Tips for Excavating and Setting Septic Tanks

Proper evaluation and preparation when installing a septic tank helps prevent later tank movement and other problems

Tips for Excavating and Setting Septic Tanks

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Potential tank movement after installation is measurable, predictable and preventable. Proper evaluation of the original soil, bedding materials, depth to groundwater, backfill materials and potential stress loads reduces the likelihood of later problems. When installing any tank, follow any manufacturer-specific instructions that may apply. 

To properly prepare the site for the excavation, the location for the tank should be staked. Verify the elevation and orientation of the tank(s) relative to the design. If the installer is not responsible for the plumbing stub-out, he or she should communicate with the responsible party to discuss elevation requirements that must be met for gravity flow. The slope on the collection pipe from the stub-out to the tank should be between 1% and 2% (i.e., 1/8- to 1/4-inch drop per foot of run). The elevation of a septic tank outlet is controlled by the elevation of subsequent components that receive effluent by gravity flow; the outlet must be set high enough to allow this. It is particularly important to note that in systems that rely completely on gravity flow, the elevation of the soil treatment area is the controlling elevation. All other elevations should be determined according to the elevation of the soil treatment area to ensure it is not installed too deeply. 

The elevation of a dosing tank inlet is controlled by the elevation of any preceding components that deliver effluent to it by gravity flow; it must be set deep enough. Any tank may have multiple inlets with different controlling elevations so it is important to consider the entire treatment train. To minimize soil pressure, prevent potential groundwater infiltration and facilitate management activities, keep tanks as shallow as possible. 

In areas where groundwater levels are high, water may be present in the excavation, indicating the need to dewater. These measures may only be needed during the installation process but are occasionally required as a permanent part of the system. Note that the need to dewater is an indication that the safety risk has significantly increased on the site. Appropriate precautions and OSHA guidelines and standards must be observed.

There are a variety of methods for dewatering including:

  • A hole at one end of the excavation: This is appropriate on sites with finer-textured soils where the flow through the soil is slow enough that a pump can keep pace. The pump must be removed once backfilling begins.
  • A hole with sump: A slotted pipe backfilled with washed rock serves as a sump at the end of the excavation. A pump installed inside the slotted pipe removes collected water. This method allows the backfilling process to proceed while dewatering continues. If this is a long-term scenario, appropriate management strategies should be incorporated into the plan. 
  • Well points: These are used to control a regional water table in sandy soils and must be designed and installed well in advance of the excavation. This option is chosen when the water enters the excavation faster than a sump can dewater. This application may or may not be permanent and is often tightly regulated.

In order to install a level tank, the bottom of the excavation must be level (using bedding material, if appropriate) and free of any large rocks or debris. It is extremely important that the base of all tanks be stabilized using appropriate bedding. In some cases, naturally occurring soil may serve as suitable bedding material. The installer should check local regulations regarding this. If native soil is to be used as bedding, it is important to avoid overexcavating to maintain relatively undisturbed conditions in the bottom of the hole. When overexcavation occurs and disturbed native materials are difficult to stabilize, it may be necessary to use clean granular material to reestablish the correct elevation.  

If native soil is not appropriate, tanks should be bedded with a layer of granular material (noncompactable materials such as pea gravel, washed rock or stabilized sand) to fully support the bottom of the tank and distribute the weight evenly. Regardless of the material used to construct the tank, the bedding material for all tanks should be free of clods, large rocks, frozen matter and debris. Unless a noncompactable bedding material is used, the bedding should be carefully compacted using a vibratory plate compactor or similar device. Consult the manufacturer for specifications of materials to be used for bedding nonconcrete tanks.

Exercise caution when using washed rock or pea gravel as a bedding material because with time, surrounding soil and other fines can settle into the void spaces, causing settling around the tank. Migration of fines can be managed by either purposely allowing the void spaces to fill during the installation process or taking measures to prevent fines from moving after installation is complete. One management method is to flood the gravel and surrounding area with clean water to carry soil into the void spaces in the gravel. Alternatively, washed rock that is graded so that void spaces are filled with smaller particles can be used. Choose a method that does not require entry of personnel into the excavation if at all possible. 

Indicate what material is used for bedding and the bedding depth. Four inches of sand or granular bedding is recommended for concrete tanks. In some situations, the use of a concrete pad may be necessary to effectively hold the grade and create a firm foundation. If a concrete pad is used, the tanks should be set before the pad hardens to provide a stable and firm connection to the concrete. A concrete tank with a clean bottom can bind with the wet concrete and provide some reduction in buoyancy. Although the bottom of concrete tanks is fairly level, it may not be perfect. Placing a tank with a nonlevel bottom on a dry concrete slab may result in pressure points that may break the bottom of the tank.

Be sure to confirm that the tank has been placed in the excavation in the correct orientation relative to the inlet and outlet. Be sure to verify the structural integrity of the tank after it has been set in the excavation to ensure no damage or shifting has occurred. Double-check with a laser or level that the tank is level in the excavation. This is critical to make sure that the inlet and outlet are at the correct relative elevations. 

Note that workers must be safely positioned while tanks are being set. Compliance with OSHA standards is critical. 

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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