Soakaway Systems: An Installation Guide

If a property cannot be connected to a main stormwater system, a soakaway is a practical and convenient solution; pipes direct the rainwater to a large underground pit where it can be collected then slowly released into the soil at a manageable rate.

Soakaways were once simply filled with stones, but over time these lost their efficiency as silt seeping in from the surrounding earth filled the voids between the stones that were designed to hold the water. Modern soakaways are constructed from interlocking plastic crates, wrapped in geotextile membrane to prevent silt from entering.

In this blog post, we take you through the simple yet essential steps to installing a soakaway.

Choosing the size of your soakaway

Several factors enter into determining the appropriate size of soakaway you will need for your property including the catchment area it will serve (e.g. the surface area of the roof) and the permeability of the soil.

A good rule of thumb for a soakaway crate replacing an old type filled with stones or hardcore is to allow one cubic metre of soakaway crate for every 50 square metres of roof.

However, we would always recommend seeking the advice of a civil engineering consultancy and have your design checked by your local water authority if it’s for an extension or new build.

The modular design of soakaway crates means the soakaway can be shallow and spread out over a larger area (useful if your site has a comparatively high water table) or the crates can be stacked for a deeper and more compact construction if space is limited. 

Additional space should be included in your calculations to allow for a base and backfilling on all four sides and the top of the soakaway. The depth of the top cover will depend on the weight of the vehicles expected to pass over the soakaway, ranging from a minimum of 150mm for gardens, 350mm for light traffic, 1000mm for vehicles weighing up to 9 tonnes gross and more still for heavier loadings.

Choosing where to locate your soakaway

You will need to decide where to locate your soakaway early on in a new build or extension project, as its location must be marked on the plans.

Select an area which is free of any underground services; look out for cables serving electrical power, telecommunications and street lighting, gas or water supply and drainage pipes. If there is any uncertainty about what may lie underneath your soil, we strongly recommend you request site plans from your local authority.

The soakaway should be at least five metres away from any building, more if the ground slopes downwards towards the building, to prevent water from the soakaway undermining the foundations.

You should also check the suitability of the soil for a soakaway as less porous soils such as clay or a high water table will prevent water from dispersing efficiently from the soakaway. The best way to establish this is to perform a soil percolation test  as described in our blog post.

Adding a silt trap to your soakaway

Even with modern soakaway construction, silt will build up over time and impair its efficiency. To help prevent this you can install a silt trap on the pipe leading to the soakaway.

This is a chamber that incorporates a removable bucket made of sturdy mesh on a PVC frame. Rainwater enters the bucket as it falls into the silt trap and the mesh captures any silt or debris, allowing the cleaner water to pass through to the soakaway. The bucket will require periodical emptying, but this is easily accomplished and preferable to having a soakaway failing prematurely.

Excavating a soakaway pit

The fastest way to excavate a soakaway pit is to hire a mini excavator, but if you don’t feel confident about operating one, are working on a tight budget or cannot provide site access for large machinery, digging by hand is another option. If you choose to dig out the pit manually, we recommend using the HSE website to learn how to do this safely.

If you are installing a soakaway under a lawn, you may want to carefully remove the turf by slicing it into strips and rolling them up so they can be re-laid once the installation is complete.

The pit must be larger than the soakaway by 150mm on each side for backfilling and deep enough to accommodate a 150mm base and the appropriate cover depth.

Preparing a base for your soakaway

When you have excavated the pit to the correct dimensions and depth, remove any tree roots, stones or rubble from the floor of the pit. 

Spread a 150mm-thick layer of 20mm pea shingle or sharp sand evenly across the floor of the pit, making sure it is levelled out and compacted. The pit is now ready for the soakaway crates to be installed.

Installing the soakaway crates

Before you begin assembling the soakaway, we recommend you read the manufacturer’s instructions carefully and familiarise yourself with how this is done.

The outer edges of the soakaway crates will need to be lined with a non-woven geotextile membrane to prevent silt and particles in the surrounding soil from entering the soakaway. Possibly the easiest method of doing this is to drape the membrane across the pit and then lower the crates into the pit, taking the membrane with them. 

Once all the crates are in place, check there are no gaps in the membrane around the crates or the join with the inlet pipe that can allow silt to enter the soakaway.

Depending on the make of soakaway crates you have purchased, they may have an interlocking design or clips to secure the crates together. If they have neither, use cable clips to hold them in place while you are backfilling the pit.

Connecting the inlet pipe to the soakaway

The type of entry point for an inlet pipe will vary from one manufacturer to another. Some have an entry plate moulded into the crate wall that can be easily cut or knocked out and the pipe can be inserted into the crate. 

If this is the case, turn all other crates so that the entry plates face inwards to the centre of the soakaway; this will prevent the geotextile membrane from sagging through them into the soakaway. 

If the crates don’t have removable entry plates, simply butt the pipe up close to the crate wall.

Placing the inlet pipe against the membrane and crate wall, draw a line on the membrane around the pipe as a guide to cut the membrane. Cut two lines across the circle, intersecting at the centre, and fold the quarter sections back. 

Insert the pipe through the membrane and if the crate has an entry plate, insert the pipe about 150mm into the crate.

Use duct tape or large cable ties to secure the quarter sections of membrane to the outside wall of the inlet pipe. This should hold the pipe securely while you add the backfill. 

If the inlet pipe is only butted up against the crate wall it should be immediately supported by backfill to prevent it from shifting while the rest of the installation takes place.

Backfilling your soakaway pit

The space between the soakaway crates and the pit walls can now be filled with a 150mm-wide layer of sharp sand or pea shingle. Apply this evenly around the perimeter in layers and compact them to prevent shifting during installation and add stability to the finished soakaway.

Once the backfill around the sides of the soakaway reaches the top of the crates, spread a 150mm-thick layer of sharp sand or shingle across the whole area, followed by normal soil (removing any large stones from the soil) to the required cover depth. Firmly tamp down the soil as you lay it to prevent further shrinkage.

If you are installing a soakaway under a lawn, the turf can be re-laid and watered in to encourage root regrowth. Should the earth settle in patches over the next few days, sprinkle stone-free topsoil over the low patches and the grass will grow back through, creating a level lawn again.

We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call the friendly team of drainage experts at Drainfast on 01420 555600 or email [email protected]. 

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!