Infiltration Testing

Impermeable area less than 25m2

Full details are provided in paragraphs 1.34 to 1.37 of the Building regulations H. It covers the method to calculate the Vp which is the time taken to drop 1 mm. This test is primarily for foul drainage but the Vp result can be converted to the infiltration rate (f) to use for small scale soakaway design.

Please be aware that whilst the volume for the infiltration test is only 300 mm cubed it needs to be at the base of any proposed soakaway. Usually a slightly larger hole will need to be dug to 1 m deep and then the 300 mm cubed test hole is dug in the base.

Percolation Test Procedure

  1. A hole, 300 mm square should be excavated to a depth of 300 mm below the proposed invert level of the soakaway. Where deep excavations are necessary a modified test procedure may be adopted using a 300 mm earth auger.  Bore the test hole vertically to the appropriate depth taking care to remove all loose debris.
  2. Fill the 300 mm square section of the hole to a depth of at least 300 mm with water and allow it to seep away overnight.
  3. The following day, refill the test section 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 level (i.e. a depth of 150 mm).  Divide this time by 150.  The answer gives the average time in seconds (Vp) required for the water to drop 1 mm.  
  4. The test should be carried out at least three times. The slowest figure from the tests should be taken.  The test should not be carried out during abnormal weather conditions such as heavy rain, severe frost or drought.
  5. Use the formula below to convert the Vp result to the Infiltration rate (f) for soakaway design:



    f = 0.01/3 x Vp            



    Typical values are between 1.0 x 10-3 and 1.0 x 10-6 m/s.

If the result is lower than 1.0 x 10-6 m/s then the ground would be deemed unsuitable for infiltration drainage. You will need to look at an off-site solution to manage the surface water. Please refer to What if the test fails.