• The percolation test calculation is based on the time taken for the water level to fall from 75% to 25% capacity. This is done to avoid the possible outliers of the high seepage rates when the pit is full with a large head of water, and the low rates when the pit is empty with very little head of water.
• The lowest value from the infiltration test for soakaways should be used for design, as this is likely to most closely replicate the soil conditions during a design storm event.
• The CivilWeb Soakaway Design Spreadsheet includes a tool to calculate the design infiltration rate from onsite test records.

The site should be checked to ensure that site investigation information is accurate and complete. This may include checking soil type and conditions, groundwater levels, areas of fill or contamination, condition of any nearby slopes or other geotechnical features.

Soakaway Percolation Test - Further Guidance

Typically it is important to complete a infiltration test at each soakaway location as the variation in infiltration rates across a site can vary enormously, even in fairly homogenous soil conditions.In variable soil conditions a separate infiltration test should be undertaken when the trench will be over 10m long. For long trenches, separate tests should be undertaken at roughly 25m spacings, even in fairly homogenous soil conditions. If required the results of the first test can be used to calculate a preliminary length for the trench, which will then determine how many subsequent tests will be required. If more than one test is undertaken, the average of the tests can be used as the design infiltration rate for the whole trench.

If the pit is deeper than around 3.0m it can be difficult to supply enough water to fill the pit to the design depth. If this is the case the test can still be done at less than full depth, though the calculated infiltration rates may be lower than for a full depth test. This is due to the higher infiltration rates generally achieved when there is a large head of water. For a test less than full depth, the design infiltration rate should be calculated from the time it takes for the water level to drop from 75% to 25% of the actual highest water level achieved during the test. The surface area calculation is similarly taken as the internal surface area of the pit to 50% of the actual highest water level achieved during the test, plus the base area. This can lead to a significant reduction in the infiltration rate measured. BRE 365 indicates this may be as low as 50% of the rate achieved from a full depth test.

The results of the infiltration tests can be influenced by seasonal factors. Ideally the test should be undertaken during the winter when soil moisture levels and groundwater levels are highest. Even better if the test can be undertaken after a period of rain. Where it is not possible to conduct the test under these conditions, consideration must be given to the effects of different antecedent soil conditions. The groundwater adjustment coefficient described above can also be used to judge the difference in performance during seasonal or exceptional high groundwater conditions.

Where access or depth constraints prevent an infiltration test from being undertaken or where a deep bored soakaway is considered, a falling head test can be undertaken in a borehole. This test should be done three times in accordance to the procedure in BS EN ISO 22282-2. Care should be taken when interpreting the results of these tests as they include a lot less water than the BRE 365 infiltration test. This is particularly the case if the test is undertaken in summer or dry conditions. The falling head tests could be repeated many more times in quick succession to better simulate soil conditions where there is concern or where the first three tests show a significant deterioration in the measure infiltration rate.

Results of incomplete tests should not be extrapolated. As described above the infiltration rate varies in a significant and complex manner in relation to the head of water in the pit and the soil conditions. If the water level does not drop to below 25% of the original water level this can be an indication of a high groundwater table or an issue with the infiltration rate of the soils in the lower section of the pit.

Where there is any concern regarding the veracity of the information obtained from the test, this can be taken account of through a higher infiltration rate factor of safety.

The site should be checked to ensure that site investigation information is accurate and complete. This may include checking soil type and conditions, groundwater levels, areas of fill or contamination, condition of any nearby slopes or other geotechnical features.