ᐅ How much additional cost is expected based on the structural engineer’s calculations?

Why can't I add clay to the basement wall once the basement is built?

Whether you can use contaminated soil on your property apparently depends on the region. In my case, no one took soil samples before I had the pile spread in my garden. Nobody cared about it at all.

Because stagnant water that cannot drain away is problematic, even with excellent waterproofing.
OK, one might think that clay soil also repels moisture, but it is not that impermeable. And then frost damage and undermining directly at the house wall are also problematic.

xMisterDx schrieb:

Why can’t I backfill clay against the basement wall once the basement is completed?

Because this will cause settlement around the house over several years. Usually, this is the area where walkways and similar features are installed.

In my case, a foundation base for a lean-to roof was built in an area with clay backfill underneath. The foundation shifted and tilted due to underground settlement five times in total and had to be repaired each time.

xMisterDx schrieb:

I’m always a bit surprised here. Foundations are never built directly on the existing soil, right? You always excavate to a certain depth, add gravel or sand, and then compact it. What would be the point of putting the excavated soil back into the foundation pit?

If you dig a 3m (10 ft) deep hole, you have to slope the sides. At least, unless your general contractor is like mine, who sloped the cistern walls vertically. But that’s another matter, especially when people are working inside. And anyone who builds formwork for the slab or installs basement walls also needs access from the outside. With a 1m (3 ft) working space, a 10x10m (33x33 ft) foundation becomes 12x12m (39x39 ft) – about 44% more excavation volume than just the base area, plus the volume for the slope.
Once the basement walls are in place, the space around is backfilled. But due to compaction and drainage requirements, it’s rarely done with the original excavated soil.

xMisterDx schrieb:

By the way, sometimes it’s cheaper that way. Around here, large trucks often drive by asking if you still need a certain pile of soil. They apparently load it up and sell it privately for a low price, bypassing landfill fees and such.

You have the wrong scale in mind. A 10x10m (33x33 ft) basement with 1m (3 ft) working space equals about 12x12m (39x39 ft), which at 3m (10 ft) depth is roughly 450 cubic meters (cbm). Add another 100-150 cbm for the slope. Natural soil weighs about 1.7 tons per cbm, so that’s easily 30-40 loads with a large semi-dump truck. The fill sand or gravel from the quarry costs about 2€ per ton, and they pay a high price to accept the soil first. This business model might work for topsoil but not for clay. Not to mention that excavated material is also sold privately via classifieds.