There are many advantages to gravelless systems, but they need to be installed with care.
One of the many things that has changed significantly over the years I have been involved with the industry has been the advent of products developed to replace use of rock or gravel as the distribution media. These are referred to as gravelless systems and come in a variety of forms, including open-bottomed chambers, fabric-wrapped pipes, expanded polystyrene peanuts and chips, and numerous other products.
Advantages of these types of systems include their relative ease of transportation and installation versus handling quantities of rock. In all cases, from an installation standpoint the same rules or principles for rock-filled trenches or beds apply.
This means that the bottom of the trench excavation should be level and follow the contour when there is a slope to the site. The soil should not be worked (excavated) if the soil is too wet. If a handful of soil can be rolled into a wire 1/8 inch in diameter, excavation should be postponed until the soil dries out. Excavations should keep the required separation distance from soil horizons subject to seasonal or continuous saturation. They should be kept shallow in general to utilize the most permeable part of the soil that will provide the best treatment. Finally, the site and the bottom of the excavation should be protected from any traffic that can compact or smear the infiltration surface, reducing the soil’s ability to accept effluent.
Chambers should be placed in the bottom of the excavation and connected following the manufacturer’s instructions. A note of caution here: I have observed installers walking back and forth on the infiltrative surface while connecting and placing the chambers. This should be avoided, or at least kept to a minimum. Even foot traffic can cause compaction and reduction in infiltration capacity.
Porous media, usually sand, is placed around the sides of the chambers at least 8 inches above the bottom of the trench. This is to make sure that the chambers stay in place during backfill.
Backfill should be placed to a minimum depth of 6 to 12 inches above the chambers. Maximum depth of backfill will be set by the chambers' strength and ability to withstand the weight of the soil cover. This maximum depth will be provided by the manufacturer. I have seen numerous instances where the chambers were installed too deep, causing deformation of the chambers or pushing them into the soil below and reducing their ability to accept effluent. Keeping systems shallow is better from all perspectives.
A final note here is that installers have reported that chambers offer a prime spot for pocket gophers and other burrowing animals to dig under and into the chambers, filling them with soil. This obviously interferes with the system’s ability to accept effluent. Those installers have reported they combat this by laying chicken wire in the bottom of the trench before placing the chambers. When the chamber is free of effluent this discourages the animals from burrowing into the chamber. Once effluent has reached that part of the system, the animals do not find it as attractive. This is a problem seen primarily in gravity systems that rely on biomat development for distribution.
About the author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program and is an emeritus professor in the university’s Department of Soil Water and Climate. Send him questions about septic system maintenance and operation by email to firstname.lastname@example.org.