Flooding under house, cause by leaking sewer?

[Steve35]

So we've got a semi-detached 60's ex-council house that we've been in since 2011.

Now our private sewer connections to the main public network are only about a few feet long each from a downstairs sink pipe, the toilet (down the outside wall) and the bath (which shares a pipe with next door also down the side of the house). All are perfectly sound.
The public sewer runs along the back of our house and round a near 90 degree bend, down the side and into the street. There's 3 manholes we can lift to gain access on our properly and the whole length is barely 15m total.

Literally every year (about two-three times) the public sewer pipe that runs through our garden and round the side of the house blocks, unsurprisingly thanks to idiots flushing stuff they shouldn't down the drains. The guys who unblock it for us each time are super-fast and never issues with them turning up to get the job done though.
We found out early this year than a connection on the public pipe in the garden was broken/slipped so it was leaking slightly. Scottish water came and dug up the garden and replaced it. They said it was the cause of the blockages as even tiny amounts of debris would get stuck on the misaligned pipework.
The blockages continue though and they were going to put a camera up it to check the whole length. They never did this to my knowledge.

Now though we've been away a week or so and found the place infested with drain flies and upon looking under the house (after blitzing all the drains with various chemicals all week) I've found a huge puddle under the house. It's always been a bit damp and cold under there but I've never seen a big puddle like this, and considering all the drought this year it's surprising.
My only conclusion is that it's the sewer leaking again (it isn't blocked though) and it's found a way into the space under the house. This puddle is on the downhill corner of the house, the lowest point, also nearest the part of the pipe they replaced.
The only other water pipes under there are the heating ones suspended on the joists but the heating hasn't lost any pressure to indicate a leak, and the volume of water would indicate a massive problem if it was from the heating system.

Any ideas where i stand on this? It's clearly SW's problem if the leak is from the public network. I'm just concerned about the health issues here, what with sewage water plus now the pest infestation it's caused. Also how much of a mess getting is all sorted is going to cause?

 

[Ian H]

First you need to dye test their drain to see if anything comes through. I’d recommend the green dye.

I say test theirs first because if you test yours, say from your toilet, the dye will go through your private bit and their public bit so you would be non the wiser.

If you can put the dye directly into the first shared manhole you will prove if it’s theirs.

 

[Steve35]

An update on this situation.

Scottish Water came out and put a camera down the pipes to check it all out. They said it was fine and they also put dye down there too to see if any leaks appeared.
They had a look under the house at the water and said it was not from their sewer pipes. The guy said there is a "ground water" or "soil pipe" down there that should allow any water under the house to drain out in the ground but it can work both ways he says. He reckoned water in the soil from heavy rain had come back in the pipe and now it was stagnant, hence the flies using it as home.

Now I've been down there today and it definitely looks like sewage to me, there's wipes in it and it does not look like it just seeped through the soil. The pipe which the SW guy said was the cause is literally half into the ground and it's clearly had something coming back out of it from wherever it leads. What purpose would anyone have putting such a pipe through the foundation wall?

Thankfully all of that water is at least 1.5m below the floor of the house and the damp-proofing so not a direct issue with anything upstairs. The concern for me is what to do to solve this issue. That pipe is almost 2m underground and i'd have to dig a huge hole to get to it from outside, it seem to be much lower than the sewer pipe (to the other side of the wall on the left of the image, probably half way up it by my reckoning).
Can I just block it up, assuming it's not vitally important?
If it is somehow letting sewage in then that would indicate and unknown connection somewhere even the SW guys are not aware of?
What the heck can i do to tidy up even if I do get the pipe dealt with? That's a lot of water and muck under there and even the soil is soaked?

 

[Ian H]

You would be best digging a hole to put a pump in. Would they CCTV the pipe for you?

 

[Steve35]

So I've been digging holes all weekend and it looks like what's happened is with the constant blockages we've had on this sewer over the years the pressure of water has broken the connection for the gutter downpipe onto the main drain. The clay gully/hopper part has shattered into a million pieces by the look of it so any water coming down that way just misses the drain and pours into the soil. Eventually it seeps down through the soil and into the sub-floor area, aided by the pipe that's meant to keep the area dry.

The downpipe connects to the main sewer rather than just a soak-away as the SW guys could see the connection on their pipe on the CCTV.

I'm getting someone in to repair the broken part (which is probably original from the 60's looking at it) and it'll be digging a sump hole in the flooded area and putting a pump into the water to suck it all out.

Thank's for the help though.

 

[JohnD]

ah yes, clay gullies do that.

 

[Ian H]

I’d do the gully then keep an eye on it for a while before you fit the pump.