more space for the installer to work within the trench.

Trenches should be laid out so they follow the contour of the land with the bottom of the trench level throughout the length of the trench. The proper length of trenches is determined by the estimated daily sewage flow, soil loading rate and width of trenches.

Most state codes limit the length of a single trench to no more than 100 feet. There should be at least two trenches unless the total required trench length is less than 50 feet. The thinking behind this is not to put “all the eggs in one basket,” so to speak, and rely on a single trench to accept all of the effluent. We think another factor is concern over keeping the bottom of the trench level for a run of more than 100 feet, particularly in sloping areas. With the technology available today, keeping a trench level is not a concern.

Spacing is important when multiple trenches are excavated. To allow room for operating equipment, there needs to be a minimum spacing of 7 1/2 to 8 feet on center. Wider spacing is preferable where space allows for better effluent dispersal. This is helpful from a treatment standpoint and it may also provide extra area if the system needs an addition or replacement in the future.

After the total length of drainfield trench is determined, topography dictates the choice of layout and distribution of effluent. In our view, drop boxes are the most desirable and flexible method to distribute effluent. They present a number of advantages on sloping sites, although they can be used on level sites as well.

Installation is simple: Trenches can easily be added to the sequence in the future if water use increases, as long as there is adequate good soil. Trenches do not all have to be the same length. Only the portion of the system needed to treat the effluent is actually used. Drop boxes provide an access point that allows for management by either the homeowner or a service provider.

One example of drop box utility is where a limiting soil or bedrock layer limits how far downslope trenches can be excavated. Placing a drop box between two 100-foot trenches along a single contour effectively provides 200 lineal feet of trench along that contour. Of course, this is dependent on having a lot size and shape that allows this, but it could mean the difference between being able to install trenches instead of an at-grade or mound system.

Distribution boxes have limited utility in sloped areas. Each trench is connected back to the box and the theory is that all outlet pipes are at the same elevation and will deliver the same amount of effluent to each trench. This also means each trench is expected to accept the same amount of effluent, which requires they have the same length as well as the same soils throughout. The likelihood of both these conditions being met is very small.

In addition, the installer needs to make sure the ground surface elevation at the lowest trench is at least 1 foot above the outlet elevations of the distribution box. If not, there is the potential for the final trench to receive most of the effluent, resulting in sewage coming to the surface.

All of these are reasons why we say that before the first backhoe bucket of soil is removed, the entire system should be staked and laid out to make sure it fits in the space available and meets all requirements.



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