DIY raised percolation area

RainInSummer

Hi all, not even sure if percolation area is the correct terminology, but I have a bio-crete unit that pumps treated waste up to a raised area in the back of the garden. I'd like to extend this area, but I'm not sure of the build up of the bed.

Is it stone on the existing ground then soil or just soil dumped in a mound, or alternating layers?

Extend the raised area, extend the existing pipes into the new area and cover them up is the vague plan.

Bit of context: The engineer I got to look over the house highlighted that the raised area was undersized but unlikely to cause issues.

6 years on and it's been fine, only had to get the tank emptied once with the floods around Christmas. No big deal. Roads were washed away nearby and some people were cut off from their homes or flooded out of it. Pretty exceptional weather.

The raised bed work would be one part of an overall drainage project for the site but I'm thinking to prioritise this piece first as you can't always be guaranteed to get a lad to empty the tank at short notice or during holidays if we do flood again.

Cheers!

Wildly Boaring

Ok so you should have a site assessment and design done. And possibly need planning permission to extend.

But yes it's usually stone.

The stone needs to be permeable but not overly permeabke and definitely not clean drainage stone.

Something like 6F2 or capping stone.

So generally the topsoil is removed.

Stone from subsoil up.

A layer of drainage stone with the pipes set in it on top. This aims for an even load distribution.

Then topsoil to seal.

The original system might be sand either but unlikely.

The sealing is the hardest and don't skimp on it especially near the bottom on the sides.

Raised beds are usually due to poor percolation. The effluent will want to escape out the sides and there's more pressure lower down the sides. So the topsoil would me a metre or wider down there.

Do not extend toward a water course or a close boundary or your own house. There are guidelines.

https://www.epa.ie/environment-and-you/waste-water/septic-tanks/#d.en.86141

RainInSummer

Cheers for taking the time for the detailed answer.

I'm terms of not extending toward a boundary I'm afraid that ship has sailed. It's already along the rear ditch.

House was built in 2006, perhaps there were different regs then.

What kind of engineer would I contact in terms of getting a site assessment done? Someone who specialises in drainage/waste handling?

Wildly Boaring

Below are the distances from the last COP.

So yep boundary is a bad start.

If it's working I'd leave well enough alone.

Any proper site assessment is going to involve telling you to move it.

If it were mine and I got on with my neighbour I'd have a chat, tell em it works fine but just gonna make it a bit bigger back into my own land. Then do a DIY extension back towards my own side. But itis all a risk