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(OP)
Dears,
I want to know the correct way to calculate the Soil sub-grade modulus, is the correct value is Kv = P/D where P is the allowable pressure and D is the corresponding allowable settlement or P is the ultimate pressure and D is the corresponding ultimate settlement?
Since i found in many soil reports that the Kv= FS*(B.C)/D where D is the allowable settlement

thank you

I use this graph to get a general understanding of K values (I use the K(30))
We would use this in a service level basis (not factored or ultimate loads).

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JAE's post is on point. But if you need a quick back-of-the-envelope type of calculation, which doesn't apply to rock or soils with cobbles, Bowles made an approximation in 1974 or something: Ks = 12(SF)qa k/ft3. Source: his textbook, but this article too: https://www.structuremag.org/?p=1239.

As an aside, how do you guys estimate an allowable bearing from a CBR? I used to have a handy little chart but lost it along the way!

I wouldn’t use CBR to allowable bearing pressure. One because I don’t consider it realiable and two because it would only be allowable based on shear failure. Settlement typically controls allowable bearing pressure.

JAE: what is the source of the figure you provided? I have a reference that includes a very similar figure but it is not identical to the one you posted.

Thanks Eirechch. Good to know. I think the graph I had years ago started with a known CBR value, would then approximate a subgrade modulus and from there would estimate approx allowable bearing. Can’t remember where or how I even got it!

Hokie93 - I've been asked that several times - got it from an older engineer back years ago - think it is from some textbook.

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JAE: In case you are interested, the figure appears to be taken from ACI 360R-92 (Design of Slabs on Grade), or an earlier edition. It is Figure 3.3.5 in ACI 360R-92. The original source is cited as a 1986 paper by B.C. Ringo ("Design and Construction of Concrete Slabs on Grade"), ACI SCM-11(86). The figure has been revised in ACI 360R-10.

Thanks, kissymoose. Nice reference.

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