How can I improve clay soil?

Hello,

we have a new building and we have a problem with waterlogging in the garden

We have about 50 cm of topsoil (which is already clayey) and then about 1.5 m thick clay layer

The clay layer only lets water pass very slowly; we have about 1-2 days of waterlogging in the garden
Excess water can (as long as there is a slope) drain off via the infiltration trench but the soil also settles differently.

But in the soil, for example in the first 40 cm, the waterlogging is still naturally present and the plants do not like that at all

We cannot get into the clay layer with a rototiller, how can we improve the soil?
Is it enough to simply spread about 10 tons of Rhine sand over approximately 180 sqm and till it in with a soil activator?

What can we do?

Would it help if we planted deep-rooting plants for 1 year and then tilled again next year before planting the lawn?

If I interpret your post correctly, the house is already completely finished and now it is only about the garden area of the property? Or is it about the entire property?

The house is completely finished; only the garden is still an issue, which is causing us problems with waterlogging.

What can we do?

Since you cannot improve the topsoil at all – the uppermost 50cm does not matter after the heavy rain whether it consists of gravel, topsoil, sand, or anything else, if there is a massive barrier layer underneath. Option: Buy or borrow an earth auger (a hand drill can still work at that depth) and drill drainage holes through the clay layer (provided there is actually permeable soil there). Neighbors have done this before and had great success with it. Otherwise: Is drainage possible?

We have already made a few holes like that, but then we would have to make many holes and disposing of the clay is also not so easy. Drainage pipes could be laid and connected to the soakaway to dilute. How does the drainage behave in summer? Does the lawn turn brown and the flowers dry out in the places where the drainage is? Sand still has to be added anyway to loosen the topsoil.

How does drainage behave in summer? Does the lawn turn brown and the flowers dry out where the drainage is located?

Of course nothing happens (because of the drainage). Drainage does nothing else than remove excess water. If you had "normal" soil, excess water would also be removed - just into deeper layers. Normal lawn roots only go 15-30cm deep. When it gets hot in summer, it will turn brown anyway (unless you water properly).