Foundation over sewer. Piles close to the sewer.

[Patrick]

Yesterday the guys from Wessex Water came over to check the Sewer Pipe that runs underneath the property and give their ok/opinion on it.

Good news is that the Pipe are all fine , not cracked or broken , so we will be able to go ahead with building over the sewer.

Bad news is :

Our rear living room wall sits more or less exactly on top of the sewer pipe.(see Plans )

I dont want to change the size of the building and we cant move the building elsewhere , as the plot is too tight to move it significally.

I was told that Wessex Water requests at least 500mm distance between any Piles and the sewer.

So how do I support this back wall?

 

I know I have to give this too my SE eventually to get it sorted, but unfortunately my SE does have the creativity of a Strawbale so every "creative" or "alternative" solution that was developed throughout the planning, I had to come up with and SE was just twisting the numbers to see if it works.

 

On the other side , my Architect is VERY creative , and I could give it to him, but chances are that he comes up with a plan that involves a curved bridge over the property that  is build out of carbon and supports the entire house via electric magnets , available for just under 9million £ (to be calculated by SE)

 

My best Idea so far would be to build a kind of Foundation-Balcony

Like this:

 

 

 

 

Where the Piles sit back from the end wall for aprox. 600-700mm (giving the sewer the requested 500mm space)

and the Concret RC BEAM just extends over the end of the piles to form this little "blacony"

 

Fortunately, our house has a small flatroof, single story "extensions" on that end , so the weight on that overhang will be minimal.

 

Any other , alternative solutions?

 

Does this make sense or is it rubbish?

And any other flaws in the drawings?

 

 

[Jeremy Harris]

I can't see why a cantilever like that won't work OK.  Plenty of designs around where part of the house is cantilevered out over a void, like this Grand Designs one:

 

[Patrick]

4 minutes ago, JSHarris said:

 cantilever

That's the word I was missing. ??

[Declan52]

Why not just move the sewer. 

 

[Patrick]

1 minute ago, Declan52 said:

Why not just move the sewer. 

 

That would be great, but  there is a stream  which would need to be redirected or  dug under, so even more expensive than cantilever  over the existing sewer I believe.

[ToughButterCup]

59 minutes ago, JSHarris said:

I can't see why a cantilever like that won't work OK. .... like this Grand Designs one [...]

 

By chance I know a little bit about that particular build.

Looking that GD episode up, and paying special attention to the issue of the cantilever will repay the effort many times over.  The lesson I took away from viewing that episode and talking to some of the people directly involved in the build is - get an SE who knows both the material involved in the build and has substantial experience with cantilevers. 

[Jeremy Harris]

I agree, but it can't be that hard to do, as we still have houses with cantilevered floors locally that date back to the 16th century, and they seem structurally OK even after all that time.

[joe90]

Perhaps it’s time to change your SE, as others have said it’s not  rocket science and any SE should be able to design it with simple calks especially as its single story , I could design it just can’t do the maths to prove it ?.

[Mr Punter]

We built a 4 storey terrace next to a river. We piled down 25m but the nearest we could safely install the piles to the river wall was 3m, so the engineer cantilevered the ground beams. Lots of reinforcing near the cantilever, but this was a much bigger engineering challenge. The rear of the houses is cantilevered about 2m and the flood wall a further metre.

 

 

 

 

[Patrick]

1 minute ago, joe90 said:

Perhaps it’s time to change your SE, as others have said it’s not  rocket science and any SE should be able to design it with simple calks especially as its single story , I could design it just can’t do the maths to prove it ?.

My SE will be able to design it. She is a good Engineer. I just wanted to know if that is the best method or if there where alternative ideas out there. My SE definitely won't come up with any creative alternative solution (should it exist) and will just go with what I tell her to do or, if in doubt, bang  a few tons of steel in.

[joe90]

2 hours ago, Patrick said:

but unfortunately my SE does have the creativity of a Strawbale so every "creative" or "alternative" solution that was developed throughout the planning, I had to come up with and SE was just twisting the numbers to see if it works.

 

This is why I mentioned a new SE.

[Patrick]

32 minutes ago, joe90 said:

 

This is why I mentioned a new SE.

Ok, fair enough . I was under the assumption that this was fairly typical for a SE .

Following a lot of others on here, this seems quite a wide spread criticism with the SE profession .

But I am sure there are obvious exceptions out there .

[Declan52]

2 hours ago, Patrick said:

That would be great, but  there is a stream  which would need to be redirected or  dug under, so even more expensive than cantilever  over the existing sewer I believe.

How fast flow rate is the stream. You could dam it a few metres back and use a pump and long hose and  divert the flow into the culvert. Then dig down and reroute it.

Is it already underneath the stream where it crosses it???

 

[Patrick]

1 minute ago, Declan52 said:

How fast flow rate is the stream. You could dam it a few metres back and use a pump and long hose and  divert the flow into the culvert. Then dig down and reroute it.

Is it already underneath the stream where it crosses it???

 

Yep, it already is going underneath the stream.

Digging down and rerouting the stream and putting a new Sewer pipe in definetley will be more expensive than redesigning the foundations, as we are on piles anyways.

This will be the Plan B , if Plan A should be not possible .

 

 

 

[joe90]

1 minute ago, Patrick said:

Yep, it already is going underneath the stream.

Digging down and rerouting the stream and putting a new Sewer pipe in definetley will be more expensive than redesigning the foundations, as we are on piles anyways.

This will be the Plan B , if Plan A should be not possible .

 

 

 

 

Plan A is very doable just needs an SE to design, especially as you are already using piles.

[bassanclan]

Might raft foundations work - depending on the depth of the sewer?

[Mr Punter]

1 hour ago, bassanclan said:

Might raft foundations work - depending on the depth of the sewer?

 

No as it transfers the building loads to the ground underneath. I imagine there is a depth where this could be ignored but it would be very deep and you would likely need an engineer / soil survey etc. and a large safety factor.

[Patrick]

OK. Just cause I always prefer to read the ending to stories, I give you the result :Emailed

 my SE, asked if she could do cantilever calcs for what I sketched up at the top of this thread.

Answer was

1. Not a problem, takes 2 hours, cost about 60£

2. Probably don't need any adjustment (subject to calcs.), as the existing piles and RC beam are massive anyways.

 

Excellent result.

Thanks everyone as well for chipping in.

[Russell griffiths]

Hi @Patrick I have 4 cantilevered ringbeam sections, the piles are set back from the corners by about 6-750mm, I will see if I can find a pic. 

Design all done by piling company. 

[Russell griffiths]

@Patrick

have a look at this, you can see the end of the ringbeam closest bit to you, if you look at the wire coil you can see a vertical bar poking up, this is the last pile in the row that is set back a good metre from the corner. 

[Patrick]

@Russell griffiths

I think I can just stop posting on the forum and start texting you with the questions I have directl,  as it seems so far that your built (at least the ground works) is pretty much what I am going to do. Copy&paste foundation ?

[Russell griffiths]

I questioned the cantilevered sections and was told by the engineer that it was stronger than having the pile in the corner as it acts like a counterbalance, with pressure pushing down being equalised by pressure pushing down on the other side of the pile. 

[Jeremy Harris]

1 minute ago, Russell griffiths said:

I questioned the cantilevered sections and was told by the engineer that it was stronger than having the pile in the corner as it acts like a counterbalance, with pressure pushing down being equalised by pressure pushing down on the other side of the pile. 

 

 

Makes sense, as that's the principle used in cantilever bridges, like this one: