Replace sewer before building over?

[HughF]

Would it be wise to replace the sewer that we are about to build over given it’s age and the fact we’re the start of the run?

 

Some background, we’re an end of terrace, with the poo stack at the front corner of the house. It travels down the side of the house where the toilet and surface gulley trap from the outhouse join, then turns a 90 and runs along the back of the terrace.

 

We’re planning a single storey rear extension this year, and are planning to knock down the existing outhouse (single skin, cast it situ roof) and replace with a 2 storey side extension in the future. As such we’ll end up bridging the rear run with out footings in two place and then pouring our concrete floor slab directly over the top of the run which is about 3ft out from the back wall of the house.

 

The existing sewer is salt glazed and dates from ‘46. It’s working fine but the manholes need rebuilding and the pipe is D sectioned due to buildup.

 

Is this a prime case of ‘if you have to ask the question then you already know the answer’?

Edited February 4, 2023 by HughF

[HughF]

I guess we’ll stick with things as they are then…

[Conor]

How deep is the pipe compared to your footings? If the pipe will run through the footings, chances are the digger will end up smashing the pipe anyway, so have a couple lengths of PVC sewer and a flexible couple on hand anyway. But if it's deeper then where you are digging, then just leave it.

[HughF]

Footings will be 1m deep, probably and we’ll be building over them in two places. Everyone else on this terrace has built over theirs and bridged them in line with the Wessex/Bristol water requirements, even though we’re on a private (Aster) treatment plant.
 

They’ll be hand dug around those areas.

 

My builders (who used to live next door and did a rear extension 2-3yrs ago) usually hand dig their footings on extensions although I’ll be doing the bulk of the excavation with my small machine on this one, mainly to save a bit on their day rate, and to save their backs. A mixture of stone and clay.

[Mike]

On 04/02/2023 at 19:28, HughF said:

pouring our concrete floor slab directly over the top of the run

If you really mean 'directly', don't do that.

 

If the top of the pipe will be within within 300mm of the underside of the floor slab, you should replace it with a new portion encased in concrete so that it's held rigid as an integral part of the floor slab, with flexible jointed 'rocker pipes' to accommodate any movement where it emerges from beneath the building. Take a look at the Hepworth Clay Technical Handbook.

 

Your BCO will need to inspect this.

[HughF]

It will be at least 1000 below the finished floor level. It’s probably at 600 below ground now and I’ve got 400 to come up, at least.

 

My main concern was, given the age, is it better to disturb it, replace with new then have one flexible connection buried at our boundary. Or is it better to leave it well alone, don’t disturb the joints between the salt glazed and worry about replacement if it collapses in the future?

Edited February 8, 2023 by HughF

[PeterW]

Dig a parallel trench, lay it in 110mm uPvC and then cut this into the stack and main connection and just crush the old clay with the machine and backfill accordingly. 

[HughF]

33 minutes ago, PeterW said:

Dig a parallel trench, lay it in 110mm uPvC and then cut this into the stack and main connection and just crush the old clay with the machine and backfill accordingly. 

Yep, sensible. Given that we have to replace the manhole/inspection chamber at one end of the run anyway… it would be a lot easier to work that manhole in new 110mm pvc.

[HughF]

A quick site visit yesterday morning prior to commencing works in early June, we’re going to rip it out and install fresh…