Soakaway

[junglejim]

Looking at building the soakaway for new build timberframe. Our engineer has recommended 5.6 x 1.2 x 1.2m .

Comments from builders are that this is excessive but we do have slow draining clayish soil so likely going to work close to this.
I'm ok with digging the trench and the general construction but wondering how far below the surface of the lawn the top of the crates should be. We're also going to install a silt trap so not sure whether this will also affect the depth at all. 

 

Any advice greatly appreciated.

 

Thanks

[saveasteading]

19 minutes ago, junglejim said:

Comments from builders are that this is excessive

General builders generally don't know much theory.  They are simply thinking it is bigger than others they have done.

 

The pipe outfall needs to be at the top of the crates, otherwise your drains sit full of water and don't work properly. 

 

Why do you need a silt trap?

[junglejim]

12 minutes ago, saveasteading said:

General builders generally don't know much theory.  They are simply thinking it is bigger than others they have done.

 

The pipe outfall needs to be at the top of the crates, otherwise your drains sit full of water and don't work properly. 

 

Why do you need a silt trap?

Thanks @saveasteading

 the engineer specified a silt trap presumably to improve efficiency of the soakaway?

 Any thoughts on how much distance from top of crates to surface? Thanks

[saveasteading]

27 minutes ago, junglejim said:

presumably to improve efficiency of the soakaway?

Why will there be any silt? From gutters of a yard? I'll reply on this . Its easy.

28 minutes ago, junglejim said:

 Any thoughts on how much distance from top of crates to surface?

Yes. The Engineer will give you the pipe depth, based on fall rate and distance. 

 

Say it is 10m and the slope is 1:50.  The pipe will have dropped 200mm but from what starting level I can't say. 

About 500mm is likely., so the crates will be in a 1.7m deep hole.

Scary. Maybe ask if you can use a different shape.

[saveasteading]

Silt trap. They are available as preformed gullies, like the ones in roads. From jdp pipes or similar.

Or build a manhole and add a bulkhead across the middle with an overflow at the top. Silt will settle on the entry side, and you must clear it occasionally.

Or a manhole with a sump.( three is depth below the outlet pipe.)

 

Will you lose height? With some yes.

[Alan Ambrose]

How do these traps get emptied and how often typically?

 

BYW re shape, you can almost certainly arrange these in any shape providing you stick to the right invert level for the inlet pipe.

Edited July 18 by Alan Ambrose

[joe90]

Even a bottle gully collects silt, I had two on my drive that I emptied occasional. 
https://www.drainfast.co.uk/products/surface-water/land-drainage-road-gullies/gullies/600-x-300mm-yard-gully-110mm-outlet

[saveasteading]

They only get really dirty when muck is sent into them, eg brushing a yard or a gutter.

I don't think I ever had to clean one out.

 

Except for a slighyly different case.   I have a recessed gully for a downpipe, with a grille over it. It fillls with leaves regularly but is very easy to clear.

Grilles over gullies stop the crud going into the pipe in the first place.

[joe90]

My drive gully used to fill with soil/muck as when I cleaned the car I pressure washed the wheel arches which living in a farming area was full of shit/mud.