Small-strain vs. large-strain shear modulus - General Bearing Capacity Equation

[Eugene Lim]

The general bearing capacity equation requires a check of the foundation's rigidity index (I[sub]r[/sub]) against critical rigidity (I[sub]rc[/sub]). Shear modulus (G) value is required to determine I[sub]r[/sub]. Is it the small-strain or large-strain shear modulus that should be used?

 

[Okiryu]

I would use the large-strain shear modulus for foundation design. My understanding is that small-strains are used mostly for seismic applications.

 

[EireChch]

Which code requires this check?

"The general bearing capacity equation' to me is Terzahgis bearing capacity (modified by Meyerhoff or others), I have never seen G in a bearing capacity calc?

 

[Okiryu]

I am assuming that the OP is working on pile foundations and using the cavity expansion theory from Vesic...

 

[EireChch]

Ok thanks, still I dont think there is a general bearing capacity equation for piles!

OP might reply

 

[BigH]

Search Malcolm Bolton - he's written a number of papers on use of small strain . . .

https://www.up.ac.za/media/shared/1...ons-on-sand-masters-degree-report.zp58597.pdf

(Archer's masters thesis - see below for paper)

https://pdfs.semanticscholar.org/b639/5a1b6f8bb0536cbbbca912d24d0a489ab299.pdf

(paper by Archer)

 

[Eugene Lim]

Thanks for your responses!

EireChch: General BC Eqn. only works for the failure mode of "general shear failure." To check against other failure modes i.e. "punching shear" and "local shear failure," most codes require a check of the foundation's rigidity. I have seen 2 methods to undertake this check; 1 from Braja M. Das and 1 from Harry Poulos.

Okiryu: I understand why you prefer l-s G over s-s G. After consulting with Harry Poulos, I decided to use an average value. Normal ‘working’ load value, as being representative of somewhere between elastic and fully plastic behaviour. The large strain value would approach zero, while the small strain value would relate to the early part of the loading process.

Okiryu: This post is talking about footings not piled foundations. It refers to the rigidity index of footings. Not the cavity expansion theory for piles. Interestingly, Vesic addressed this issue i.e. the rigidity of footings and associated failure modes. If you have "Principles of Foundation Engineering" (2016) by Braja M. Das, refer to Figures 4.2 and 4.3.

BigH: Thanks for the links!!! Very pertinent. Thank you.