Bridging a Private Sewer Pipe?

[iDIY]

Hi Guys,
I have used an excavator to dig my trench for the footings on my extension (1m Deep) and just in the process of tidying it up by hand before pouring the specified 225mm of concrete. I knew i have a sewer pipe crossing about 300mm below the concrete footing so this will need bridging. Its on my land and not a shared feed to the main sewer which is well off my land.

Before I get the BCO in to sign off on the trench prior to pouring, i want to make sure i have done the bridging correctly.

I am thinking of doing it as per the attached picture so the whole thing can be done in one pour. Basically i would dig 350mm either side and 150mm below the pipe, put in some shuttering either side leaving a 150mm space all around the pipe and filling it with pea gravel. I would then sit concrete lintels across the shuttering with a 200mm overlap either side then pour the concrete encasing the lot in one go.

This would provide support for the lintels (200mm either side) and leave a level continuous run or concrete throughout the trench with the lintels encased inside.

 

[^woody^]

The whole strip needs to go down to invert level

 

[iDIY]

The whole strip needs to go down to invert level

Sorry woody, no idea what that means. Can you explain please ?

 

[^woody^]

LOL. This does not bode well

The whole length of that strip foundation on the side of the extension that goes over that pipe, needs to be at a depth which is level with or below the bottom of that drain. Preferably the strip goes under the pipe and you bridge over it in the blockwork

You can't just drop a section like you have done and then bridge over the pipe.

Plus there needs to be clearance (50mm or so) above the pipe to allow for settlement.

 

[^woody^]

Oh, and you would normally have engineering bricks starting at 225mm (3 course) below ground level on the external skin - inside skin can be blocks as per your drawing.

 

[iDIY]

Given the run on that side of the extension is about 9m long, surely I can step down the foundation on either side of the pipe, keeping the stepped down section clear of thd 45 degree line either side of thd pipe? Like the pic below?

 

[^woody^]

It would be better to have at least 2-3m of foundation at the lower level, but if the pipe is nearer to one end then half the strip would normally be at the lower level.

There are several factors to influence all this and you really should be getting the advice from whoever is designing the extension, and remember that building control need to inspect the trench before the concrete is poured.

 

[iDIY]

It is out at one end.

I don’t want to pay the architect to come back and answer questions that this forum has proven itself able to address with past experiences. That’s why I adked here.

As I mention in the first post, I want to get the trench right before my BCO comes to sign it off prior to poring, hence the questions here.

Thanks for the steer.

 

[^woody^]

You should not have to pay the architect for this, it's part of his job and he should have already done it.

 

[John D v2.0]

Also make sure the base of the trench is flat and any step really is a step, to stop any risk of things sliding down hill.

 

[freddiemercurystwin]

You should not have to pay the architect for this, it's part of his job and he should have already done it.

Well only if the OP paid for regs drawings.

 

[Footsoldier888]

But unless you are on some sort of fixed price contract it always comes back to the client to pay in the end.

Out of interest, why is a private drain laid to that level? I thought 600 - 1200 was the norm, with some shallower.

Is filling the drain with concrete and designing a new drain run possible??? If the cost implication is huge could you even think about a septic tank instead???

 

[iDIY]

Out of interest, why is a private drain laid to that level? I thought 600 - 1200 was the norm, with some shallower.

Its hard to describe but if nothing was done years ago my back garden would slope downwards from the house ground level to an access road at the back which is a good 3mts lower. The sewer main runs 2mts below that access road. The back gardens all have a retaining wall effectively creating a 3mtr step in the garden so the sewer pipe from the house to the main sewer has a very steep fall for its first part to an inspection chamber then another shallower fall to the main sewer. the main sewer being at a depth that will create a fall for the houses at the rear of ours which are a good 3m lower.

 

[Footsoldier888]

What I am thinking is what went in previously might not conform to current regs anyway. I thought in this situation there is supposed to be a tumbling device to take the water from a high to low level so as to stop drains being too steep, as I have read the solids can get left behind if a fall is too steep.

Don't you need to look at all aspects of the drain design at this stage to avoid some horribly expensive situation happening once you start building?

 

[iDIY]

It’s been draining fine since the 1930’s so I can’t see why it would need changing now. I will prep the bridge as woody suggested and see how the BCO likes it.