How to build a soakaway for garden drainage.

 

Soakaways are part of a garden drainage system and this article is best read together with my other drainage related pages linked from here 

 

lawn and garden drainage 

Diagram of a Soakaway system
Diagram of a Soakaway system

 

Soakaways are a simple way of dispersing surface and storm water where connection to the domestic drain system is impractical, however soakaways rarely work when dug out in heavy clay as clay soils do not allow water to pass through, instead the water collects rather like a pond.  

Most gardens in London, Surrey, parts of Berkshire and surrounding counties are built on clay.

Other options.If you can't connect into the flood water system and the garden soil is mostly clay another option to disperse water is to install a pump and sump system.

Protecting property foundations. To avoid water collecting in the soak away, leaking into and damaging the foundations, it should be situated at least 5m away from building.

The size of the soak away will need to be worked out in advance, generally a soakaway built on non clay soil to take and disperse water from a gravel French drain system  draining a 120 m2  waterlogged  garden will not need to be larger than 1m square and 1.5m deep.  

I have included a formula below to calculate the volume of a soak away constructed to take roof water and more extreme garden drainage.

Positioning the soak away. The soakaway should be sunk lower than the area being drained.

Materials that can be used to construct a soakaway

Pre-fabricated soakaway baskets. A cost effective alternative to the traditional gravel soak away, also saves wheel barrowing tons of gravel or hardcode into your garden.  The crates look a bit like milk cratesthey measures 800x500x540mm

The Crates should be wrapped in the geotextile fabric listed here which allows water to soak through into the crates but keeps soil out and can be used in exactly the same way as traditional gravel filled castaways.The baskets can be laid side by side and on top of each other to fill the hole, fill any gaps with clean gravel and cover with soil and turf.  Click the picture for more information and buying options.

Constructing a basic soak away is fairly simple you dig a hole in the ground and fill the hole with gravel available from most builders merchants. (The 20m flint gravel from amazon is Ideal). Connect up the drainage pipes or French drains to carry the water to it and the water will gradually seep away into the surrounding soil. Over time unless the gravel is wrapped soil will eventually clog up the soak-away.

 

 

Soakaway using wrapped gravel.  A simple way to stop the soil and other particles that are carried in the water clogging up the soak away, is to line the hole with the heavy duty fabric on the left before shoveling in the gravel and covering it before topping up with soil.


What's the best position for a soakaway

Position and dig out the soakaway  lower than the area being drained and at least five meters from any property, dig the trenches that are discharging into the soakaway ensuring that there is an adequate fall along the entire length, including the side drains if you are using a herringbone pattern. This page on my website explains how garden drainage works and is installed.

Connect the trenches to flow into the hole. Tidy up the soak away and trenches by removing any lose soil that has fallen in.  Line the hole and trenches with the weed control fabric and fill with clean 20mm gravel to about 5" from the top.

Wrap the fabric over and shovel in topsoil, slightly overfilling to allow for settlement.

You're done, leave the earth to settle for two or three weeks and turf or sow grass seed over the top.

If you are using the pre wrapped soak away crates featured above in place of gravel, line the bottom of the hole with gravel to get it level and fit the crates, back fill any gaps around the edge with gravel and cover with earth.

Calculating the size of a soakaway.

The types of soakaway I have described here can also be used to disperse roof water and water from flooded gardens.  Getting the size of the hole correct to be able to handle the volume of water is important.

This formula is generally used to calculate the volume of the soakaway and should be suitable for most situations.

  • C= (AxR)/3

Where C is the capacity or volume of the soakaway in m3
A is the area of roof of the building in m2
R is the rainfall expressed in m/h (metres / hour)

For the rainfall a standard 50mm per hour is usually used which equates to 0.05m/h

Using this formula we can quickly calculate the size of the soakaway that needs to be installed. Say we have a roof area of 60 m2 and we us the standard rainfall of 0.05 m/h the calculation is as follows:

  • C=(60x0.05)/3
  • C=(3)/3
  • C=1 m3