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How Drain Fields are Constructed

The drain field is also known as leaching beds or absorption field. It is an essential part of the sewage disposal system. The septic tank may last for 20 to 30 years but drain fields do not normally last that long unless they are well-designed, treated with a bacteria supplement and the gravel structure and internal PVC remain intact. People pay very little attention to how drain fields are constructed, but in this article, we will discuss the important aspects to consider in building a reliable and lasting drain field.

First, it is very important to check with your local health department or county office if a permit is required or if the drain field needs to be inspected while it is being built or after it has been built. Second, determine the soil’s absorption capacity. The most important thing that needs to be considered in building a drain field is the permeability of the soil. A soil scientist can test the ground with a so-called “perc” test (percolation test). This test determines the absorption rate of the soil. It can also check other aspects such as ground slope, system capacity and depth of the bedrock before deciding the size and where to put the leaching bed. Obviously, a drain field should not be located in a poor drainage area. Ideally, it should be at least 10 feet away from a body of water and 10 feet from edible plants.

The next step will be digging the trenches. A conventional drain field is built with piping. It runs a straight line and should not be longer than 100 feet. The trench can be dug either by hand or using a machine like a trencher or a backhoe. The trench depth should always be kept in mind while digging. Having a yardstick or ruler handy is very helpful. Ideally, trenches are roughly 18 inches wide and with a flat bottom. The location of the piping for the drainage system is often determined by the local regulatory agency, but usually it is placed 1 to 3 feet or more below the surface. Also, bear in mind that trenches should slightly slant downward for no more than ¼ inch per 8 feet of pipe. Too much slope could create a back up in your system. The trenches and piping are placed in parallel with each other, wherein the distribution box is at the head of the design.

After the trench and the piping have been set up, the trenches will now be filled with gravel. The gravel should be up to about 6 inches from the top. Afterwards, a layer of tarpaper or any other fabric barrier will now be placed on top of it. The barrier keeps the soil out of the gravel. Finally, the soil shall cover the fabric. The mounds will not be visible as the earth eventually compacts.

Always remember that the amount of water being put into the septic tank is the same amount that flows into the leaching beds. Here are simple reminders to keep your drain field from early failure:

  1. The drain field must be constructed in an area with good ventilation and sunlight.
  2. Avoid constructing anything above the drain field.
  3. Remove trees that are planted near the drain field area as its roots may damage the distribution pipes.

If you plan on taking this task on, seek out as much information as possible and consult professionals who have constructed these types of systems. Once the drain field is laid out, little can be done to change what’s in place so proper planning is the key to a successful project.

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