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Desai Yield Surface:

I have a hard time understanding the parameters of the model for soils. If I have three principle stresses, σ

_{1}, σ

_{2}, σ

_{3}. And R is defined as the

__Tri-axial strength in tension__, how would I define R.

Lastly can I just say that

I

_{1}= σ

_{1}+σ

_{2}+σ

_{3}

J

_{2}= 1/6 * ((σ

_{1}-σ

_{2})

^{2}+(σ

_{2}-σ

_{3})

^{2}+(σ

_{3}-σ

_{1})

^{2})