Underground drainage design : drop shaft & ensuring future compatibility for moving bathroom

[Undaunt]

The original cracked/leaking clay drainage of the house has been disconnected and partially removed, with an inspection chamber installed as described in this thread (thank you to those who so kindly offered advise) We have temporary drainage set up & need to figure out the next step.

This is the original rough plan :

We now have a 450mm inspection chamber where the Buchan trap was, with its invert at approximately 110mm below ground level.

1. Drop Shaft/Back drop connection to Inspection Chamber to deal with overly steep fall (~1in5)

The fall from the current soil pipe location to the inspection chamber inlet is excessive at around 1 in 5. I have found the suggestion on Paving Expert about a back drop connection to the IC, which would enable us to get the 1 in 40 fall up to this point.

Questions :

• Is this the best option?
• Are they prone to blockage? I also worry that waste might regularly overshoot the junction/drop?
• Is a rest bend the preferred option? One image on paving expert shows a 90° knuckle bend, the other connecting to a plastic IC shows a rest bend. Another option is a 'ramped connection' as shown in the second image, although this will make it even closer to the house, which might be a problem.

2. Rainwater gully into soil stack above rest bend? (combined sewage system)

The house is a Victorian terrace & has very shallow foundations. The corner next to the soil stack has already sunk quite a bit due to saturation/washing away of soil from undetected foul water leaks hidden under concrete, potentially over many years & the ground still feels soft here.

We created a quick fix for the initial leak from a cracked clay pipe collar embedded in concrete by angle grinding off the collar then inserting a McAlpine Drain connector like so :

We suspect there may be further damage to the clay pipe joints underground so need to replace with plastic pipe back to the IC. We would like to minimise disturbance and be cautious about digging close or too deep near the shallow victorian foundations.

We have ascertained that the clay rest bend shown in the photo below goes directly underneath the rainwater gully, running perpendicular - right next to these foundations.

Initially I thought we would need a pipe run a little distance away, perpendicular to the wall with 3 x 45° junctions : one for the gully, one for a soil stack rest bend and another gully for the kitchen wastes - possibly with a rodding eye further along :

However, this is only going to be a semi-permanent installation (see below) and whilst I need it to work well & give time for the soil/wall to dry out, it would be good to minimise number of fittings required - since ultimately they are unlikely to remain here.

I hit upon the idea of connecting the gully to a junction above the rest bend like this :

Questions :

• Is this allowed (building regs)?
• Is it a good, bad or terrible idea?
• I have put a single socket access pipe just above ground level - this would allow rodding access and a socket for kitchen 40mm waste to go into. Useful or unnecessary? (could just use solvent weld strap boss)

3. Future plans for moving bathroom to front of house, understairs toilet & rear extension.

I hope to run drainage under the ground floor, mainly to enable moving the bathroom to the front of the house. I am envisioning a soil stack in the hallway boxed in (possibly slightly chased into wall to reduce its volume) and probably a stub stack under the stairs for toilet & basin (possibly washing machine too if I can fit it in) - the kitchen sink waste could drain into a stub stack here too, since I hope to maybe build a side-return extension at some point.

Might this drainage run arrangement work - with a 250mm sealed access chamber in the kitchen? Any thoughts or suggestions?

I am obviously keen to reduce likelyhood of it blocking - I recognise it is a long run... & also want to ensure there is good rodding access if it does. It potentially will be buried under a limecrete slab in due course (we would probably lay it in the subfloor under the suspended timber floor for now, to get the new bathroom up and running - although we would have to cut a channel through the solid concrete kitchen floor)

Because the bit we are connecting up now will not be there permanently, I am wondering whether we should connect it up to a side inlet of the 450mm inspection chamber - but I know that is poor practice (since you are supposed to have the main channel in use). However, doing so would allow us to put in a drop shaft connection at the same time for the eventual main run through to the soil stack in the hallway, meaning we wouldn't have to dig down so deep (1.2m) again around the IC - we would only have to expose the drop shaft 60cm down or so.

I also realise that the drop shaft in itself could almost act as a small change of direction - in other words the rest bend connecting to the inlet would stay static, but the top pipe and rodding access could 'rotate' slightly. Any thoughts on this? I'm guessing this is poor practice again and could cause problems. Doing so would enable us to 'switch' the run above the drop shaft over from the side return to the hallway soil stack when everything is in place.

Anyway - sorry for massive post. Any thoughts appreciated.

 

[oilboffin]

There's a lad on this forum called hertsdrainage and he's very knowledgeable on this sort of problem
so keep your eye out for when he's on.Bob

 

[Hugh Jaleak]

Will try and answer your points.

Backdrops are fine, provided the approaching drain is laid to a sensible fall, the waste wont be travelling fast enough to overshoot the drop. Yes, they can block in the vertical section, but any drain can block. Depending on depth of drop, would determine which bend to use, anything more than about 3-4 feet, I'd use a longer radius bend. If waste drops too far, and hits a knuckle bend, there is a risk of it spattering and fouling up the opposite benching of the chamber.

Gulley connected into stack as in your pic will be fine. Rodding eye in the stack is fine, can never have enough access points!

Internal chambers are best avoided if at all possible. Ideally, bring out the drains from the bathrooms separately, and common them into a chamber once outside the building. Should the worst ever happen, its filthy enough clearing drains externally, (at least you can wash down), internally doesn't bear thinking about. Any reason then why the kitchen drainage cannot continue to drain into the proposed gulley(ies) as already mentioned?

Wont hurt to connect to a lateral on the chamber, again, if laid to a proper fall, the waste will disappear as it's supposed to. Leaves your 'straight through' connection free for the proposed future works. It'll get a flush through each time it rains heavily, so should help keep everything clear.

Lastly, with the backdrop, as long as the rodding access in in a straight line with the main run, it wont hurt is its slightly out of kilter with the connection into the chamber. Often seen the branch angled into direction of flow in the chamber whilst the incoming sewer is at 90deg to the main channel.

Every job is different, remember it's ultimately its the BCO who will have the final say.

 

[Undaunt]

Thanks Hugh - I'm going to take those points on board & do some sketches models over christmas (i've measured levels etc a bit more accurately with a laser level now).

Backdrops are fine, provided the approaching drain is laid to a sensible fall, the waste wont be travelling fast enough to overshoot the drop. Yes, they can block in the vertical section, but any drain can block. Depending on depth of drop, would determine which bend to use, anything more than about 3-4 feet, I'd use a longer radius bend. If waste drops too far, and hits a knuckle bend, there is a risk of it spattering and fouling up the opposite benching of the chamber.

Perfect - we will work back from FFL then to ensure a sensible fall (I'm assuming 1 in 40) & use a rest bend on the backdrop rather than a knuckle bend.

Gulley connected into stack as in your pic will be fine. Rodding eye in the stack is fine, can never have enough access points!

Great - that way of connecting the bottle gully makes life simpler.

Internal chambers are best avoided if at all possible. Ideally, bring out the drains from the bathrooms separately, and common them into a chamber once outside the building. Should the worst ever happen, its filthy enough clearing drains externally, (at least you can wash down), internally doesn't bear thinking about. Any reason then why the kitchen drainage cannot continue to drain into the proposed gulley(ies) as already mentioned?

Good point. I'll try draw some other designs in sketchup that avoid an internal access chamber. Agreed - future kitchen drainage could maybe use the run we lay to the current soilstack/gully - it might need extending slightly to create a stub stack in the kitchen island location, when extension is built (we are considering rebuilding the entire back of the house with a timber frame structure, highly insulated with hempcrete, since the back of the house is in a structurally pretty bad & incredibly cold state!)

Wont hurt to connect to a lateral on the chamber, again, if laid to a proper fall, the waste will disappear as it's supposed to. Leaves your 'straight through' connection free for the proposed future works. It'll get a flush through each time it rains heavily, so should help keep everything clear.

Great - makes more sense to me to use one of the side inlets on the inspection chamber like you say to keep the main channel free for main future run. I'm not sure how it is going to work to do with the back drop connections however - presumably we would need one for each - and that will make one of the risers full of holes for rodding access!

 

[Hugh Jaleak]

May be worth putting 2 chambers in, fit a second at a higher level to common all the connections, then the outlet from this into a backdrop into the original chamber, to get down to existing invert level. Would make things less crowded on the one chamber.

Alternatively, look at the 'Marscar bowl', if these are still available. Bloody expensive, but will allow all connections to be made on one level, with the bowl outlet dropping into the existing connection you've made, and negate the need for 2 chambers.

 

[John D v2.0]

Building regs don't recommend connecting a branch into a stack right at the rest bend, you'd be better with a few inches difference if possible. However if it's a short stack only upstairs and downstairs with open vent, there's less likely to be big pressure differences on the pipe.
If it's easy enough, connect the rain gulley into the drain rather than the stack.
This also protects the rain gulley from any unlikely blockage at the base of the stack