Significant water damage in the new building - but caused by what?

Good evening!

After living in our new house for almost 2 years now without any notable problems or defects, we had to unexpectedly discover a water damage in the basement the day before yesterday.
It "started" with a small, wet spot by the heating room door, meanwhile we know that water must be standing on the entire floor slab, i.e. under the screed, and it has already slightly risen into almost all walls.
I will try to describe the situation as compactly as possible but as detailed as necessary.

First, a few bullet points:

- Soil sample from summer 2020 showed that there is no groundwater or pressing water present
- The floor slab was made of 25cm waterproof concrete. Since it is a residential basement, the entire floor was additionally welded shut from the inside
- Because the basement is below the sewer level, we have a lifting system that pumps wastewater from a small bathroom (not in use) as well as from the basement hallway
- The drainage pipes lie under the floor slab and are led through it. The same applies to the vent pipe of the lifting system
- This consists of a sealed chamber, also under the slab, as well as a second chamber accessible from above. Normally there may be no water standing here at all, since everything is fed into and pumped out of the lower chamber. Ultimately, for us, it is purely an inspection opening
- The said bathroom in the basement is still in a shell condition because it is currently not needed. The shower area, roughly 1x1m, is completely free so that one can see the floor slab or tar paper
- Furthermore, the heating room and hallway are not yet tiled, so the screed is visible

Now, briefly about the sequence:

- After the spot on the screed was noticed and it was clear that it was not coming "from above," I opened the lifting system, which was half full of water. It was dripping in at the side. After we ruled out that it was a defective pipe (underfloor heating and wastewater from the descaling system run nearby), we found that water must be inside the screed or the insulation layers
- We then noticed that moisture had already risen on ALL walls, whether interior or exterior walls, but always only slightly above the screed
- Bad enough, but frightening was the view into the nonexistent shower of the bathroom, where the water stood nearly 10cm high
- We pumped everything out, initially into buckets, removing at least 150-180 liters of water
- More water kept coming; at the beginning, you could really see it flowing, fortunately it has decreased now. At this point, I think we are talking about approximately 250 liters of pumped water
- Both the plumbing company and the shell builder were on site, but so far we have not found the fault. A leak detector is supposed to come, but only at the beginning of the week

To exclude:

- Tap water -> The water pipes in the basement apartment are not connected at all and otherwise, the meter does not move if no consumer is running. Other water pipes are visible in the heating room and are not leaking etc.
- Heating water -> Probably not that much on the underfloor heating pipes in the whole house and the system keeps its pressure perfectly
- Lifting system -> Works perfectly. Pumps reliably at about half fill level and would sound the alarm and log everything precisely in case of an incident. There are no faulty entries in the last months
- Defective wastewater pipes -> We suddenly introduced a large amount of water and also switched off the lifting system and let the pipe system fill up. This took a really long time, but once it was full, we watched the level through a wastewater pipe for a while and it did not drop. So no leak
- Sewer backup -> Would already have to be noticed elsewhere and the water does not smell or anything

Unfortunately, it couldn’t be made much shorter, sorry...

In my opinion, slowly only groundwater remains, which presses up from below through the floor slab, which actually should not happen due to the double sealing.
Since the walls themselves are not wet, except just at floor level, I consider a defect at that spot unlikely.

After reviewing some pictures from the construction phase, we noticed that the pipes run through, except one, apparently have no sealing. On one, you see a kind of black collar, on the other two nothing is visible.

Do you think it is conceivable that water pushes up past the pipes? And can this really lead to such masses of water and the distribution over the entire floor slab? Since all walls are wet, the water must have reached everywhere.
Of course, it is also questionable whether all the water really stood ON the floor slab or if we brought along incoming water from the soil while pumping. Currently impossible to say.

By the way, the lifting system also has a rubber collar, which is inside the concrete. However, a corner was cut out here for the vent pipe.

Of course, we know that it has rained incredibly much in recent months and the groundwater has certainly risen strongly as a result. But as a layman, I just cannot explain this situation. Do 250 liters of water fit between the floor slab and screed?! Simply insane.

Thanks in advance for reading and possible tips. Best regards!

Without leak detection it will be difficult

Wow, how similar our stories are…
In my case, the feeder pipe of the pressure hose of the lifting system was not installed, and therefore groundwater was pressing into the basement there. Fortunately, they installed this feeder pipe in your case.
It actually looks as if the wall collar (that's what this sealing sleeve is called) is missing on the right (longer) green KG pipe. I'm not sure, because they also come in the same green as the KG 2000 pipe, so they can easily be overlooked.
The water can theoretically only come through one of the penetrations of the base slab. What about the white pipes, how deep do they reach into the base slab?
That one pipe where you suspect the missing sealing, where is it located today? I suspect that the screed is blocking the view of the bare slab there? Then I would have it opened up first. If that is already the cause, you can think about a remediation method. In our case, three methods were applied right away.

Base slab made of watertight concrete. And how is the connection of the walls? Could it perhaps simply be due to the rainfall of the last week that it leaks through the connection of the walls on the base slab?

What do the basement walls look like? Also WU concrete?

Are there photos of the finished (poured) basement from the outside?

Hello!

Oh my goodness, you really have a serious problem.... but still, happy and good new year!

Do you think it’s conceivable that the water is pushing up past the pipes? And can that really lead to such masses of water and distribution across the entire floor slab? Since all the walls are wet, the water must have gotten everywhere.

Yes, without any doubt. Groundwater can build up considerable pressure, it will come through every little crack. In my parents' house, I’ve seen water coming through the basement walls multiple times, it came like through a small pipe! That was the reason I uncompromisingly decided to build with a white tank, I don’t feel like pumping out the basement anymore.

At first glance, I just can’t understand how anyone could come up with the idea to put the connections through the floor slab in a basement. But the planner must have had some reasons....

If you were already able to pump out 250 liters of water, I assume that there is several times more stuck in the screed insulation. Getting that dry again will be a major challenge.

What kind of screed do you have? Cement or anhydrite? The latter would have to be completely removed anyway. With cement, you probably won’t avoid having to expose all the penetrations of the floor slab and reseal them. If you have underfloor heating in the basement, that ultimately means renewing the entire screed in the basement as well. In other words, you should prepare yourself for putting the basement back to its shell construction state.

Those were the bad news, now the good:

Since you’ve only been living in the house for 2 years, this is not your problem, but your general contractor’s. This should be covered under the warranty. Insist absolutely on the “major” renovation, even if it initially seems worse and more frightening to you (= basement back to shell state, possibly house uninhabitable for a few weeks).

He will resist with all his might (without knowing further details, I would estimate the damage sum somewhere between 25,000 and 50,000 euros, it could also be significantly more), but you have to fight that battle.

If it isn’t perfectly dried out and newly sealed again, sooner or later you will have massive problems with moisture and mold in the basement. And that after the warranty period has expired.

Good luck!

Andreas