Guide to a Driveway Soakaway
By its name you can probably tell that a driveway soakaway – ‘soaks’ – water away. They are an important part of some driveway drainage systems.
Usually, this is where water cannot drain to bedding areas and especially for driveways on steep gradients where surface water gushes towards a house.
If there are existing drainage problems after rainfall (easily identified by waterlogged areas) then the construction of a soakaway is a good idea!
Driveway Soakaway – Sub Surfaces
Depending on which region you live in the sub surface under your driveway will be chalk, limestone, clay, sand or a mixture of all these.
Soakaways are not for heavy clay soils – How could they be?
Water takes forever to drain through thick, heavy and dense clay!
Driveway soakaways are for soils made up of sands, limestone or chalk.
Glad that’s out the way!
But be sure to conduct a water drainage test….more on that below.
Driveway Soakaway – How big?
A simple answer is 2 cubic metres!
That’s big and will more than handle 98% of Driveways that need a soakaway in the UK.
For many 1 cubic metre will do.
In terms of installation they have to be at least 5 metres from a building.
Good, nice and easy.
But what if you want a complicated answer?
We could attempt to dazzle you with volume flows, velocity, capacity and area?
Throw in a couple of equations?
There’s a little one below, but only little.
For the moment, forget about velocity flows and making forecasts for ‘worst case scenario’ deluges.
Don’t even start to think about Ice caps melting and tropical hurricanes…..until they do because nobody really cares, do they?
So, instead to help me and you I asked one of my children to provide a drawing. She completed it in 3 minutes and made 1 spelling mistake.
But, she got the idea, and we’re sure you will.
So, whilst the drawing is not to scale you get the idea, right?
Things to consider? Do you need a soakaway in the first place?
Is it required under SUDs regulations?
Local authority building guidelines?
Does the water from your roof drain onto your driveway?
Or does it have its own drainage?
Sub – soil suitability? Clay? Sand? Limestone? Chalk?
How much of driveway area will drain into soakaway?
Driveway Soakaway Drainage – Technicals
IF surface water from roof drains to driveway – calculate roof area
So 10 (l) x 6/2 (w) = 30 m2 is roof area of one side of roof.
Then multiply this by ‘roof pitch factor’ – varying degrees of pitch affect this, as shown in table below:
Assume roof pitch in example is 45 degrees.
30 m2 (roof area) X pitch angle factor (1.5) = 45 m2
In a prolonged downpour your roof will be taking 45 m2 of surface water and this will be going to your soakaway, where else?
Okay, that’s the roof bit out of the way.
Remember, if your roof drains to your driveway and doesn’t have its own drainage – you will need to add in the drainage from your roof area to calculate the size of the soakaway needed.
Driveway Soakaway size – without roof drainage (only driveway surface water drainage)
In comparison, you may not need to take roof surface water drainage into account at all – maybe it drains to the back of your house, maybe along guttering to your neighbour.
Ah, what a relief!
So what size soakaway will you need then?
Assuming your linear channel drainage is taking all the water from your driveway, what size soakaway will this need to ‘pipe’ into?
As mentioned at the very beginning, 1 cubic metre (1 m3) will normally do but a larger area (with steep gradient) may need 2 cubic metres (2m3.)
But, this is rule of thumb talk or common sense approach.
If you want to get all water flow calculationish and start trying to predict rain fall/flow rates – go ahead.
Send us your calculations and we’ll send you a paper hat!
Seriously, if you have a steep gradient driveway and the area is more than 200 m2 ensure engineering type calculations are made for drainage!
Eden district council provide a link to one of the most authoritative papers on Soakaways – if you visit this government site click on the link to Soakaway design Digest 365 (PDF: 477Kb / 12 pages) – a good night time read. Also worth reading is BS 8301: 1985 Code of practice for Building Drainage
On this note – looking up your local councils ‘Building Control’ guidance on drainage for driveways is always a jolly good thing to do!
Driveway Soakaway- Installation with crate or just fill with rubble?
Many soakaways are constructed by digging a hole during the sub base preparation stage and then infilled with available rubble – slabs, crazy paving, bricks etc.
Whether you are a DIYer or driveway installer it’s wise to line with a geo textile non woven membrane which you can buy here and here. This membrane stops soil from getting in the gaps of the infilled soakaway and capacity is therefore retained.
Alternatively, soakaway crates can be used. These are usually delivered flat pack and are easy to set up. Rubble is then filled within the crate.
Best price for a decent soakaway crate SAVE ££££s Dirt cheap (pardon the pun!)
In summary, soakaways are useful and necessary in some instances. Their construction and cost is minimal. Ensure piping into the soakaway is angled so water flows easily from the channel drainage and also ensure a water test is completed.
Percolation Test Method For Rain Water Disposal – or Water test in English
- Hole 300mm square to depth of 300mm below where pipe will go into the soakaway. For deeper excavations please seek specialist advice.
- Fill the hole with water and come back and check it the next morning.
- Refill the hole and time how long the water takes to seep away from the 75% point to the 25% point (this is 150mm). Time it.
30 mins to drain. Calculate average time in seconds.
30 minutes divided by 150 milimetres = 12,000 s/m
To clarify, 150 (mm) x 12 = 1800, then divide by 60 (mins)
= 12 s/m
Complete this test at least 3 times with two trial holes. Then average figures from tests!
Quicker the better, like most things.
Keep in mind – the permeability of the sub grade is necessary or a soakaway will be (more or less) useless.
If your local authority/district council insists, ensure you have a detailed breakdown of why the installation of a soakaway is not feasible. Don’t be tempted to ignore building regulations because increasingly local authorities can and do use various tools at their disposal, to ensure compliance.
But wait….lets not end it like this…far too heavy.
Driveway Soakaway – Alternative options
Why not opt for a permeable driveway surface such as:
Permeable Block Paving?
Good for the environment, no drainage issues and cost saving in the long run!
Vising the Drivewaywise Directory