Joining a raft foundation to an existing building.

Depending on how soft the ground is you may be able to keep the attached edge of the raft foundation from moving with reinforcement, but the outside edge away from the joint may settle anyway no matter what you do unless you put in more extensive foundation work, piers or something similar or you may end up with an angled floor and structural cracks in the enclosure.

When you attach a raft foundation to a fixed wall with rebars you are in affect cantilevering the raft section. The rebars may keep it from breaking off but it will likely crack.

Don't do it! At least not if you can help it. A contractor did that to me through carelessness. My old house had a pier foundation. During rconstruction of an addition and pouring a new raft foundation he was supposed to cut and remove the old piers.

He failed to do so and every year one side of the house rose about 3" with the frost heave. There was a 17 foot thick claybelt underneath the foundation. The frost went into the ground about 4 feet and created considerable expansion each year.

I ended up with cracked walls and a buckled floor. Cost to correct the problem was considerable. The contractor declared bankruptcy so it was impossible to do anything financially.

If you allow differential settlement of the extension, you should construct proper deformation joint continuously running through the slab, floor, walls and roof. Any framing, joists or roof trusses adjacent to the joint should be doubled and a proper gap should be filled and/ or covered with expandable/movable details.

If you try to prevent differential settlement, you should build the extension foundation the same way/depth that your main house foundation had been built (and than of cause connect slabs with dowels)

Good Luck

Len

You would dowel the footings together, not the walls.

NO! Don't try to secure the two types to each other. You are going to need some serious reinforcing to try to stop any differential setlement, and then, it would only be localized, transferring any potential failure somewhere else, but you will get some kind of failure if there is any differential settlement. You WILL end up with a failure somewhere if you try to do this. Make provisions for the different foundation types with the construction methodolgy. You should be able to calculate the expected diff. settlement and make sure you account for it. The loads are likely small, and differential settlement is likely very small, as well, but I would not pin the two together in the concrete.

Differential settlement is a consideration, though usually substantially less than total settlement. The older foundation of the existing structure has likely completed much of its natural consolidation and settlement. The newer foundation will likely settle after construction. this means at the line of connection over time there will be an abrupt differential along those members that connect to the older structure e.g., doors, walls, floors, and roof. If the two structures are totally independant of each other such that they are fully separated, there is no cross connection and/or a change in vertical separation is not a consideration, then tying the slabs together should not be considered. However, if you want to connect the structures, you should mitigate for differential settlement and connect the foundations. With regard to reinforcement, it is typically more cost effective to try to mitigate excessive differential settlement geotechnically in the subgrade than stiffen the footings.

Hello guest,

You have not provided enough information to make an informed recommendation. We don't know where you are located geographically. Could you please respond to the following:

1. Do you have frost in your area?
2. Depth of frost penetration?
3. Are you in a tropical area?
   
4. Soil type below raft foundation...clay, silt, sand, gravel, other?
5. Has soil been disturbed?
   
6. Foundation type of adjoining building...raft, strip footing, pile, other?
7. What kind of structure will rest on the raft foundation...one level, two level, carport, garage, other?
   
8. Will the proposed structure be rigidly connected to the existing structure above the foundation?

Please respond to the above and CR4 may be able to offer some useful advice.

The existing building is on a raft foundation and I would like to extend this building using another raft.

1. We do have frost but very mild in South Africa

2. On the surface only

3. Semi Tropical - temperature range -4 deg C to 35 deg C

4. Mostly clay.

5. Soil was disturbed when the initial building was built.

6. The existing building is a raft foundation and the extension will be raft as well.

7. We will be extending size of the existing building - which is 8 one level semi-detached flats. The extensions are also one level.

8. Yes, preferably the extension will be rigidly connected.

I believe that the responders to your question appreciate the informative feedback from you.

It has been, however, nearly twenty years since this discussion was active.

I would also believe that, if this is a revived project and you are still looking for some advice, you will get more attention and more responses in a new discussion thread.

It would probably help if, within that new thread, you referenced this deferred discussion for reference.

Thank you very much for information - my apologies I seem to have missed the last posting date!

Thank you for the additional information. Joining a raft foundation to an existing structure can definitely require careful planning, especially when considering load distribution and long-term settlement differences between old and new sections.

In some projects, combining proper foundation design with a Bespoke Retaining Wall system can also help improve structural stability and soil retention around the extension area. It’s always interesting to hear how different engineers approach these types of foundation connections.

I posted on this forum originally because I had previously added a wall to an existing building which had a raft foundation. The technique used was to dig out the clay soil where the wall footing was to be made, filled then with contruction soil and digging the foundation in that. The area that was filled was approximately 200mm bigger than the footing of the wall. The wall footing was pinned to the foundation and the wall to the existing building with rebar. After about a year - the wall cracked between it and the existing building. Now that I wanted to extend the existing building, I knew that the technique would not work.
No matter where I looked for information, there was no definitive answer, and so I elected to employ a crude principle of keeping the two foundations and buildings seperate, and using expansion joints between the buildings - covered with aluminium cover strips (mounted on the existing building only) so that the expansion joints are hidden.