How can I lower the floor in the basement?

[Hibbi]

Hello!
I live in a 1.5-story brick house with a basement built in '58.
I am in the process of replacing the drainage pipes in the basement due to broken cast iron drains and also taking the opportunity to lower them as much as possible.
I am also planning to re-drain the house in the future and insulate with isodrän.

Now I'm considering insulating the basement slab too, but reality does not match the drawings.
According to the drawings, the slab should be "floating" and the outer walls + the heart wall should rest on cast beams. Attached are pictures of the drawings.
There is no reinforcement in the slab.

The basement walls "gråsuggor" are directly built on the basement slab, but slightly thicker where the outer walls are.

I wonder how I can reinforce them to be able to get more insulation under the slab without losing ceiling height.
I've read that some people underpin, excavating a bit at a time (max 1000 mm) from both sides of the wall and underpinning first a footing similar to those on the drawings, then casting the final part between the new footing and the wall with expanding concrete.

Is it risk-free to do so to avoid future settlements? There's only river sand under the slab.
Or do you have other solutions?

As I understand from the drawings, my brick walls are load-bearing since there are no wooden studs in the outer walls except those that support the insulation.
I have no drawing for the basement itself except what is shown in these.
The reinforcement drawing is for the entrance floor, stating a bit about the concrete quality used, same for the beam details.

 

[Charlotte Andersson]

It is quite a complicated operation to do what you are considering. It involves both ensuring the durability of the finished construction and making sure everything stays in place during the construction period. This is definitely something you need to consult a structural engineer to get a grasp on. If something is done wrong, it can have dire consequences for the building. This type of intervention in a load-bearing structure also usually requires a building permit due to the significant risks if done incorrectly.

 

[cpalm]

I think this page gave very good concrete tips:
https://www.husgrunder.com/renovera-husgrund/grava-ur-kallaren-och-isolera/

We only excavated one room. We consulted a foundation expert before we started digging under the walls and got the thumbs up - perfect ground conditions and no directly load-bearing walls, so it was trivial, which is probably not very representative.

We underpinned according to the "one piece at a time" method, and there weren't really any issues. What you have to keep in mind is that it takes a very long time (calendar time). We did one segment a week. It's also a challenge to achieve proper reinforcement when there are many short segments to be tied together.