difficult ground for construction - soil replacement?

Hello,

we have found a nice building plot, but have not yet purchased it. We are already in negotiations with construction companies (mostly general contractors).

This week, the soil surveyor was on site and carried out two drillings. We already knew that a building pit had been excavated there 15-20 years ago. The construction project apparently died, and the pit was refilled with the excavation material from other construction sites in the residential area. According to measurements, the depth of the backfill is between 1.70 m and 2.00 m. Below and outside the former pit, starting at 10 cm, there is clay soil mixed with stones, so a very firm building ground.

The (verbal) recommendation is a foundation with strip footing or construction with a basement. The latter should be executed as a [Weisse Wanne] due to possible groundwater.

The written soil report will not be available for another 1-2 weeks. However, another builder in the same residential area (building with basement) told me that he would also recommend soil replacement without a basement and without an old excavation. The soil is probably unsuitable for backfilling because of the high impermeability to water. He also installed drainage. His costs were €15,000.

My question now is whether you would also do a soil replacement including drainage if building without a basement?

Thank you very much

Marko

I would wait for the written expert opinion and the founding recommendation... in case of doubt, I would consult a structural engineer and not the neighbor -.-

But does the founding recommendation usually also say something about soil replacement? I thought it only said something like strip foundation and the depth for it.

For us, it said "good soil conditions" and "soil replacement 50 cm for boreholes A, B, C, for D 80 cm until topsoil is removed" or something similar - the type of foundation was determined by the structural engineer.