Septic drain field installation.

What is a drain field?

A drain field is a set of trenches that spreads the liquid from a septic tank into the soil, where it is treated. Under Missouri's construction standards, a standard trench is 18 to 36 inches wide, with its bottom 18 to 30 inches below the finished grade.

Solids stay in the tank. The liquid flows to the field through a distribution box or drop boxes, seeps out of perforated pipe into a bed of stone, and soaks into the soil below. The soil does the final cleaning, which is why the soil decides whether a system works.

How big is a drain field?

Missouri sizes a standard drain field by bedrooms and by how fast the soil absorbs water. The trench bottom area runs from 150 square feet per bedroom in fast soil to 600 square feet per bedroom in slow soil.

That is trench bottom area, not yard area. Trenches are spaced at least three times their width apart and no closer than 5 feet on center, and no trench runs longer than 100 feet without approval. The yard also needs room set aside for a replacement field. Your soil evaluation and the approved design give the real numbers for your lot.

What stone goes in a drain field?

Missouri's standards call for clean gravel or crushed stone sized between 1½ and 3 inches, at least 12 inches deep in the trench. At least 6 inches sits below the pipe and 2 inches over it.

Crushed limestone has to be washed and at least 1½ inches. Dust and fines clog the soil, so driveway gravel and crusher run do not belong in a drain field. The stone is covered with untreated building paper, drainage fabric, or another approved barrier before any dirt goes back, so soil does not wash down into it.

Gravel or chambers?

Some designs use plastic chambers or wrapped gravelless pipe in place of stone. Missouri's standards allow both and give each a trench width it counts as. We build what the approved plan specifies.

What dirt goes over a drain field?

In most soil, the dirt that came out of the trench goes back in over the barrier. In slow soil, where water takes more than 45 minutes to drop an inch, Missouri's standards call for sand, loamy sand, or sandy loam as backfill when it is available, capped with 2 to 4 inches of loamy soil.

The backfill is left loose and is not packed with equipment. The surface is shaped so rain runs off and no low spot holds water, and grass is started as soon as possible.

Fill dirt and drain fields

Some sites need fill to build up enough soil before a field can go in. Missouri's standards treat that as part of the design, with rules for the dirt and for how it is placed:

• Sandy texture. The fill should be sandy, with no more than 20 percent clay.
• Dry work only. Fill is not hauled or worked wet, and the ground is loosened first while it is dry.
• Tilled in layers. Each layer is tilled into the one below, so there is no hard boundary between the fill and the natural soil.
• No traffic. Equipment stays off the loosened ground.
• Shaped and seeded. The site is graded to shed water, and turf grass is established after the system goes in.

Ordinary fill dirt is clay loam, sold for raising grade and backfilling foundations. It is not the sandy fill a drain field design specifies. Tell us what your design calls for and we will quote the material with the job. For regrowing grass over a finished field, screened topsoil is the right cover.

Repairing a drain field

Wet ground over the field, a sewage smell in the yard, and slow drains through the whole house are the usual signs of trouble. A single broken part can often be repaired. A field that has stopped absorbing water has to be replaced, and the new one usually goes in fresh ground.

Our septic replacement page covers the warning signs and what to do first. If the yard has no room for another standard field, the design may call for a low pressure pipe system or a drip system.

How we build a drain field

1. Layout. The trenches are staked from the approved plan, along the contour of the ground. Buried utilities are located first.
2. Trenches. Each trench is dug to the plan depth with a level bottom.
3. Stone and pipe. Washed stone, perforated pipe, and more stone go in to the depths on the plan. Chambers go in where the plan calls for them.
4. Barrier. The stone is covered before backfill.
5. Inspection. The field is inspected before it is covered.
6. Backfill and grade. The right dirt goes back loose, and the surface is shaped to shed water and left ready for seed.

Rock is common here. J1S owns its hydraulic hammer for breaking rock when a trench hits it.

Where we install drain fields

J1S is based in Festus and installs and replaces drain fields in Jefferson County, Ste. Genevieve County, St. Louis County, and nearby counties. In Ste. Genevieve County that includes Ste. Genevieve, Bloomsdale, St. Mary, and the rural lots between them. See all the septic systems we install.