Storm/Surface Water Drains

[Barney12]

Hi All

 

Just finalising my groundworks jobs. 

I was interested in what people (especially those with a passive slab) have done about surface water drainage? 

Did you install all your pipe runs and gully's etc before the slab went down?

Or after the slab?

Or perhaps even only once the house was up?

 

I can see pro's and cons for before and after.

 

Edited April 18, 2017 by Barney12
Typo

[Gone West]

I did all mine after the slab was laid and the frame was up. I couldn't see any advantage to laying surface drainage pipes before the slab as they would probably just be in the way or could be damaged.

[Jeremy Harris]

We made the area of sub-base (which was type 3) bigger by around 300mm all around and fitted a 100mm diameter perforated land drain around two sides (the North and West sides).  This drains to a large storm surge "tank", more from 20 Aquacell crates, buried under the drive.  The same "tank" takes the rainwater run off from both the house and garage roofs and the drive.  We're on hard gault clay, but luckily there is a band of relatively free draining soil in one corner of the tank area.

[Gone West]

2 minutes ago, JSHarris said:

We made the area of sub-base (which was type 3) bigger by around 300mm all around and fitted a 100mm diameter perforated land drain around two sides (the North and West sides).

That's interesting because that's what I wanted to do. The BCO said I couldn't because he wouldn't allow drainage into the soil within 5m of the house. He said the land drain was drainage.

[Jeremy Harris]

Our BCO had never seen a passive slab before, so seemed to pretty much accept that whatever we were doing was OK!  He didn't want to inspect the house related ground works at all; he came on the day we did the foul drain pressure test but didn't do another inspection until the day before we poured the concrete, by which time the land drains were buried.

[Ian]

2 hours ago, PeterStarck said:

That's interesting because that's what I wanted to do. The BCO said I couldn't because he wouldn't allow drainage into the soil within 5m of the house. He said the land drain was drainage.

The Building Regs Approved Document says no surface water soakaways within 5m of the building. The concern is that the soakaway can affect the house foundations. It's something I've seen myself several times when rwp connections have failed (split) underground next to a house. The water can wash away the fines in hardcore leading to subsidence.

 

Edit: The land drain that @JSHarris will have installed isn't classed a soakaway as I'm guessing that the pipes he's talking about are designed to remove water from the ground rather than to act as a soakaway (the perforations in his pipe will have been at the top of the pipe and not the bottom)

Edited April 18, 2017 by Ian

[Jeremy Harris]

Yes, in our case there's no drainage, as in allowing water to collect under the slab, in fact the whole idea is to prevent this, by allowing water to drain away from the under-slab area.  If you don't do this, then on soil like ours (impermeable hard gault) the slab ends up sitting in a swimming pool.  The idea was for the free-draining layer under the passive slab to be given somewhere to drain to, to prevent the build up of water.

 

 

Edited April 18, 2017 by JSHarris

[Barney12]

This is broadly what I'm planning. Note that we have a level change in our slab and surrounding ground, the shaded yellow area is 600mm higher.

I've had to pull the soil pipes further under the slab than I would have liked but this is due to the fact we have a 'cat slide' roof at the rear that drops to a single story. This makes the runs to the first floor a little more tricky (difficult to explain in 2d!).

I've also added provision for brick slot drains in front of all the the low threshold doors.

 

Orange: Foul

Blue: Surface

Black: Ducts for power, water etc.

Blue: Insulated flow and return to ASHP.

 

[Ian]

@Barney12

Just one small point regarding your foul run:

If you are intending using pre-formed plastic manholes then you would be better always having the main run of pipe connecting on the axis of the manhole and not on a side branch pipe connection as this sketch. The reason is that the pre-formed benching of the main axis run of the plastic manholes are set slightly lower than the branch incomers. If you don't connect to the main axis, waste will sit trapped in the dead end of the main branch run.

 

[Ian]

@Barney12

 

Also, I'm not sure what these 2 manholes are for. If you're not connecting any side branches into them you'd be better off without them with the added bonus of saving a bit of money.

 

[Barney12]

15 minutes ago, Ian said:

@Barney12

Just one small point regarding your foul run:

If you are intending using pre-formed plastic manholes then you would be better always having the main run of pipe connecting on the axis of the manhole and not on a side branch pipe connection as this sketch. The reason is that the pre-formed benching of the main axis run of the plastic manholes are set slightly lower than the branch incomers. If you don't connect to the main axis, waste will sit trapped in the dead end of the main branch run.

 

 

Thanks Ian. Really useful. I'll change the drawing.

[Barney12]

11 minutes ago, Ian said:

@Barney12

 

Also, I'm not sure what these 2 manholes are for. If you're not connecting any side branches into them you'd be better off without them with the added bonus of saving a bit of money.

 

 

This is at the 600mm level change.

I assumed I need a drop shaft connection?

[Ian]

2 minutes ago, Barney12 said:

 

This is at the 600mm level change.

I assumed I need a drop shaft connection?

apologies - I see what you mean.

 

Edit: Have you compared the cost of your 2 back-drop manholes compared to just digging your drain runs a bit deeper?

Edited April 18, 2017 by Ian

[Barney12]

Just now, Ian said:

apologies - I see what you mean.

 

BTW: I wish there was another way! I hate manhole covers.

I was trying to see if I could create a drop shaft but without a chamber but i've not found anything online that refers to it being possible?

Theoretically I could get a rodding point into the face of the step?

[Ian]

5 minutes ago, Barney12 said:

 

BTW: I wish there was another way! I hate manhole covers.

I was trying to see if I could create a drop shaft but without a chamber but i've not found anything online that refers to it being possible?

Theoretically I could get a rodding point into the face of the step?

Not sure why you need a drop shaft for the surface water sewer. You can go for a steeper gradient for the SW sewer run as you are not bound by the same gradient restrictions as you need for the FW. You'll still need a manhole where you have a change in gradient but it doesn't need to be a back-drop.

[Barney12]

7 minutes ago, Ian said:

Not sure why you need a drop shaft for the surface water sewer. You can go for a steeper gradient for the SW sewer run as you are not bound by the same gradient restrictions as you need for the FW. You'll still need a manhole where you have a change in gradient but it doesn't need to be a back-drop.

 

I did consider that but with a 600m drop the gradient would mean I'm too low at the soakaway end.

I assume its not permissible to add a swept bend which would still be roddable? Something like this:

 

 

 

[PeterW]

Is that last one foul or storm...? If so, storm it's fine, foul it's not..!

 

600mm is nothing when you are digging out, a 450mm manhole can be used up to 1200mm with a standard cover and 3m with a restricted cover so you could just drop all the drains accordingly. 

[Barney12]

Just now, PeterW said:

Is that last one foul or storm...? If so, storm it's fine, foul it's not..!

 

 

Storm.

[PeterW]

So storm can drop at a steeper angle - no real need for rodding points if you filter the crud and leaves out before it gets to the pipes. 

[Barney12]

1 minute ago, PeterW said:

600mm is nothing when you are digging out

 

Alas it is in our ground. it goes like this: Place bucket in ground. Watch digger go up in the air!

Well unless its the 20T machine in which case its: Place bucket in ground. Create hugely larger trench as the rocks are pulled out.

[PeterW]

Ahhh ok ..! Any chance you can go deeper below the slab though ..?

[Barney12]

49 minutes ago, PeterW said:

Ahhh ok ..! Any chance you can go deeper below the slab though ..?

 

I was trying to avoid runs under the slab. Probably paranoia!

[Onoff]

Reminds me of whoever it was on eBuild who basically had to trench through ROCK!