Percolation Test Method

BS 6297 2007 + A1 2008 is the British Standard that covers the design of a treated effluent soakaway drainfield. It advises details of methods of determining percolation rates and Trial Site Assesment Hole (TSAH) proceedure.

The first test that must be carried out is the TSAH. This is a hole, at least 2.1 metres deep in the proposed soakaway area. If bedrock or the water table is found at this depth, then the site is not suitable for a soakaway and a soakaway cannot be installed.

If the above hole shows no problems then the BS 6297 + A1 2008 advises carrying out the following percolation test to estimate the area of the drainage trench and the length of the land drain.

This test should be completed before purchase or installation so that the suitability of the site and soil can be assessed for sub-surface irrigation and effluent disposal. However, other factors should also be taken into account, for example local knowledge about ground conditions, water table levels and seasonal variations. The following information is provided to aid your testing or please contact us direct.


 Excavate a hole 300mm square to a depth 300mm below the proposed invert level of the land drain. This percolation test hole cannot be below 1 metre below ground level to its base. 

Fill the 300mm square section of the hole with water to a depth of a 300mm (minimum) and allow the water to seep away overnight. Next day, refill the test section with water to a depth of at least 300mm and observe the time taken, in seconds, for the water to seep away in the middle 150mm. section, i.e. do not include the first or the last 75mm. of the hole in the timed test.

Divide the time by the depth (150mm.) of water in the hole. The answer gives the average time required for the water to drop 1mm (Vp in s). Take care, when carrying out the test, to avoid abnormal weather conditions such as heavy rain, severe frost or drought.
This test should be repeated at least three times, and an average calculated. If any of the results are 50%+/- the average, carry out a further three tests and calculate a further average.
A satisfactory result is an average value, Vp of below 30s/mm or less. If the measured values exceed this, then carry out further tests at a minimum of three different locations on the proposed land drain route, or at least three tests on separate days on the site proposed for the soakaway.
Make water level observations referring to a fixed datum using a ruler or pre-marked stick.
The value found In this way is called the percolation value (Vp in s) of the soil and can be used to determine the area of drainage trench bottom required to disperse effluents. If the percolation value is less than 12s or exceeds 100s then BS6297 considers that the soil is not suitable for a soakaway system.

At =P x Vp x 0.25

At = Floor area (m2) of subsurface drainage trench

P = The number of persons served by the tank
Vp = The percolation value in sec/mm

The area determined should be used to calculate either the total floor area of the drainage trench and therefore the length of land drain, or alternatively the floor area of one or more shallow soakaways.

To obtain the linear pipe length, divide the drainage area (At) by the proposed trench width (normally 0.6 metre or 0.9 metre).
Interpretation of Results
The following table provides typical irrigation trench length for septic tanks in metres assuming a trench base width of 600mm.
Number of Bedrooms1 to 345678
Rate of Fall Vp (sec/mm)Minimum required length of soakaway drainfield pipework
0-12Percolation Test Failure
13 - 20344047546067
21 to 305060708090100
31 - 40678094106120134
41 - 5084100116134150167
51 - 60100120140160180200
61 - 70117140164186210233
71 - 100167200234268302336
Over 100Percolation Test Failure

We do suggest, irrespective of results and calculations, that a minimum trench length of 60m is installed.
BS EN 6297 2007 advises that it is not desirable to position a foul water soakaway nearer than 15m to a building, or position it so that ground beneath foundations is adversely affected. Further that if, in the winter the groundwater level rises to within 1.2m of the proposed invert level of the irrigation system, then sub-surface irrigation should not be used.


The trenches should be between 300mm and 900 mm wide and 1 m wide strips of undisturbed ground should be maintained between parallel trenches. The pipes should be laid on a 300mm layer of clinker, clean gravel or broken stone of 20-50mm grade. 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 longest pipe length allowed in one run is 30 Metres.

The remainder of the trench can be filled with normal soil. Pipes should be laid at a minimum depth of 200mm below the surface.

For longevity the soakaway should be designed with facilities for inspection and maintenance. The life of a soakaway will be reduced if its waterways become clogged by silt or floating material.
Access points enable the point of discharge of the drain to be viewed and material cleared from the soakaway. Sub surface irrigation systems should have at least two inspection points on each length of straight pipe i.e. at least at both ends.
We would also suggest that the drainage system should be constructed from lengths of 110mm perforated clay or slotted rigid plastic pipe, preceded by approximately 3 metres of unperforated pipe connecting to the tank outlet. Corrugated pipes designed specifically for land drainage are not allowed. The layout should ensure even distribution throughout the absorption field, avoiding steep gradients on sloping sites. A closed circuit system is required. Please also refer to Building Control regulations part H for full details.