Rising damp at the base

Hello, I have the following problem:

On the east side of our house, the terrace was built higher and is one meter above the garden. Accordingly, one meter of gravel was filled in there. Now the base is somewhat damp over a length of three meters. In other words, two years after the completion of the house, slight efflorescences form on the plaster and the plaster chalks.

The terrace slopes down towards the garden, and since the roof has an overhang, rain actually never comes closer than half a meter to the house wall. So I suspect that the moisture is rising from below. The efflorescences are just slightly above the terrace level. A dimpled membrane was installed in the earth against the wall, and there is a 20 cm wide and deep gravel strip between the wall and the pavement. There is no basement in this area, only a deep strip foundation. My suspicion is that the dimpled membrane does not seal properly at the very bottom and that moisture penetrates somewhere through the gravel up to the base and then rises.

Now I do not want the terrace to be torn up and one meter of gravel to be excavated, thus ruining my garden.

My plan therefore: remove the gravel bed so that it lies about 10 cm below the terrace level. Use an angle grinder to cut a horizontal slot in the plaster until the insulation boards (Styrodur) are reached. Then fill the slot with bitumen so that the moisture can no longer rise. Refill the gravel bed = nothing visible of the bitumen anymore and then repaint the base. I know this sounds adventurous, but would this work?

Two years after completion? Contact your craftsmen, your developer, or [Generalunternehmer].

That's exactly the point. I have spoken with both the landscaper and the plasterer. The plasterer believes that the landscaper must have damaged the dimpled membrane, while the landscaper thinks the plasterer did not properly connect the dimpled membrane. After a long back and forth, both would be willing to step in, tear up the terrace, excavate the gravel one meter deep, and repair it from below. Problem: You can no longer reach that area with a mini excavator, and there is no space to store all that stuff. There are beds everywhere that would be damaged. Therefore, my idea, which would mean 1.5 days of work for me and cost nothing except some facade paint and bitumen mass, would be my favorite.

If this option made sense, the craftsmen obligated to ensure it would have suggested it! They wouldn't propose such a costly solution for them just for fun.