Evaluation of a soil report for weak subsoil

Hello, I am new here and would like to introduce myself and my problem and hope that I am not asking for too much.

I am a German living abroad and live in Belgium. Here we now want to buy a plot of land and build a house.

A plot that we really like unfortunately does not have the best soil survey. It is one of the last free plots in the street (approx. 500m long and on both sides of the plot there are already houses, built in the 70s and 80s. There are also other newer houses in the street.

We are now considering whether the soil report advises against the purchase or whether everything can be satisfactorily solved for a few thousand euros more. We plan two floors with approx. 150m² on the ground floor and 120m² on the upper floor, of which 20-30m² are to be open space.

I have copied below the Google translation of the relevant part of the soil report from Dutch and adjusted it to (hopefully) better and correct German.

We do not plan a basement and the question that interests us is how much more expensive the construction of the foundation slab would be in such a case. I am very aware that many factors and also the location play a role here, but we are not looking for a precise quote but a rough estimate.

Requests to a structural engineer here on site are also running, but experience shows that this usually takes weeks rather than days and nice plots here go quickly. Therefore, we would be very happy about any reasonably well-founded estimate (x thousand euros extra for the foundations/foundation slab compared to a normal foundation slab).

More details or the entire report can be sent via private message if needed.

Thank you very much in advance,
Dirk

Legend:

R0 = street
Level of sounding borings:
S01 = R0 + 0.8m
S02 = R0 + 0.64m
S03 = R0 + 0.75m

Groundwater approx. 1.50m below the surface

4. Foundation advice
The foundation advice is based on a house with a full basement, a ground floor and one floor.
We suspect from the site observations that the original zone has been filled and/or disturbed. Since excavation and disturbed soil are not suitable for foundations, the presence and thickness must be visually checked before the start of work (e.g. by test trench or manual drilling).

4.1 General foundation slab at R -2.00
We consider a general foundation slab of 15 m x 10 m set at a relative level R: -2.00 m. The corresponding depths below the current ground level are:
S01: 2.80 m
S02: 2.64 m
S03: 2.75 m
We expect theoretically the compensation effect of the excavated soil with no settlement up to a load of 45 kN/m². This would theoretically be sufficient for an average house with a full basement, ground floor and one floor.
BUT the foundation slab would probably be situated just above the very weak clay-rich zone. It is not foreseeable whether a foundation slab can be properly executed on such soft soils. In any case, due attention must be given to reinforcement to absorb possible differential settlement.
In addition, during construction, the groundwater must be temporarily lowered. We refer to the "Guidelines for Drainage" Sept. 2009 (Drainage Working Group and BBRI). Lowering the groundwater in clayey or clay-rich soils is on the one hand difficult to implement in practice and on the other hand can lead to significant settlement with possible damage to adjacent houses.
We therefore strongly advise against building a house with a basement at this location. (Note: Not planned anyway, so that is good)

4.2 General foundation slab at R -0.20
Because it is not appropriate to place the foundations in the filled soil, we alternatively consider a general foundation slab set beneath this disturbed soil at a relative level R: -0.20 m.

Depths below the current ground level are:
S01: 1.00 m
S02: 0.84 m
S03: 0.95 m
If disturbed and/or filled soil is still present at these depths, further excavation must be carried out and refilled with suitable backfill material.
A settlement calculation theoretically results in the following results:
Table 4.1 - Settlement calculations for the general foundation slab at R: -0.20 m
kN/m²
Settlements in cm
at kN/m² S01 S02 S03
20 0.27 1.26 0.60
25 1.82 2.39 1.75
30 3.00 3.43 2.86
35 4.04 4.33 *3.84
40 4.98 5.14 *4.66

*: The effect of settlement theoretically goes deeper than the reached CPT depth.
The final settlements at a load of 35 kN/m² are expected according to usual standards for a general foundation slab to be a maximum of 5 cm settlement. This is sufficient for an average house with a crawl space, a ground floor and a floor.
Since the foundation slab is also located over a weak zone here, we recommend giving sufficient attention to reinforcement in order to compensate possible differential settlements and avoid damage.

Hello, first of all I am not an expert. I think the foundation slab needs to be reinforced with steel beams. +-15-20,000€ additional cost.

Thank you for your assessment. I had also thought about reinforcement, but it was and is difficult for me to estimate the amount of the price.

WARNING: If you don't know yourself, you should definitely not trust the statements of . More details can be found in topics he has also participated in.

Furthermore, the expert report states that further excavation and backfilling/compacting may be necessary. Just as an example: Back then, excavating 50 cm and backfilling/compacting alone cost me an additional 12,000 EUR – without a reinforced base slab. However, the house is also significantly smaller than your planned one, so except for temporal and regional price fluctuations, it is likely to become correspondingly more expensive. You can only be certain when you ask a deep foundation specialist.

Hello,

surely I do not rely solely on any statements here, and Einradbrot has clearly written that he is not an expert.

That was not what I meant. But feel free to read his other posts.