Soakaway - How to connect everything together

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So my structural engineer has sized the required soakaway for my house (inc new extension)

22m long, 0.6m wide, 1.5m deep (starting from 1m down). 20m3 in total

I'd like to dig and fill the soakaway myself as I have an old digger

The soakaway is to take water from 300m2 of roof, as the current surface water soakaway has failed, it is going to be bypassed.

My intention is to dig out the existing 110mm drainage that runs to the existing soakaway and extend that to my all encompassing soakaway.

It will be a long skinny trench soakaway. hole dug, terram installed, then 40-60mm granular filled. The inflow would be at one end of the soakaway as its on a natural slope.

The engineer had asked for 110mm twinwall fully perforated pipe to run the length of the trench
but that's all the detail I have.

I've scratched around the internet and found mixed info, but bre365 seems to suggest that installing an inspection shaft either end of the trench, with a horizontal distribution pipe is the way to go, sketches I have seen show a 225mm inspection shaft all the way to the base of the soakaway connected to the 110/150mm twinwall.

I cant figure out how I could connect up the 110mm drainage pipe to the 225mm inspection shaft and that to the 110mm twinwall.

I've drawn a picture of what I'm thinking and a picture from bre365 digest.

anyone got any advice?
 

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Drill a 110mm hole in a bit of 225mm twinwall.
or
You can get unequal T's for 150 225 so you could use a reducer down from 110 to 150. Or equal T and use a 110 to 225mm.

20m3 seems very large, if you are having to muck away and bring in fill then it would be a lot cheaper to use crates. You only get about 20-30% void with rubble so that's 6m3 of crates.
 
Certainly the voids are important as a buffer but does the surface area not make a difference as well? So in less well draining land you'd need a longer shallower soakaway but in quicker draining land a rounder one would do?
 
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It does yes but you can lay crates in what ever shape you want to, does not need to be a cube it could be a long line of them. But really you won't fill a soak away unless it's during heavy rain, prolonged rain can water log a soakaway.
 
Hi there, thanks for replying

I live on land that's clay down to 2 meters so I need to dig out regardless, that's a non issue, and I have land that I can dump the spoil on, so it's really just time.

Stupidly never thought on just cutting a hole..

Any comment on the design of one entry point at one end of the soakaway with a 90 degree turn at it, does it matter?
 
Any comment on the design of one entry point at one end of the soakaway with a 90 degree turn at it, does it matter?
Long time ago, and never had problems, I did a 4 meter perforated pipe run, laid level, then a 90° turn to a normal (non-perforated pipe) 2 meters long laid with a small fall, then another 90° turn to another 4 meter perforated pipe run, laid level, to another 90° 2 meter pipe (non-perforated) laid with a small fall, to another 90° turn to another 4 meter perforated pipe run level. I think I may have then added a fourth 4 meter perforated pipe run, but it was so long ago I cannot remember.
Obviously, all laid on gravel, covered with gravel, surrounded by a suitable membrane.

Also, in addition, my current soak-away field from the septic tank is also three connected runs, side by side but maybe 2 meters apart, connected via 90° turns.
This run has inspection chambers at beginning of first run, and at end of last run.
 

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