Drainage - Height Differential

[Mulberry View]

Our build seems to have a fairly simple drainage requirement, well, it would have been had it not been for our foundations f*uck up. I won't go into that now, but the legacy of it results in my having to pass the majority of my drainage under the foundations. Believe me, this is a small price to pay for how deep in the doo-doo we were a few weeks ago.

 

In this image, you'll see a trench partly dug. The red line denotes the main drainage run, heading from far to near. There are 2 chambers denoted by white rings on the pic. I'm happy with the depth of these, they line up with where I need to bore under the foundations (the 2 lines you can see coming into the chambers), more about that later.

 

 

The majority of the run you see in this pic, including both chambers, deal with 3 Bathrooms and a Utility Room, so in my eyes takes priority. The soil pipe you can see coming in from afar serves ONLY the Kitchen, so no WC's. I really can't put that one under the foundations, it's just too much work there and would be more disruptive than I think worthwhile. Consequently, there is a 500mm height difference between the pipe and the connection point on the next chamber (marked '2') on the pic. That chamber is 2.2m from the wall from which the offending soil pipe emerges, hence I do not want ANOTHER chamber.

 

What's the best way to drop the Kitchen pipe down? Do you think I could get away with 45°/45° or 30°/30° drop just after it's gone over the foundation, a brief downwards ramp if you will, totalling no more than 1.5m and connected straight into the chamber.

 

As far as going under the foundations. We are on hard chalk, which my SE seems to suggest gets me away with a lot of stuff that other ground conditions might not allow (I am awaiting a response from my SE to back up my proposed solution). I'm trenched both sides, to about 200mm below the bottom of concrete. My idea is to core under the foundations and insert a 160mm soil pipe sleeve, then pass the 110mm through that. Any thoughts?

 

Thanks in advance.

[Mr Punter]

24 minutes ago, Mulberry View said:

Do you think I could get away with 45°/45° or 30°/30° drop just after it's gone over the foundation, a brief downwards ramp if you will, totalling no more than 1.5m and connected straight into the chamber.

 

Yes no issue

[Russell griffiths]

Drop it as steep as you like, the old wives tale of leaving solids behind is more for clay pipe, plastic runs fast. 

Slope it down straight into chamber , get a bit of speed behind it and it will keep the whole run clean and shiny. 

[saveasteading]

Pipes passing through solid walls or footings should be isolated by wrapping in something flexible. eg boxing round with polystyrene or foam sheet.

This is so that any movement of the building does not damage the pipe.  

In your case, on chalk, the building will not move, but the pipe run might, in theory.

Not worth arguing with the bco.

 

is it not possible to go through the eps blocks before they are filled?

 

Once the hurt has passed, it would be interesting for everyone to know what went wrong. We can all learn.

[Conor]

You can have whatever fall you want on a grey water run. I had similar, two 45s and a metre of pipe between them. Have the second bend within a metre of the chamber so you can deal with any issues of they arise. BC was happy with this run from kitchen / washing machine.

[Tosh]

30 minutes ago, saveasteading said:

is it not possible to go through the eps blocks before they are filled?

my thoughts as well.

[Mulberry View]

3 hours ago, Russell griffiths said:

Drop it as steep as you like, the old wives tale of leaving solids behind is more for clay pipe, plastic runs fast. 

Slope it down straight into chamber , get a bit of speed behind it and it will keep the whole run clean and shiny. 

 

That's a great opinion. Thanks. I agree that the grey water from the Kitchen will ball in and push the you-know-what's on their way.

[Mulberry View]

3 hours ago, saveasteading said:

Pipes passing through solid walls or footings should be isolated by wrapping in something flexible. eg boxing round with polystyrene or foam sheet.

This is so that any movement of the building does not damage the pipe.  

In your case, on chalk, the building will not move, but the pipe run might, in theory.

Not worth arguing with the bco.

 

is it not possible to go through the eps blocks before they are filled?

 

Once the hurt has passed, it would be interesting for everyone to know what went wrong. We can all learn.

 

As an update to this, both BCO and my SE have said that the drainage run needs to be encased in concrete, owing to the fact that it is less than 1m away from the foundations. Happy days. If I encase the whole thing in concrete, surely that's going to take out any scope for movement in any of the pipe, hence the whole flexible joint thing is nonsense.

 

I had hoped to have my drainage and beam & block inspected at the same time, but this isn't really practical, so I think I'll need to get the drainage all laid and set, then get BCO out, they'll advise what needs concrete and what can have pea shingle, then I can move on. I like bedding pipes into pea shingle, you can fine tune the fall and get it just so, I feel with concrete that's going to be really hard. What can I seat the pipes on that won't get blasted out when the concrete goes in?

[Mulberry View]

3 hours ago, Conor said:

You can have whatever fall you want on a grey water run. I had similar, two 45s and a metre of pipe between them. Have the second bend within a metre of the chamber so you can deal with any issues of they arise. BC was happy with this run from kitchen / washing machine.

 

That illustrates perfectly what I want to do. Very helpful indeed. Thanks.

[Mulberry View]

2 hours ago, Tosh said:

my thoughts as well.

 

In a nutshell, the catalogue of errors in the way the foundations were dug (covered roughly in this post) resulted in some urgent decisions about rectification and a change of plan on concrete heights/volumes etc. Keep in mind that my building has 3 internal level changes and is built on a slope with a height differential of over a metre on both planes from one side/end to the other.

 

Consequently, the decision about 'top of concrete' height has, in my opinion resulted in the concrete being too high, thus passing my pipes over the top in the area shown will result in them only being about 200mm under the soil. I'm not at all comfortable with that. In fact you can see one of the air-bricks and the height of the pipe as it emerges over the foundation to get an idea of the distance we're playing with. Hence I want to get the drainage pipes as low as possible, as soon as possible after we're over the foundation. Make sense?

 

Luckily this all adds up with the onward drainage run which was laid well in advance, in fact it probably prevents the need for a backdrop in the onward chamber.

[saveasteading]

13 minutes ago, Mulberry View said:

hence the whole flexible joint thing is nonsense.

No, these are 2 separate issues.

Drains are encased in gravel normally, but the gravel is replaced by  concrete under loads such as roads.  This is a small amount  of concrete to spread any loading away from from the pipe and prevent it from squashing.

 

I would suggest that you mix up some concrete and use it to bed the pipe to level with control. Then the surround and cover can be poured from above, though still with care. You can use a semi-dry mix so it is almost like gravel, and will stay in place.

For total control, you can then mix and hand place all the concrete. Slower work but you don't need much concrete.  For economy place a plank parallel to the pipe and concrete against it. A contractor will want ready mix to pour in the trench, at your expense of course.

 

The bco and se are making the standard assumption that loads spread down at 45 degrees from the edge of the load. so if your drain is well below the footing then that would apply.

Except that you say you are founded  on chalk, so the loads are not going to spread through the fill at all, but into the chalk. You might point that out, and save the concrete bed and surround.

 

This drain concrete surround would not prevent differential movement, hence i still think you are to allow movement through the wall.

 

It is low risk in your situation, so just check what they want and do whatever. But try first to return to gravel.

[Mulberry View]

On 06/01/2023 at 18:03, saveasteading said:

No, these are 2 separate issues.

Drains are encased in gravel normally, but the gravel is replaced by  concrete under loads such as roads.  This is a small amount  of concrete to spread any loading away from from the pipe and prevent it from squashing.

 

I would suggest that you mix up some concrete and use it to bed the pipe to level with control. Then the surround and cover can be poured from above, though still with care. You can use a semi-dry mix so it is almost like gravel, and will stay in place.

For total control, you can then mix and hand place all the concrete. Slower work but you don't need much concrete.  For economy place a plank parallel to the pipe and concrete against it. A contractor will want ready mix to pour in the trench, at your expense of course.

 

The bco and se are making the standard assumption that loads spread down at 45 degrees from the edge of the load. so if your drain is well below the footing then that would apply.

Except that you say you are founded  on chalk, so the loads are not going to spread through the fill at all, but into the chalk. You might point that out, and save the concrete bed and surround.

 

This drain concrete surround would not prevent differential movement, hence i still think you are to allow movement through the wall.

 

It is low risk in your situation, so just check what they want and do whatever. But try first to return to gravel.

 

That's a very helpful post, I really appreciate it. Thanks.

 

 

I've made one of the passes under the foundation. Here you'll see that I've dug down to about 200mm or so under the bottom of the concrete (which is about 400mm thick, spec was 300mm) and bored through solid chalk. I opted to use 127mm which will take the 110mm pipe. Unless anyone here has a better idea, I'm likely to lay all the drainage, including the slope I spoke of (which looks like I can achieve what I need with 22.5° bends) then get the BCO out to check, at that time, I'm hoping we can have a practical conversation about backfill for it all.

 

In the meantime, I'll await a reply from my SE with his opinion on how to deal with the backfill and my method of passing under the foundations.

 

Furthermore, in the grey water run from the Kitchen, am I able to add a T-Piece midway and use the vertical branch to come up in the island?

 

Thanks for all your help so far.

[Mr Punter]

I would not run a pipe under a foundation.  In fact, where we have had a pipe go slightly deeper to fall to an outlet we have increased the foundation depth so it is below the invert of the pipe.  Make sure BC are happy with your proposed setup before you spend time laying the drains.

[Mulberry View]

OK, so I've managed to agree a solution that both BCO and SE are happy with. I am going under, I have no practical choice. Apparently, I'm OK because I'm on hard chalk and the perpendicular run of drainage is not within the 'interaction zone', in other words a 45° angle downwards from the bottom of the foundation. My SE recommended a flexible joint on each side of the foundation to be sure. So far, so good.

 

Here's my proposed layout... (this is position 2 on the original photo at the top of this post).

 

 

I have an SVP that is designed to run up to the upstairs Bathrooms. It needs to be positioned directly on the other side of the wall that we're passing under, so I need a Rest Bend as close as possible to the outside face wall in order the put this SVP in a corner. This means I cannot nicely install a flexible joint between the pipe that passes under the foundation and the Rest Bend because it'll push the Rest Bend so far into the room that I cannot bring it back again in the height I have spare. I can put the flexible joint on top of the Rest Bend, that's apparently acceptable and is shown by the green blobs on this sketch. However, if I bed the foot of the Rest Bend in concrete (which I understand is supposed to be the case?), then this will negate the effect of the inner flexible joint, will it not?

 

Any words of wisdom as to how I can fish myself out of this muckin' fuddle?

 

Edited January 18, 2023 by Mulberry View
Typo

[saveasteading]

A core drilled hole under existing foundations? Do you simply intend to fit a 6" masonry cutter to a drill? That should work n chalk well enough. Gtting the sope right will be a challenge. 

Get your SE to specify what fills the gap after the pipe is in. Anything will do, but you done want it queried by the bco.

 

I would dig a trench in the chalk to both sides of the wall, wide enough to get your drill in, then core out under the found, then backfill however instructed. (a specialist would charge many hundreds to drill 2m)

[Mulberry View]

8 hours ago, saveasteading said:

A core drilled hole under existing foundations? Do you simply intend to fit a 6" masonry cutter to a drill? That should work n chalk well enough. Gtting the sope right will be a challenge. 

Get your SE to specify what fills the gap after the pipe is in. Anything will do, but you done want it queried by the bco.

 

I would dig a trench in the chalk to both sides of the wall, wide enough to get your drill in, then core out under the found, then backfill however instructed. (a specialist would charge many hundreds to drill 2m)

 

Yes, if you go up 3 or 4 posts on this thread, you'll see what I've already cut through one of them. I used 127mm because that's the biggest I have. SE asked for low PU foam or similar around the pipe, pea shingle in the trench.

 

I have pushed the pipe through, I'll check the fall today, but keep in mind it's a very short run and directly after a vertical pipe/rest bend, so it's never going to block.

 

Any thoughts on my rest bend bedding?

[saveasteading]

Great. Downhill all the way though. Otherwise silt, as we will call it, may build up in the flat.

[Mulberry View]

11 minutes ago, saveasteading said:

Great. Downhill all the way though. Otherwise silt, as we will call it, may build up in the flat.

 

I'm about 90% of the way through the second penetration and the arbor has just broken off in the SDS. Fun times! Now to find some Araldite.