Excavation and over-excavation when installing septic system components

February 10, 2022

 By Sara Heger, Ph.D.

Unless the site conditions require over-excavation, it should be avoided as natural soil typically provides the most stable base for system components. Use a laser and verify proper elevations throughout the excavation process to limit the need for imported backfill material for pipe and tank support.

There are many reasons over-excavation can occur on a job site. It may be intentional due to the removal of materials such as bedrock, trees or rocks. In addition, organic, loose or rapidly settling soils do not provide a stable base for onsite wastewater treatment system components. Work should only occur when the soil is at the appropriate moisture content to limit the impact to the soil structure in the soil treatment area. 

Equipment with low ground pressure should be used to remove the materials described below: 

  • Shallow bedrock is a problem because it does not provide a level base and can damage system components. Fractured bedrock is a problem because it allows effluent to pass quickly to the ground water with no treatment. Impermeable bedrock prevents water movement and can cause systems to fail to the surface. 
  • Organic soils (peat) decompose over time and settle, resulting in unsupported onsite wastewater treatment components. 
  • Large diameter trees may be removed so the system can be installed as designed.  Determination of whether trees need to be removed can be done in the design or installation process. Consider consulting an arborist when evaluating trees near a system installation. In making this decision, the type of tree and corresponding root system, the site and soil characteristics, the system being installed, and local regulations should be considered. Roots that are in a compacted ball or a taproot (i.e., hickory, walnut, butternut, white oak and hornbeam) are easier to remove from a site than large fibrous roots systems (i.e., red oak, honey locust, basswood, sycamore, pines, birch, fir, spruce, sugar maple, cottonwood, silver maple and hackberry). Remember that for some onsite wastewater treatment components, such as mounds, trees should be cut off at ground level rather than removed. If there are too many trees or boulders, a different site should be selected.
    • Stump grinding is the quickest method to remove the upper portion of tree roots but may not be deep enough for STA installation. Hand digging the stump can be very difficult, even for stumps of small trees. If hand removal is considered, cut the trunk of the tree as high as possible to provide leverage to loosen and break the roots as the digging progresses. Tree removal may be more effective and efficient using large equipment such as an excavator, but be careful to limit compaction of the STA. It is not practical to remove all the roots of a tree. In most cases, the main part of the stump is ground or dug out, leaving the smaller lateral roots in the ground to decay. The stump should be removed to a depth 12 to 36 inches below the infiltrative surface elevation depending on effluent quality and local codes. Clean sand is often used as the backfill and treatment media, but be sure to consult with the designer as to the depth and recommend backfill/treatment material. Some stump grinding machines can grind the stump to 24 inches deep or more.
  • Large diameter rocks and stones may be removed during the installation resulting in an area of over-excavation. 
  • Loose fill material does not provide a stable base, and removal may be the best option. Depending on the type and depth of loose fill, a 1:1 to 3:1 ratio of horizontal to vertical removal depth may be needed.
  • Soils that settle rapidly (dry silts and sands of low density) or swell (expansive clays) under wet conditions do not provide a stable base, and removal may be recommended.
  • Construction mistakes result in the need for bedding material to be installed to get systems to the appropriate elevations and provide a good support base.

Regardless of the reason for over-excavation, stability is essential and the appropriate bedding material must be selected and installed. In some cases, if an area is over-excavated, the resulting surface may need varying amounts of backfill material as shown in the figure below. Be careful because areas with more backfill (B) will settle more then areas with less (A). In some cases the entire excavation may require over excavation and a consistent backfill to support the tank properly at the proper depth.

Figure showing septic tank, area of excavation, and area of backfill.

If the infiltrative surface of a trench or bed is over-excavated, do not install compactable backfill as the compacted material will have reduced water flow in comparison to the rest of infiltrative surface. The installer needs to determine, in consultation with the designer and local permitting authority, if the over-excavation resulted in an unacceptable vertical separation distance to any limiting condition or impacted other design parameters such as the infiltration rate.

If the existing soil and separation to limiting condition is unacceptable, the designer or regulatory authority may allow sand media to be installed in lifts, or layers, to the required depth for treatment and a level infiltrative surface. If the vertical separation has not been negatively impacted, washed rock can be placed in the areas of over-excavation to create a level base for the remaining distribution media.