Soakaways - Size claculation

[Jodie-Sue]

Hi everyone

I am currently studying a National Certificate in Civil Engineering (slightly bizarre as I am an office based QS, but hey ho!).

Could someone please explain how you would calculate the size of a soakaway - having never dealt with drainage I am a bit lost!

[Tony McC]

Rule number one - read the flaming website!!!

The Soakways page might be a good place to start.


'kin students! Pah!

[TheRobster]

BRE (British Research Establishment) digest 365 covers the basics of soakaway design and operation:

http://www.brebookshop.com/details.jsp?id=844

If you need further information you could check out CIRIA's (Construction Industry Research and Information Association) report: Infiltration Drainage: Manual of Good Practice (Report R156).

http://www.ciria.org/acatalog/R156.html

Alternatively there are any number of computer programs that can perform the calculations.

http://www.webcomsystems.co.uk/soakaway_design.htm
http://business.virgin.net/malcom.wearing/crmmainframej.htm
http://www.microdrainage.co.uk/
http://www.wallingfordsoftware.com/products/infoworks/

[TheRobster]

Assuming the trial pit has proved suitable, the next task is to calculate the size of soakaway that will be required. There are a few formulae used to calculate the required storage capacity for a soakaway, but the one we normally use is...

Vol = A × (rainfall rate/3000)

I'm not sure how you derived this....is it empirical? I.e. based on experience of soakaway operation in general?

If the design storm is assumed to be 50mm/hr for a duration of 1 hour and assuming no infiltration during the storm then the volume to store this would be given by:

Vol = A x (rainfall rate/1000)

So I assume the equation divides by 3000 rather than 1000 to take into account the fact that infiltration is occuring during the storm?

[Tony McC]

It's an empirical equation I first learned back in the 1980s. It's almost 7 years since I wrote that page and I would have to check the archives to find out just where I checked and verified what was in my little black book.

[TheRobster]

Ah ok. Well no need to look it up on my account. I just wondered if it was a rule of thumb equation based on observation.....which is what it seems to be.