Calculated foundation behavior single-family house - tolerance range

Hello everyone,

Due to a complicated soil (backfilling of a former fire pond with construction material in the 60s) and the associated expected settlement, we are facing the decision for a new building whether to work with micropile foundations (approx. €23,000) or soil replacement (depending on depth and soil classification for disposal).

In both variants, different settlement behavior is expected (calculated):

1. Soil replacement with 0.5 m gravel cushion approx. 3.1 - 5.1 cm
2. Soil replacement with 1.0 m gravel cushion approx. 2.8 - 4.5 cm
3. Soil replacement with 1.5 m gravel cushion approx. 2.3 - 3.6 cm
4. Pile foundations approx. 1 cm

How much settlement is experience-based tolerable? We are building a solid wood house with 96 m2 floor area on a slab foundation.

If soil replacement in terms of settlement behavior would be within the tolerance range, we would have to try to specify the costs through a soil classification.

I am grateful for any assessment/experience.

Regards
Stefan

Option 4 seems best to me since the costs are clear. What do options 1-3 cost?

Option 1 would be almost cost-neutral except for gravel, as we need the same amount of excavation for Option 4.

Option 2 would be about 96 tons of excavation after deducting 0.5 m of excavation, with estimated disposal costs between €20-80 per ton, totaling approximately €1,920 - 7,680.

Option 3 with about 192 tons of excavation would have disposal costs of approximately €3,840 - 15,360.

Gravel costs about €20 per ton. With a density of 1.5 tons per m3, this adds roughly €1,440 for Option 2 (about 72 tons) and €2,880 for Option 3 (about 144 tons).

If the calculation is correct, soil replacement would always be more economical - provided the higher settlement does not matter. What do you think?

I would now calculate the whole thing in cm

for example Option 2; worst case 7,680 for 4.5 cm = to reach about 1 cm we would be at €34,560

then there are certainly estimated costs for the earthworks because you also have quite a range in Options 2 + 3

who guarantees you that it’s not 8 cm? in the end you have a house that sinks everywhere