Soakaway - How big do I have to build it?

[steadyonbob]

Hi,
I need to build a soakaway for our extension/rebuild but I am unsure how big it needs to be.
Our roof area at the front is approximately 41.5 sq mtr.

I can't seem to quite understand the calculation. Do I just divide by 40 which would mean the soakaway needed is just over 1 cubic metre below the inlet pipe? Also can the hole be filled with rubble? This would reduce the volume etc...

I will buy some of those plastic crates (aquacell) and fill the hole with those to maintain the volume if that is what is necessary. But they are quite expensive

Can anyone set me straight on this as it seems to me that the days of a little hole filled with rubble are gone, if you need to meet the building specs.

Please advise. Thanks,
Bobby


P.S. - (I will need to do something similar for the back of the house, in fact a bit bigger as the rear roof area is about 55 sq mtr).

 

[jono_h]

It's probably worth talking to Building Control at the council to see if they have a 'standard' design.

Around here they look for a metre cubed hole, 5m from the house, filled with clean hardcore.

You're right in that filling it with hardcore does reduce the effective volume, but it's just a case of providing a space forctge water to percolate away.

 

[Static]

Design method for soakaways are based on area drained and permability of the soil.. if i remember correctly its all about balancing the size of the hole to allow a heavy rainfall to half drain the soakaway within 24hrs..

What ground do you have to drain into? If its gravel or chalk then chances are a small soakaway will be fine.. if its thick clay then you may need to rethink things.. if unsure dig a big hole... and fill it with water.. and watch how quick it drains..

 

[garyo]

I think it might depend on your local BCO. My extension roof is 10m^2 and the BCO has requested a 1.2 x 1.2 x 1.2 cube filled with rubble.

I'll be speaking to him about it this morning hopefully as failed to get any specific requirements re: capping it off with concrete/slabs to prevent soil ingress, and whether it needs a lining of some description.

Edited to say - I'm on clay.

Gary

 

[jeds]

For roof areas less than 100m³ it's exactly as you said. 41m²/40 = 1.02m³ - i.e. a fraction over one cubic metre. No percolation test is required. The nonsense part is that this assumes the ground drains reasonably well - but how anybody knows that without a percolation test is anybodys guess? If there's any doubt, more is better than less. You don't want a water logged garden.

Dig a pit 1mx1m and 1.25m deep to allow for capping off. Fill with crushed stone or rubble (no fines) and put a sheet of poly over the top (250mm from the surface) finish off with whatever the topping will be. If possible include some means of inspection and cleaning - maybe a rodding eye.

 

[steadyonbob]

Thanks for the replies I must admit to being a bit confused.

What surprises me about these rubble filled soakaways, is that they don't actually meet the volume requirements from what I can see.

It seems that the true soakaway volume of a rubble filled hole is only about 30% of the size of the actual hole.

I can't understand how this meets the regs as most average soakaway holes seem to be just over 1 metre cubed.

If they are filled with rubble, why don't you have to build a hole with a volume of 3 cubic metres so that once it is filled with rubble or stone, it comes back to a useable volume of about 1 cubic metre.

I am trying to avoid having to shell out for 6 or 7 Aquacell or Polystorm plastic crates, which will run to £300 give or take.

I will give Building Control another ring, but I need to know the facts a bit more. They will probably prefer the plastic crates method ;-(

Bobby

 

[Static]

Soakways are mostly designed on a 30% void ratio, and single sized stone wrapped in some teram or permiable fabric.. so a basic hardcore filled soakaway should be fine.. in the end its not a massive area your draining..

Aquacell / Stormcell products are damned expensive and normally used in larger situations for storm water storage.. i dont think ive ever seen them used as private soakaways... maybe if you were draining 30 odd houses..

 

[jeds]

Soakways are mostly designed on a 30% void ratio.

Exactly. You don't have to worry about the actual volume of the soakaway. it has allready been allowed for.

 

[theoldun]

Bob Afraid, that in all good faith you are being given some slightly misleading advice here. Soakaway design is very clearly laid out in Building Reg H2.26 and certain BC areas have taken it to heart, as we have found out to our expense.
The secret is to design your soakaway for catchment areas of 25m2 or less. This will then allow you to design for a 10mm rainfall in 5 minutes. Any area over 25M2 comes under BRE Diges 365, and you need to be a mathematician to work this formula out.
The standard B.Regs formula, at first looks difficult, but in fact is very easy.
If it will help you, I will dig out our last design, and some notes and post them in couple days time. Will also tell you how some people slightly cheat on this.
old un.

 

[theoldun]

Bob Just looked at this post again and saw your original post was 16 April 2010.
I got confused, as I looked at the last post by Jeds of to-days date, and carried on from there.
If you are sorted will not bother to post design. old un

 

[freddiemercurystwin]

H2.26

3.26

 

[Static]

The secret is to design your soakaway for catchment areas of 25m2 or less. This will then allow you to design for a 10mm rainfall in 5 minutes. Any area over 25M2 comes under BRE Diges 365, and you need to be a mathematician to work this formula out.

The main complication with the BRE365 design is getting the infiltration correct and with the right units..

 

[JayCeeStorm]

Why oh why would anybody want to dig out a big hole and then fill it up with CGA (Coarse Granular Aggregate) - some kind of zen exercise?

The 30% figure bandied about refers to the voids in the granular surround to a concrete or brick lined soakaway.

If you want a little tool that'll design it for you go to:

http://www.uksuds.com/

look about halfway down on the left under TOOLS and click on Infiltration Design. You can download spreadsheets to design lined and crated soakaways as well as trench soakaways all as set out in BRE 365.

If you need to do a percolation test then:

1. Dig a hole 300mm square to a depth 300mm below the proposed invert level of the incoming pipe. Where deep drains are necessary, the hole should conform to this shape at the bottom, but may be enlarged above the 300mm level to enable safe excavation to be carried out.

2. Fill the 300mm square section of the hole to a depth of at least 300mm with water and allow it to seep away overnight.

3. Next day, refill the 300mm square section of the hole with water to a depth of at least 300mm and observe the time, in seconds, for the water to seep away from 75% full to 25% full level (i.e. a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp) required for the water to drop 1mm. For example 60mins divided by 150mm = 24 seconds (Vp = 24 seconds) this will give you an answer in mm/s divide by 1000 for m/s

4. The test should preferably be carried out at least three times with at least two trial holes. The average figure from the tests should be taken. The test should not be carried out during abnormal weather conditions such as heavy rain, severe frost or drought.

Sorry that's all a bit wordy but hope it helps (and keeps you dry) - oh, and in 40 years as a qualified drainage engineer I've never seen a soakaway work in clay

 

[theoldun]

Why oh why would anybody want to dig out a big hole and then fill it up with CGA (Coarse Granular Aggregate) - some kind of zen exercise?

I do not really know why this post has been brought up again, but to answer your question the reason we dig a big hole and fill it back up again is that it is the easiest and most cost effective method that we know to design and install an infiltration drainage system to comply with Building Regulations for domestic premises with a catchment area of 25ms.

3. Next day, refill the 300mm square section of the hole with water to a depth of at least 300mm and observe the time, in seconds, for the water to seep away from 75% full to 25% full level (i.e. a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp) required for the water to drop 1mm. For example 60mins divided by 150mm = 24 seconds (Vp = 24 seconds) this will give you an answer in mm/s divide by 1000 for m/s

With regard to your percolation test, we assume this is a copy taken from a Local Authority web site. As you quite clearly state the time for the water to fall 150mm is taken in seconds, however in your example you use 60minutes divided by 150 to give a (Vp) of 24 seconds.
This not correct as 60 minutes divided by 150 would give you a (Vp) of 0.40.
The correct calculation is 60 minutes multiplied by 60 seconds=3600 sconds, divided by 150 gives a (Vp) of 24 seconds
oldun

 

[gazza176186]

Hi,

I am having to dig a soakaway for a patio that is 4m by 3m (12m2)

I have dug a hole (so far) 500mm deep, 400mm long and 300mm wide.

Is this going to be big enough? I have looked at the math sum for volume using 50mm average rainfall a hr (found online) v=12(50/3000) to get 0.200m3. But his does not tell me the size of the hole I need.

Thanks for advice/help

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