Preparation for paving terrace/driveway on clayey soil

Hi, on our property we have about 0.7-0.9m of topsoil and then comes cohesive soil (clay soil). I currently have two questions: 1. Is it necessary for the planning of the terrace and driveway/parking spaces to completely remove the organic soil? Or is it enough if I excavate to the extent that I have a proper frost and base layer + bedding and paving? That would already be a lot of soil that I would likely have to dispose of. 2. The surfaces are supposed to be permeable with a slope towards the lawn area. The cohesive soil is only conditionally permeable. Do you see a problem there and should I ensure targeted drainage of the rainwater? Our infiltration shaft is only designed for the roof areas.

We have similar soil, 1. depends on the terrace covering, the more paving stones the deeper (30-40 cm). We have WPC as a barefoot decking, 30 cm above the lawn, only 10 cm was removed after construction and pedestals. 2. slope towards the lawn

No idea how to give a qualified answer there.
0.7m topsoil, from where? From the top edge of the slab? From the bottom edge of the slab? From the top edge of the finished floor?

Just between the top edge of the finished floor and the bottom edge of the slab, there are already 30, 40 cm. And the house is usually founded on a gravel cushion, which extends 1-2m beyond the strip foundation and therefore ends at the bottom edge of the slab...

Basically, you should found frost-free, i.e. 60-100cm deep depending on the region, especially with clayey soil. The water does not seep away, it freezes, expands. Bad.

For a mailbox that may be overkill, for curbstones too. But for a terrace and especially parking spaces where you park an electric car fully loaded weighing up to 3.5 tons. There I would make a proper substructure.
The (more economical substructure with 20-30cm gravel) may all work, as long as there is only light frost. But woe if we experience a winter with 2-3 weeks of -10 or -15°C. Then everything is ruined.

Hi,

Regarding 1: If you reach 70–90 cm of topsoil, then for load-bearing or permanently stressed areas (like driveways or parking spaces) you definitely need to excavate down to the load-bearing soil. Topsoil is humus-rich, retains a lot of water, and constantly changes its structure with frost and moisture, which can ruin your pavement or even the driveway in the long run. So yes, for car parking spaces the entire organic soil should be removed, even if disposal is really annoying. For a terrace, depending on the construction, you might be a bit more lenient, but even there: if you want it to be stable long-term, better do it properly.

Regarding 2: Clayey soil and infiltration is a tricky thing. It can absorb some water, but very slowly, and once it’s full, it’s full. Especially during heavy rain, not much goes away. If you pave the surface water-permeably and slope it towards the lawn, that’s okay as long as your lawn can absorb it. But in the long run, it can lead to waterlogging or muddy areas. Directed drainage, for example via a swale or soakaway, can help here, especially if you notice the soil can’t keep up after heavy rains.

In short: Either “do it right” and lay a proper foundation plus water drainage from the start, or live with the risk that you might have to do rework in a few years. It depends on how permanent and load-bearing you need it to be.