Installing a Soakaway for Surface Water Drainage

Installing a soakaway provides a means by which rainwater from a building can be collected and dispersed into the soil in a suitable location. It’s important to note that this method of drainage is not suitable in clay soils as these do not allow water to pass through them. The ground it’s dug into should be lower than the building if possible but certainly not higher. Also, to avoid possible excess water damaging the foundations, it should be at least 5m away from buildings.

Originally, soakaways were quite simple arrangements where a hole would be dug in the ground and backfilled with hardcore and gravel. Drainage pipes would be laid to this pit from the surface water collection manhole and the water would gradually seep away into the surrounding soil. However, this arrangement can be problematic because any fine soil particles travel with the water and can eventually clog up the soakaway.

Modular soakaway cell Inlet blank on modular cell

A far better system is employed nowadays using what are known as modular attenuation cells. This is a very grand name for what on first inspection appear to be something like old milk bottle crates. The cells are lightweight plastic structures with a high void ratio. This allows them to be buried in the ground providing a storage area for water while it percolates into the surrounding soil. Because they are also fairly strong, they can be covered with a layer of soil and grassed over.

The required size of the soakaway will need to be worked out in advance. The following 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

We now know that we will need enough modular cells linked together to give us at least 1 cubic metre. Obviously the size of each unit varies from manufacturer to manufacturer so check the product sheets to see how many cells will be needed.

It’s generally easier to lock the cells together before placing them in the ground. Assuming that our cells each measure 1000mm long by 500mm wide and 500mm deep, we would need four of these to provide a 1m3 capacity. When the cells are locked together, we will therefore have a unit measuring 1000mm in each direction. Beneath the unit and on all sides, we need 100mm of gravel and, on top, we need a total of 500mm cover including a 100mm gravel layer. From this we can see that the size of the hole needs to be 1600mm deep and 1200mm square. One of the inlet blanks will need to be removed for connection of the drain feeding into the unit. Determine which of these is in the most suitable position for your drain run and cut the retaining lugs to remove it.

Trench for drain run to soak away

Once the position of the soakaway has been established, dig a trench from the house for the drain pipe to run in. The depth of this will be dependent on the various levels but ensure that there is an adequate fall along its entire length to ensure proper drainage.

Hole being dug by hand for soakaway

Mark out the overall size of the hole on the ground and cut away the grass in 50mm thick strips so that they can be stored away and reused later. In reality, the size of the hole will benefit from being a little larger than this on plan as you will probably need some working space to enable you to get the cells in position easily. Unless you’re lucky enough to have access to a mini digger, you are probably going to dig the hole by hand. If the ground is hard going, you can use a fork to loosen it before shovelling it out. You might find it easiest to shovel the excavated soil straight into a barrow for disposal, rather than having to double handle it. Try to keep the sides straight and vertical – obviously this will depend on the nature of the soil and you may have to rake back the sides to prevent them falling away.

Hole dug for soak away

Once the hole has been dug, a 100mm layer of gravel blinding should be laid over the base. Make sure that this is levelled out so that you have an even base for the cells to sit on. The next job is to lay out the permeable fabric used to wrap the interlocked cells. This close weave material is very important as it stops any fine soil or gravel particles from getting into the soakaway and clogging it up whilst still allowing water to flow through it. Make sure that the fabric is large enough to cover the entire unit – in our example, this will be 4m x 4m. Lay the fabric over the hole so that it sits centrally, then ease the middle down to the bottom of the hole so that it lays as flat as possible. Lower the interlocked soakaway cells into position and connect up the drain pipe via the inlet cut-out. Draw the fabric up round the sides. Try to keep it as flat as possible here as well. Work the fabric around the inlet pipe so as to ensure there aren’t any gaps. Use tape if necessary here and on any joins to keep the material in place.

Drain connected to soak away Soak away partially backfilled

Backfill the sides with gravel and a 100mm layer over the top. You can then use previously excavated material to build the level back up on top of the unit. Make sure that this soil is well compacted to minimise any settlement later. Finally, relay the grass that you put to one side earlier and tamp this down well.

If there is any settlement of the ground over the next few weeks, simply make it up with good quality topsoil and allow the grass to grow back through.

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