Rainwater Soakaway Design???

[iSelfBuild]

I'm a little confused on this one. I have done a fair few percolation tests in the past in order to design drainage fields for treatment plants.

 

Area = Population x Percolation Value (Time from 75% down to 25%) X 0.2 (secondary treatment) / A = P x Vp x 0.2

 

Calculating the VP is a dead easy process.

 

I have attached the results of one of my rainwater soakaways on a site, I never did the calculations for this or the soil infiltration test for them.

 

How is the soil infiltration (f) derived/performed?

 

Once I have the figure how do I replicate this sheet I have too?

 

 

[the_r_sole]

There's a BRE guide for rainwater soakaways it's all slightly different inputs and the output is in m3 too rather than m2 for a drainage field...

Scottish building regs have a simplified calc too that you can use on a single dwelling...

[iSelfBuild]

Thanks is this the simplified formula? I'm assuming I calculate VP the same way as in drainage fields?

 

The soakaways may be sized using the following simplified formulae derived from BRE Digest 365:

(A x 0.0145) – (a x f x 900) = S

Where - A is the area to be drained in m².

• a - is the internal surface area of the soakaway to 50% effective depth, excluding the base in m². This has to be assumed for initial calculation purposes.

• f - is the soil infiltration rate, in m/s, determined in accordance with clause 3.9.1. This calculation produces Vp in secs/mm [conversion = (1/Vp) /1000].

• S - is the required storage in m³.

 

I know the VP was 29.7 for the drainage field, so I do a test like that and convert it into a f value? = (1/Vp) /1000].

 

a is the internal surface area of the soak away? I'm a bit confused by it being assumed?

[the_r_sole]

Yeah, it seems you can pretty much make it up ?

[Temp]

2 hours ago, iSelfBuild said:

How is the soil infiltration (f) derived/performed?

 

It's measured by digging a hole and filling with water. As I recal the BRE guide has details of the procedure.

[iSelfBuild]

9 hours ago, Temp said:

 

It's measured by digging a hole and filling with water. As I recal the BRE guide has details of the procedure.

 

Yes there is a long winded way for f value, then there is thankfully the method of doing a VP value and converting it.

 

I just don't get the whole a = assumed bit as that completely changes the final outcome so is open to manipulation?

 

 

[the_r_sole]

I set this up as an excel thing before but I'm stuck to find it - the a actually doesn't change the end result hugely (unless you change it by a huge amount!) But put something realistic in, then you can tweak it to what you think - my guess is the simplified calc has a lot of room for manoeuvre!

[iSelfBuild]

6 hours ago, the_r_sole said:

I set this up as an excel thing before but I'm stuck to find it - the a actually doesn't change the end result hugely (unless you change it by a huge amount!) But put something realistic in, then you can tweak it to what you think - my guess is the simplified calc has a lot of room for manoeuvre!

Thanks. So a - is the internal surface area of the soakaway to 50% effective depth, excluding the base in m².

 

So say 1m x 3m is... 0.5 x 1+1+3+3. So 4m2? Assuming the top isn't considered as well as the base of the pit.

[Timedout]

Try using CIRIA R156.

[iSelfBuild]

On 24/07/2020 at 14:54, iSelfBuild said:

Thanks. So a - is the internal surface area of the soakaway to 50% effective depth, excluding the base in m².

 

So say 1m x 3m is... 0.5 x 1+1+3+3. So 4m2? Assuming the top isn't considered as well as the base of the pit.

 

 

Can anyone confirm I am working out the area correctly? 1m x 3m - 50% depth would be 0.5m, the base is excluded so it is 0.5 x 1+1+3+3 = 4m2

 

f = (1/VP) / 1000     or    0.00003367 = (1/29.7) / 1000

 

For 100m2 area to be drained:

 

The soakaways may be sized using the following simplified formulae derived from BRE Digest 365:

(A x 0.0145) – (a x f x 900) = S

 

(100m2 x 0.0145) - (4 x 0.00003367 x 900) = 1.328m3

 

 

So how do I then take this 1.328m3 area and size the soak away considering it will be filled with clean rock.

•  

[Temp]

6 hours ago, iSelfBuild said:

Can anyone confirm I am working out the area correctly? 1m x 3m - 50% depth would be 0.5m, the base is excluded so it is 0.5 x 1+1+3+3 = 4m2

 

Correct

 

[iSelfBuild]

52 minutes ago, Temp said:

 

Correct

 

Any ideas how I take the m3 area and convert that into a real life sized drainage pit?

 

I can't get my head round why the one I posted initially was specified at 1m x 10m

 

 

[Temp]

As I recall there are two things that have to be right:

 

* The volume S is important because it has to be big enough to store the storm surge.

 

* The area a is important to ensure it drains away before the next one.

 

I think the area a appears in the equation for S because water can flow out while it's flowing in and taking that into account allows S to be smaller than it otherwise would be. So I believe it's safe to ignore the -a*f*900 term if necessary but that would make the volume bigger.

 

If you want to include it I think you should calculate the required a first then put that in the equation for S.

 

Then when you have both numbers you can work out a size of soakaway that meets both.

 

I should add I've not looked at the BRE doc for ages but that's how I think it works.

 

 

[iSelfBuild]

On 31/07/2020 at 13:41, Temp said:

As I recall there are two things that have to be right:

 

* The volume S is important because it has to be big enough to store the storm surge.

 

* The area a is important to ensure it drains away before the next one.

 

I think the area a appears in the equation for S because water can flow out while it's flowing in and taking that into account allows S to be smaller than it otherwise would be. So I believe it's safe to ignore the -a*f*900 term if necessary but that would make the volume bigger.

 

If you want to include it I think you should calculate the required a first then put that in the equation for S.

 

Then when you have both numbers you can work out a size of soakaway that meets both.

 

I should add I've not looked at the BRE doc for ages but that's how I think it works.

 

 

 

Thank you, I'm still a bit confused. This calculation seems to suggest 1.32m3 is big enough but it doesn't say if this is for crate systems where there is a big void or if it's for granular rock infilled soakaways. 

[Ferdinand]

I haven't really followed this thread.

 

However I would expect the number to be the volume of space for water, which would be minus the volume of solid gravel.

 

Unless it has voids in the gravel, in which case the thread needs to continue ?.

[iSelfBuild]

32 minutes ago, Ferdinand said:

I haven't really followed this thread.

 

However I would expect the number to be the volume of space for water, which would be minus the volume of solid gravel.

 

Unless it has voids in the gravel, in which case the thread needs to continue ?.

 

That makes 100% logic ?

 

But how do you calculate the volume or should I say displacement of gravel?

 

The chap at the top (screen print) had Aggregate void at 30%...

 

So 1.32 m3 x 3.33333* = 4.39 m3

 

I would say that is a very suitable sized soakaway???

[Ferdinand]

2 hours ago, iSelfBuild said:

But how do you calculate the volume or should I say displacement of gravel?

 

The chap at the top (screen print) had Aggregate void at 30%...

 

If you thought about this for 5 minutes with a coffee you would get it - it is an experiment we all did at school: Find the volume of an irregular solid by seeing how much it raises the level in a container of water.

 

In this case we only need an answer to about +/- quite a lot, so personally I would guestimate a third for any reasonably sized sort of gravel, but obvs it takes up more of the volume if it smaller. But the tolerance means we don't need a big sample. If i really wanted to know I'd do it a la probable Jeremy:

 

1 - Get a big measuring cylinder or a bucket or anything of which you know the volume. Find out where say the 5l line is (put in 5l of water and make a mark or measure the depth).

2 - Empty water and fill up to your mark or depth with gravel. Do the hokey-cokey and shake it all about to max the amount of gravel.

3 - Refill with water and see how much it takes to the same line. Steal a measuring jug from the boss.

4 - The difference as a fraction or % allows you to calculate the allowance you need to make.

 

This method even allows for any voids inside your pieces of gravel.

 

Personally I would add 25% to the whole thing afterwards just to allow for uneven rain to give me a buffer.

 

Ferdinand

 

Edited August 2, 2020 by Ferdinand