Foundation recommendation after demolition of a semi-detached house from 1880

Hello everyone!

I would really appreciate an assessment of the following topic:

---- Initial situation ----

Semi-detached house bought from the family. Year of construction 1880. Outdoor area. Founded on rock stones. No slab foundation. Planned demolition of the semi-detached house and new construction without a basement (timber frame construction / then clinker brick facing (requirement by the building authority) on the same footprint. Not wider, not longer, not higher. (Requirement by the building authority, outdoor area!) Living space after the new construction: 126 sqm, 1.5 stories

--- Soil survey ---

2. Subsoil
The subsoil was examined through four driven core soundings (BS1 to BS4) each to a depth of 5.0m below ground level (GOK). The locations of the drillings can be found in the site plan, the drilling results are shown in the column profiles (Appendix 2). The starting heights of the soundings range from -1.81 m to -0.59 m relative to the auxiliary benchmark (HFP = manhole cover at the location marked in the site plan). From the sounding results, it appears that humus topsoils and fill soils as well as softened clay soils are present down to depths of 3.2m below GOK (BS1), 2.6m below GOK (BS2), 0.2m below GOK (BS3) and 0.6m below GOK (BS4). Below these are glacial soils (glacial clay, glacial marl) as well as locally medium dense sands. The glacial soils range in consistency from "soft plastic" (BS4: 2.4-2.8m below GOK), "soft to stiff" (BS3: 2.2-3.6m below GOK; BS4: 1.2-1.8m below GOK) and "stiff plastic" (BS1: 3.2-5.0m below GOK; BS2: 2.6-5.0m below GOK; BS3: 3.6-5.0m below GOK; BS4: 1.8-2.4m below GOK and 2.8-5.0m below GOK).

3. Groundwater
After completion of the sounding work, water levels from 1.14m below GOK to 2.28m below GOK were measured. These are near-surface groundwater levels subject to weather-related fluctuations.

4. Soil assessment / foundation
The near-surface soil layers in the area of the planned building represent a highly settlement-prone subsoil, so that deep foundation on small piles is recommended. The load-bearing soils (glacial marl with at least stiff plastic consistency) suitable for a deep foundation are present from depths of HFP -4.8m (BS1), HFP -4.5m (BS2), HFP -4.4m (BS3) and HFP -3.4m (BS4). The embedding of the piles must be carried out depending on the required pile load and considering the negative skin friction to at least 3m into the load-bearing subsoil.

For the protection of the slab against soil moisture or pressing water, DIN 18533-1: 2017-07 must be observed. Without drainage, a design water level of the top of terrain is to be assumed. In the transition area between the adjacent semi-detached house and the planned new construction, the guidelines of DIN 4123 must be observed for building protection. The excavation can be carried out with an open water management system. Sufficient disposal of surface water must be ensured.

---Uncertainty---

The structural engineer (not the structural engineer of our new building) who is supposed to certify the "structural stability of the remaining semi-detached house" required by the building authority for the demolition, has now stumbled over the planned pile foundation. Quote: "Since 1880 this house has stood on the rock stones and has settled. If a strip foundation is installed in the same place, that might also suffice and an expensive pile foundation might not be necessary. This should at least be examined, especially because it is going to be 'only' a lighter house in timber frame construction with clinker bricks."

What do you think about all of this? Many thanks for your feedback.

-The Project Landliebe-
instagram.com/projekt.landliebe

The soil expert has given his recommendation. If a structural engineer disregards the recommendation, he bears the responsibility. Ask the structural engineer (not the one from the house) if he can justify that to you. Because a deep foundation certainly also carries risks for the existing semi-detached house.

Did you demolish the foundation of the old house? You should be able to use it for the new building with the same dimensions, shouldn’t you? That is not only the best solution for the neighboring house.

The additional partition wall to the neighbor will probably be the problem.

The soil expert has given his recommendation. If a structural engineer overrides the recommendation, he bears the responsibility. Ask the structural engineer (not the one from the house) if he can calculate it safely for you. Because a deep foundation definitely also carries risks for the existing semi-detached house.

Did you demolish the foundation of the old house? You should be able to use it for the new building with the same dimensions, right? That is surely the best solution not only for the neighboring house.

The structural engineer sees it exactly the same way. Possibly a foundation on the existing rock stones because the house will have the same dimensions. The general contractor has just initiated the calculation with him. I am curious.

Unfortunately, your plan cannot be read on Instagram. Please upload the plan here as a jpg format. Is a concrete slab on strip foundation planned? Who supplies it? The timber house builder?