Soakaways or rainwater drainage pits are part of a garden drainage system. It's a large hole that collects water from a French drain system, allowing the water to soak-away into the ground.
This article describes how to construct a Soakaway for draining water away from a domestic garden and is best read together with my other drainage related pages linked from here. lawn and garden drainage
Soakaways also known as drainage pits are a simple way of collecting and dispersing surface and storm water when connection to the domestic drain system is impractical. However, Soakaways rarely work when dug out in heavy clay. 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, The soakaway should be situated at least 5m away from any 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 1m square and 1.5m deep. I have included a formula below to calculate the required volume of a soak away constructed to take roof water and garden drainage.
Positioning the soak away. The Soakaway should be sunk in the lowest part of the area being drained, with the French drains sloping toward the Soakaway. I have included a. Drainage guide lower down this page.
Scroll on down for; Best position to build a soakaway, How to build a soakaway, Calculating the size.
FOLLOW construcion info on this page.
This Crowbar is an essential tool for digging deep holes If you need to remove stones, roots and rubble as you are digging the soakaway,
This "Best of British "solid forged, square mouth shovel from Bulldog's Premier range, is ideal for Digging out and shovelling clay from soakaways.
Spear & Jackson Neverbend Professional Treaded Digging Spade. Digging deep holes can hurt the sole of your feet (as well as every bone in your body) The tread on this digging spade helps a lot.
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.
The types of soakaway I have described here can also be used to disperse roof water on non-clay soil. 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.
Where C is the capacity or volume of the soakaway in m3
A is the lawn area or 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 an 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
Garden and lawn drainage D.I.Y. guide
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