Soil Subgrade Modulus "K" Value ?????
Soil Subgrade Modulus "K" Value ?????
(OP)
I am designing a raft/slab foundation on grade for a G+1 light gauge steel hospital building in SAFE software. The soil bearing capacity is 50 KN/m2 and soil is soft clay soil but to analyze this foundation in SAFE I need to know the subgrade modulus “K” value so any one can suggest the conservative K value range for this type of soil and bearing capacity????
One thing I would like to shear that I am planning to replace the existing soil with new special granular compacted material of 30 cm thick layer for soil improvement.
One thing I would like to shear that I am planning to replace the existing soil with new special granular compacted material of 30 cm thick layer for soil improvement.





RE: Soil Subgrade Modulus "K" Value ?????
Your better undercut replacement material might run 300-500.
RE: Soil Subgrade Modulus "K" Value ?????
RE: Soil Subgrade Modulus "K" Value ?????
K = pressure / settlement. Crude, but will get you in the ballpark. If 2,000psf and 1" settlement, then K = 14pci.
You cannot estimate K with only allowable bearing pressure.
My experience is that structural engineers over-estimate this property.
More complicated because of the stiffness of the mat foundation, but order of magnitude is what you're interested in.
After you run your initial SAFE analysis, check your settlement results. Do they match within 20% of the settlement value you used for your K calcs? If so, your K estimate was reasonable.
K is not constant across the entire mat (although that would make the modeling easier).
Also, short term K is different (higher by 50% to 150%) than long term K.
RE: Soil Subgrade Modulus "K" Value ?????
RE: Soil Subgrade Modulus "K" Value ?????
Bowles equation (120xqa) give me the subgrade modulus value is about 22pci which is very less then as suggested above (50 pci) so which value is best approach to design this raft/mat footing on grade.
i have one more question , as i mentioned above the bearing capacity is 50KN/m^2 and when i run the analysis on SAFE after applying the loads the maximum ground pressure is coming about 75KN/m^2 which is more then allowable for this reason i am planing to improve the soil by adding 300mm thick granular material layer with good compaction at site so does this approach is reasonable how much tentative percentage of bearing capacity will increase after adding the 300mm special granular material thick layer and with compaction.
slab is on grade
thickness of slab is 220mm
Area of slab is about 10000 sqm
RE: Soil Subgrade Modulus "K" Value ?????
The slab thickness is small which leads me to believe that it might be better to simply use spread footings for the columns (which I would assume would be at 20 to 40 ft spacings??) and then use a typical floor slab on a layer of sand and gravel bedding at least 500 mm thick. I do not think, personally, that I would look at your situation as a "unified" slab - either in theory or practice.
And I think you are incorrect in thinking about the suggested layer of granular in the concept of increasing the bearing capacity - it is there principally to offer a flexibility to the slab. But, again, I wouldn't be looking at this as a slab as you are.
In slab design, you might wish to look at the work of Poulos and others on raft stiffness and supporting deformation modulus (E) values. You can find numerous articles that discuss the design of rafts using the elastic approach which is, again, my personal view, a better way to approach than "k" values which are almost "pulled out of the air".
RE: Soil Subgrade Modulus "K" Value ?????
the maximum UDL load i have 60KN/m which give me soil pressure 75KN/m^2 which is more the the allowable for this reason i am improving the soils at site by compaction and adding new layers . up to 3 m i have the same kind of soil as i mentioned above.