Soakaway query

[KTB]

We are at the stage of completing our site rainwater drainage with the construction of a rather large soakaway (6.2 X 8.7 X 1m deep, with 450mm soil above). We have been spec'd "40mm  single sized gravel" instead of crates by the engineer, but he hasn't said what depth of gravel is needed. All the drawing says is "40mm single sized gravel fill to soakaway".

 

A quick google says 150mm deep, some other sites say 300mm deep.

 

Can anybody point me to the regs, so I can cover myself with building control? 

[Ben1984]

If your soakaway needs to be 1m deep and you aren't using crates then the 40mm gravel needs to be 1m deep... if you only filled the hole with 150mm gravel your soakaway would be 6.2 x 8.7 x 0.15m. 

My understanding of your design would be dig a hole measuing 6.2m x 8.7m and 1.45m in depth - Fill the first 1m of the hole with the 40mm gravel and the cover it in 0.45m of soil. You would need to use some kind of membrane/fabric around the gravel.

[KTB]

Thanks, that's exactly what I was afraid of. That would need 90 tonnes of gravel, which I've been quoted at nearly 3 grand, then labour, pipes and membrane.

 

Scary stuff for a hole in the ground.

[Redbeard]

Have not looked at crate prices recently but it will be worth a comparison, or do you have some vehicular loading issue?

[JohnMo]

1 hour ago, KTB said:

90 tonnes of gravel, which I've been quoted at nearly 3 grand

Is that by the lorry load or by the 1 tonne sack. Direct from quarry is about half the cost of a building merchant.

[KTB]

17 minutes ago, JohnMo said:

Is that by the lorry load or by the 1 tonne sack. Direct from quarry is about half the cost of a building merchant.

Local quarry, 21.70 plus VAT a loose tonne, plus 100 + Vat per 30 tonne load delivery for 4 miles - Joke 

[KTB]

28 minutes ago, Redbeard said:

Have not looked at crate prices recently but it will be worth a comparison, or do you have some vehicular loading issue?

Would need new engineer work and a building warrant resubmission, apparently. I personally think the submitted design is vastly over engineered for a 190 m sq roof.

[JohnMo]

That sounds huge for a rain water soakaway, thought the size was for a treatment plant soakaway.

 

Our roof area is about 300m²

 

This what our drawings show, with the calculation 

 

[KTB]

Here's the engineering calculations. I might try resubmitting them tomorrow to see if this is over engineered.

 

 

[Ben1984]

22 hours ago, KTB said:

Here's the engineering calculations. I might try resubmitting them tomorrow to see if this is over engineered.

 

 

Unless I'm missing something those calculations state that the required soakway volume is 14.85m2 - your proposed dimensions: 6.2 x 8.7 x 1.0 give a total volume of ~54m2

[KTB]

Here's the warrant drawing. I think that it is far too big. But the calculations above say 9.9 X 5 X 1m, pretty much the same. Adapted to be 5m from all boundaries, apparently hence 8.7 X 6.2 X 1m.

[Stratman]

In anticipation of getting our own soakaway sized, I've been trying to put together a spreadsheet based on the methodology set out in BRE Digest 365 (2016), so I can do it myself.  It's a work in progress and I've gone down the rabbit hole with it over the last few days.  I've noticed that the soakaway construction type has a huge influence on the size of the hole you need.  Your calculations note a 'percentage free volume' of only 30%, i.e. 70% of your hole is taken up by your gravel fill. Plastic soakaway crate are mostly void - about 95% free volume, so less of your hole is taken up by the crate material itself so you can get away with a smaller hole.  If your 15m3 hole is costing £3k in gravel, a smaller (say 10m3?) hole might cost £2k in crates (e.g. 10 x 5 crates/m3 = 10 x £40).  Plus there's the difference in excavation cost and muck away.

[Stratman]

Correction: your current design is for a 50m3 hole filled with gravel to obtain 14.85m3 water storage. By my rough calculation you may only need a 15.5m3 hole to get the same storage from crates. So you might spend £3k on crates but much less excavation.