Botched earthworks - excavation pit reaching groundwater

Hello everyone,

I am a bit desperate and have been racking my brain lately regarding the earthworks for a foundation slab.

Unfortunately, I was too trusting and believed an earthworks contractor who said that 1.7m of excavation was necessary (according to the soil report). But this turned out to be incorrect!!! He even dug deeper because he thought the clay soil was not load-bearing. Now groundwater has been encountered.
This too deep section (1/3) of the excavation pit was then filled back with soil!!! which, of course, came out again because luckily it was noticed. Afterwards, rubble was placed and on the remaining part (2/3) of the still moist pit, mineral concrete 0/32 was applied and compacted. Now a geotextile was laid, then ring earth electrode and another layer of mineral concrete/gravel 0/32. That is the current status. On average, now another 1.5m-2m of gravel must be applied.

Thus, the excavation pit had an estimated slope of 10 degrees at the base… but it is now almost leveled.
I have since terminated the cooperation with the earthworks contractor due to lack of expertise. However, this has cost me quite a bit of money because the down payment is lost.

Now my questions:

1. Is the slope at the base a problem? Or should it have been completely level?
2. Could the groundwater encountered underneath the rubble cause problems regarding settlements? (Clay/silt)
3. The subgrade was not evenly scraped off and was moist… Problem regarding compaction of the 0/32 gravel and settlements?
4. The excavation pit will remain open for 1-2 months. Problem regarding rain?
5. Can it be left as is because a huge gravel layer will come on top anyway or what can I do? Bring in a geotechnical engineer who also did the report?

I am already at my wit’s end and have just started.

Thank you very much in advance.

Best regards Steven

I was unfortunately so gullible and trusted an excavator that 1.7m of excavation was necessary (according to the soil report).

The excavator would have been a fool if he had accepted less against his better knowledge! And you would have been stupid to insist on that - because you do not have a degree to produce a soil report.

He even dug deeper because he thought the clay soil would not be stable.

The soil expert should have been consulted there.

Now my questions:

No matter what you ask, the answer is: get the soil expert. Because all individual questions can be answered with yes/no/maybe (except the topic of rain ==> it is normal for construction pits to remain open for many months).

Hello, it would be interesting to see what the soil report says about this. I myself am a soil surveyor, so my statements should of course be taken with caution since I have not seen the construction site, the entire process, or the report!

1. no (rainwater/surface water will accumulate more according to the slope, but it should not cause any problems) 2. no (is it really permanent groundwater or temporary layer water that occurs after rainfall? However, the "coarse aggregate" that was installed is the right choice.) 3. no (generally, the base should be dry and firm before the gravel is installed. With soft soils, you can also excavate more and additionally "press" gravel into the ground. The actual subgrade can then be built on this.) 4. no (a soil subgrade made of clay should never be left exposed; as soon as a layer of gravel is installed on it, it is considered "protected." If necessary, after the waiting time, it may need to be recompacted before installing the next layers.) 5. yes (in my opinion, it can stay as is; replacing clay soil with gravel is always better. With the cushion thickness in the end, I wouldn't worry about a single-family house. To be 100% sure, it is always advisable to consult the surveyor.

In general, the earthworker's approach does not necessarily seem wrong. If there is water in the soil, the soil often gets worse with depth rather than better. However, this should be clarified by the soil report. Soft soils generally need to be replaced. But if the earthworker did not know how the soil continues at depth, then of course they keep digging even when they are actually too deep. As I said, that is what the soil report is for: to adapt the foundation to the soil conditions.

... I am a bit desperate and have lately been racking my brains about the earthworks for a base slab.

You have already received competent information and answers from a professional, which I could not have given you as a non-expert. Regarding your description of your soil, it seems to me that it is "approximately" like our soil or the soil in our construction area. Clay, water at about 1.50m, etc. Maybe it will relieve you a bit to know that none of this is a problem. It is all solvable. Basically, and I had to learn this too, everything on a construction site is always solvable because there are usually several ways to the goal or different techniques.

Our soil survey and what our earthworks contractor did will probably not help you; otherwise, I would give you the documents.

I'm already at my wit's end and have only just started.

It will be fine. You can ask questions here anytime/day or night and will always receive helpful answers. Building is an excellent opportunity to learn how to deal with challenges in life. And it is also a lot of fun.

Who is building the house or the base slab? I know it like this: the house builder ensures that the base slab guarantees the safety of the house, and the base slab builder, in turn, ensures that the earthworks are carried out/have been carried out so that the base slab meets all safety and quality requirements set by the house builder.

Okay, thank you very much for your assessments, they mean a lot to me. Then I can be calm and continue to build on it. It would be best if I do bring the expert out after all.

The soil report shows up to 1.7m of fill - silt, slightly clayey, and afterwards silt, clayey - basically almost the same.

Thanks for the reassuring words, at the beginning not everything is easy, especially if you are not familiar with it. I tend to worry easily about things that might not be necessary.

Yes, I have a project manager from Glatthaar/foundation slab who reviews it and also gave the instructions. I probably decided too early to end the collaboration with the earthworks contractor, but I lost trust due to a few incidents. It will be alright :)

Thanks again.