In the last two articles, we touched on the issue of how to install piping properly, in ways that avoid pipe damage and dips in the pipe. Installation should follow the ASTM-2321 Standard. But what exactly does that mean?

Let’s look at the general piping installation process and three rules to follow during excavation and backfilling.

Build a base

All plastic pipe material needs to be installed on a solid base. Ideally, the base should consist of the natural, unexcavated soil — except where organic or peat soils are present.

However, the reality is that installation usually requires placement of some type of granular base material. If the soil is excavated and then backfilled, the base should be compacted in 6- to 12-inch lifts. To enable proper backfilling around the pipe, the width of the trench bottom should be about three times the pipe diameter (12 inches wide for a 4-inch pipe).

The excavation should follow the designed pathway for the pipe. This is especially critical in pumping situations, such as the supply pipe to a manifold in a mound- or pressure-distribution system. If the trench is over-excavated, or if organic soil material is present, then use an inorganic base, such as sand or pea rock, to bring the base to the required elevation before laying the piping. The key for any sub-base material is that there must not be any settling or bellies in the pipe.

Rules to live by

Now for the three simple rules to follow during excavation and backfilling to ensure that piping is installed correctly and effluent will flow where you want it to.

Rule 1. Avoid over-excavating the trench if possible. If wastewater is to flow properly, the pipe must be supported. Unsupported pipe can clog and freeze. By having unexcavated soil under the piping, you reduce the risk of developing low spots or dips in the pipe where solids can settle or water can collect and freeze.

If the trench is over-excavated, you must bring in material and compact it to the elevation necessary. This means extra work, and it costs time and money. If the excavation is in organic soils, the pipe will be subject to settling and heaving during wetting/drying or freezing/thawing of the soil. This also puts pressure on pipe joints and connections, causing leaks. In addition, the pipe will not maintain its proper slope.

Rule 2. Support the pipe over deep excavations. This simply means that if you have over-excavated the trench, or if the pipe has to cross an excavated area to enter or leave a sewage tank or other pretreatment device, the pipe will be at least partially on fill that can settle.

To minimize problems, use a granular fill and compact it to support the pipe. Sand or rock is typically used. The rock should not be too large. If it is, topsoil can filter down into the rock spaces, causing a depression at the surface that can collect surface water. This in turn can create a drain around the system components, potentially allowing water to enter the system.

Where the pipe crosses such a fill area, use at least Schedule 40 piping to minimize bowing and the need to provide support. Where 2-inch piping is used for pressure supply lines, the pipe can be placed inside a 4-inch pipe. The 4-inch pipe should then be sealed to keep the pipe from becoming filled with groundwater or soil.

Rule 3. Backfill with a good material. The backfill material has two jobs: protecting the pipe system, and maintaining the pipe slope. Protection means the pipe is surrounded by and covered with the material. This provides some insulation to prevent freezing in cold climates. Another way to avoid freezing is to use insulated pipe at points where freezing may be an issue.

Large stones should be avoided during the backfilling process. The strength of the pipe is related to its round shape. Objects such as large rocks or dry, heavy clay soil clods can deform the pipe and lead to failure. To minimize the weight of the cover, apply granular backfill to at least the midpoint of the pipe.

Plastic pipe needs to be protected from UV rays and other surface conditions that can cause failure. Lawn mowing and traffic over excavations can break and crack the pipe. You can prevent this by creating a backfill system with granular material to support the pipe.

Finishing should be done with a soil that will allow establishment of a good vegetative cover. Make sure there is a crown on the finishing material to allow for settling. Good topsoil should contain:

• Less than 5 percent particles larger than 2 mm (#10 sieve).

• No material larger than 2.5 cm (no large rocks).

• No highly organic soil (only 1 to 5 percent organic matter to promote healthy plant growth).

Install piping correctly and you have made an important contribution to a long-lasting and high-performing treatment system.