How to build a soakaway for garden drainage.

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What is a soakaway/drainage pit?

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.

Materials needed 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 crates, they 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 .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 before shovelling in the gravel.

Tools to make digging out and building a soakaway easier

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 leverage tool makes it easier.

This "Best of British "solid forged, square mouth shovel from Bulldog's Premier range, is ideal for trench work, Digging out and shovelling clay from soakaways.

Bulldog Tools have been made at Clarington Forge in Wigan, England, for over 200 years. Generations of Farmers, Contractors and Professional Landscapers stand testimony to a quality of product upon which their livelihoods depend.

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.

What's the best position to build a soakaway

If the soakaway is being used as a standalone drainage system it should always be positioned in the lowest part of the area being drained, so that water will flow into it and at least five meters from any property to avoid water soaking into the foundations.
If the soakaway is part of a French drain system, again it should be positioned at the lowest point, lower than the drains so that water from the French drains will flow into it. This will avoid having to use pumps to transfer the water from the trenches into the soakaway. The fall along the entire length needs to be just enough to keep the water flowing. Again, position the soakaway at least five meters from any property.

How to build a soakaway

To construct a basic stand alone soak away or one to use as a dispersal point for a French drain system where the water will gradually seep away into the surrounding soil.
Dig a large hole in the ground, line the sides with heavy-duty landscaping fabric, to keep the earth from over time clogging up the gravel. (fabric available above).
Next dig the trenches that are discharging into the soak-away 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 loose soil that has fallen in.
Fill with large chunky clean rubble or 20mm gravel to about 5" from the top.
Wrap the fabric over the top 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 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

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