All about the Soakaway
A soakaway is a system to take storm and surface water and to disperse it into a hole in the ground (soakaway) which will then empty through infiltration. A soakaway is used wherever it is not possible to connect to the sewer system.
Over the last few decades with the increase in home owners opting to dig up their front gardens and install driveways, together with the increasing popularity of the patio garden, the main sewer systems have become over burdened. This has led to new regulations being brought in called SUDS. A soakaway deals with water at source so it will fulfill many peoples obligations under the regulations.
To work, a soakaway must be cited where the water table is low enough for infiltration to take place, bearing in mind that water tables often rise and fall depending on the season, although there are some situations where a soakaway can still be installed with a high water table after carrying ground investigations.
Where to site a soakaway.
There are a few things to consider when deciding where to site the soakaway.
- It must be at least 5m away from the building
- It must be sited so that the base of the system is lower than the area being drained
- It must not saturate the foundations of any structure
- It must not become contaminated by pollutants
- It must be permanently above the water table
- It must be far enough away from any other infiltration devices so as not to compromise its capacity
Once the area has been confirmed, a trial pit must be dug to determine if the area has a sufficient rate of percolation (vp) or simply the correct depth to the water table. BS6297 gives the advice of digging a hole 300mm square in plan and at least 250mm deep then filling the hole with water and measuring how long it takes to drain of water. A simpler approach is to excavate a full trial pit to determine how likely the soakaway will work, this pit will need to be at least 1.2m deep. At the bottom of this pit dig a further trial hole as described earlier. The pit can then be left for 24 hours, and if the trial hole has no water after being left then a soakaway is highly likely to work in that area. If there is water in the hole then the water should of stabilised at the level of the water table and can be measured from ground level to determine if a soakaway is possible using the following calculations.
What size of soakaway?
Assuming that the above test was successful we now need to carry out a calculation to determine the size of the soakaway needed.
Vol = A × (rainfall rate/3000)
A is for the area to be drained multiplied by the rainfall rate which is 50mm in the UK divided by 3000. This gives you the storage volume in cubic metres that the soakaway is required to be. If the soakaway is to be filled with old bricks or an aggregate, this will reduce the storage volume of the chamber. once you have the size of the soakaway in M3 you will need to carry out the next calculation to determine the depth.
M3 / (width x depth of chamber)
You will then need to add onto this figure, the depth of the invert of the pipe from ground level, which carries the water into the chamber. If you do not have sufficient depth from ground level to the top of the water table (assuming the earlier test revealed the water table) then the problem can be overcome by increasing the plan size of the soakaway.
As you can see there are various things to consider when installing a soakaway and I have not even covered the various types available. If you require any help in Tunbridge Wells, Kent and East Sussex then please contact us through our website www.johnsonlandservices.com