Proper Backfill for Septic Tanks

Proper Backfill for Septic Tanks

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After a septic tank is set, it must be appropriately backfilled. All tanks should be backfilled with successive tamped “lifts” or depth increments of uniform gradation.

The installer should verify that the backfill material is free of clods, large rocks, frozen matter and debris that can result in voids in the backfill that may allow settling over time. Crushed rock or pea gravel 1/2-inch in diameter is preferred if native materials are not appropriate. 

Each layer should be uniform, no greater than 24 inches thick, and of nearly equal heights around the perimeter of the tank. However, compaction under the haunch (bottom curvature of some tanks) is best done in 6- to 12-inch layers. If the material used is compactable, it should be compacted to prevent settling of the soil around the tank. Be sure that the method used to compact the material does not compromise the structural integrity of the tank. Backfill with granular material to at least the midseam of the tank to be sure that the settling is limited. Flowable fill or native soil free of deleterious material may be used above the midseam. 

Compacting fill around a septic tank
Compacting fill around a septic tank

All pipe penetrations through all tanks must remain watertight after backfill. Thus, it is critical to ensure there is minimal movement of the outlet pipe during the backfill process, as this can alter the working liquid elevation in the tank and may damage or displace the effluent screen. Tamp the backfilled soil under the pipe to give it a firm foundation. The section of pipe that runs across the excavation from the tank to undisturbed soil should be rigid (Schedule 40 PVC or stronger) to eliminate deflection. The pipe joints should be over native soil rather than in the excavation to ensure they do not settle. Settling may result in leaks or cause related components like tees and/or effluent screens to shift out of their proper (plumb) orientation. Pipes that may run over the top of the tank or in excavated areas (such as electrical conduit and/or return lines) can be sleeved in larger pipes to provide extra support. If this is done, it is always a good idea to take a photo for documentation.  

Manufacturers of nonconcrete tanks may recommend or require simultaneously filling the tank with water to just above the backfill level to avoid uneven or excessive pressures on the tank walls during the installation process and to minimize the risk of the tank shifting position. Follow the recommendations of the manufacturer, as this approach is not always appropriate. A tamping tool may be necessary to provide good contact of backfill against and between tank ribs, but care must be taken to avoid damaging the tank.

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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