Percolation Test

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Septic System Percolation Testing.

Once you have decided to put in a new system then the first thing you have to sort out is where you are going to get rid of the water that it produces.

The simplest way is to discharge to a local ditch / stream / pond or river.  If you can't do that or don't have one handy then the only other way of getting rid of your water is by using a soakaway.

The following test procedure is taken from the © Ireland Environmental Protection Agency 2000 Manual and is a better test than the British Standard BS6297: 1983

The percolation test comprises the measurement of the length of time for the water level in the percolation test hole to fall from a height of 300 mm to 200 mm above the base of the hole. 

PERCOLATION TEST (T Test) PROCEDURE:

Day 1:

  1. Two percolation test holes should be dug in the proposed percolation area. Each hole should be 0.3 m x 0.3 m and 0.4 m deep below the proposed invert level of the distribution pipe (Figure 25).  The bottom and sides of the hole should be scratched with a knife or wire brush to remove any compacted or smeared soil surfaces and to expose the natural soil surface. 
  2. Clear water should be carefully poured into the hole at about 10.00 am so as to fill it to the full height of 0.4 m. The water should then be allowed to percolate.
  3. At about 5.00 pm the hole should once again be filled to the full height of 0.4 m and allowed to percolate overnight. 

Day 2:

  1. The hole should be filled with clear water at about 10.00 am and the water should be allowed to drop such that there is 0.3 m of water in the hole.
  2. Thereafter, the time in seconds required for the water to drop 100 mm, that is from 0.3 to 0.2 m, in the hole should be recorded.
  3. The hole should then be refilled to the 0.3 m level again and the water allowed to drain to the 0.2 m level and the time again recorded.
  4. The filling and measurement of the percolation rate through the hole should be repeated two times – three tests in all. 

Divide the average time in seconds by 100. The answer gives the average time required for the water to drop 1mm (Vp in s). Take care, when carrying out the test, to avoid abnormal weather conditions such as heavy rain, severe frost or drought.

 
A satisfactory result is an average value, Vp of 24s/mm or less. If the measured values exceed this, then carry out further tests at a minimum of three different locations on the proposed land drain route, or at least three tests on separate days on the site proposed for the soakaway.

 
Where deep excavations are necessary a modified test procedure may be adopted using a 300mm earth auger. Bore the test hole vertically to the appropriate depth taking care to remove all loose debris.

 
Make water level observations referring to a fixed datum using a dipstick or some suitable alternative water level indicator.

 
The value found In this way is called the percolation value (Vp in s) of the soil and can be used to determine the area of drainage trench floors required to disperse effluents. If the percolation value exceeds 140s then BS6297 considers that the soil is not suitable for a soakaway system.

At =P x Vp x 0.25

At = Floor area (m2) of subsurface drainage trench

P = The number of persons served by the tank

 
Vp = The percolation value in sec/mm

 
The area determined should be used to calculate either the total floor area of the drainage trench and therefore the length of land drain, or alternatively the floor area of one or more shallow soakaways.

To obtain the linear pipe length, divide the drainage area (At) by the proposed trench width (normally 0.6 metre or 0.9 metre).

 
Interpretation of Results

 
The following table provides typical irrigation trench length in metres assuming a trench base width of 600mm.
 

 
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

 
9

 
19

 
30

 
46

 
54

 
81

 
6-10

 
17

 
38

 
59

 
92

 
108

 
162

 
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, you should consider not using a septic tank and soakaway system.  The use of a Biokube system that allows discharge directly to a ditch will be a much more economic method of disposal.
 
We do suggest, irrespective of results and calculations, that a minimum trench length of 30m is installed.

BS EN 752-4 1998 advises that it is not desirable to position a soakaway nearer than 5m to a building, or position it so that ground beneath foundations is adversely affected. Further that if, in the winter the groundwater level rises to within 1m of the proposed invert level of the irrigation system, then sub-surface irrigation should not be used.

 
IRRIGATION SYSTEM DESIGN

 
BS EN 752-4 1998 advises that a sub-surface irrigation system should be very carefully designed and constructed using porous or perforated pipes laid in trenches with a uniform gradient not steeper than 1:200.

 
The trenches should be between 300mm and 900 mm wide and 2 m wide strips of undisturbed ground should be maintained between parallel trenches. The pipes should be laid on a 150mm layer of clinker, clean gravel or broken stone of 20-50mm grade. The trenches should be filled with the same material to a level 50mm above the pipe and covered with plastic strip or geotextile material to prevent the entry of silt.

 
The remainder of the trench can be filled with normal soil. Pipes should be laid at a minimum depth of 500mm below the surface.

 
For longevity the soakaway should be designed with facilities for inspection and maintenance. The life of a soakaway will be reduced if its waterways become clogged by silt or floating material.

 
Access points enable the point of discharge of the drain to be viewed and material cleared from the soakaway. Sub surface irrigation systems should have at least two inspection points on each length of straight pipe i.e. at least at both ends.

 
We would also suggest that the drainage system should be constructed from lengths of 110mm perforated clay or perforated rigid plastic pipe, preceded by approximately 3 metres of un-perforated pipe connecting to the septic tank outlet. Corrugated pipes designed specifically for land drainage are not recommended. The layout should ensure even distribution throughout the absorption field, avoiding steep gradients on sloping sites. A closed circuit system is preferable.
 

 

Percolation Test Hole
 

 

 

  

Have your Soakaway Designed for One Single-owner Household

Package includes:-


Full design calculations as outputted from Visual Soakaway Design,
Typical AutoCAD drawings of Soakaway in paper or digital format as required.

 

Contact:

WebCom Systems
PO Box 21045
Alva
Clackmannanshire
Scotland
FK12 5YE
Tel 07050 184360
 
 
 

 
     
 
All enquiries should be made to:

Chris White
Biotank Ltd.
Birch Farm
Southery Road
Feltwell
Norfolk
IP26 4EN.  UK


Telephone: 01277 889333

International Telephone +44 1277 889333   

Fax: +44 (0)1277 889388

email: enquiries@biotank.co.uk  web: www.biotank.co.uk

Please note that our offices are closed for our summer holiday
from Tuesday 1st July until 9th July 2008.  

 

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