Design raft foundation under core in the absence of subgrade modulus

[ENG_33]

I am going to design a raft foundation for the core walls in FEA software but subgrade modulus is not provided by geo report. How do I analysis and design it without it?
I can use hand calculation to analysis and design like a pad footing but my question is how do I get the moment applied to the foundation? Do I simply sum up the moments of each wall in the core obtained from my 3D model?
Thank you.

 

[Rabbit12]

I'd start by asking the geotech.

If that is not an option, you likely know the type of soil, find a reference with a range of values for your soil type and analyze on each end of that range (and maybe a little more and less) and look at your slab forces and bearing forces. I've done this for less critical installations. If this is something critical proceed with caution.

 

[JoelTXCive]

I have done on less critical slabs too. I use Staad, or Visual Foundation and the subgrade modulus is just 1 input that easily changed. You can do a bunch of runs at differing subgrade moduli and see what the results are.

I still have bearing capacity from the geotech though, and a soil report telling what the soil type is the foundation is sitting in.

 

[ENG_33]

Thank you.
The founding material is weathered rock. Bearing capacity and elastic modulus are provided.
I will try to ask the geo consultant to provide subgrade modulus.
If it is not obtained, for simplified calculation what do you recommend? I was trying to treat it like a rigid plate and apply the axial force, moment in both directions. Only question I have here is how do I sum the moment? Do I simply get local Mx and local My from each wall in my 3d model and then covert to global Mx & My and sum them up?

 

[JLNJ]

You could try to envelope the solution as suggested. Weathered rock will be pretty stiff which drives you bearing values up and your mat flexure down. A softer subgrade drives the flexure up and the bearing down.

That said, your design may end up pretty inefficient. Best to discuss with a good Geotech.

As far as the application of the loads goes, I would be tempted to model it all together since the raft behavior might affect the load path. If you are confident it does not, then just apply the reactions to the raft.