Building over a public sewer

[cjconway]

Hi all,

I'm looking for advice regarding a new extension we are building over a public sewer at the back of our house.

When we moved in there was a 2m deep conservatory running the full 7m of the back of our house. We have demolished this with aim of building a proper extension of 3m deep and with a gable ended 1st floor extension above.

The conservatory was built with full foundations and concrete floor slab which we have left in place. The new foundations trench was simply dug a meter or so further from the back of the existing house and at the boundary fence we dug out the small strip of earth between the existing foundations (which were set slightly back from the boundary) and the boundary with our neighbour, alongside the old foundations.

On the advice of our engineer we dug a 450-500mm wide trench about 900mm deep into virgin boulder clay. Building control duely signed off the trench and we have filled with 6m3 concrete.

Since then we have received a letter from Building Control alerting us to the fact that there is no build over agreement in place for the public sewer running parrallel at 1.3m from the back of our house, underneath the old conservatory. As there was no requirement for a Building Control certificate for the conservatory the previous owner was never aware of the need for a Build Over agreement and as a result we now need to put one in place to cover the new building and the old foundations.

So, Anglian water have said that we must dig down to the sewer pipe where it goes under the foundations and protect with concrete. This involves us digging down below the pipe which is at 1.5m below the surface and then excavating good hard boulder clay from under the foundations, which in one spot is about 600mm wide, and then replacing this with concrete.

I'm hoping someone can shed some light on the logic of this for me as to a lay man this seems counter intuitive. From my view it seems I have a concrete slab sitting in solid boulder clay, 500mm above a pipe encased in boulder clay. Surely the big concrete foundation spreads the weight it carries evenly over its footprint and then the clay underneath does the same, the result of which is that there would appear to be no particular pressure on the pipe.

The structure we are building is a brick clad timber frame building.

Apart from the fact that this will be a major undertaking and in doing what Anglian are asking will involve some dubious health and safety practices, I simply do not understand why it is necessary.

Please can someone shed some light on this or if you agree that this seems unnecessary please let me have any suggestions of what to do instead?

Thanks,
Chris

 

[luisdesign]

I would have thought that they would have wanted you to provide softer compactable material to allow for ground movement of the new extension without potentially breaking the sewer as a result?

You should query it with the water company, but ultimately, you are building over their sewer and as such you will need to build in accordance with their requirements.

As a note, the ownership / responsibility of sewers was only recently (in the last 3 or so years) changed so that the vast majority of sewers are now in the hands of the water companies even where they were previously classified as private sewers... It is likely that when the previous owner done their work a build over agreement would not have been needed.

 

[noseall]

This is one heck of a miss by the architect!

Now you have the problem that your existing foundations are not deep enough in relation to the pipe invert.

How come the pipe was not spotted or at least the change in soil conditions when you were digging across the pipe trench?

Was everyone blind that day?

Were the drawings submitted to building control or are you doing this on a building notice?

 

[luisdesign]

This is one heck of a miss by the architect!

Now you have the problem that your existing foundations are not deep enough in relation to the pipe invert.

How come the pipe was not spotted or at least the change in soil conditions when you were digging across the pipe trench?

Was everyone blind that day?

Were the drawings submitted to building control or are you doing this on a building notice?

They are building in addition to the full foundations that already existed for the conservatory, so they would not have dug out the ground to find the sewer themselves.

 

[cjconway]

Thanks for this, just to be clear we did not dig down to anything like the depth of the pipe. The only new foundation we put in that crosses the pipe was 700mm deep and tide in to the existing foundation. The pipe is supposedly at 1.5m.

Does asking for a second opinion from the water company seem a reasonable request? There is debate certainly over whether we put hard stuff or soft stuff in there.

Really I dont want to do anything to it, the sewer runs under 5% of the foundations that bridge it, even less of the foundations as a whole. The building is timber framed and the steel is loaded to the foundations on over the sewer pipe.

Ultimately I have to do what they say, I just want to make sure its the right thing!

Thanks

Chris

 

[Tipper]

I would have thought the existing slab is protective enough on its own.

Can you not get your structural engineer to review and suggest a suitable solution and to get it agreed by the water company?

 

[maltaron]

You are lucky that they are just asking you to protect the pipe. I had to replace my sewer with malleable iron pipe and had a 12 month fight with the water authority to get them to pay for it. For your own peace of mind, you should get a CCTV survey done and if the pipe is in poor condition try to get it replaced at their expense. Consider what will happen if the pipe fails in the future, its possible you will to have the pipe diverted around the extension at your expense (or possibly relined)

 

[noseall]

The portion of the foundations that cross the pipe should not effect the pipe as a result of settlement. This usually means digging down to below the pipe invert and then bridging the pipe with lintels etc.

Building control and the water company may accept a steel bridging arrangement whereby you load the outermost part of the foundations and use a suitable steel beam 'bridge' back to the house or onto foundations that are greater than 45 degrees rising away from the pipe invert.

We would typically dig down to below the pipe and shutter-stop each side of the pipe, pour the conc' then bridge the gap with pcc lintels after the conc has gone off.

In most cases we would replace the pipe with new plastic.