Septic drainfield on a steep slope Guide to Septic Drainfield Installations on Steep Slopes

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Steep slope septic drainfield systems. This article describes the installation of septic drainfields or leach lines on difficult terrain such as on steep slopes where a property disposes of wastewater using a septic tank and drainfield.

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Guide to Septic Installations on Steep Slopes

Steep terraced slope septic (C) D FriedmanAlso see Sewer or Septic Line Installation at Steep Sites, and for people conducting a septic system inspection or test, also see DIFFICULT SEPTIC SITES. Technical reviewers are welcome and are listed at "References." Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author.

Our photo (left) illustrates terracing used by the excavator during a septic drainfield installation. In addition to the text of this article (below), readers concerned with septic installation advice for steeply sloped or rolling sites should see these articles:

  • Sewer or Septic Line Installation at Steep Sites which describes sewer piping between a building and septic tank or sewer main at steep sloped sites which slope down significantly between the building and the septic tank.

  • DOSING SYSTEMS PRESSURE which describes pressure dosing systems that may be useful for disposing of septic effluent at steep sites where the drainfield must be located either uphill or downhill from a septic tank or building.

  • GRAVELLESS SEPTIC SYSTEMS- some gravelless systems can handle gentle curves necessary to follow rolling slope lines

  • Pumps Septic pumps, sewage ejector pumps, grinder pumps, effluent pumps, sump pumps, & septic pumping stations, septic pump alarms - systems that will be needed if a building or septic tank are lower than the drainfield or sewer main.

  • STEEP SLOPE DESIGNS (text located just below) which describes installation of septic drainfields on steep or rolling sites.

Guide to Installing Steep Slope Septic Drainfield Systems

Installing Septic Drainfield Piping on Steep Slopes Located Downhill from the Septic Tank - Uniformly Sloped Land

The sketch at page top shows an installation approach for septic drainfields across a steep or rolling slope.

  • D-box for septics on steep slopes: Clarified septic effluent leaves the septic tank (or an effluent pumping station) and flows into a large distribution box. This D-box will need to be larger and have more connecting ports than those used for a conventional flat-area drainfield: each effluent line to be located across the hillside needs to connect directly to the D-box.

  • Individual effluent lines for steep slope septics: Connected to the distribution box are individual septic effluent drain lines, each of which is routed to an individual gravel trench and perforated drainfield

  • Effluent line feeder pipes for steep slope septic systems: Piping between the distribution box and the drainfield trench is solid, not perforated.

  • Effluent lines for hillside or rolling land drainfields: Each of the drainfield lines is installed along the slope, not uphill or downhill, sloping gently (1/8" to 1/4" per linear foot) from the inlet end of each drainfield trench to its lowest point.

Installing Septic Drainfield Piping on Steep Slopes Located Downhill from the Septic Tank - Rolling Land

Gravelless septic may curve on hilly sites

  • Flexible distribution piping or gravelless systems for steep slope septics: Where land is rolling or contoured, flexible piping may be easier to install than rigid pipe systems since flexible effluent lines can be routed in a trench which needs to curve in order to remain parallel to the fall line of the slope of rounded hilltops or rolling land.

  • Pressure dosing for steep slope septic systems: Other septic effluent handling systems such as pressure distribution systems may be more tolerant of installation on steep or uneven sloped land and can employ either rigid or flexible perforated piping.

Drip line septic system sketch

  • Drip lines which also use flexible distribution piping for septic system effluent dispersal may also work on some hilly sites.


Why Drainfield Effluent lines need to be properly sloped

Septic drainfields that treat and dispose of clarified wastewater effluent using gravity need to be properly pitched and never sloped too steeply. Otherwise effluent will run too quickly to the low end of the drainfield line and gravel trench where it is likely to simply break out to the property surface.

If you have to install a conventional perforated pipe and gravel trench drainfield on a steep slope you'll need to run the trenches along the slope or parallel to the fall line, stepping down the slope from trench to trench.


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