Soil Subgrade Modulus "K" Value ????? 1

[TekEngr]

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.

 

[oldestguy]

My reference says it can run between 50 and 150 pounds per cubic inch. I hope your allowable stresses in the building are selected with a better system than by making use of a general cook book value like this K value.

Your better undercut replacement material might run 300-500.

 

[Ron]

I would use about 50 pci for the K value if design a mat foundation. If pavement, I would use 100 to 200....but it doesn't make much difference in pavement design even though it is used.

 

[ATSE]

You can calculate the K yourself. What settlement do you expect at the center and the edges and the corners? What is the average total load?
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.

 

[Okiryu]

How thick is your soft clay layer? Depending on the size of your raft, stresses will extent to deep locations. Although you will be using the "k-value" to design the slab, you may also need to check settlements throughout the depth of influence to verify the overall performance of the raft (i.e. the slab can be well designed [structurally], but if excessive settlements can occur, then the performance of the raft will not be adequate).

 

[TekEngr]

after drilling the wells up to 3m at different location on site , same kind of reddish soil extracted with bearing capacity 0.5 to 0.7 kn/m^2
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

 

[BigH]

Thinking outside your "original question" - you have indicated a soft clay with an Allowable bearing capacity of 50 kPa - this implies an undrained shear strength of about 25 kPa. It also implies that your clay is normally consolidated. What is the loading your mat is having on the clay? How thick, as someone asked, is the clay? You may have mat settlement issues - with 10,000 m2 of floor (say 50m x 200 m) - this is a huge slab area - two football pitches in length and one in width or if 100 m x 100 m, a football pitch in both directions. The variability that you would expect across a site this large might in the soil properties could be huge (say a portion of the site was a fill dump for a while where the area might have been preloaded - hence portions might undergo larger settlements than other areas).

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".

 

[TekEngr]

thanks for your valuable reply. i don't have any point load all loads are UDL loads. loads transfers through light gauge walls not through columns on foundation slab.
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.