Soakaway Crates

Soakaways have been introduced to relieve the burden on the countries ageing drainage network which already operates at near full capacity. New developments and buildings have to offer rainwater mitigation on site, (we have to deal with the extra water produced from our new build on site without letting it affect the main sewage network).

The main elements for any soakaway are means to capture rainwater (gutter downpipes, channel drains, etc.), silt capture (means of capturing any silt or debris (leaves, dirt and rubbish) (this can be achieved from a simple bottle gully to a recommended silt trap or catchpit), underground drainage pipe and fittings to transport the water from source to silt capture and finally to soakaway.

At soakaway point we will require soakaway crates, geotextile wrap and waterproof gaffa type tape.

Ok this point I have to highlight that the following is only a general guide and not necessarily exact requirements for your project.

The first thing we need to ascertain is the cubic capacity of the soakaway required (this calculation is only a starting point) we will need to know the square metres of all areas that will be flowing into our soakaway (this includes roofs, patios and paths etc.)

Next we need to decide on the location we are going to put our soakaway (please note there is a requirement where possible for this location to be at least 5 metres from both lived in buildings and the boundary of the property. As a general exception it usually has to be a minimum of 3 metres where the above is not possible.)

Next we need to perform a percolation test at this location (scary terms and requirements may vary in different areas and with different engineers or professionals, the purpose of a percolation test is simply to discover how well the soil drains at our proposed location for our soakaway. (To simplify we are trying to find if the soil is good, medium or poor draining as this will help us calculate the size of our soakaway).

At BP Plastics we have an absolute minimum test requirement of digging a hole approximately 100mm (post hole size using an auger maybe) to a depth of 1.2 metres deep, we should start the next part of the test early in the day, we fill this test hole with water to the top. We record the time. 6 hours later we go back and check the hole. If the water has disappeared we have a result. Our soil is good draining and we can proceed to calculate the required size of our soakaway.

However, if the water has not totally receded we need to move to part 2 of the test and come back the next morning and check our test hole, if the water has all gone we can then move on to calculate the size of our soakaway as we have medium draining soil. If the water has not drained by this time then the location we have tested is not suitable for a soakaway, at this point we can explore trying a test at a different location to see if results improve or to accept we are going to look at moving to attenuation solution which is a slightly more complicated option to soakaway and often more expensive.

Ok so now we have the main information we need to calculate the size of soakaway we need (again I must remind you this is only general and may vary).

There are 2 more important things we need to take into account when we calculate our soakaway : climate change allowance and 100-year storm scenario.

At BP Plastics we work on a general rule of 15% climate allowance and 35% 100-year storm allowance, giving a resilience level overall of 50%.

So our calculation for standard good draining soil is, we require 200 litres of capacity for each 10 square metres of area of capture progressing to our soakaway. This equal 0.2 cubic litres per 10 square metres. (to keep this easily understandable to the readers I am going to calculate on a 10m x 10m roof only) we work out our square metres 100 in this case we divide this by 10 to give us the number of 0.2 cubic metre requirements in this case 10 giving us a total basic requirement of 2 cubic metres for our soakaway. We now need to add 50% for climate change and a 100-year storm giving us 3 cubic metres. That is the volume requirement for our soakaway (please do not assume that is the actual size of the soakaway it does not quite work like that it will not be 3m x 3m x 3m) if our percolation test result was medium (water drained overnight) (also look at us we knew what the percolation test meant) we have to add 50% to our original calculation so 2 cubic metre becomes 3 cubic metres we only need to add the original storm allowance so our soakaway size will be 4 cubic metre in total

Ok so now we have the cubic volume we can look at size, there are way too many options at this point for me to fully offer so I will offer one example to show how to calculate.

There are 3 main soakaway crate sizes in the industry we can offer all variants at BP Plastics Ltd

Polypipe / Polydrain 1000mm x 400mm x 200mm (0.2 cubic metre)

Pennine Aquastorm 1000mm x 1000mm x 400mm (0.4 cubic metre)

Brett Martin 1200mm x 600mm x 347mm (0.25 cubic metre)

BP Plastics offer crates in different load bearing from light duty 20 tonne to heavy duty 65 tonne, we can also offer many specialist options (but that can of worms we don’t want to open here in our simple guide).

Ok so we previously calculated the volume we need for our soakaway now to calculate the size (we calculated a volume of 3 cubic metres which I will offer 2 different sizes below)

Option 1 single layer of crates (this will require a larger area but a shallower dig depth of hole), we will need 15 (1000mm x 500mm x 400mm = 0.2 cubic metres) crates to give us a volume of 3 cubic metres. So our soakaway size will be 3 metres x 2.5 metres x 0.4 metres. (please see basic installation instructions below for hole size)

Option 2 double layer of crates (for this option I have increased the number of crates to 16 giving us 2 equal layers of the same size and 3.2 cubic metres of volume) so our soakaway size will be 2 metres x 2 metres x 0.8 metres (smaller hole at deeper depth)

Hopefully my previous comment that soakaway size and soakaway volume are two different things, now makes sense.

Firstly, please note we at BP Plastics Ltd are not drainage engineers or SuDs specialists, the information offered has been gained from over 5 years of experience of providing the products required for soakaways

My name is Shaun and I have 20+ years of experience in the garden and building trade and I am here to help you from start to finish of our soakaway project me and my colleagues at BP Plastics Ltd are always only a phone call away, I am happy to speak to your drainage engineer, architect, building control and your building contractor so everyone is on the same page and understand in their unique ways what the aim is and how we are going to achieve it. I can also offer suggestions for minor alterations to your plans that will offer the opportunity to save money using more easily accessible options

The information is offered only as advice to help you understand the soakaway principle and concept. Although you will hear technical terms I will try to offer a simplified explanation for these

Hopefully, I can make it easy to understand and I am at the end of the phone or email along with my colleagues at BP Plastics should you have any more questions or would like to progress with ordering.

if you have any feedback to offer or need help, please email me

[email protected]

our general contact details are

https://www.plasticdrainage.co.uk/

[email protected]

I look forward to joining you in your project and working together to achieve the best results.

Please take care Kind Regards Shaun.