Natural gravel layer beneath the base slab

I had the subsurface conditions on our property investigated. Four soundings were carried out using a light percussion probe. The inspection report now states that a 20 cm natural gravel base (natural gravel layer) is to be provided beneath the slab to improve the soil. The total cost for this is €3,500 gross.

Since I am not familiar with this, I would be interested to hear what you think about it. What speaks for it, what against?

I cannot assess the term "nature" in "natural gravel layer" vs. just "gravel layer"

Arguments in favor
- that I don’t know of any concrete slabs without gravel at all (but I’m not a real expert)
- that the gravel can act like a drainage and surface water can fairly easily percolate through the gravel layer without the concrete slab being permanently submerged
- which is of little use if your groundwater level already reaches just below the edge of the lawn
- that the gravel basically represents a leveling layer in terms of subsidence and erosion
- that an expert was specifically called in who considers the gravel layer necessary
- no construction company will dare to do anything else if this report is available

Arguments against
- it costs money and is useless if the expert wants to rip you off
- do you believe that? if not, then the question about the purpose of the gravel layer is automatically obsolete and virtually already answered

I cannot assess the costs; for us, the gravel layer was included in the offer/in the concrete slab.

I don’t know it at all without it and wouldn’t build without it either.

The question is rather why the contractor doesn’t automatically include it in the contract? Conversely, I could imagine that many other things are also not included and you will probably have to expect additional expenses in several areas. For example, topsoil removal, trench excavation, excavation and transport of material, depending on soil class with surcharge, etc.).

Secondly, I wonder why a gravel layer should cost 3,500 euros. I find this amount extremely high.

... what do you think about it? What speaks in favor, what against?

What could possibly speak against it? You have a geologist prepare a subsoil report, he issues a foundation recommendation, and the builder tells you that this foundation will incur an additional cost of 3500 gross.

And now you ask us builders in this forum what we think about this professional advice.

Since I am not familiar,

That is exactly why you enlist the help of a geologist. The builder must also be able to rely on such a report—any construction that is built inferior to the geologist’s recommendation risks being permanently unstable.

You need to start trusting the experts. Opinions in a forum may vary, and occasionally fundamental expert knowledge can be encountered, but with such engineering tasks, you can’t simply say: "I’d skip that—you won’t see it later anyway... it’ll hold up fine without natural gravel... in my crystal ball, I see good soil and a geo-fool..."

And if we were all structural engineers and geologists here, we couldn’t say anything conclusive either, because we don’t know the soil!

You are right. I will have it done that way. After all, it’s about the foundation, and if an independent expert says it is necessary, then that’s how it is.