is this sewer bridge ok?

[biryani_boy]

rear extension, I made my own drawings, we're working with building control. I found out there's a public drain in my back garden, foundation will go over it (perpendicular). I am looking to apply for a build over agreement, will this be ok?

basically the invert level is at 0.7m (surveyed), but the foundation will be around 1.0-1.2m deep (based on the builder's experience from local works).

would this work or do I need to make any changes? I left 200mm each side, as opposed to the minimum 150mm required by thames water.

1. should I have more courses of engineering bricks under ground level?
2. should the lintel be right under the first row of engineering bricks and the rest filled with pebbles? isn't that too much pebble?
3. is anything needed UNDER the pipe? do I fill that bit with concrete (up to 200mm under invert level), then fill with pebbles 200mm above, then lintel?

thanks

 

[Summer Breeze Block]

Assuming the diagonal shading is concrete, couldn't you sit the lintel on top just below ground level and get rid of five brick courses?

 

[biryani_boy]

that's what I had in mind, but wouldn't that be too much pebble around the pipe? is that ok?

 

[biryani_boy]

this is what I came up with, is this a "standard" solution to this problem? all I need is for thames water to be satisfied we'll properly protect the pipe, the builder and the building control officer (and my wallet) won't allow anything else to happen.

 

[^woody^]

The foundation should go under the pipe and the lintel close above the pipe maintaining the minimum distance around the pipe - no advantage with greater distances.

As it is you have effectively two separate foundations that can (and do) move separately.

 

[biryani_boy]

right, that makes sense. I can pour some concrete under the pipe, let's say between -1.00m to -0.85 (leaving 150mm minimum clearance under the pipe).

what about above the pipe? should I just increase the number of engineering brick rows to end up with ~150mm between lintel and the top of the pipe?

on a similar note, can I pour concrete above the lintels too?

 

[Summer Breeze Block]

This leaflet suggest 100mm / 4" of granular fill above and below pipe, so you won't need a huge amount of pea shingle: https://idoc.pub/documents/wickes-underground-drainage-1d4795y0w7l2

If drain is old clay pipe and there is a joint under your new wall, take care. It's easy for joint to start leaking if knocked or exposed too long. Can't help with other questions, sorry.

 

[biryani_boy]

right, I managed to get it this far. can someone please shed some light on the most practical way to fill that bridge? it's a very easy problem when the pipe is just under the ground level, but when burried at 700mm, I'm not sure whether the whole foundation has to start above the lintel or whether I can fill that gap with something and have a "normal" 3 or 4 rows of engineering brick.

 

[^woody^]

 

[biryani_boy]

Thank you, I’ll adapt my drawings and submit a build over agreement application. Where it says compressible material, is pebbles ok?

 

[^woody^]

Word it as "compressible material". This would be something like Claymaster or a simialr foam product, not gravel