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Septic system drain fields

Q&A: GMA Techline | October 1, 2023 | By:

Q: Should I sock the perforated pipe of my septic system drain field and eliminate the gravel rock/bedding around the pipe?

A: Unfortunately, only socking perforated (and hopefully corrugated) pipe for a septic system drain field works for very few instances over time. We have seen it work in alluvial deposits of free-flowing sand and gravel with perk test less than1 inch/minute (2.5cm/minute) (i.e., this is a very fast draining material). In most cases, homeowners are not so lucky when it comes to hydraulic-geotechnical conditions at the site. Traditional drain fields for septic systems are typically made up of geotextile-wrapped trenches that are filled with gravel. Perforated pipes are laid within the gravel-filled trenches. Generally, a geotextile filter surrounds the gravel to keep soil from infiltrating into the trenches. The geotextile filter surrounding the gravel trench needs to remain free flowing over time (i.e., it cannot clog). It usually is a needlepunched nonwoven polypropylene geotextile with an ASTM D4751 AOS of 70–80 and an ASTM D4491 permittivity of greater than one (1) sec-1. 

Contemporary drain fields are made with a synthetic infiltrator system, which is an alternative to traditional gravel and pipe drain fields. They use a total geosynthetic-engineered modular design. The gravelless systems are made up of preassembled units of a perforated-corrugated drain pipe surrounded by polystyrene aggregate and held in place with a knitted geonet, which can be placed vertically or horizontally. It should be noted that these contemporary drains do not respond well to heavy construction traffic. They need to be buried with at least 18 inches (45 cm) of soil cover prior to experiencing any traffic or they risk collapse.

Both of these systems need to be sized properly and should be designed against clogging and infiltration. Again, the site hydraulic and geotechnical conditions of each scenario need to be considered. You probably want to reach out to a site civil or geotechnical engineer familiar with your project. One needs to also consider codes that govern septic drain field designs at either the state or local level.

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