[mod 2002/07/23]
	     NOTE:
             Plasticity yield criterion updated July 23, 2002 to include
             fluid pore pressure effects explicitly.
             Plastic yielding uses the Drucker-Prager criterion
             
                       1/2
                  (J ')    = p(1-lambda) X sin(phi) + c
                    2

             where J ' is the second invariant of the deviatoric stress
                    2   
                   p   is the pressure in the solid
                   phi is the internal angle of friction
                   c   is the cohesion
                   lambda is the pore pressure ratio (p      / p     )
                                                       fluid    solid
                                                       
	     In subroutine viscosities1, this appears as
	     
	       splas = epressure*(1.0-lambda_pore_prs)*dsin(phi) + coh
	     
	     (Old version was  splas=epressure*dsin(phi) + coh)
	     
	     phi, lambda_pore_prs, and coh are material properties
	     specified in input for each material.  phi,
	     lambda_pore_prs and coh can be specified to vary with
	     the second invariant of the deviatoric strain, here
	     termed "strain".