Cost planning for earthworks

Hello Pinkiponk! Thank you for your detailed answer and sorry for my late reply...

In the meantime, a lot has happened on our side as well. So much that I would immediately accept your 15,000 Euro estimate. More specifically, we have a soil report for our building plot. One of the many drillings was carried out right next to our property (I know that might not say anything about our specific building ground, but it serves as a guideline).

The fact is that the difference between OKFmin and the current level is currently 1.30 meters. The soil has 30 cm of topsoil, which must be removed immediately. Another 50 cm are root residues, after which there are only "medium-firm" and "firm" layers, which are suitable for building (?). After consulting with the expert, at least the top 80 cm need to be removed and replaced (this is called soil replacement, right?), and the foundation is planned as strip foundations. With the 1.30 meter terrain elevation (minus 40 cm floor slab, so "only" 90 cm), I therefore have to fill up 1.7 meters with Z0 gravel/crushed stone (water protection area, no RCL). Is that correct, assuming the report applies to our property?

So I need 1.7 meters x 120 m2 = 204 m3 of Z0 fill material? And additionally, for the remaining 255 m2 of our property, an elevation of 1.30 meters, for which parts of the topsoil can be used.

Our architect, who unfortunately proved to be very uncommitted very early on, only said: Let it come as it may, you really can’t say or calculate all that. Please, dear readers, now seriously: We are planning about 25,000 Euros for the earthworks, 5,000 Euros more is of course annoying but not a "big" problem. But: Will this become a money pit? What will such soil replacement cost, the "special" Z0 material for water protection, the extra foundations with strip footing? Or am I totally exaggerating? Don’t get me wrong. I’m not asking for a price exact to the Euro, but an honest assessment from people who are not my architect.

We are building on a slab foundation, without a basement, in the south of Duisburg.

Thanks for reading and assessing.

I would like to add the following: The purchase agreement for the property (seller: City of Duisburg) explicitly states that additional foundation work is to be expected and that the buyer is aware of this at the time of signing. So a disclaimer by the city that I am responsible for everything that still needs to be done before I can start building...

Are you allowed to build a basement? Maybe just a crawl space? Would you have an upper limit for the additional costs (far from a money pit)? Is there an obligation to fill the remaining property with soil?

Hello Pinkiponk! Thanks for your detailed reply and sorry for my late response...

No reason to say "sorry, ...", it's clear that you have a lot going on at the moment. :)

...

Unfortunately, I cannot comment competently on the other figures and questions. Except maybe regarding the strip foundation. I don’t know the technical background, but in our house construction (currently) a strip foundation was initially discussed and then the builder switched to a "full slab (?)". I don’t know the reasons for this, I lack the experience. This at least resulted in the builder crediting us about 5,000 euros, but the excavation contractor charged about 15,000 euros more. That would be positive in your case. I don’t know the exact interplay, but with strip foundation versus slab it seems that more savings are made on earthworks with a strip foundation than vice versa. As for your height specifications, unfortunately I am a bit slow, I don’t even understand that on our own property. ;-)

I would like to add the following: The purchase contract for the property (seller: City of Duisburg) explicitly states that additional foundation costs are to be expected and that the buyer is aware of this at the time of signing. So a disclaimer from the city that I am responsible for everything that still needs to be done before I can start building...

Hello Hafenstraße, I have about 250 m3 of Z 0 soil to offer. The property is very close to you. You can reach me via gtgeorge79@gmx.de. Unfortunately, I cannot send you a PM.

Hello , hello to everyone else!

Great offer, thank you very much! Unfortunately, I cannot commit (yet) at the moment because we are just ordering the surveyor. I think our civil engineering work will only start in summer, even though it really hurts right now not to be able to accept such an offer immediately...

In the meantime, I have also received further information from the soil expert. Maybe you could help me with the individual formulations:

"As already explained in the phone call last week, the founding risk is already minimized with a load-bearing base slab.
1. QUESTION: Is a base slab always load-bearing?? Mine is from Weberhaus (Weberith), will be 20 cm thick, has 15 kg reinforcement per sqm, 120 mm perimeter insulation, and frost skirts (80 x 40 cm).

By building up about 90 cm on the current ground surface level, the need for any further soil replacement is eliminated.
2. QUESTION: Is that really the case? Does silty (= worse) soil not have to be removed if I just "simply" pile the good gravel, which I need anyway for raising the terrain, on top?

The topsoil must be removed in any case beforehand and can be reused in later green or garden areas. After removal of the topsoil, the terrain raise can take place on the ground surface level.
3. QUESTION: See question 2. The removal of the topsoil is clear, afterwards the terrain raise can take place on the silt (partly described as stiff, partly as medium dense) without me having to make any real excavation pit (except the topsoil)?

Before placing the fill, a geotextile should be laid to prevent the fill material from settling into the cohesive soil.
4. QUESTION: See questions 2 and 3. So the cohesive (= bad) soil actually remains and does not need to be replaced?
5. QUESTION: What costs does a geotextile cause?

It is especially important during construction to avoid unnecessary mechanical stress on the soil structure, i.e. it should be driven on as little as possible. Furthermore, the exposed formation on which the fill is to be placed should be protected as much as possible from moisture or waterlogging. The lowest layer of the fill should only be statically compacted to avoid activating capillary-bound water."
6. QUESTION: What does "statically compacted" mean?

Overall, this answer sounds very relieving to me. I like to remind you of my original fear, which probably hits every builder at some point. We are planning with 30,000 euros for the earthworks including the soil report. From your point of view, could this still become 50, 60,000 euros or more? Or can I now put this concern aside for the time being? I know there is no legally binding advice here. I do not want that either! But maybe someone has built on a similar plot and knows how much/little effort the above mentioned (especially financially) will mean...

Wishing you a good start to the week!