Drainfield & Trenches

Notes

• A trench longer than 100 ft does not work, it just looks like it does. Effluent enters at one end and the near end takes almost all of it. The far end stays dry for years, then the near end clogs and the whole run fails at once. 750 ft² at 2 ft wide is 375 ft of trench, so it goes in as 4 runs with a distribution box splitting the flow.
• The distribution box is the cheapest part and the most common failure. It has to be dead level, and it settles. One trench then takes more than its share and fails while the others sit idle. Level it on undisturbed soil or a concrete pad, and fit flow levellers in the outlets — a few dollars against re-digging a field.
• Absorption area is measured on the trench floor. 750 ft² ÷ 2 ft of width = 375 ft. Some states let you count sidewall area as well, which cuts the length substantially; most do not. Ask the county which convention applies before laying anything out, because the two answers differ by a third.
• Reserve a second field of the same size. Most counties require it on the plan and will not permit without it, and the ones that do not require it still leave you nowhere to go when the first field reaches end of life in twenty to thirty years. Do not put the driveway, the pool or the shed on it.
• Nothing heavy drives on a drain field, ever. Compaction closes the soil pores the whole system depends on, and it cannot be reversed by digging it back up. Keep vehicles, stacked materials and livestock off it, and plant grass — not trees, whose roots find the perforations and fill them.