Is this how to build a soak away under a concrete slab?

DazlerD · 14 Jan 2025 at 16:02

Hi

As per the picture really...

I'm creating a 4' by 16' concrete slab that I am sloping away from the garage but it's going against the slope of the garden which runs towards the garage. After digging down 5"-6" from my final height (under garage DPC) Im left with an area of stones that is at least a couple of feet deep, and clay. The area doesn't drain when it rains. Does the image show the correct procedure to add a soakaway crate?

Thanks

Nb obv picture not to scale...

SeatIbiza · 14 Jan 2025 at 16:40

This seems a receipe to cause subsidence...

DazlerD · 14 Jan 2025 at 17:16

SeatIbiza said:
This seems a receipe to cause subsidence...

Why would that be? The rain falls across the whole patch at the moment. I'm moving the rain further away from the garage.

Can you explain your reasoning please?

SeatIbiza · 14 Jan 2025 at 17:27

Well it looks like you're wanting to dig under the foundations? You're creating a weak point that will have water flowing into it.

Maybe I've misinterpreted the images, but digging under like this is how you'd underpin a building i.e. fill that void with concrete.

An annotated picture of the actual building would help...

DazlerD · 14 Jan 2025 at 18:18

Hi,

Ah right I think i see what you mean. However, on the left is the garage wall with DPC marked. The concrete slab will be 15' long so the acu drain on the right will be over 14' away from the garage. The soakaway will be under this.

Rob_B · 14 Jan 2025 at 18:25

Not an expert but could you move the soakaway to the side and have a gutter at the end of the drive for it to be connected to?

DazlerD · 14 Jan 2025 at 18:26

Not to Scale lol
In my mind in just moving 14' x 4' of rain and putting it back in the ground further away from the garage. Cant see how it can any worse than before !!

DB_SamX · 14 Jan 2025 at 21:19

A soakaway should be at leat 5m away from a building because it can cause localised soil washout which can lead to subsidence.

The G&W Guide to... Soakaway Offset - Ground & Water

Learn why soakaway offset must be factored into drainage scheme design to avoid potential damage to building foundations and public areas

www.groundandwater.co.uk

DazlerD · 14 Jan 2025 at 21:46

Actually the end blocks are exactly 196 inches away from the garage. which is 16'4".

Anyway, the post was really about the right way to do the soakaway, not where it was or what would happen.

Anybody got any comments on the actual process, thanks.

DB_SamX · 14 Jan 2025 at 22:05

https://www.pavingexpert.com/drain08 inatallation details in here. A membrane to is important to wrap the crate.

Personally i wouldn't build your retaining wall on top of the soakaway, and i would provide a catchpit before the crate or some form of acceess hatch for maintenance.

DazlerD · 14 Jan 2025 at 22:19

Thanks, there is certainly a lot of info there ! Maybe i need to place the soakaway further up the garden. There is certainly less rocks further up.