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Posted: Wed Jan 04, 2006 11:14 am
by 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!
Posted: Thu Jan 05, 2006 4:46 pm
by Tony McC
Rule number one - read the flaming website!!!
The Soakways page might be a good place to start.
'kin students! Pah! 
Posted: Fri Jan 06, 2006 4:18 pm
by 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/
Posted: Wed Jan 11, 2006 11:01 am
by 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?
Posted: Wed Jan 11, 2006 11:36 am
by 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.
Posted: Wed Jan 11, 2006 12:35 pm
by 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.