Foul water drain plans.

[Russdl]

I'm after your collective wisdom again!

 

On the attached image the architects foul water plan in dotted lines (I hope the quality is good enough to see it) goes clockwise around the outside of the proposed property to the existing sewage connection.

 

A developer friend of mine suggested the route marked in red from the two stacks, the opposite way round to the architect.

 

I'd considered the route in blue on the diagram (the red and blue lines converge where the stacks will be)

 

It would be under a passive slab foundation and the width of the slab is 7.5m.

 

The Invert Level of the existing drain is 60.66m OSBM and the finished floor level needs to be 61.88 OSBM, so, takeaway about .450m for the floor and slab leaves 61.43 OSBM, or .77m fall between the underside of the slab and the existing Invert Level. On my blue lines the longest distance between stack and existing drain is about 25m so (if I'm right) there is sufficient fall. 

 

I hope I've used all the correct terminology...

 

Are there any reasons why one proposal trumps the other?

 

 

 

 

[Bitpipe]

No expert, but the established practice tends to be minimising long foul runs under your slab to enable maintenance, rodding etc. Both your architect and builder seem to be following this convention.

 

I have a similar arrangement, basically a square footprint with fouls front and back, main sewer to front. The rear fouls exit the house and follow the house footprint with an IC at each turn.

 

Felt like a bit of overkill but there was enough fall to make it work and wasn't that expensive in the grand scheme (also worked for us as we had a caravan at the rear of the site during the build that used that foul run also).

 

I've had a few blockages since and have been glad that rodding the whole run is straight forward.

 

N.B. best practice is also to have ICs every 8-9m as the rodding kits are typically 10m long. If you use Clarke style covers then they blend in very well with paving or other hard surfacing.

 

You seem to have plenty of fall (1:40-1:80 is the recommended range) so no need to minimise the length of the run - what's your motivation?

[Russdl]

5 minutes ago, Bitpipe said:

what's your motivation?

 

Good question, well phrased...

 

I think it was to get everything pointing in the right direction, i.e. heading towards the existing drain and keep it simple but I take your points re minimising long foul runs under the slab.

 

Thanks for the input.

[MikeSharp01]

You will need intermediate chambers as you can only go 11m (IIRC) between chambers so maybe thats what the architect was thinking as these chambers will have covers and perhaps you won't want them in some places, eg the middle of a lawn!

 

[Russdl]

@MikeSharp01 Another good point that I should probably have been aware of, thanks!

[Bitpipe]

Having just completed hard landscaping, I've had to deal with a few poorly located chambers - two were partially covered by my path to the driveway and another was too close to the house.

 

ICs can more or less go anywhere so do some thinking on where hard and soft surfaces will be, boundaries (fences, walls etc)  and make sure your chambers are well clear of any interface.

 

Can't really move them once they're in!

 

Agree with Mike that IC covers in lawn are tricky - you can try and keep them low so a mower will go over them (paint green ) - turf tends not to do well on top of them. 

 

[Russdl]

2 minutes ago, Bitpipe said:

so do some thinking on where hard and soft surfaces will be

 

I'd considered that, and the first IC would be under decking/patio and disguised as such. I hadn't considered @MikeSharp01's comment about the max 11m run, which would indeed place an IC chamber in the lawn or a flower bed so your point is taken, thanks.

 

 

[Russdl]

So far it seems like the architects plans or my developer friends plans are winning. The developers version seems much simpler than the architects, any ideas why the architect would have chosen the long way round save for some comments already posted?

[Thedreamer]

I was actually standing outside with our digger driver (and a million midges) deciding on the placing of inspection chambers tonight. 

 

The red route has an 90 degree angle so your definitely need a inspection chamber there and it would appear to be slightly below the vehicle access which is handy.

 

I would however use the blue route.

[Russdl]

7 hours ago, Thedreamer said:

The red route has an 90 degree angle so your definitely need a inspection chambe

 

Of course! And the architects route would need at least 3 IC's for the same reason, plus 1 more for where the two drains meet and yet 1 more for the max 11m run (if that is the limit)

 

So assuming @MikeSharp01's correct r the max 11m (I'll get on to checking that right now) the IC count would be:

 

5 for the architects route

3 for the developers red route

2 for my blue route 

 

If the 11m rule is correct, all plans will have an IC some point on the lawn between the hose and remaining drain.

 

So on the IC count the blue route would win. Any other views on running foul drains that far under a passive type slab construction?

 

~~~~~~~

 

According to Approved Document H, if I've interpreted Table 13 correctly then the stack to the first IC can be up to 22 meters (or bananas, it actually doesn't mention meters anywhere on that table) and then from IC to IC/Manhole maximum 45.

 

So that would reduce at least 1 IC from all the options.

 

Doc H does say "The layout of the drainage system should be kept simple. Changes of direction ... should be minimised" The architects plan does the opposite.

 

Ant thing else I'm missing?

Edited June 20, 2018 by Russdl
Checked Approved Document H

[Bitpipe]

Thought exercise:

 

On my system, there is an IC directly where the internal run meets the external run for each stack (I have four - utility, kitchen, front stack, rear stack). Then ICs where runs meet or to break up long linear runs.

 

If I get a blockage in the external run (and I've had a few, obviously some debris found its way in during construction) it's easy to rod / powerwash the drains from IC to IC and clean everything out from the house to the main sewer. 

 

In your blue run, how would you do this? If you have the first IC on the terrace , you'd be able to flush from there to sewer but how would you clean the section behind it (without pushing blockage further back up the pipe).

 

Not sure if its a good idea or not, but you could extend the blue runs further back to a rodding eye (one next to bin store, one next to parking) and solve for this issue. We have these for rain water runs - no idea if they are acceptable for foul runs.

 

Its tempting to save money on ICs, pipe, excavation etc at this stage but think carefully on any unintended consequences - you will not be able to remedy later.

[epsilonGreedy]

12 hours ago, Russdl said:

The Invert Level of the existing drain is 60.66m OSBM and the finished floor level needs to be 61.88 OSBM, so, takeaway about .450m for the floor and slab leaves 61.43 OSBM, or .77m fall between the underside of the slab and the existing Invert Level. On my blue lines the longest distance between stack and existing drain is about 25m so (if I'm right) there is sufficient fall. 

 

 

I have been following your thread because I have to make the same decision soon and face a similar situation with different people proposing an external foul drain route v. direct through foundations (suspended beam & block in my case). I also have to be careful to maintain a minimum viable gradient.

 

I am posting to suggest a small tweak to the through foundation 450mm drop before you calculate your remaining gradient. Page 463 of the Building Services Handbook has a diagram for a collar boss discharge stack which confirms a minimum drop of 450mm from FFL to the top of the horizontal foul drain pipe. However page 461 shows a traditional stack where the minimum radius at the bottom of the stack is 200mm.

 

If your discharge pipe runs vertically through your passive slab will you also have to add 200mm to account for the final bend before the horizontal run starts? I do not know and I am quoting minimum figures from different discharge stack systems.

 

 

[Russdl]

1 hour ago, Bitpipe said:

Not sure if its a good idea or not, but you could extend the blue runs further back to a rodding eye

 

That would be the plan. I think I got the idea from here, did you post a picture of your rainwater solution? If so that was my inspiration but I also don't know if its acceptable for foul runs.

Failing that then i would plan on access at the base of each stack prior to finished floor level. Neither of those solutions have been properly thought through yet...

 

1 hour ago, Bitpipe said:

Its tempting to save money on ICs, pipe, excavation etc at

 

That wasn't the driving force for me but would be an unintended consequence if I opted for my blue route.

 

@epsilonGreedy  Pointed out the obvious requirement for the rest bend at the base of the stack - so obvious that I hadn't taken it into account. The remaining fall could start to get tight now and the shortest route may be the only option (apart from all the other options that I have no knowledge of!)

[Temp]

I'm sure you all know this but make sure your layout has pipes meeting at the correct angles. Joints will accommodate a small error in the angle but it's a pain to have to make a connection if the angles are way off standard.

[Russdl]

@Temp Thanks, another thing that had been on my mind.

[MikeSharp01]

6 hours ago, Russdl said:

first IC can be up to 22 meters

Sorry - my bad. Not sure why I used 11m.

 

[Russdl]

@MikeSharp01 No problem! It got me looking at things I hadn't properly considered and hopefully my interpretation is correct.

[epsilonGreedy]

On 20/06/2018 at 10:11, Bitpipe said:

If I get a blockage in the external run (and I've had a few, obviously some debris found its way in during construction) it's easy to rod / powerwash the drains from IC to IC and clean everything out from the house to the main sewer. 

 

In your blue run, how would you do this? If you have the first IC on the terrace , you'd be able to flush from there to sewer but how would you clean the section behind it (without pushing blockage further back up the pipe).

 

 

I am in a similar position to the OP and found a solution. It is possible for the drainage run to start with an inspection chamber. The OP could have a couple of ICs under the car park side of the house, these would run under the foundation and the soil stack would join as a branch into the existing drain pipe.

[PeterW]

You don’t need an IC to start the run, just use a rodding eye on the end of the main run and come in on the branches with your flows. 

 

 

 

[Russdl]

@PeterW I like the idea of a rodding eye on the end of the main run, however it would appear simpler to having a rodding access point at the base of each stack. Apart from ensuring they were easily accessible what are the draw backs with such a solution? (Stinking the house out when it comes to rodding just sprung to my mind).

 

Any other reasons why the rodding access shouldn't be inside the house at the base of the stack?

[Mr Punter]

12 minutes ago, Russdl said:

@PeterW I like the idea of a rodding eye on the end of the main run, however it would appear simpler to having a rodding access point at the base of each stack. Apart from ensuring they were easily accessible what are the draw backs with such a solution? (Stinking the house out when it comes to rodding just sprung to my mind).

  

Any other reasons why the rodding access shouldn't be inside the house at the base of the stack?

 

You will need access at the base of the stack up to the main run plus access to rod the main run.

[epsilonGreedy]

4 minutes ago, Russdl said:

Any other reasons why the rodding access shouldn't be inside the house at the base of the stack?

 

 

The rodding access point design I mentioned is a solution to the problem of two toilets joining at a Y junction under the foundations of a house but to me it feels like it goes a long way to mitigating the downside of through foundation foul drainage runs even without a Y junction.. According to the NHBC standards such a Y junction is not allowed without an external rodding point at the start of the run. You are not proposing such a Y junction so maybe your idea is legit?

[epsilonGreedy]

13 hours ago, PeterW said:

You don’t need an IC to start the run, just use a rodding eye on the end of the main run and come in on the branches with your flows. 

 

 

Que voice of Captain Mainwaring: I was waiting to see which man would spot that deliberate error. ?

[PeterW]

42 minutes ago, Russdl said:

@PeterW I like the idea of a rodding eye on the end of the main run, however it would appear simpler to having a rodding access point at the base of each stack. Apart from ensuring they were easily accessible what are the draw backs with such a solution? (Stinking the house out when it comes to rodding just sprung to my mind).

 

Any other reasons why the rodding access shouldn't be inside the house at the base of the stack?

 Getting a rod around a swept bend at the foot of a stack would be a challenge, and once used, I wouldn’t want rods coming back into the house ..!! Other issue is buildup - if your blockage is at the foot of the bend on the junction, opening a rod point on a vertical stack with a WC above you will be........ interesting ........ 

[Russdl]

20 minutes ago, PeterW said:

Getting a rod around a swept bend at the foot of a stack would be a challenge, and once used, I wouldn’t want rods coming back into the house ..!! Other issue is buildup - if your blockage is at the foot of the bend on the junction, opening a rod point on a vertical stack with a WC above you will be........ interesting ........ 

 

? Two very good points there @PeterW along the lines of my concern over the pong but much more tactile. Outside rodding it is!