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Author: Subject: building near a public sewer.
smart51

posted on 22/4/16 at 07:04 PM Reply With Quote
building near a public sewer.

We're just about to replace a lean to at the side of our new house with a proper extension. The planning office at the council has pointed out that there is a public sewer in the back garden perhaps within 3m of the house. They want either proof that it is more than 3m away or they want permission fromSevern Trent, which will mean a bill of nearly a grand.

I've dug the start of a trench from 1m to 3m from the new wall. I'm 600mm downso far, or about lLev with the next door neighbour's garden. He has a 4m long rear extension whichmust be over the sewer if it is near the house. HHow far towards the centre of the earth must I dig before getting the inspector in?

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sdh2903

posted on 22/4/16 at 07:18 PM Reply With Quote
Just built over a sewer. 2 in fact. Have you contacted the water board for a map of where the sewer lie? Where are you getting the £1000 charge from? I've had them out to map out a sewer that was unmarked on their maps and obtained a build over agreement. My only costs were the architect drawings of 200 quid and 75 quid for a cctv survey. The biggest issue is time. They are slow as anything. From first contact to getting an agreement was 14 months.

Key is getting direct contact with someone rather than generic email addresses or phone numbers.

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Sam_68

posted on 22/4/16 at 07:18 PM Reply With Quote
quote:
Originally posted by smart51
How far towards the centre of the earth must I dig before getting the inspector in?

I generally get the contractor to keep on digging until he hits something valuable like coal or oil, or failing that until you've got a good source of geothermal energy.

But you do know that you can request a water and sewer map for a fairly modest sum that will hopefully give you a better idea of the location of the sewer, and will also hopefully give cover and invert levels?

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smart51

posted on 22/4/16 at 07:25 PM Reply With Quote
We had a map from ST as part of the searches. Its a bit large scale though. The sewer looks to be just over 2m back from the new wall. I'm told ST want £200 to process the application, £250 each for before and after cameras down the sewer plus the council want a more expensive full plans application rather than a simple building notice.

What's an invert level BTW?

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Sam_68

posted on 22/4/16 at 07:33 PM Reply With Quote
quote:
Originally posted by smart51
What's an invert level BTW?


Invert level is the measurement to the bottom of the pipe, taken when you lift a manhole cover (cover level obviously being the level of the manhole cover).

They should be marked as CL and IL with numbers beside them on drawings/maps. They can be used to work out the approximate depth of the sewer (remembering that sewers slope, so in an ideal world you need to use some basic trigonometry to work out the depth at intermediate points between manholes).

Both should be to Ordnance Survey datum levels.

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smart51

posted on 22/4/16 at 07:46 PM Reply With Quote
quote:
Originally posted by Sam_68
quote:
Originally posted by smart51
What's an invert level BTW?


Invert level is the measurement to the bottom of the pipe, taken when you lift a manhole cover (cover level obviously being the level of the manhole cover).

They should be marked as CL and IL with numbers beside them on drawings/maps. They can be used to work out the approximate depth of the sewer (remembering that sewers slope, so in an ideal world you need to use some basic trigonometry to work out the depth at intermediate points between manholes).

Both should be to Ordnance Survey datum levels.


Thanks for that. It's useful to know. It might also be useful o find where the manhole covers are. We don't have one and neither do the neighbours.

I might have to get back in touch with ST then. They just said fill in the application form and return it. A proper map will be useful. Trig is no prob. It might help target the hole before it becomes open cast.

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smart51

posted on 23/4/16 at 04:44 PM Reply With Quote
Found the sewer - can we build?

So I dug a trench and found the sewer. It is a 7" OD clay pipe, the top of which is 750mm below ground level.

However, the centre of the pipe is 2965mm from the outside of the new outside wall we plan to build, 2875mm to the nearest edge of the pipe. We could switch from 140mm blockwork to 100mm, saving us 80mm. That gives us 3045mm to the centre line or 2955 to the edge. The big question is, do we need to measure to the centre line of the pipe or the nearest edge? If it's the centre line, we're in. If it's the nearest edge, we have to disturb Severn Trent before being allowed to build.

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Sam_68

posted on 23/4/16 at 06:39 PM Reply With Quote
quote:
Originally posted by smart51
The big question is, do we need to measure to the centre line of the pipe or the nearest edge? If it's the centre line, we're in. If it's the nearest edge, we have to disturb Severn Trent before being allowed to build.


Theoretically, it's nearest edge, I'm afraid. Centreline would be a bit nonsensical, given that some sewers are big enough to drive a car down.

Have a word with them, though - they might be willing to cut you some slack, particularly since it's so shallow that there's no way your foundations will be loading it (basic rule of thumb is that you project a 45 degree line down from the lower corner of the foundation, and anything within that zone might be influenced by the foundations loads; in your case, that line will project well below the sewer, so there's no real risk to it).

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smart51

posted on 23/4/16 at 06:54 PM Reply With Quote
quote:
Originally posted by Sam_68
quote:
Originally posted by smart51
The big question is, do we need to measure to the centre line of the pipe or the nearest edge? If it's the centre line, we're in. If it's the nearest edge, we have to disturb Severn Trent before being allowed to build.


Theoretically, it's nearest edge, I'm afraid. Centreline would be a bit nonsensical, given that some sewers are big enough to drive a car down.

Have a word with them, though - they might be willing to cut you some slack, particularly since it's so shallow that there's no way your foundations will be loading it (basic rule of thumb is that you project a 45 degree line down from the lower corner of the foundation, and anything within that zone might be influenced by the foundations loads; in your case, that line will project well below the sewer, so there's no real risk to it).


Yeah, the council wants a 1m deep x 600mm wide concrete foundation for a 1.3m long single story wall. The base of the foundation will be below the sewer pipe. I guess we'll get the inspector back in. If he says go ahead, we'll fill in the hole and forget about it. If he says no, we'll switch to a timber framed wall with a rendered finish. That will be about 90mm thinner so will give us a few cm of breathing space. Studwork is more expensive than blockwork says the builder, but it won't be by much.

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