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Standard chamber sections can be perforated with 75mm diameter holes for use as soakaways, providing a minimum area of exfiltration of 50,000mm2 per metre of nominal diameter per metre of depth.
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Shaft/chamber section
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Standard number of perforations per section
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| DN |
0.50m deep |
0.75m deep |
1.00m deep |
900
1050
1200
1350
1500
1800
2100
2400
2700
3000
3660
4000
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6
8
8
8
10
12
12
14
16
18
-
-
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9
12
12
12
15
18
18
21
24
27
33
36
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12
16
16
16
20
24
24
28
32
36
44
48
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Note: DN 1350, DN 3660 and DN 4000 are not covered by the European Standard/not Kitemarked.
Soakaway design
Soakaway sections can be installed individually or by combining several together to provide sufficient storage and area of exfiltration. The effective storage capacity should be calculated using the depth of chamber below the invert of the incoming pipe and the base of the chamber or to the top of the water table whichever is the highest.
Prior to installing any Soakaway, the proposed location should be investigated thoroughly to ensure soil infiltration, geotechnical and hydrogeology aspects are reviewed and appropriate. BS 8301:1985 recommends in ground with low permeability, the storage capacity should equal 12mm of rainfall over the area to be drained (reference to BRE Digest 365 is recommended).
In suitable ground conditions the soakaway can be placed on a ring strip foundation thus providing additional exfiltration through the base of the chamber. If ground heave cannot be ruled out, the units should be sited on a concrete foundation slab. The excavation should be backfilled with a band of 40mm nominal single size stone, stone rejects or hardcore to provide permeable surround to the soakaway chamber. BS 8301:1985 also recommends that soakaways should be constructed not less than approximately 5 metres from a building or in a position where the ground below the foundations is likely to be adversely affected.
The following graph is based on the formula:
Where h = effective depth of soakaway chamber (invert of incoming pipe to base or water table) A = area to be drained (m2).
To use graph:
1. Take a vertical line from the “Area to be drained”.
2. Take a horizontal line from the “Effective depth of chamber section”.
3. At the intersection read off diameter of section required.
Example
Area to be drained = 500m2.
Effective depth of chamber section = 3.0m.
Intersection falls between 1500mm and 1800mm diameter.
Therefore use 1800mm diameter section.
If it is considered that 12mm rainfall is insufficient then increase “Area to be drained” accordingly, e.g. 20 minute retention of 50mm per hour rainfall.
500 x 50 = 694
12 x 3
Intersection falls between 1800 and 2100mm diameter. Therefore use 2100mm diameter section.
Alternatively:
To calculate the “effective depth of chamber section”:
1. Take a vertical line from the “Area to be drained”.
2. Take the diameter of section.
3. At the intersection read off the “Effective depth of chamber section required”.
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