By Sara Heger
Many regulations across the U.S. and Canada limit gravity trench lengths to 100 feet. Is this length justified in science or practices? Let’s look at the possible reasons.
According to the Manual of Septic Tank Practices (1957/1967), “A soil absorption field consists of a field of 12-inch lengths of 4-inch agricultural drain tile, 2-to-3-foot lengths of vitrified clay sewer pipe, or perforated, nonmetallic pipe. In areas having unusual soil or water characteristics, local experience should be reviewed before selecting piping materials. The individual laterals preferably should not be over 100 feet long, and the trench bottom and tile distribution lines should be level. Use of more and shorter laterals is preferred because if something should happen to disturb one line, most of the field will still be serviceable. From a theoretical moisture flow viewpoint, a spacing of twice the depth of gravel would prevent taxing the percolative capacity of the adjacent soil.”
This is a historical document that served as the basis for many state codes that have held this number in their regulations. But this still leaves us wondering how or why it got in the regulations to start with. Therefore, I reached out to a few retired legends in the industry to see their thoughts.
From Jim Anderson: “The whole rationale for trench length was to make sure there were at least two trenches so if there was a problem with one the other was still there to function. Also, there was access to two trenches which at least allowed the potential to do some system management. It had nothing to do with whether an installer could keep it level and on the contour for longer distances.”
If you only have one trench and a blockage occurs the system will have no other option but to back up or surface. There are some sites where it is simply difficult to find a contour longer than 100 feet to places trenches and remain on the property.
From Jim Converse: “It probably got there because it is a nice round number. I have a hard time thinking that there was research done on it to prove that it is a correct number.”
From a scientific perspective the rationale that comes to mind for me is the development of a biomat in a gravity trench. As septic tank effluent flows into a soil treatment trench, it moves vertically through the distribution media to the biomat where treatment begins. The biomat is a biological layer formed by anaerobic bacteria, which secrete a sticky substance and anchor themselves to the soil, rock particles or other available surfaces. The biomat develops first along the trench bottom, where effluent begins to pond. The biomat develops along the soil-media contact surfaces on the trench’s sidewalls. When fully developed, the gray-to-black sticky biomat layer is about 1 inch thick. Flow through a biomat is considerably slower than flow through natural soil, allowing unsaturated conditions to exist in the soil beneath the soil treatment trench. The flow through the mature trench bottom is based on the long-term acceptance rate of the soil, which is influenced by both the soil type and the biomat. When trench length is limited, then the biomat can develop more quickly across the length of the trench. The longer a trench is the more time it will take for a mature biomat to develop the length of the trench; therefore limiting trench length could improvement treatment. It may take time but eventually Stage 4 will be achieved regardless of the distribution media.
There may be some sound reasons to keep gravity trench lengths under 100 feet, however it is not a magic number. Therefore, a maximum trench length of 100 feet is not a number that should be rigidly held to if the site and soil conditions require a longer length to meet the sizing requirements. In addition, some regulations and designers will factor in the contour/linear loading rate of a site to account for groundwater mounding, which may result in a system that is narrower and longer. The use of pressure distribution is another tool a designer should consider if even distribution across the length of the trench upon startup is needed.