ᐅ Is a concrete slab foundation impossible without a geotechnical engineer?

Hello everyone,

For our project, a significant amount of fill was required, meaning the house will rest on an 80 cm (31.5 inches) layer of gravel, with an additional layer of up to 1.40 m (4 ft 7 in) of recycled concrete gravel underneath in some areas. The team did an excellent job compacting it and performed a dynamic plate load test. Result: Evd = 37.3 MN/m².

The concrete slab company (a major basement builder starting with K) is now refusing to proceed and is demanding values for frost depth, subgrade modulus, and stiffness modulus, which apparently only a geotechnical engineer can determine (???). I’m wondering why I would need a soil survey for a 2 m (6 ft 7 in) thick gravel layer? Can anyone shed some light on this, or is a soil report really unavoidable?

Best regards

Without being a professional, I can already imagine that this is true.

On the one hand, they have to provide a warranty in accordance with legal requirements, and on the other hand, you need a basis to be able to assert warranty claims.

The company is not entirely wrong. By the way, the compression plate test must be carried out every 30cm (12 inches). The reason is, of course, that someone takes responsibility to guarantee that there will be no settlement. Of course, you can also find a company that builds the foundation slab that way because "they have always done it like that." But the risk remains, especially with these construction heights. Just ask an engineering office what it costs. I think it will be in the low three-digit range.

Pixelsurium schrieb:

I’m wondering why a soil report is necessary for a 2 m (6.5 ft) layer of gravel? Can someone clarify this, or is it really unavoidable?

Who takes responsibility that the house won’t settle later and you don’t end up making warranty claims against the foundation contractor?
Of course, you could also build with just any local construction company, but that’s like buying cheap—twice. Be glad you have such a professional company by your side. I really appreciate how they work.

Hi everyone,

First of all, thank you for the quick responses. I already suspected that it’s a bit more complicated than I initially thought. I was under the impression that the soil surveyor examines the natural ground and then makes a recommendation on how much gravel is needed to ensure frost protection and stability. Since we have 2 meters (6.5 feet) of gravel on top, I assumed the soil wouldn’t matter anymore – so what’s the point of an additional report? It wasn’t about being overly precise, but rather about the usefulness of it. Basically, I thought that in this case, the surveyor wouldn’t do much more than plate load tests, which have actually already been done.

Best regards

As far as I understand, it is also important for the structural engineer, who is ultimately responsible for ensuring that the building does not "tip over."

Edit: haha, I just have to think of a rather famous tower. It was probably built without a soil survey. Most people know the outcome.
By the way, the tower is located in Pisa.