Can drainage pipes be laid on clay?

Because mold forms in our basement despite ventilation and the walls are damp from time to time, the installed sands above the drainage were checked. Only partially, non-permeable clay was found in two test drillings; otherwise, the sand is permeable. What makes me suspicious, however, is that according to the layer protocol, the drainage was laid directly on the existing natural clay soil. The fleece was laid directly on it and only very little gravel was placed around the pipe on the inside – is this correct? So far, the drainage has never carried water in the three years since construction, which is also clearly visible on the pipes in the pump shaft. How can one test the drainage?

Hello,

Because mold forms in our basement despite ventilation and the walls are damp from time to time, the built-in sands above the drainage were tested. ...

What sustainable sense does that make, since water is known to only flow "downhill" voluntarily!
If walls become damp, this can fundamentally have two causes:
1. Internal moisture due to room use, condensation on the walls caused by insufficient ventilation.
2. External wetting due to lack of vertical sealing.

Regarding 2., there are usually two load cases: pressing and non-pressing water.
Depending on the load case, the necessary vertical seals are planned and executed accordingly!

One can certainly have different opinions about drainage systems.
Example:
The surrounding native soil is significantly clayey, thus little permeable to surface water.
At the construction site, a drainage with a permeable gravel layer is installed around the building. For surface water, a welcome "sink." The rising walls are considerably stressed by this.

Regards.

A drainage should also be cleaned regularly (flushed), otherwise it can become clogged and will no longer be effective. Do you also have enough slope in the drainage?

For the planning and execution of drainage, the rules of DIN 4095 from 1990:06 apply. The laying of a drainage pipe takes place within a filter layer, which on the one hand ensures that the incoming water and any carried solid components are not washed out together, but only the water is drained. On the other hand, this filter layer naturally also ensures that the drainage pipe does not become clogged during the work.

Whether a drainage system works can be easily determined by flooding the backfill of the excavation with a garden hose over a longer period of time and then checking whether any water is being discharged at all.

But one thing is very important: the drainage is not responsible for the tightness of the building. This is the task of the waterproofing.

However, if the moisture load in the basement is not permanent but only occurs at certain times, this could also indicate a dew point problem and not a defective waterproofing. Supporting this would also be the lack of water load on the drainage.

This moisture load would then occur more in spring/summer and rarely in winter.