DIY Etude de Sol Instructions for Calculating Soakaways Drains in France

The “Etude de Sol” in France, calculating dimensions for a soakaway in France (tranches d’épandage).

You may be encouraged to have a costly “étude de sol” as the first step when replacing a fosse sceptic in France, however, if you wish to benefit from the reduced ecological impact, lack of odeurs, and convenience of a packaged domestic wastewater treatment plant in France the “etude de sol” becomes a little superfluous and with a little patience you can calculate the permeability of the soil yourself. A DIY etude de sol! This should save you between 4-500 euros too, which can go towards your domestic sewage treatment plant . . . . .

Domestic wastewater treatment plants in France should ideally have a drainage field or soakaway to disperse the water produced by the WWTP as this is what the relevant authorities prefer, and it saves your local council being as meticulous at keeping the roadside drains clear.

The infiltration zone around the sub surface irrigation pipe becomes biologically active and enables additional treatment of the effluent too. The unsaturated zone beneath this provides attenuation and a pathway for oxygen diffusion for further wastewater treatment. A saturated zone provides further dispersion and dilution of the treated effluent.

The siting and design of the drainage field depends on many factors, the most important of which is that there is sufficient suitable land with subsoil capable of absorbing the daily effluent discharge on a long term basis.

In the absence of any recent French regulations for septic tank or wastewater treatment plant installation we have based the following document on BS 6297: 2007 which is a new code of practice for the design and installation of drainage fields for use in wastewater treatment. It provides recommendations and guidance to aid preliminary planning, detailed site investigation to identify suitable drainage field locations and the assessment of site characteristics. It provides systems designs and layouts including percolation testing and determination of trench area.

Site assessments must be made before equipment is purchased or installed as not all land is suitable for drainage field use. A percolation test establishes the length and area of infiltration trench required to disperse the effluent. Soil porosity can vary across a site and the percolation test should be carried out at the intended location of the proposed soakaway. It should not be carried out in extreme weather conditions such as drought, frost or heavy rain.

In order to assess the infiltration zone of your soakaway for your wastewater treatment plant in France a percolation test is required and a deeper trial hole should be used to identify the soil and conditions beneath the drainage field, including the position of the seasonally highest water table as ideally you need a minimum of 1.2 m of unsaturated soil above this position.

Etude de sol, the Soakaway Percolation Test and calculations in France

  1. Excavate at least two holes, 300 mm square to a depth at least 300 mm below the proposed invert level (bottom of the infiltration pipe), spacing them along the proposed line of the subsurface irrigation system or soakaway for your wastewater treatment plant in France. While digging the hole, note and record changes in soil characteristics at measured depths and the position of the water table if reached.
  2. Saturate the local soil by filling each hole with water to a depth of at least 300 mm and allow this to seep away completely.
  3. If the water drains rapidly, within 10 minutes, the hole should be refilled up to a maximum of 10 times. If the water continues to drain away rapidly, you may well have dug into a rabbit warren or mole hole! Try again a metre or so away.
  4. If the water has not soaked away within 6 hours, the area is not suitable and you should consider direct discharge of your wastewater treatment plant to the nearest surface water drain, preferably on your property, even if this means digging a small trench which can run into a nearby ditch. This avoids any requirement for you to ask permission to discharge direct.
  5. Determine the percolation rate by refilling each hole with water to a depth of at least 300 mm and observe the time in seconds for the water to seep away from 75% full to 25% full (i.e. a depth of 150 mm).
  6. Divide this time in seconds by 150. This gives the average time in seconds required for the water to drop 1 mm.
  7. Repeat the test at least three times in each hole.
  8. Take the average figure from the tests to produce the percolation value Vp (in seconds).
  9. Obtain the average figure for the percolation value (Vp) by summing all the values and dividing by the number of values used.
  10. Retain the results, these may be required later.
  11. Where the Vp results vary widely (50% above or below the average figure), make further tests on a minimum of three different locations in the area of the proposed drainage field.
  12. Drainage field or soakaway drains can only be used when percolation tests indicate average values of Vp between 15 and 100 and the preliminary assessment of the trial hole tests has been favourable.
  13. The minimum value of 15 ensures that untreated effluent cannot percolate too rapidly into the ground potentially resulting in the pollution of groundwater when using a fosse septique in France. Where Vp is above the limit of 100, effective treatment or soakaway is unlikely to take place in the drainage field as there will be inefficient soakage leading to wastewater ponding on the surface.
  14. If the Vp is between one and 15, or greater than 100, the outflow from the wastewater treatment plant should be output direct to a surface water drainage ditch as being the only option. If this is located on your property in France, no permission is needed, even if this water subsequently runs onto public property.

Testing the Soakaway

How to Calculate Soakaway trench area and trench lengths

The Vp is used to determine the total floor area of the drainage trenches and therefore the total length of irrigation drain. For domestic premises, the floor area of the drainage field required may be calculated as follows.

  A = p x Vp x 0.25 for septic tanks
Or A = p x Vp x 0.20 for package wastewater treatment plants
  (i.e. 20 % less, because the effluent has received additional treatment)

The calculated area A should be converted to an amount of linear trench based on the width of the trench which is usually between 0.3 m to 0.9 m. The layout of the trench network will depend upon the soil porosity and the availability of land but the legs of the trenches should be connected so as to form complete loops.

Trench length table

Where the calculated result indicates the need for a long drainage trench length (200 metres) serious consideration should be given to the use of a packaged wastewater treatment system which has been CE certified to EN12566-3 to produce a better quality of effluent. This effluent may be fed into a water course or open culvert. All equipment (for less than <50 pop equivalent) should meet the requirements of their relevant standard, i.e. EN 12566 part 1 for septic tanks, or EN 12566 part 3 for package treatment plants.

Effluent processed through a wastewater treatment plant in France contains far fewer pollutants and fine solids than that from a septic tank in France. Therefore the irrigation system is better protected and less likely to block and can be smaller. The standard recognizes this fact and differentiates. As an example: Irrigation systems for domestic applications

Trench area to Population Table

In all cases treated effluent

  • will have a lower long term impact on the site as fewer pollutants are discharged
  • will generate fewer solids thus preserving the life of the irrigation system
  • requires a smaller irrigation system which is
    • less expensive to install & easier to locate on a restricted or poorly draining site


Your recommended certified Installer is the ideal person to design the soakaway or drainage field for your specific site. BS You can work to some basic guidelines for your French wastewater soakaway. e.g. not closer than 7m to a building. Drainage fields for septic tanks should ideally be a minimum of 10 m away from a water course or ditch, although soakaways for WWTPs can be closer. 3m away form site boundaries, away from trees and plants with extensive root systems, and from existing supply services, access roads and other drainage fields. The detailed preliminary assessment should have identified other limiting criteria such as wells, rivers containing pisciculture etc.

In brief, after the septic tank or treatment unit, the effluent pipe should be connected to an inspection &/or distribution chamber which leads to the drainage field. This should be designed as a closed circuit with facilities for inspection and maintenance. The layout should ensure even distribution throughout the absorption field, avoiding steep gradients on sloping sites. An inspection chamber at the furthest point from entry on each leg or loop is advisable.

The drainage / sub-surface irrigation system should be very carefully constructed using 110 mm downward facing perforated pipes laid in trenches with a uniform gradient not steeper than 1:200. The trenches should be between 300mm and 900 mm wide and minimum 1m wide strips of undisturbed ground should be maintained between parallel trenches. The pipes should be laid on a 200- 300 mm layer of clean gravel granular fill material graded either 16-32mm or 20-50mm. The trenches should be filled with the same material to a level 50mm above the pipe and covered with geotextile material to prevent the entry of silt. The remainder of the trench can be filled with normal soil. Pipes should be laid at a minimum depth of 200mm below the surface. Corrugated pipes designed specifically for land drainage should not be used.

We also supply a packaged irrigation system (Ezy Drain) that replaces the traditional construction methods. Based on an innovative design that has been used overseas for more than 20 years the system combines pipe work and an integrated filter. The integrated filter utilizes modern recycled material and provides a high performance, well voided, freely draining irrigation medium. Ezy Drain is supplied in three meter length and replaces the stone / gravel and perforated pipe work traditionally used for drainage field soakaways therefore is much quicker and simpler to install, especially for DIY applications. For further information on Ezy Drain see here, or for a quotation for the Ezy Drain solution click here. We hope you found our page on DIY Etude de sol in France useful, you can always give us your feedback here.