Subgrade Modulus

[ztengguy]

Working on a mat foundation on rock. This is preliminary, and I was going to assume a k value of around 200psi. Im not sure if this is the best thing to use since its on rock. Due to the loading, I will get some negative bending in the mat, and im not sure this is the best approach to this design.
Any thoughts?

 

[dik]

I would wonder why you would have a mat foundation on rock... simple spread footings might be more appropriate.

Dik

 

[ztengguy]

its a detention type structure, so need a slab for sealing off the cells.

 

[dik]

I'm missing something... Can you have spread footings with granular material to an elevation slightly above the top of footing and a concrete SOG over? or if concrete walls, can you found these on rock, again with a SOG for the floor? A raft on rock, due to the stiffness of the rock will essentially be transferring loads directly from the walls/columns directly to rock.

If you really need a modulus of subgrade reaction... then check with a geotech... could be 1000 or greater...

Dik

 

[ztengguy]

Yea, I am leaning towards just some spread footings with a paved invert type thing under the cell. I was trying to get some prelim numbers out, and it would make sense the k value would be high.
Thanks