Septic tanks provide an effective on-site wastewater removal system for buildings that are not connected to a municipal sewer line. Gravel and rock is a traditional construction method for installing wastewater sewer effluent lines designed to provide overburden support for the ground surrounding the system.
Leach field sewer lines for a septic system make up part of the home plumbing system, which also includes the sewer line from the house to the septic tank, the septic tank itself, outlet sewer pipes and the treatment field. The effluent outlet pipe and leach field must be positioned so they do not impact the water supply, including groundwater wells. Sewer lines must be located at least 100 feet from wells less than 100 feet in depth and 50 feet from wells over 100 feet in depth. The installation of sewer lines must be completed by digging a level trench; otherwise, low spots or uneven areas in the trench can cause the effluent to collect and freeze.
Gravel-filled trenches for septic sewer lines must be at least 18 to 36 inches wide and 6 to 48 inches in depth, according to North Dakota State University. Field gravel of between 3/4- to 2 1/2-inches in size should be first washed before being used in sewer line applications; washing gravel prevents particles of clay and soil from seeping into the gravel and causing blockages. Once the sewer line is installed, the surrounding gravel should be covered with materials to avoid seepage, such as geotextiles or rosin paper. Rock and gravel trenches should be installed in environments with a soil percolation rate of less than 60 minutes per inch (MPI) and can be used in areas with rates of between 61 to 90 MPI.
Pipes used in gravel-filled sewer trenches are commonly manufactured from a plastic material, such as high density polyethylene (HDPE). These pipes can be manufactured as smooth-walled available in rigid forms or corrugated piping that can be curved to fit into a property boundary. Smooth-walled pipes are available in sizes of 4 or 6 inches for residential septic tanks in 2- or 3-foot wide gravel-filled trenches; corrugated piping is manufactured in sizes of 3 inches and larger. Each pipe is perforated with large holes at least 1/2-inch or larger to avoid problems with effluent clogging the pipes.
Septic sewer lines are usually installed with a backhoe for digging the trench; once the trench is completed, plastic pipe can be installed quickly and easily due to its lightweight nature. Other options for septic sewer lines include the use of gravel-less trenches created with larger sizes of corrugated plastic pipes wrapped in geotextiles. These trenches require the use of native soils to surround the sewer lines.