Help with construction contract

Hello.
My construction contract was agreed upon based on the Construction Code. The amount x was fixed in the contract as a lump sum price, which is considered a fixed price.
Now I have the following clause in the contract: "The lump sum price further assumes that the framework conditions for structural engineering and civil engineering works included in the scope of services have been taken as a basis. Insofar as the actual scope of services for structurally necessary work and/or the quantity of excavation deviates from the scope in the scope of services, the contractor is entitled to reasonable additional remuneration from the client."

Now it is the case that more cubic meters of gravel were needed for the civil engineering than was estimated in the scope of services. I have now received an additional agreement that I am supposed to sign, in which the additional gravel will be charged to me.
Furthermore, a new calculation of the statics has shown, without changing the floor plan, that more steel supports than agreed are now necessary. I assume that I also have to pay for this due to this clause.

I have done some research and found out that in the case of a lump sum (fixed) price, the contractor has to bear the costs for additional effort, even if it deviates from the scope of services.

I would be grateful for your advice.

Why shouldn’t what you signed in the contract apply - namely that the fixed price is not flat-rate like that?

Exactly for this reason, there is repeatedly the note that enough reserves should be available for additional expenses in case of unfavorable ground conditions. If this is not needed, then it can certainly still be used in the additional sample selection or for making the first special repayment.

Is there a soil survey report that was available to the construction company before accepting the contract? Otherwise, I also see it that way: if the contract was signed with this clause, then it is binding for both parties.

What do the underlying conditions look like?
Was the additional effort reported by the contractor before execution and confirmed by you?

That's actually not quite legal. Whether he needs 5 or 50 supports, fills a puddle with gravel, or is a quarry is his problem. The risk lies with the contractor here.

But in the end, what do you want to do... if you don't pay, he simply won't continue building, and you certainly don't want a legal dispute.