Percolation tests

What is a percolation test?

A  percolation test is a test to determine the absorption rate of soil for surface water soakaways or septic drainage fields. There are two common types of percolation test, they are different and not to be confused.

1. Surface water soakaway percolation test.

Soakaways have been the traditional way to dispose of stormwater from buildings and paved areas remote from a public sewer or watercourse. In recent years, soakaways have been used within urban, fully-sewered areas to limit the impact on discharge of new upstream building works and to avoid costs of sewer upgrading outside a development. Soakaways are seen increasingly as a more widely applicable option alongside other means of stormwater control and disposal. Soakaways must store the immediate stormwater run-off and allow for its efficient infiltration to the adjacent soil.

Percolation tests for surface water soakaways are carried out in accordance with the design procedure set out in BRE Digest 365.

The basic test method is:

1 Excavating a trial pit of sufficient size to represent a section of the design soakaway.

2 Filling the pit several times in quick succession whilst monitoring the rate of seepage, to represent soil moisture conditions typical of the site when the soakaway becomes operative.

3 Examining site data to ensure that variations in soil conditions, areas of filled land, preferential underground seepage routes, variations in the level of groundwater, and any geotechnical and geological factors likely to affect the long-term percolation and stability of the area surrounding the soakaway have been assessed. Groundwater should not rise to the level of the base of the soakaway during annual variations in the water table. Local building control and/or planning authorities should advise where fluctuations in groundwater level may cause a problem in the long-term for any proposed depth of excavation.

Percolation test

The percolation results are then used to design a suitable surface water soakaway. We will design the soakaway based upon these results, considering the ground conditions and suitable construction methods for the size of soakaway required.

Soakaway design

2. Drainage field percolation test.

Percolation tests for septic system soakaways and drainage fields are slightly more complicated. They are actually carried out in conjunction with two other tests, a Groundwater Source Protection search and also a Trial Site Assessment Hole. These tests are carried out in accordance with various legislation and in particular Building Regulations – Approved Document H and BS 6297 2008.

The basic test method is:

1 Excavate a test hole 300mm square x 300mm deep below proposed invert level of the drainage field trench bottom.

2 Fill the test hole with water and allow drain away over night.

3 Refill to a depth of 300mm and note time taken in seconds to drain away from 75% full to 25% full (i.e. 150mm drop in level from 225mm to 75mm).

4 Repeat the procedure in two more test holes and calculate the average of the three results.


Drainage field percolation test

Percolation tests are complicated and need to be carried out by experienced contractors. The above descriptions are only an indication of what the tests actually involve and we strongly recommend that further advice is taken. Failure to carry out correct percolation tests for both surface water soakaways and drainage fields will result in the system not working and potentially expensive installation work being ineffective and non-compliant.

DJ Wilkinson can carry out these tests for you and also carry out the design of the required soakaway too. Not only that we can give you a quotation to install the soakaways and drainage fields or provide you or your architect with a bill of quantities to enable an effective tender.

Get in touch see how we can help!


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