Percolation Test

A percolation test is an in-situ test to assess the permeability of soil to determine its ability to drain water. They are an essential step in the design of drainage fields for septic tank systems, helping to determine the size and location of the drainage field to discharge wastewater to the ground safely. Septic systems and drainage fields are used when it is not reasonable to connect to the main sewage network and need an on-site wastewater treatment system.

If you require testing to determine the infiltration rate of soil to aid in the design of a surface water drainage system, such as soakaways, attenuation tanks and other Sustainable Drainage Systems (SuDS), please visit our BRE 365 Soakaway Test page.

Sub Surface provides percolation testing nationwide, as described in BS6297:2007+A1:2008 ‘Code of practice for the design and installation of drainage fields for use in wastewater treatment’, and in The Building Regulations 2010: Part H ‘Drainage and waste disposal’. Established in 1972, we offer a professional service with fast mobilisation and quick turnaround results to meet project demands.

Preparing for a Percolation Test

There are two steps to complete before performing a percolation test:

1. 1. Groundwater Source Protection Zone (SPZ) Search: to establish that the proposed drainage field is not within 10m of a watercourse or permeable drain, not within 50m of a groundwater abstraction point, and not within a Zone 1 SPZ. This is necessary to ensure that the proposed drainage field will not risk contaminating any groundwater sources.
   2. Site Assessment Trial Pit: A trial pit should be excavated adjacent to the proposed drainage field to determine the localised groundwater table and to characterise the profile of the soil. This is recommended to be taken out to a depth of at least 1.50m. This is necessary to ensure that the proposed drainage field will not be at risk of failure due to changes in groundwater levels.

The findings of the above assessments may determine that a drainage field is not suitable, in which case a percolation test would not be necessary. If the findings are favourable, a percolation test should be conducted.

How to Conduct a Percolation Test

An independent and qualified engineer should perform a percolation test to ensure accurate results and regulatory compliance. The steps involved in a percolation test are as follows:

1. 1. Excavation: Hand-excavate a 300mm x 300mm square trial pit to a depth of 300mm below the proposed invert level of the drainage pipe. This is usually a maximum of 1.00m below ground level. (BS6297:2007 states that the invert level of the drainage pipe should be no greater than 700mm below ground level to ensure aerobic contact between the effluent and the soil.)
   2. Filling: Fill the trial pit with water to a depth of at least 300mm.
   3. Testing: Monitor the water level at regular intervals in order to observe the time taken for the water to seep away from 75% full to 25% full level.
   4. Results: A percolation value (Vp) is calculated, which is the average time, in seconds, required for the water level to drop by 1mm.
   5. Repeat: The test must be completed three times in each trial pit and in at least two locations in the proposed drainage field. If the results vary widely between the two locations, a third location should be tested to obtain an average Vp value.

Subject to site conditions and percolation rates, Sub Surface can typically undertake between three and five percolation tests per day. We offer fast mobilisation and a professional, reliable, and competitively priced service nationwide.

Interpreting the Results of a Percolation Test

The Building Regulations 2010: Part H ‘Drainage and waste disposal’ states that drainage field disposal should only be used when the average value of Vp is between 12sec/mm and 100sec/mm. For example, if the test pit is filled with 300mm of the water, it must soak away from 75% full to 25% full (150mm) in a time greater than 30 minutes and less than 250 minutes.

If the water soaks away faster than this, untreated effluent would percolate too rapidly into the ground, potentially polluting the groundwater. If the water soaks away slower than this, there would be a risk of the septic tank overflowing or wastewater ponding on the surface in the drainage field.

It is important to keep in mind that unusual weather circumstances, such as intense rain or snow, or a period of drought can affect the regular groundwater level and the permeability of the ground. Ideally, the percolation tests should be carried out during a period of close to average rainfall to give the best chances of a Vp in the correct range.

If the average Vp is not in the range above, then a septic system with a drainage field would not be possible and other solutions would need to be explored. These may include a drainage mound or a small sewage treatment plant that discharges to a suitable nearby ditch or watercourse.

Request a Quote

Established in 1972, Sub Surface are experts in Ground Investigation testing and services. We would be happy to assist if you have any questions regarding Percolation Tests or if you would like a quote. Please do not hesitate to contact your Regional Sub Surface Office.

Other In-situ Tests and Services: CBR Testing, Soakaway Tests, Ground Investigation

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