Lagrangian Equation of Equilibrium

As shown by (2.116), the equation of equilibrium in terms of the total stress for the reference configuration can be written as 

Let us decompose II into an effective stress II and a pore pressure P = PI such that 

Note that P is already given in (6.21). By substituting (6.27) into (6.24), we obtain the following Lagrangian equation of equilibrium under a finite strain field  

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