System Site Plans: Review of Soils and Site Layout

Reviewing these critical variables allows the installer to be confident the system can be installed as designed

System Site Plans: Review of Soils and Site Layout

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The next major step in the planning process is to conduct a site review after the design plan has been reviewed. The soil conditions, protection of the soil treatment area and layout of the system starting from the structure are needed for an accurate bid.

Soil conditions

Soil conditions such as wetness and texture may limit not only constructability but the type of equipment that may be employed at the site. If the site is wet at a certain time of the year, it may limit when the onsite wastewater treatment system can be built. Depending on the texture of the soil, smearing and compaction cannot be avoided or mitigated when the soil is wet. Soil wetness may limit the type of equipment to be employed at the site, favoring tracked equipment with less pressure per square foot on the soil. If possible, it is better to wait for a time of year when the soil is drier to minimize risks of smearing and compaction that can seriously affect system performance.

Soil treatment areas

The soil treatment area should be roped off and marked for protection from heavy equipment, stockpiles and traffic before construction begins. Everyone involved at the site should be informed of the importance of leaving the area intact and that the roped-off area should not be breached for any reason. It is a good business practice to inform all contractors at the site in writing so if the soil treatment area is compromised, someone can be held responsible. 

Plumbing stub-out

The elevation of the plumbing stub-out is critical in the hydraulics of the onsite system because it is the controlling elevation for pipe inverts in all down-gradient components. As part of the site review, the building stub-out elevation should be noted and compared to what is shown on the design plan. 

Keep in mind that there may be multiple stub-outs. If the stub-out is at a lower elevation than is indicated on the design, all elevations in the system must be adjusted accordingly. Many components are limited in how much elevation can be changed because of limitations on burial depth. If the treatment train includes only gravity components, the vertical separation requirements for the soil treatment area must be met to ensure proper treatment. In this case, a stub-out elevation that is too low will require the addition of a dosing tank and a pump to maintain the required elevation in the soil treatment area. 

Extreme caution should be taken when developing a bid for a system that does not have an existing stub-out. Any variation in the stub-out elevation from what is designed will likely result in modifications that may increase the cost of the system.  

Site layout

One of the most important things during the site review is to evaluate the site layout. Check that all setbacks can be met. The layout might hinder site accessibility in cases where natural features (stream, wetlands or large trees) are present in the road frontage. Special permits may be required to access parcels of land that require wetland or stream crossings. It is usually the responsibility of the designer to acquire these special permits. As discussed above, some existing structures or landscape features may also hinder accessibility to the construction site and may limit the options for equipment to be used for system construction.

If proposed structures limit the accessibility or constructability of the onsite treatment system, careful planning with the rest of the contractors is needed to build the system and structures in a sequence that allows the project to come to a satisfactory end. In this case, protecting a system installed prior to home construction by roping off the area and posting signs is imperative 

In addition to ensuring that the system can be built, ensuring that the system can be maintained is also an important consideration. If a septic tank is installed down-gradient from the road or driveway, the site has too steep a slope or is too far for a pumper truck hose to reach, then the tank will not likely be maintained properly. A possible solution to a problem like this is to provide a transfer tank at a location between the septic tank and the road. This allows pumping from the septic tank to the transfer tank; then the pumper truck can take the effluent from the transfer tank. The important point is that the system needs to be serviced and maintained. Accessibility to the system for maintenance is thus critical. 

If the design plan is for an existing site, the installer should verify that all features match the plan. Note any features that are absent or marked as ‘proposed.’ If the site is under construction, it is possible that the site may change by the time of job staging. Other contractors may stockpile materials or equipment in areas that the installer planned to use for access or materials storage. Good communication with other contractors working on the site is critical. 

An on-site review of the soil and building sewer depth are critical variables that can impact the system. Laying out the system allows the installer to be confident the system can be installed as designed. 

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to

This article is part of a series on site planning:


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