A homeowner recently sent in a question about the installation of an onsite treatment system. The system was going to be installed quite a distance downslope of the house and the homeowner wondered if special precautions should be taken in terms of the piping from the house to the septic tank and the system.

The answer is yes, precautions should be taken when laying piping between components from the house to the septic tank. Some of them are just part of good pipe installation, and a few precautions are directly related to the steep slope.

In terms of good pipe installation, the trench the pipe is laid in should be at least twice the pipe diameter. In the case of 4-inch PVC sewer pipe, the trench should be at least 8 inches wide. This is usually not a problem because backhoe and mini-excavator buckets are commonly at least a foot wide. This allows the pipe to snake from side to side in the trench to account for natural pipe expansion and contraction.

PROPER BEDDING

Any debris from excavation should be removed from the trench before the pipe is laid. If there are rocks, bedrock or soil hardpans in the trench bottom, the pipe should be bedded in a well-graded, compacted granular material. This not only supports the pipe, but also keeps trench-wall soil from entering the trench. Check your local regulations for any required bedding specifications and, of course, follow those requirements.

In general, if coarse granular material is used, it should be placed below and up to the midpoint of the pipe to provide support below and on the sides of the pipe. This also ensures support is not lost over time by migration of finer material into the bedding. Depth of the bedding material from midpoint of the pipe to the bottom of the trench is usually 6 inches. The trench is then backfilled with clean soil free of rocks and debris — usually in at least two stages — taking care not to deflect or damage the pipe during the backfill process.

The recommended slope for 4-inch diameter PVC pipe carrying raw sewage is between 1 inch in 8 feet to 1 inch in 4 feet. Having pipe slopes in this range ensures enough slope on the pipe to provide the required velocity through the pipe to carry both water and solids. If the slope is not steep enough, the velocity will not be high enough to scour the pipe, which could lead to constant pipe blockage. If the slope is too steep, water will essentially flow ahead of the solids, leading to pipe blockage. Under either condition, the homeowner will be faced with the continued expense and exasperation of having someone clean the pipe.

For the condition described by the homeowner, in which there is a steep slope from the house to the tank, the pipe will require one or more slope breaks to maintain flow and drop the elevation. This is typically done using elbows. At each of these break points, a clean-out should be installed that is readily identifiable and brought to the surface in case there are plugging problems.

CLEAN-OUT CLUES

A couple of comments about clean-outs: They should be the same size as the sewer-line pipe, up to 4 inches, and at least 4 inches for larger diameter pipe. For 4-inch PVC pipe, the distance between clean-outs should be no more than 100 feet. Clean-outs can either be one directional or two directional. From our perspective, two directional is a better choice because the pipe can be jetted or snaked in two directions. However, site characteristics may dictate the choice.

We always recommend that a clean-out be installed near the outside of the residence where the sewer exits. It should be made with a full Y fitting. This allows the service provider to clear blockages in the line without having to enter the residence.

Riser pipes for the clean-out should be the same size in terms of pipe-wall thickness and material. All necessary fittings should also be of the same material and consistent sizes. As indicated earlier, any clean-out should be brought above the surface so it is easily accessible to the service provider. The cap on the clean-out should be threaded, and some codes require that the cap be provided with a raised nut or recessed socket for removal. Access to the clean-out should be in a protective enclosure such as a valve box.