Installing Plastic Soakaways

In times gone by, soakaways consisted of large pits filled with hardcore, stones and gravel. The problem with this arrangement was that over time, soil particles were inevitably washed down into the spaces between the stones, filling the voids and rendering the soakaway useless.

In recent times, much better soakaways have been constructed using modular water storage cells, known as soakaway crates due to the fact they closely resemble the old-style plastic milk crates. With water management an ever-increasing factor for developers, homeowners and Local Authorities, water crates are now an everyday item found in almost all types of building projects. When correctly installed, plastic soakaways are vastly more efficient than their hardcore predecessors.

The following guide will first explain how deep a hole you should dig depending on the size of your crates, before showing you how to install plastic soakaway crates in an excavated hole.

Soakaway Depth

We have a number of options. Our Polystorm cubic metre tank is 400mm deep, but can be stacked to increase depth or laid next to each other as a shallow tank. For this crate, you should allow for 150mm base, side and top fill, plus a minimum additional 150mm top cover for garden applications (350mm for light traffic applications).  This gives a total depth of 700mm for garden applications or 900mm for light traffic applications when laid next to each other.

Our most popular Rainsmart soakaway is the Rainsmart Ellipse Double Tank Module (900004M3). The crates themselves have a depth of 860mm. Using the same top and backfill as above this gives a total depth of 1310mm for garden applications or 1510mm for light traffic applications.

If you have a particularly high water table or if you would simply prefer to install a more shallow soakaway, check out our Shallow Dig Options 90003F and 90003FV2. The depth of the crates in these options are only 440mm and even allowing for the base, side and top fill the total depth is 890mm for garden applications or 1090mm for light traffic applications.

In heavier traffic applications when using our 65 Tonne Heavy Crates, the amount of top cover varies depending upon the type of traffic above. For car parks with vehicles up 9 Tonnes, we recommend a minimum cover depth of 1m. For specific advice on cover depths for heavier loadings / HGV’s please contact us.

How to Install Soakaway Crates Step-by-Step

1. If you have purchased flat-packed soakaway crates, you can assemble them on site without the need for any specialist equipment; only a rubber mallet is required to tap and secure each of the plates in place. Our soakaway assembly instruction video shows how a cubic metre crate can be assembled in less than 10 minutes. If you have purchased assembled crates, you can skip this step.
2. A 150mm base layer of sharp sand should be laid in the base of the excavated hole – the crates will sit on this.
3. The individual crates should be held together temporarily with cable ties or clips. This is purely to hold the crates in position whilst the side file is shovelled into the excavation – no ties or clips are ever used to serve any structural purpose in a soakaway design. All of our cubic metre sets include ties or clips.
4. All outer faces of the crates should be wrapped in a non-woven geotextile membrane. This is a membrane that allows water to permeate through into the void whilst preventing any soil particles from being washed in. Our cubic metre soakaway sets all include sufficient membrane for this purpose. You can either wrap the crates as a whole once you have connected them together, or it may be easier to place the membrane centrally over the pit and gradually lower the crates down on top of it, allowing the membrane to be pushed down by the weight of the crates. If you choose this method, draw the membrane up carefully around the sides to make sure that you do not leave any gaps.
5. Before connecting a drainage pipe to the crate, a small cross should be cut with a sharp knife in the membrane where the centre of the entry point sits.
6. Once the wrapped crates have been lowered into the pit, the soakaway is then connected to the surface water drainage system. Depending upon the system, the stormwater pipe is generally introduced into the side of any of the crates using pre-formed knockout sections or pipe entry points, although it can just be butted up to the side of the crates if required. Carefully peel back the cross in the membrane that you cut, insert the pipe and tape or clamp the membrane tightly around the outside of the pipe to stop the ingress of any soil particles.
7. If butting the pipe up, it should be immediately supported by the side fill and, for best practice, a small amount of readymix concrete poured around the joint. Domestic soakaways are usually used in conjunction with 110mm or 160mm pipe, or even with 100mm flexible land drain if required. If you are introducing the pipe via a preformed knockout hole on the create, you can skip this step.
8. All that remains is to backfill the excavated hole, this should be done using 150mm of sand or pea shingle all around the sides and above the top of the crates. A further amount of a minimum of 150mm of the soil that was excavated can then be shovelled back in and firmly tamped down.

If you have any further questions about the process of installing plastic soakaways, please don't hesitate to get in contact with our team of experts.