Why do a Percolation Test?
A soil percolation test is required to be carried out on the land where the
septic tank system or treatment plant soakaway is intended to be built. It is a test that determines the rate in which soil absorbs a known volume of water. This is to ensure that the land (soil) is suitable for a soakaway, and to ensure that the soakaway is designed properly.
A percolation test is ALWAYS required for septic tanks.
How do you carry out a percolation test?
A percolation test should not be carried out during abnormal weather conditions and if done in Summer, then the times should be increased by 30% to 50%, depending on the recent weather.
The percolation test method
1) Excavate a hole 300mm square and 300mm below the proposed outlet from the septic tank or sewage treatment plant.
2) Dig the test hole vertically to the appropriate depth. Remove all loose debris.
3) Fill the test hole with water to a depth of at least 300mm. Allow to seep away overnight.
4) Next day, refill the test section with water to a depth at least 300mm. Observe the time, in seconds, for the water to seep away from 75% full to 25% full.
5) Divide this time by 150mm.
6) The answer gives average time in seconds (Vp) required for the water to drop 1mm.
7) Carry out the test at least 3 times, with at least 2 trial holes.
8) The average figure from the tests should be taken.
9) The average value of Vp should be between 12 and 100 and the preliminary site assessment report and hole tests favourable to use drainage field disposal.
10) The minimum value ensures untreated effluent cannot percolate too rapidly into groundwater before tertiary soil treatment has taken place. Where Vp is outside these limits, effective treatment is unlikely to occur. However, where there is an alternative form of tertiary treatment to treat the effluent, it may still be discharged into a soakaway.
IMPORTANT Some Councils insist that Percolation Tests are carried out by 'qualified persons' and do not accept your own test results. You may be required to contact a contractor to carry out the percolation test. Please check with your Local Authority.
A percolation Test is deemed to have failed if the results are under 12 or over 100 seconds/mm. drop.
The Soakaway Design
1.27 Part H2 of the Building Regulations 2000: Drainage and waste disposal (go to page 32) states that
A drainage field or mound serving a wastewater treatment plant or septic tank should be located:
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at least 10m from a watercourse or permeable drain.
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at least 50 m from the point of abstraction of any groundwater supply and not in any Zone 1 groundwater protection zone.
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at least 15m from any building.
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sufficiently far from any other drainage fields, drainage mounds or soakaways so that the overall soakage capacity of the ground is not exceeded.
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At least 1.2 metres above the highest winter water table level.
Remember, the pupose of a soakaway is not only to dispose of the effluent into ground, but also to further treat it, using aerobic soil
bacteria, before it mixes with groundwater.
|
Tank Capacity
|
2800
|
3800
|
4600
|
6000
|
7500
|
9000
|
|
No. of Persons
|
4
|
9
|
14
|
22
|
26
|
39
|
|
Rate of Fall, Vp (sec/mm)
|
Required length of irrigation drain, in metres
|
|
0-5 FAIL
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|
|
|
|
|
|
|
6-10 FAIL
|
|
|
|
|
|
|
|
11-20
|
34
|
75
|
117
|
183
|
217
|
325
|
|
21-24
|
40
|
90
|
140
|
220
|
260
|
390
|
|
25-30
|
50
|
113
|
175
|
275
|
325
|
487
|
Where the calculated result indicates the need for a drainage trench longer than 200 metres, serious consideration should be given to the use of a more economic alternative sewage treatment system such as a Crystal ECO. A Crystal will produce a better quality of effluent which, with permission, may be fed into a water course or open culvert. Please contact us for further details.
