Existing rainwater Soakaway proof

[Yvonne]

Hello 

Unsure how to approach this, been asked to prove existing rainwater Soakaway is suitable for build, site is an old property which has existing sewerage tank/Soakaway, Council have no issues with the septic tank and it’s Soakaway but need to *prove* existing rain/roof water Soakaway is suitable ? Anyone come across this ? Thank you. 

[kandgmitchell]

The first two matters are:

 

Will the existing soakaway be at least 5m from the new dwelling, and is the roof area of the new dwelling larger than the original. It needs to be yes to the first and no to the second. If is isn't then really you've fallen at the first hurdle.

 

However, usually old soakaways are a rubble filled pit. Over the years both the pit and the drain to it get silted up, sometimes completely. To be honest I'd be considering replacing the old with a new which tend to be based on rigid plastic perforated crates. These hold more water whilst it percolates into the ground and are thus more efficient.

[Kelvin]

Yes they are very particular about rainwater soakaways especially if in area prone to flooding. 
 

First thing is identify where the soakaway is then read up on the rules about them. You then need it sized for your new building. As said above though I’d be looking at replacing anyway. Cost shouldn’t be too great. Better to do it right now than have a problem after the house is up. I’d also get your foul soakaway and septic tank checked too. If you plan on selling the house at some point I’d consider swapping it for a treatment plant. 

[saveasteading]

It would be difficult to prove an existing soakaway without pouring in a day's worth of rain, then seeing if you could do the same next day.

Unlikely to pass anyway after years of silting up. There is no harm in incorporating it if that works.

Agree with Kelvin about the capacity of crates, although they are quite expensive. 

If you have lots of permeable ground then use French drains. If not, then crates are worth looking at, but it all still has to go somewhere.

 

I have a strong suspicion that bcos don't look closely at soakaways. Avoidance of blame perhaps?

[kandgmitchell]

As saveasteading says it'll be almost impossible to prove an existing soakaway still performs in the same way as a new one. If you want to design a soakaway then you need BRE Digest 365, carry out the percolation test and use the results in the formula's provided. The truth is that owners don't want that bother and builders want it even less, so the approach was "dig a hole, fill it with hardcore and run a drain to it". 9 times out of 10 it just worked and no one thought any more of it. These days with flood risk assessments, specialist designs for all parts of the build and everything certificated, that approach doesn't work any more. So the cop out for BC is to say "yes you can use it but prove it's OK". Ultimately you'll be making a new one unless you have a very understanding BCO.

[saveasteading]

On 09/04/2023 at 13:24, Yvonne said:

Unsure how to approach this

What is your ground like?  

Big site ?

Sloping?

Assuming you know where the soakaway is, does it get wet there?

 

Doing percolation tests is easy if you have a spade and water, and the inclination. It is worth getting this right, becayse apart from perhaps causing flooding, you want your rain to disappear. Your site already deals with the rain, you just have to spread out what your new house drainage will concentrate.

[Kelvin]

The guys above know more about it than me. But think about it logically. During the groundworks phase you’ll have the right equipment on-site and the site will be turned over due to all the other works. It’s far better and cheaper to get your drainage done properly now than end up with a problem after the house is up when access is harder and you mess up the ground again. We’ve taken a lot of care over the drainage so far and even though the soakaway isn’t in yet we have a well drained site. Consequently it’s easy to work in as it’s dry even after heavy rain. When the house goes up, with its big roof added to the garage with its equally big roof, all that water needs to go somewhere. I wouldn’t be trusting some unknown soakaway that’s been there for years even if it saved a few thousand. 
 

 

[Yvonne]

Thank you for all your detailed replies. It fell at the first hurdle - so new Soakaway it is. Thank you again. 

[saveasteading]

16 minutes ago, Yvonne said:

new Soakaway it is.

You should still have to prove that the ground can absorb it. Will you be doing a percolation test? It is easily done diy, requring a spade,  a bucket or hose, and a watch.

[ProDave]

46 minutes ago, saveasteading said:

You should still have to prove that the ground can absorb it. Will you be doing a percolation test? It is easily done diy, requring a spade,  a bucket or hose, and a watch.

And a tape measure.

[saveasteading]

2 hours ago, ProDave said:

And a tape measure.

Of course. 

And i should have included water supply. An empty bucket is no use. If the ground is permeable it will need 2 buckets, or the same bucket refilled.