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Raft foundation in different levels

Raft foundation in different levels

Raft foundation in different levels

Hi all

The attached capture is shown my structure which have a basement under the part of it and the raft foundation slab (lower foundation) is considered as foundation system for this part while as the other part of structure is supported by raft foundation (upper foundation) in different level and this foundation is rested on the soil and there is no connection with the basement walls.

My inquiry is :

Should I do a vertical joint in the superstructure at the encounter location of the upper foundation with basement walls ? ( Please take a look to the attached capture )

Knowing that the primary geotechnical investigation is referred to that the soil in different level have the same properties and the allowable bearing capacity is about 350 KN/m2 .

Thanks in advance.


RE: Raft foundation in different levels

Without any details of the basement retaining wall or soil profile, I'd suspect a joint is required- very rarely are the soils at different levels so uniform as to preclude some differential movement (whether settlement or heave).

If you put the joint in and nothing moves, so be it. If you don't put the joint in and everything moves, big problem.

All the best,

RE: Raft foundation in different levels

Please don't post in more than one room. We look at many of these.

RE: Raft foundation in different levels

Hi @ Mad Mike

Thank you so much . I get your point .

Best Regards

RE: Raft foundation in different levels

Hi @oldestguy

I really apologize . But I thought this thread needs to clarified from both geotechnical and structural .

Thank you to draw me attention to that

Best Regards

RE: Raft foundation in different levels

Hi all

Actually, I expected that the excavation at the site will be also two levels. So the upper part will be rest on the nature soil .
I am thinking about delaying the upper part construction until casting the basement floor slab and the lateral pressure from the upper part should be considered when I design the basement wall. Exactly as Destru side .


RE: Raft foundation in different levels

I think you may or may not have a differential settlement issue.

How deep is the basement? One storey (3.5m deep approx?). You are unloading the soil in this area. Removing approx 60kPa of overburden(assuming a unit weight of 17KN/m3. If you are in a normally consolidated soft clay then that 60kPa difference in net loading may be an issue.

If you are sitting in a over consolidated stiff clay, then you may be loading along the recompression consolidation curve and settlement will be alot less.

As with every post on this forum, if you give more info regarding soil type, site investigation etc you will get better answers.

If architecturally you can get away with a construction joint then go for it. Like MM said, better to have it and not want it than to need it and not have it

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